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Targeting ERK, an Achilles' Heel of the MAPK pathway, in cancer therapy

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The mitogen-activated protein kinases (MAPK) pathway, often known as the RAS-RAF-MEK-ERK signal cascade, functions to transmit upstream signals to its downstream effectors to regulate physiological process such as cell proliferation, differentiation, survival and death. As the most frequently mutated signaling pathway in human cancer, targeting the MAPK pathway has long been considered a promising strategy for cancer therapy. Substantial efforts in the past decades have led to the clinical success of BRAF and MEK inhibitors. However, the clinical benefits of these inhibitors are compromised by the frequently occurring acquired resistance due to cancer heterogeneity and genomic instability. This review briefly introduces the key protein kinases involved in this pathway as well as their activation mechanisms. We also generalize the correlations between mutations of MAPK members and human cancers, followed by a summarization of progress made on the development of small molecule MAPK kinases inhibitors. In particular, this review highlights the potential advantages of ERK inhibitors in overcoming resistance to upstream targets and proposes that targeting ERK kinase may hold a promising prospect for cancer therapy. © 2018 Chinese Pharmaceutical Association and Institute of Materia Medica, Chinese Academy of Medical Sciences.
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REVIEW
Targeting ERK, an Achilles' Heel of the MAPK
pathway, in cancer therapy
Feifei Liu, Xiaotong Yang, Meiyu Geng, Min Huang
Division of Antitumor Pharmacology, State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica,
Chinese Academy of Sciences, Shanghai 201203, China
Received 17 September 2017; received in revised form 11 December 2017; accepted 8 January 2018
KEY WORDS
Mitogen-activated protein
kinases;
Extracellular signal-regu
lated kinase;
ERK inhibitor;
ERK kinase;
Cancer therapy;
Drug resistance
Abstract The mitogen-activated protein kinases (MAPK) pathway, often known as the RAS-RAF-
MEK-ERK signal cascade, functions to transmit upstream signals to its downstream effectors to regulate
physiological process such as cell proliferation, differentiation, survival and death. As the most frequently
mutated signaling pathway in human cancer, targeting the MAPK pathway has long been considered a
promising strategy for cancer therapy. Substantial efforts in the past decades have led to the clinical
success of BRAF and MEK inhibitors. However, the clinical benets of these inhibitors are compromised
by the frequently occurring acquired resistance due to cancer heterogeneity and genomic instability. This
review briey introduces the key protein kinases involved in this pathway as well as their activation
mechanisms. We also generalize the correlations between mutations of MAPK members and human
cancers, followed by a summarization of progress made on the development of small molecule MAPK
kinases inhibitors. In particular, this review highlights the potential advantages of ERK inhibitors in
overcoming resistance to upstream targets and proposes that targeting ERK kinase may hold a promising
prospect for cancer therapy.
&2018 Chinese Pharmaceutical Association and Institute of Materia Medica, Chinese Academy of Medical
Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND
license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Chinese Pharmaceutical Association
Institute of Materia Medica, Chinese Academy of Medical Sciences
www.elsevier.com/locate/apsb
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Acta Pharmaceutica Sinica B
https://doi.org/10.1016/j.apsb.2018.01.008
2211-3835 &2018 Chinese Pharmaceutical Association and Institute of Materia Medica, Chinese Academy of Medical Sciences. Production and hosting by
Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Corresponding author.
E-mail address: mhuang@simm.ac.cn (Min Huang).
Peer review under responsibility of Institute of Materia Medica, Chinese Academy of Medical Sciences and Chinese Pharmaceutical Association.
Acta Pharmaceutica Sinica B 2018;8(4):552562
1. Overview of the MAPK pathway
The mitogen-activated protein kinases (MAPK) pathway, often
known as a cascade of protein kinases composed of RAS, RAF,
mitogen-activated protein/extracellular signal-regulated kinase
(MEK) and the extracellular signal-regulated kinase (ERK), is a
highly conserved signal transduction pathway in all eukaryotic
cells. The MAPK pathway is one of the best-characterized
signaling cascades that regulates a variety of normal cellular
functions, such as cell proliferation, differentiation, survival and
apoptosis, by transmitting signals from upstream extracellular
growth factors to diverse downstream effectors located in the
nucleus
1
. The activation of the MAPK pathway initiates from a
conformational change of RAS. Stimulated by upstream receptors,
guanosine diphosphate (GDP)-bound RAS (inactive) switches to
guanosine triphosphate (GTP)-bound RAS (active), which causes
the recruitment and activation of RAF. Activated RAF phosphor-
ylates and actives MEK, whose activation directly leads to the
phosphorylation of ERK. Activated ERK phosphorylates multiple
substrates ranging from kinases to transcription factors, and is
positioned as a key kinase that controls a large number of cellular
processes due to its rather broad nature of substrate recognition.
1.1. Components of the MAPK pathway
The principal upstream factor of the MAPK pathway is RAS
protein, a member of the small GTPase (guanosinetriphosphatases)
superfamily composed of more than 150 members. Members of
the RAS superfamily are divided into families and subfamilies
based on their structure, sequence and function. The ve main
families are RAS, RHO, RAN, RAB and ARF GTPases (Fig. 1).
The RAS family itself is further divided into 6 subfamilies (RAS,
RAL, RAP, RAD RHEB, and RIT) and each subfamily shares the
common core G domain, which provides essential GTPase and
nucleotide exchange activity. RAS is the most frequently studied
protein in the RAS subfamily. In humans, four RAS proteins have
been identied, including HRAS, NRAS, KRAS4A and KRAS4B.
The KRAS4A and KRAS4B are two isoforms of KRAS, produced
by alternative splicing of the same gene
2
. RAS is a GTP-binding
protein and serves as a molecular switch in a cycle between
inactive RASGDP and active RASGTP
3
. The GDP/GTP cycle
is regulated by RAS guanine nucleotide exchange factors (GEFs)
such as Son of Sevenless (SOS1) protein that catalyze the
formation of RASGTP
4
. Meanwhile, GTPase hydrolyzes RAS
GTP to RASGDP and consequently terminates the RAS signal-
ing, accelerated by the interaction of GTPase with GTPase-
activating proteins (GAPs) including p120GAP and neurobro-
min
4
. In response to extracellular stimuli, inactive RASGDP
converts to active RASGTP to promote the activation of several
downstream effectors. Activation of RAS stimulates various
signaling pathways, which includes the MAPK pathway, the PI3
kinase (PI3K) pathway and the Ral-GEFs pathway; among them
the MAPK pathway is the best characterized
5
.
RAF is the rst protein kinase of the MAPK pathway and has
three isoforms which are the paralogues ARAF, BRAF and CRAF,
sharing a high similarity of domain organization
6
. Active RAS
GTP binds to the N terminus RAF and leads to activation of RAF
7
.
RAF activation involves an array of complex processes that
include recruitment of RAF from plasma, the formation of a
RASRAF complex in the membrane, dimerization of RAF
proteins, phosphorylation or dephosphorylation of different
domains of RAF, disassociation from RAF kinase inhibitory
protein (RKIP) and association with scaffolding complex such as
kinase suppressor of RAS (KSR)
4,8
. The precise sequence of these
events has not been well elucidated, but the RAF dimerization is
widely considered a pivotal step
9
. After being activated, RAF
protein phosphorylates and activates MEK, followed by the
positive phosphorylation of ERK1/2. All three isoforms of RAF
are able to active MEK1/2 through phosphorylation, while B-RAF
shows the most potent activity
9
.
MEK is a central component in the MAPK signaling cascade.
MEK1 and MEK2 are tyrosine (Tyr) and serine (Ser)/ threonine
(Thr) dual-specicity kinases and share approximately 80% simi-
larity. Within the MAPK pathway, MEK1/2 are phosphorylated and
activated by RAF
10
. However, RAF is considered only a subset of
MEK1/2 activators, as MEK1/2 are also activated by multiple MAP
kinase kinase kinases (MAP3Ks) including MEKK1 (mitogen-
activated protein kinase kinase kinase 1), MEKK2/MEKK3 com-
ponent, MAP3K8 and the mixed-lineage kinases (MLK14; also
known as MAP3K9, MAP3K10, MAP3K11 and KIAA1804)
1114
.
The signals of these upstream activators converge at the level of
MEK1/2. On the contrary, MEK1 and MEK2 are the exclusively
specic activators of ERK1/2. Since ERK1/2 have been discovered
to regulate a large number of substrates, MEK1/2 serve a unique
role as crucial gatekeepersin MAPK cascade
10
.
ERK1 (p44) and ERK2 (p42) are the proteins encoded by two
splice variants of the same gene, and members of the MAPK
superfamily also include ERK3/4, ERK5, ERK7/8, Jun N-terminal
kinase (JNK)1/2/3 and p38, α,βand γ(ERK6) and δ
15,16
, all of
which have been shown to play roles in cancer. ERK1/2 can be
positively activated by MEK1/2 through phosphorylation of Thr
and Tyr residues, namely Thr202 and Tyr204 of ERK1 and
Thr173 and Tyr185 of ERK2, respectively. Previous studies have
suggested that ERK1 and ERK2 may have distinct functions for
proliferation, and RAS-induced transformation requires ERK2
rather than ERK1. ERK1 has been discovered to antagonistically
compete with ERK2 for MEK, which results in a weakening of
ERK2 signaling. It still remains unclear whether ERK1 and ERK2
have different substrates
17
. Inactive ERK1/2 are associated with
microtubules in the cytoplasm. Upon being phosphorylated by
MEK1/2, they translocate into the nucleus and activate a sub-
stantial variety of transcriptional factors
18
. ERK1 and ERK2 have
hundreds of substrates, many of which participate in key physio-
logical processes that control cell proliferation, differentiation,
survival and death
19
. The phosphorylation and activation of
transcriptional factors, including CREB, ELK-1, ETS, NF-κB,
c-Myc, and the stimulation of the 90-kDa ribosomal S6 kinase
(p90RSK) (Fig. 2). The association with scaffold PEA-15A to
enhance ERK activity is one of a few examples showing that
ERK1/2 regulates their substrates to carry out normal cellular
functions
1922
. Additionally, the activation of ERK substrates can
lead to feedback loops, which in turn regulate the ERK signaling
pathway either positively or negatively depending on the
substrate
23
.
1.2. Activation of the MAPK pathway
The activation of the canonical MAPK pathway is triggered by a
stimulating process in which the growth factors (such as EGF)
bind to the cell-surface receptors mainly tyrosine kinase receptors
(RTK, such as EGFR), resulting in the dimerization and trans-
phosphorylation of RTK
24
. This binding of growth factors to cell-
Targeting ERK, an Achilles' Heel of the MAPK pathway, in cancer therapy 553
Figure 2 The major downstream targets of ERK1/2 in the MAPK pathway. ERK regulates both cytosolic targets and nuclear transcription
factors, thus promoting proliferation, survival and other malignant phenotypes.
Figure 1 The major RAS family numbers. The RAS GTPase superfamily is composed of ve main families, RAS, RHO, RAN, RAB and ARF.
The Ras family itself is further divided into 6 subfamilies, RAS, RAL, RAP, RAD, RHEB and RIT.
Feifei Liu et al.554
surface receptors causes the formation and activation of receptor
complexes which contain adaptors including SHC (SH2-contain-
ing protein), GRB2 (growth-factor-receptor bound protein 2) and
GAB (GRB2-associated binding) proteins. Among these proteins,
those containing SH2 domains are recruited to specic phospho-
tyrosine residues. One of these SH2-containing proteins, GRB2,
constitutively binds to the RAS activator SOS
5
. Adaptor proteins
associate with the RTK-phosphorylated intracellular domains,
which can recruit GEFs to cell membrane, increasing the level
of RASGTP bound protein
4
. RASGTP is demonstrated
to directly bind to RAF protein, recruiting RAF from the
cytoplasm to membranes, which enables the RAF to be an active
kinase
8,25
. Activated RAF subsequently carries out a chain of
phosphorylation reactions to its downstream substrates, namely,
MEK and ERK, initiating the activation of the canonical MAPK
pathway
7
.
The ERK cascade is under extensively homeostatic control by
feedback loops (Fig. 3), the effects of which can be broadly
divided into two different types: rapid short-term effects and a
delayed long-term effect
26
. The rapid feedback mechanisms refer
to activated ERK1/2 in turn stimulating inhibitory phosphorylation
of their upstream factors and kinases such as MEK, RAF, SOS and
RTKs, which prevents signal propagation of this pathway
and maintains stable cellular functions
18,27
. Specically, ERK1/2
phosphorylate BRAF and CRAF to inhibit the phosphorylation
of MEK
28
. The delayed feedback mechanisms can be briey
described as the de novo expression of Sprouty (SPRY) proteins
and dual-specicity phosphatases (DUSPs) that regulate the
MAPK pathway by dephosphorylating ERK1/2. Namely,
SPRY proteins interfere with a RAF catalytic domain to inhibit
RTKs and SOS, while DUSPs dephosphorylate the p-T-E-pY
motif to inactive ERK
29
. In short, the feedback loops control
the ERK signaling pathway in multiple ways, and play an
essential role in maintaining cellular homeostasis in physiological
conditions.
2. Aberrations of the MAPK pathway in cancer
It has been widely appreciated that aberrant activation of this
pathway is closely linked to various kinds of cancers. Dysregu-
lated MAPK signaling leads to the occurrence and progression of
cancers via multiple mechanisms, particularly gentic altera-
tions
1,26
.RAS has been identied as an oncogene and is
mutationally activated in approximately one-third of all cancers,
with pancreas (90%), colon (50%), thyroid (50%), lung (30%) and
melanoma (25%) with high-ranking prevalence
30
.RAS mutants
encode mutated proteins that harbor single amino-acid substitu-
tions primarily at glycine 12 (G12) and glutamine 16 (Q16) in
human cancers (Table 1). These mutated proteins are GAP-
insensitive and constitutively GTP-bound, leading to stimulus-
independent and persistent activation of the downstream effectors.
Among the RAS family, KRAS is the most frequently mutated
isoform and occurs in more than 20% of all human cancers,
followed by NRAS (8%) and HRAS (3.3%), and no other
RAS mutation has been found
30
. The mutation types of RAS
Figure 3 Activation and feedback regulation of the MAPK pathway. The classical MAPK pathway is activated in human tumors by upstream
receptor tyrosine kinases (RTK) or by mutations in RAS, BRAF, and MEK1. RTKs activate RAS by recruiting adaptor proteins (e.g., GRB-2) and
exchange factors (e.g., SOS). RAS activation promotes the formation of RAF dimers, which activate MEK-ERK cascade through phosphorylation.
ERK pathway activity is regulated by negative feedback at multiple levels, including the transcriptional activation of DUSP proteins that
negatively regulate the pathway. ERK also phosphorylates and thus regulates CRAF and MEK activity directly. ERK, or its immediate substrate
RSK, also phosphorylates SOS at several residues, inhibiting its activity and thus negatively regulating RAS activity.
Targeting ERK, an Achilles' Heel of the MAPK pathway, in cancer therapy 555
proteins may be associated with tumor types; the NRAS mutations
have been identied in approximately 20% of melanomas, for
example
31
.
BRAF mutations have been widely identied in tumors, with a
signicant percentage (7%) of all human cancers. This mutation
is highly prevalent in hairy cell leukemia (100%), melanoma
(50%60%), papillary thyroid cancer (40%60%), colorectal
cancers (CRC, 5%10%), pilocytic astrocytoma (10%15%) and
non-small cell lung cancer (NSCLC, 3%5%)
32
.Themost
common mutation of BRAF refers to BRAF-V600E, which is
a point mutation at valine 600 to yield glutamic acid. The
BRAF-V600E mutation is notably prevalent in melanomas
(63%) and papillary thyroid carcinomas (more than 50%)
33
.
Oncogenic BRAF mutations lead to overactivity of its down-
stream effectors MEK and ERK
32
.
In terms of downstream kinases in the MAPK pathway, MEK
mutations have been mainly identied in melanoma
10
, and also in
ovarian cancer cell lines and gliomas
34,35
. Generally, all of the
upstream mutations can lead to ERK protein hyperactivation,
which is responsible for a series of ERK-signaling-regulated
substrate activation and consequently related to a wide range of
tumors
36
. For instance, overexpression of ERK can induce
modulation of anti-apoptotic molecules such as BCL-2, a protein
that is linked to drug resistance in some types of breast cancer
37
.
3. Inhibitors targeting the MAPK pathway
Targeting the MAPK pathway has attracted signicant interest in
cancer therapy. Efforts directly targeting RAS protein are believed
to be very challenging in spite of the promise shown by a few RAS
inhibitors in the early development stage. Clinical benets
achieved by BRAF and MEK inhibitors have shown that targeting
these downstream RAS effectors is a very promising approach for
therapies of cancers harboring oncogenic mutations in this path-
way
1
. However, patients treated with RAF or MEK inhibitors
frequently develop drug resistance. The resistance involves very
complicated mechanisms, including gene mutations occurring in
the targeted proteins, MAPK signaling interaction with PI3K
pathway, loss of functions in MAPK signaling feedback control
and abnormal alterations of tumor suppressor genes
38
. It has been
believed that single drug resistance can trigger multi-drug resis-
tance
39
. Given this situation, researchers continue to pursue
approaches that can reverse drug resistance, and develop combi-
natorial strategies to obtain therapeutic efcacy
40,41
.
3.1. Efforts in targeting RAS protein
RAS protein is a central regulator of growth factor-induced cell
proliferation and survival in cells, and is the upstream factor of the
ERK signaling pathway, PI3K pathway and RalGEFs pathway
41
.
The aberrant activation of RAS is closely linked with various
kinds of cancers. It is a signicant challenge to develop RAS
inhibitors, and three decades of this effort has not generated
clinically effective molecules so far
31
. This difculty is rstly
attributed to the tertiary structure of RAS protein, which is very
smooth and oppy, thus hardly providing a binding pocket for
small molecule inhibitors
42
. Moreover, oncogenic mutant RAS
proteins are insensitive to GTPase-activating protein-catalyzed
GTP hydrolysis, resulting in constitutively active GTP-bound
protein. The afnity of RAS protein for GTP is extraordinarily
high, and it is almost impossible to develop a competitive binding
strategy as a result
43
. These characteristics of RAS make directly
targeting it very hard to achieve, so that recent targeting strategies
are mainly on proteins that regulate RASGTP interaction or RAS
mutants. Other efforts also explore the therapeutic opportunity of
targeting SOS
44
, which plays a role in regulating the rate of GDP/
GTP exchange. Small molecules have been discovered to be able
to bind a unique pocket on RASSOSRAS complex, which
facilitates SOS-catalyzed nucleotide exchange and interferes with
MAPK signaling
45
.
Recently, a strategy that targeted KRAS mutation G12C gained
increasing interest. The G12C mutation refers to a glycine replaced
with a cysteine and it occurs frequently in lung cancers, roughly
50% of RAS-driven lung adenocarcinomas. Lim et al.
46
reported
that small molecule compounds, SML-8-73-1 and SML-10-70-1,
can selectively inhibit KRAS G12C mutant. Biochemical analysis
has shown that the inhibition of KRAS G12C with SML-8-73-1
restrains KRAS protein in an inactive state. Crystallographic
studies demonstrate that the inhibition of KRAS G12C disrupts
the effector binding regions switch-I and switch-II, which subverts
the native nucleotide binding preference from GDP to GTP. This
study validates KRAS-G12C as a targetable mutant.
Recruitment of RAS to the membrane is a crucial step of RAS
activation. This process requires a post-translational modication
Table 1 MAPK mutations in different cancers.
Cancer type Mutation type and rate (%) Major mutation site
Prostate cancer KRAS (90%) G12D, G13D, G12V, G12S, G12C
NSCLC NRAS (35%) Q61K, Q61R, C186F, Q61L, Q61K,
CRC KRAS (45%) G12D, G12V, G13D, G12C, A146T, F566L
BRAF (12%) V600E
Pancreatic cancer KRAS (90%) G12D, G12V, G12R, G12C,
Melanoma NRAS (15%) Q61R, Q61L, Q61K, Q61H
BRAF (66%) V600E
Bladder cancer KRAS (50%) G12V, G12D, G12C,
AML NRAS (30%) G12D, G13D, G12V, Q61H, A59E, A164T
Ovarian Cancer BRAF (30%) V600E, A747V, G464E, V226M
Papillary thyroid cancer RAS (60%) KRAS:G12R, NRAS:Q61R
BRAF (35%70%) V600E
NSCLC, non-small cell lung cancer; CRC, colorectal cancer; AML, acute myeloid leukemia.
Feifei Liu et al.556
such that farnesyl transferase adds a lipid tail on RAS, enabling it
to attach to the cell membrane. Targeting this transferase to
prevent RAS membrane recruitment was originally considered a
promising approach. This approach eventually failed as no farnesyl
transferase inhibitors present positive clinical effects
47
. An expla-
nation proposes that different isoforms of RAS protein such as
NRAS and KRAS, can be geranylgeranylated if the farnesyltrans-
ferase is inhibited, and these prenylated RAS proteins retain the
ability to localize to the cell membrane and are still functional
30
.
Recently, it has been revealed that correct localization and
signaling of farnesylated KRAS is regulated by prenyl-binding
protein phosphodiesterase δ(PDEδ), which sustains the spatial
organization of KRAS by facilitating its diffusion in the cyto-
plasm. The strategy of targeting tPDEδhas shown activity in
suppressing RAS activity and anticancer activity has been
observed in animal models
48
.
In short, RAS protein was believed to be undruggabledue to
the aforementioned reasons. Recent novel approaches that target
RASGTP interactions or the KRAS mutation are considered
promising strategies
49
. However, the effect of suppressing MAPK
signaling by these approaches is still limited to laboratory models
and is far from showing clinical effectiveness.
3.2. BRAF inhibitors and the BRAF paradox
As the downstream kinase of RAS in MAPK cascade, the RAF
family proteins play an important role in this signal transduction
9
.
Among the three RAF isoforms, CRAF was rst identied as an
oncogene and considered a potential target, as CRAF is docu-
mented as an important RAS effector
50
. Efforts in targeting CRAF
have yielded numerous pre-clinical compounds. In particular
Sorafenib (Nexavar, Bayer/Onyx), an orally-available compound
that was originally discovered as a CRAF (also BRAF) inhibitor
and lately identied to be a multikinase inhibitor, was approved for
renal and hepatocellular carcinoma
51,52
, for its anti-angiogenesis
effect.
Later efforts have highlighted the promise of targeting BRAF
for the treatment of BRAF-mutant melanoma, which validates
BRAF as a therapeutic target. Selective BRAF inhibitors, such as
vemurafenib and dabrafenib, have achieved clinical benets
53,54
.
Vemurafenib (Zelbraf, Roche/PLexxikon) was the rst BRAF-
V600E-selective inhibitor that entered clinical trials. It was
approved for metastatic and unresectable BRAF-mutated melano-
mas
55
. Dabrafenib (Tanlar; GlaxoSmithKline) was approved for
BRAF V600K-mutated metastatic melanoma in 2013
53
. Vemur-
afenib and dabrafenib gained desirable clinical efcacy in the
treatment of patients suffering BRAF-V600E and BRAF-V600K
melanomas, which yielded a signicant improvement in disease-
free progression and overall survival of these patients.
Though BRAF inhibitors have achieved clinical benets in the
treatment of melanomas, the extent and duration of treatment with
BRAF inhibitors is variable and the extremely high frequency of
emergence of drug resistance eventually leads to failure of the
treatment using BRAF inhibitors
56
. For example, recent studies
have shown that all ATP-competitive RAF inhibitors, including
vermurafenib, dabrabenib and sorafenib, could lead to paradoxical
activation of the MAPK pathway in BRAF wild-type cells. The
paradoxical activation of MAPK pathway is an intriguing phe-
nomenon that primarily results from conformational changes such
as RAF dimerization and transactivation enhanced by RAF
inhibitors
57,58
. This paradox causes various degrees of adverse
effects, which are mainly benign skin tumors including squamous
cell carcinomas and keratoacanthomas
59
. The paradox-induced
skin tumors have an uncharacteristically high incidence of RAS
mutations, raising the concern that the same mechanism might
accelerate progression of other RAS-driven cancers. Also, studies
demonstrated that paradoxical activation of ERK signaling can
promote tumor growth in both RAS-mutated tumors and BRAF
tumors
60
.
Furthermore, efcacy of selective BRAF inhibitors is limited to
BRAF-mutated metastatic melanoma, despite the fact that the
BRAF mutation has been identied extensively in carcinomas
such as CRCs, thyroid cancers, glioblastoma and NSCLC
60
.
Facing the challenges of drug resistance and paradoxical activation
encountered in the treatment of BRAF inhibitors, researchers
attempt to seek solutions with combination usage of inhibitors of
BRAF and other pathways. There are reports suggesting that the
insensitivity to RAF inhibitors could be driven by the EGFR-
mediated MAPK signaling reactivation in BRAF-mutant CRCs
61
.
Combination of EGFR and BRAF inhibitors in BRAF mutant CRC
cells have been shown to be able to block the reactivation of
MAPK signaling and improve the therapeutic efcacy in vivo
62
.
3.3. MEK inhibitors
The MEK1/2 kinases in the MAPK pathway were not considered
potential targets in the past, as MEK1/2 are rarely mutated in
human cancers
63
. Lately, with the emergence of the paradoxical
phenomenon of the rst generation of RAF inhibitors, targeting
MEK1/2 has attracted growing interest among pharmacological
researchers
10,64
. The rst MEK1/2 inhibitor PD098059 was
reported in 1995, which was shown to inhibit the dephosphory-
lated form of MEK1 and MEK1 mutant (S217E, S221E). This
compound is an allosteric inhibitor and its discovery indicated that
MEK1/2 are valuable cancer drug targets
65
. Trametinib (MEKi-
nist, GlaxoSmithKline/Japan Tobacco) is the rst MEK inhi-
bitor to reach the market, approved as a single agent for BRAF
V600E metastatic melanoma in 2013
66
. Furthermore, targeting
KRAS mutant cancers with dual inhibitors is under extensive study
and recently shown to be effective
67
. The combination of MEK
inhibitors and PI3K-AKT pathway inhibitors was found to
increase progression-free-survival (PFS)
10,67
. Also, the combina-
tion of MEK inhibitors and rst-generation BRAF inhibitors are
better tolerated than the respective mono-therapies, because the
paradoxical activation of MAPK signaling in BRAF wild-type
tissues antagonizes the inhibitory functions of MEK inhibitors, and
in turn limits this paradoxical activation
67
. FDA also approved the
combination of dabrafenib and trametinib for BRAF-V600E/K-
mutant metastatic melanoma in 2014. The combination of the RAF
inhibitor with the MEK inhibitor cobimetinib
40
(GDC-0973,
Genentech) led to simultaneous suppression of both the melanoma
and RAF-inhibitor-induced leukemia.
However, as with other small molecule inhibitors, patients
treated with MEK inhibitors also develop drug resistance within
several months
10
. The primary sensitivity of MEK inhibitors
correlates with the decreased expressions of cyclin D1 (CCND1),
p27 (KIP1) and cell cycle arrest
63
. In the presence of MEK
inhibitors, feedback reactivation of MAPK signaling seems to be
Targeting ERK, an Achilles' Heel of the MAPK pathway, in cancer therapy 557
consistently stronger in RAS mutant tumors than BRAF-V600E
tumors, with reasons that are not entirely understood. ERK
feedback regulation is also considered to be related to intrinsic
resistance to MEK inhibitors in oncogenic KRAS-mutant cells
68
.
3.4. ERK inhibitors
Compared with the progression and clinical application of RAF
and MEK inhibitors, the development of ERK1/2 selective
inhibitors lags far behind. This might be due to an earlier
assumption that ERK is the only downstream target of MEK,
and so no additive benets were expected over MEK inhibitors
69
.
However, discovery and development of ERK inhibitors has
gained an increasing interest with the difculties faced by RAF
and MEK inhibitors, as well as with the deeper understanding of
the MAPK pathway.
3.4.1. The advantage of targeting ERK kinase
Despite the exciting anti-tumor activities and survival benets
achieved by the approved RAF and MEK inhibitors, the unavoid-
able drug resistance has become the main challenge of designing
and developing novel inhibitors targeting MAPK pathway
69
. The
underlying mechanisms, which generally stem from cancer hetero-
geneity and genomic instability, are mostly related to the com-
pensatory activation of upstream components. More extensive
exploration of the biology of the MAPK pathway has elicited the
proposition that targeting the downstream kinase ERK, as well as
combining ERK inhibition with RAF and MEK inhibitions would
be benecial.
ERK sits at a unique position in the MAPK pathway, as the
upstream molecule RAF has very few effectors besides MEK,
which has no substrate other than ERK; and ERK is the only
activator that is able to stimulate the wide variety downstream
substrates
1
. ERK1/2 inhibitors can reverse the abnormal activation
of MAPK pathway induced by upstream mutations including RAS
mutations
61,70
. ERK is not only downstream of RAF, but also a
negative inhibitor of RAF
23
. Furthermore, accumulating evidence
suggests that subtle differences in the spatio-temporal activation of
ERK generate variations in signaling outputs that regulate biolo-
gical responses. Moreover, crosstalk between ERK and other
pathways has been shown to be crucial for determining cell fate
18
.
For example, a recently discovered oncogenic factor O-GlcNAc
was linked to classical ERK signaling which is essential for the
maintenance of the malignant phenotype of cancers
71
. All these
have emphasized the potential benets that can be gained by
targeting ERK kinase in mutant MAPK pathway cancers.
More importantly, ERK inhibitors are able to overcome the
acquired drug resistance induced by upstream kinases inhibitors
(Fig. 4). Substantial studies have indicated that the reactivation of
the MAPK pathway is a crucial event of acquired resistance of
BRAF and MEK inhibitors
38,61,62
. The occurrence of acquired
drug resistance to MAPK pathway inhibition involves a series of
complicated mechanisms, which mainly include wild-type BRAF
amplication, BRAF-V600E amplication and MEK1/2 amplica-
tion or mutation
38,64
. Selective ERK inhibitors were reported to
reverse acquired resistance to MEK inhibitors
70
as well as double
drug resistance to BRAF and MEK inhibitors
40,64
. It is believed
that ERK inhibitors may be less sensitive to resistance mechanisms
than inhibitors of the upstream molecules in MAPK pathway.
Thus, targeting ERK is considered to be more effective than
Figure 4 Therapeutic potential in cancers that are resistant to MEK and BRAF inhibitors. Resistance to BRAF inhibitors can occur through
various mechanisms, including activating BRAF mutations and BRAF amplication, which can be overcome by both MEK inhibitors (MEKi) and
ERK inhibitors (ERKi). ERK inhibitors have the advantage to further overcome resistance to MEK inhibitors that occurs upon MEK mutation.
Feifei Liu et al.558
targeting BRAF or MEK in various forms of acquired resistance
64
.
As such, combinational usage of ERK inhibitors and upstream
inhibitors has become an important strategy to overcome acquired
resistance and optimize therapeutic efcacy. For example, one of
the selective ERK inhibitors, SCH772984, was found to re-
sensitize tumor cells after the emergence of resistance to BRAF
inhibitors or MEK inhibitors, offering the rational for combination
of ERK inhibitors with inhibitors of BRAF or MEK
72
. Another
study demonstrated that dual inhibition of MEK and ERK
synergistically inhibited the emergence of resistance and overcome
acquired resistance to MEK inhibitors
40
. Indeed, apart from the
MAPK pathway per se, in many cases of drug resistance induced
by upstream inhibitors, ERK inhibitors was discovered to be
capable of retaining their ability to suppress the MAPK pathway
and overcome drug resistance. It has been shown that acquired
resistance to PLX4032 developed by mutually exclusive PDGFRβ
(also known as PDGFRB) upregulation or NRAS mutations, is
mediated by the reactivation of the MAPK pathway, and can be
reversed by downstream inhibition
73,74
.
3.4.2. Current status of ERK inhibitors
According to the data from Thomson Reuters Integrity (Table 2),
only two ERK inhibitors that are BVD-523 and GDC0994, have
reached clinical trial until very recently a few more inhibitors are
reported. Considering the fact that ERK1 and ERK2 were founded
three decades ago, the design and development of ERK inhibitors
has lagged far behind compared with the upstream inhibitors.
BVD-523 (Ulixertinib, BioMed Valley) is an ATP competitive,
kinase selective inhibitor. The IC
50
against ERK1 and ERK2 are
300 and 40 pmol/L, respectively. BVD-523 showed its anti-
proliferative activity in cell lines with activating BRAF mutations
such as Colo205, as well as KRAS mutant colorectal and
pancreatic cell lines. Importantly, BVD-523 is effective in several
models that show intrinsic or acquired resistance to other MAPK
pathway inhibitors. BVD-523 inhibits growth in wild-type cells
and a RAF/MEK cross-resistant cell line bearing a MEK1-Q56P
mutation with similar potency. Single-agent BVD-523 inhibits the
growth of a patient-derived tumor xenograft harboring cross-
resistance to dabrafenib, trametinib, and the combination treatment
following clinical progression on a MEK inhibitor. BVD-523 was
found to be efcacious in patient-derived xenografts resistant to
vemurafenib
75
. Being consistent with its mechanism of action,
strong pharmacodynamic effects were observed against p-ERK
and downstream substrates. BVD-523 is currently the most
advanced ERK inhibitor in clinic. BVD-523 is in the recruitment
stage of two phase I/II clinical trials for solid tumors and
hematologic malignancies, which have completed dose-escalation
studies. It is also in phase I/II stage for acute myeloid leukemia
(AML) or myelodysplastic syndromes. A phase I/IIa trial results
recently released at the annual meeting of the American Society of
Clinical Oncology (ASCO) in June, 2017 observed 2 patients with
partial responses (3/27, 11%) during dose escalation and an
additional 11 partial responses (13%) were observed during the
expansion part of the trial, including 1 patient with melanoma
resistant to BRAF/MEK inhibitors. Side effects are comparable to
those seen with MEK inhibitors. BVD-523 received Food and
Drug Adminstration (FDA) Fast Track designation in September
2015. Phase I clinical trials in pancreatic cancer in USA are
underway as well.
GDC-0994 (RG-7842, Genentech/Array) is a very potent dual
inhibitor of ERK1/2 (with IC
50
's of 1.2 and 0.3 nmol/L,
respectively). GDC-0994 has shown promising combination activ-
ity with cobimetinib in RAS-mutated cancer cell lines and animal
models. GDC-0994 is now in the recruiting stage of a dose
escalation trial in patients with locally advanced or metastatic solid
Table 2 Current status of ERK inhibitors
a
.
Phase Drug name Organization Indications
Biological Testing FRI-20, ON-01060 Onconova, Temple University Cancer
Biological Testing VTX-11e National Institutes of Health (NIH) Cancer
Preclinical 25-OH-D3-3-BE, B3CD,
bromoacetoxycalcidiol
Aphios, Boston University School of
Medicine
Neuroblastoma; Cancer,
prostate
Preclinical FR-180204 AstellasPharma Rheumatoid arthritis
Preclinical AEZ-131, AEZS-131 AEternaZentaris Cancer
Preclinical AEZS-136 AEternaZentaris Solid tumor
Preclinical SCH-772984 Merck & Co. Cancer
Preclinical AZ-13767370 AstraZeneca Cancer
Preclinical BL-EI-001 Sichuan University, Tsinghua University,
Shenyang Pharmaceutical University
Cancer
Phase I LY-3214996 Eli Lilly NSCLC, pancreatic cancer, CRC,
melanoma
Phase I LTT-462 Novartis NSCLC, melanoma, ovarian
cancer, NSCLC
Phase I KO-947 Kura Oncology, Araxes Pharma Cancer
Phase I (Terminated) CC-90003 Celgene Cancer
Phase I (Terminated) GDC-0994, RG-7842 Genentech; Array BioPharma Solid tumor, NSCLC, CRC,
melanoma
Phase I MK-8353, SCH900353 Merck Sharp & Dohme CRC, NSCLC, melanoma
Phase I/IIa BVD-523, Ulixertinib Biomed Valley Discoveries, Vertex Acute myeloid, solid tumor,
melanoma
NSCLC, non-small cell lung cancer; CRC, colorectal cancer.
a
Source from Thomson Reuters Integrity.
Targeting ERK, an Achilles' Heel of the MAPK pathway, in cancer therapy 559
tumors
76
. Combination of GDC-0994 and MEK inhibitor cobime-
tinib shows enhanced anti-tumor activity in KRAS and BRAF
mutant tumor models
40
.
SCH-772984 is a highly potent (ERK1/2 IC
50
are 4 and 1 nmol/
L, respectively) kinase-selective compound. Similar with VTX-
11e, SCH-772984 was found to inhibit p-RSK, yet unlike other
ATP competitive ERK inhibitors, it also showed a distinct ability
to inhibit phosphorylation of the activation loop of ERK by MEK
in A375 melanoma cells. It is noteworthy that SCH-772984
inhibits both the kinase activity of its target as well as prevents
its phosphorylation by upstream effectors, analogous to the ability
of RO5126766 to both inhibit MEK and prevent its phosphoryla-
tion by RAF
77
. In additional-cell based studies, SCH-7772984 was
found to strongly inhibit the growth of a vemurafenib-resistant cell
line derived from A375 cell line with an acquired KRAS-G13D
mutation
78
.
AEZS-136 is a highly potent and selective ATP-competitive
ERK inhibitor, which can overcome the RAF inhibitor-induced
paradoxical cell activation and acquired resistance to MEK
inhibitors in tumor cells. Compared with common RAF inhibitors,
AEZS-136 shows a more potent capability to prevent BRAF wild-
type, BRAF-V600E mutant, RAS wild-type and KRAS mutant
tumor cell line proliferation. AEZS-136 is efcacious in MEK
inhibitor resistant HCT-116 and MDA-MB-231 cells which have
been well characterized in terms of the MEK-F129L allosteric
binding pocket mutation, as well as with varying degrees of KRAS
amplication, and in cellular proliferation assays and MAPK
pathway phosphorylation studies
79
.
(S)-14k is an ATP competitive ERK inhibitor and an orally
bioavailable agent which inhibits tumor growth in mouse xeno-
graft models. On the basis of its in vivo efcacy and preliminary
safety proles, (S)-14k was selected for further preclinical evalua-
tion
72
.(S)-14k (Array) and VTX-11e (Vertex) are both analogs
with structural similarities to RG-7842. The binding mode of
VTX-11e was determined by X-ray co-crystallography with ERK2
and involves key interactions between the hinge NH and main
chain carbonyl of Met108 with the amino pyrimidine. Identied
using structure-based drug design, VTX-11e was selected for more
extensive proling due to its superior potency against ERK2 (K
i
o2 nmol/L) and high selectivity (4300-fold) against GSK3,
CDK2, and Aurora A
80
. Despite the competitive position of this
series from a timing standpoint, it does not appear that a member
of the series has entered the clinic.
4. Perspective
MAPK cascades were discovered more than three decades ago and
new functions are continuously revealed
50,55
. With the emergence
of acquired drug resistance after treatment onset as well as the
deeper appreciation that has been obtained in exploration of ERK
kinase, there is a growing interest in targeting ERK kinase. The
combination usage of ERK inhibitors with upstream inhibition
may result in more desirable therapeutic benets
40,41,64
. It is now
clear that combination of different inhibitors of the same target, or
drugs for different targets within the same pathway, can result in
marked differences in effectiveness compared with single inhibi-
tor, depending on tumor type and mutational status. For example,
Moriceau et al.
39
has found that low concentration of ERK alone
cannot reverse BRAF-inhibitor and MEK-inhibitor double resis-
tance (DDR, double-drugs-resistance) unless co-targeting BRAF
and MEK. High concentration of ERK inhibitors can inhibit the
growth of DDR cells, but brings serious adverse effects. Of note, it
also has been reported that BRAF inhibitor-resistant melanoma
cells also often fail in response to ERK inhibition, which is
explained by the ERK-dependent feedback loop to activate RAS
and PI3K signaling. This study showed a broader targeting
strategy that combining ERK and PI3K/mTOR inhibitors showed
sufcient activity in tumors with BRAF inhibitor resistance
68
.
Considering the breakthrough of immunotherapy, the combination
of inhibition of MAPK kinases and immunotherapy may equip
doctors with new weapons in cancer treatment one day
81
.
Currently, ERK inhibition is still at its early stage in clinical
studies. Clinical studies have not reported the occurrence of
acquired resistance to ERK inhibitors. Nevertheless, it is important
to note that the possible occurrence of ERK inhibitor resistance
caused by ERK1/2 mutation prompts a new question on how to
deal with this resistance
82
.
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Feifei Liu et al.562
... Previously, the NF-κB signaling pathway had been found to be involved in hepatic stellate cell activation and liver fibrosis [38,39]. In the canonical MAPK signaling cascade, phosphorylation of RAF proteins upon activation simultaneously activates MEK, which subsequently leads to positive phosphorylation of ERK1/2 [53]. ERK1/2 has hundreds of substrates, many of which are involved in essential physiological processes that regulate cell proliferation, differentiation, and death, such as NF-κB, c-Myc, and others [54]. ...
... The crosstalk of NF-kB and ERK can lead to feedback loops that in turn regulate ERK signaling either positively or negatively [55,56]. Additionally, activated ERK1/2 can also in turn stimulates inhibitory phosphorylation of its upstream factors and kinases such as MEK, RAF, and RTKs, thereby preventing signal propagation of this pathway and maintaining stable cellular function [53,57]. This may explain why huA5 regulates ERK and NF-kB phosphorylation, but not MEK phosphorylation. ...
... The mitogen-activated protein kinase (MAPK) pathway, which includes p38 MAPK, ERK, and JNK, plays a pivotal role in signal transduction in metabolic diseases and cancer [136,137]. Accumulating evidence has demonstrated that ERK inhibitors are less susceptible to resistance mechanisms than inhibitors targeting upstream molecules in the MAPK pathway, such as RAF and MEK [138,139]. In recent years, several ERK inhibitors that have shown encouraging results for cancer treatment have been introduced in clinical studies. ...
... Ulixertinib (BVD-523), an ATP-competitive kinase-selective inhibitor that targets phosphorylated ERK2 (pERK) and the downstream kinase RSK (pRSK), has demonstrated antitumor effects on NRAS-mutant melanoma and BRAF-mutant solid tumors [140]. Notably, ulixertinib has been found to be effective in several models showing intrinsic or acquired resistance to other MAPK pathway inhibitors [138] . Moreover, the combination of ERK inhibitors with upstream inhibitors exhibits synergistic benefits [141][142][143]. ...
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... (F. Liu et al. 2018a). Curcumin had a suppressive effect on fibrosis via upregulation of PTEN, effected through PI3K/Akt/ mTOR signaling. ...
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The mammalian target of rapamycin (mTOR) is a crucial enzyme in regulating multiple signaling pathways in the body, including autophagy, proliferation and apoptosis. Disruption of these mTOR signaling pathways can lead to an array of abnormalities and trigger disease processes, examples being neurodegenerative conditions, cancer, obesity and diabetes. Under conditions of oxidative stress, mTOR can regulate apoptosis and autophagy, with tissue repair being favored under such circumstances. Moreover, the correlation between mTOR and other signaling pathways could play a pivotal role in the pathophysiology of numerous disorders. mTOR has a tight connection with NF-κB, Akt, PI3K, MAPK, GSK-3β, Nrf2/HO-1, JAK/STAT, CREB/BDNF, and ERK1/2 pathways, which together could play significant roles in the regulation of inflammation, apoptosis, cell survival, and oxidative stress in different body organs. Research suggests that inhibiting mTOR could be beneficial in treating metabolic, neurological and cardiovascular conditions, as well as potentially extending life expectancy. Therefore, identifying new chemicals and agents that can modulate the mTOR signaling pathway holds promise for treating and preventing these disorders. Curcumin is one such agent that has demonstrated regulatory effects on the mTOR pathway, making it an exciting alternative for reducing complications associated with complex diseases by targeting mTOR. This review aims to examine the potential of curcumin in modulating the mTOR signaling pathway and its therapeutic implications.
... Active RAF dimers transmit signals downstream to ERK which has numerous phosphorylation targets both in the cytoplasm and in the nucleus (Gardner et al., 1994;Graves et al., 1995;Haling et al., 2014;Lavoie et al., 2020;Lee et al., 2020;Liu et al., 2018). Our prior research showed that patient-derived xenografts harboring AGK-BRAF, ARMC10-BRAF, or SEPT3-BRAF fusions demonstrated varied ERK subcellular localization when treated with MAPK inhibitors (Turner et al., 2019). ...
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BRAF kinase fusions are a form of structural variation in the genome and are recurrent events in driver-negative melanomas. While BRAF fusions reproducibly conserve the kinase domain, there is genetic variability with 5' gene partners and specific BRAF breakpoints. We investigated how genetic diversity of BRAF kinase fusions affects dimeric signaling and ERK activation. We overexpressed BRAF fusions with 5' gene partners including AGK, ZKSCAN1, ARMC10, PPFIBP2, and TRIM24 and found fusion dependent signaling and inhibitor sensitivity. Despite the development of next generation RAF inhibitors, there was paradoxical ERK phosphorylation with multiple pan-RAF inhibitors, which was ameliorated in certain BRAF fusions with vertical RAF/MEK inhibition using trametinib and LY3009120. Collectively, we observed some fusion-dependent effects but also tumor growth suppression and resolution of paradoxical activation with vertical pathway inhibition.
... Importantly, knockdown of FGFR3 attenuated TGF-β1-induced changes in the expression of fibrosis markers, including Collagen I and α-SMA ( Figure 5D). The MAPK signalling pathway plays a critical role in governing diverse cell functions, including proliferation, differentiation, and apoptosis [29]. Through KEGG signallingpathway analysis, we found that FGFR3 is an upstream signalling molecule for the MAPK pathway. ...
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Silicosis is one of the most prevalent and fatal occupational diseases worldwide, with unsatisfactory clinical outcomes. This study aimed to investigate the therapeutic effect and related molecular mechanisms of how mesenchymal stem cell (MSC)-secreted exosomes alleviate SiO2-induced pulmonary fibrosis. miR-99a-5p was significantly downregulated in silicosis models via high-throughput miRNA screening, and was overlapped with miRNAs in exosomes from MSCs. miR-99a-5p was significantly downregulated in the lung of a mice silicosis model and in TGFβ1-induced NIH-3T3 cells. In contrast, fibroblast growth factor receptor 3 (FGFR3), a direct target gene of miR-99a-5p, was upregulated in vitro and in vivo. Furthermore, we demonstrated that MSC-derived exosomes deliver enriched miR-99a-5p to target cells and inhibit TGF-β1-induced fibroblast transdifferentiation to reduce collagen protein production. Similarly, in a silicosis mouse model, MSC-derived exosome treatment through the tail veins of the mice counteracted the upregulation of fibrosis-related proteins and collagen deposition in the lung of the mice. By constructing exosomal therapeutic cell models with different miR-99a expressions, we further demonstrated that miR-99a-5p might attenuate pulmonary fibrosis by regulating target protein FGFR3 and downstream mitogen-activated protein kinase (MAPK) signalling pathways. Our study demonstrated that MSC-derived exosomes ameliorate SiO2-induced pulmonary fibrosis by inhibiting fibroblast transdifferentiation and represent an attractive method of pulmonary fibrosis treatment.
... When the RAS proteins are activated, they initiate several downstream signaling pathways, such as the PI3K/AKT/mTOR network, RAF/MEK/ERK pathway, and RalEGF/Ral route (79,80). The dysregulation of the RAF/MEK/ERK signaling pathway is linked to tumor growth (81). ...
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Lung cancer, a common type of malignant neoplasm, has seen significant advancements in the treatment of lung adenocarcinoma (LUAD). However, the management of lung squamous cell carcinoma (LSCC) continues to pose challenges. Traditional treatment methods for LSCC encompass surgical resection, chemotherapy, and radiotherapy. The introduction of targeted therapy and immunotherapy has greatly benefited LSCC patients, but issues such as limited immune response rates and adverse reactions persist. Therefore, gaining a deeper comprehension of the underlying mechanisms holds immense importance. This review provides an in-depth overview of classical signaling pathways and therapeutic targets, including the PI3K signaling pathway, CDK4/6 pathway, FGFR1 pathway and EGFR pathway. Additionally, we delve into alternative signaling pathways and potential targets that could offer new therapeutic avenues for LSCC. Lastly, we summarize the latest advancements in targeted therapy combined with immune checkpoint blockade (ICB) therapy for LSCC and discuss the prospects and challenges in this field.
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Lung cancer is a malignant tumor with the highest morbidity and mortality rate worldwide, with nearly 2.5 million new cases and more than 1.8 million deaths reported globally in 2022. Lung cancer is broadly categorized into two main types: non-small cell lung cancer (NSCLC) and small cell lung cancer (SCLC), with NSCLC accounting for about 85% of all cases. Early-stage lung cancers often present without obvious symptoms, resulting in most patients being diagnosed at an advanced stage where traditional chemotherapy has limited efficacy. Recent advances in molecular biology have elucidated the pivotal role of gene mutations in tumor development, paving the way for targeted therapies that have markedly benefited patients. Beyond the well-known epidermal growth factor receptor (EGFR) mutation, an increasing number of new molecular targets have been identified, including ROS1 rearrangement, BRAF mutation, NTRK fusion, RET fusion, MET mutation, KRAS G12C mutation, HER2 mutation, ALK rearrangement, and NRG1 fusion. Some of these targeted therapies have already been approved by the Food and Drug Administration (FDA), and many others are currently undergoing clinical trials. This review summarizes recent advances in NSCLC treatment with molecular targets, highlighting progress, challenges, and their impact on patient prognosis.
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RAS proteins (KRAS4A, KRAS4B, NRAS and HRAS) function as GDP-GTP-regulated binary on-off switches, which regulate cytoplasmic signaling networks that control diverse normal cellular processes. Gain-of-function missense mutations in RAS genes are found in ∼25% of human cancers, prompting interest in identifying anti-RAS therapeutic strategies for cancer treatment. However, despite more than three decades of intense effort, no anti-RAS therapies have reached clinical application. Contributing to this failure has been an underestimation of the complexities of RAS. First, there is now appreciation that the four human RAS proteins are not functionally identical. Second, with >130 different missense mutations found in cancer, there is an emerging view that there are mutation-specific consequences on RAS structure, biochemistry and biology, and mutation-selective therapeutic strategies are needed. In this Cell Science at a Glance article and accompanying poster, we provide a snapshot of the differences between RAS isoforms and mutations, as well as the current status of anti-RAS drug-discovery efforts.
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K-Ras proteins are major drivers of human cancers, playing a direct causal role in about one million cancer cases/year. In cancers driven by mutant K-Ras, the protein is locked in the active, GTP-bound state constitutively, through a defect in the off-switch mechanism. As such, the mutant protein resembles the normal K-Ras protein from a structural perspective, making therapeutic attack extremely challenging. K-Ras is a member of a large family of related proteins, which share very similar GDP/GTP-binding domains, making specific therapies more difficult. Furthermore, Ras proteins lack pockets to which small molecules can bind with high affinity, with a few interesting exceptions. However, new insights into the structure and function of K-Ras proteins reveal opportunities for intervention that were not appreciated many years ago, when efforts were launched to develop K-Ras therapies. Furthermore, K-Ras undergoes post-translational modification and interactions with cellular signaling proteins that present additional therapeutic opportunities, such as specific binding to calmodulin and regulation of non-canonical Wnt signaling.
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Activating Ras mutations are found in about 30 % of human cancers. Ras activation is regulated by guanine nucleotide exchange factors, such as the son of sevenless (SOS), which form protein-protein interactions (PPIs) with Ras and catalyze the exchange of GDP by GTP. This is the rate-limiting step in Ras activation. However, Ras surfaces lack any evident suitable pockets where a molecule might bind tightly, rendering Ras proteins still 'undruggable' for over 30 years. Among the alternative approaches is the design of inhibitors that target the Ras-SOS PPI interface, a strategy that is gaining increasing recognition for treating Ras mutant cancers. Herein we focus on data that has accumulated over the past few years pertaining to the design of small-molecule modulators or peptide mimetics aimed at the interface of the Ras-SOS PPI. We emphasize, however, that even if such Ras-SOS therapeutics are potent, drug resistance may emerge. To counteract this development, we propose "pathway drug cocktails", that is, drug combinations aimed at parallel (or compensatory) pathways. A repertoire of classified cancer, cell/tissue, and pathway/protein combinations would be beneficial toward this goal.
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A central feature of signal transduction downstream of both receptor and oncogenic tyrosine kinases is the Ras-dependent activation of a protein kinase cascade consisting of Raf-1, Mek (MAP kinase kinase) and ERKs (MAP kinases). To study the role of tyrosine kinase activity in the activation of Raf-1, we have examined the properties of p74Raf-1 and oncogenic Src that are necessary for activation of p74Raf-1. We show that in mammalian cells activation of p74Raf-1 by oncogenic Src requires pp60Src to be myristoylated and the ability of p74Raf-1 to interact with p21Ras-GTP. The Ras/Raf interaction is required for p21Ras-GTP to bring p74Raf-1 to the plasma membrane for phosphorylation at tyrosine 340 or 341, probably by membrane-bound pp60Src. When oncogenic Src is expressed with Raf-1, p74Raf-1 is activated 5-fold; however, when co-expressed with oncogenic Ras and Src, Raf-1 is activated 25-fold and this is associated with a further 3-fold increase in tyrosine phosphorylation. Thus, p21Ras-GTP is the limiting component in bringing p74Raf-1 to the plasma membrane for tyrosine phosphorylation. Using mutants of Raf-1 at Tyr340/341, we show that in addition to tyrosine phosphorylation at these sites, there is an additional activation step resulting from p21Ras-GTP recruiting p74Raf-1 to the plasma membrane. Thus, the role of Ras in Raf-1 activation is to bring p74Raf-1 to the plasma membrane for at least two different activation steps.
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The extracellular signal-regulated kinases ERK1/2 represent an essential node within the RAS/RAF/MEK/ERK signaling cascade that is commonly activated by oncogenic mutations in BRAF or RAS or by upstream oncogenic signaling. While targeting upstream nodes with RAF and MEK inhibitors has proven effective clinically, resistance frequently develops through reactivation of the pathway. Simultaneous targeting of multiple nodes in the pathway, such as MEK and ERK, offers the prospect of enhanced efficacy as well as reduced potential for acquired resistance. Described herein is the discovery and characterization of GDC-0994 (22), an orally bioavailable small molecule inhibitor selective for ERK kinase activity.
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Background: BRAF(V600E)-mediated MAPK pathway activation is associated in melanoma cells with IFNAR1 downregulation. IFNAR1 regulates melanoma cell sensitivity to IFNα, a cytokine used for the adjuvant treatment of melanoma. These findings and the limited therapeutic efficacy of BRAF-I prompted us to examine whether the efficacy of IFNα therapy of BRAF(V600E) melanoma can be increased by its combination with BRAF-I. Methods: BRAF/NRAS genotype, ERK activation, IFNAR1, and HLA class I expression were tested in 60 primary melanoma tumors from treatment-naive patients. The effect of BRAF-I on IFNAR1 expression was assessed in three melanoma cell lines and in four biopsies of BRAF(V600E) metastases. The antiproliferative, pro-apoptotic and immunomodulatory activity of BRAF-I and IFNα combination was tested in vitro and in vivo utilizing three melanoma cell lines, HLA class I-MA peptide complex-specific T-cells and immunodeficient mice (5 per group for survival and 10 per group for tumor growth inhibition). All statistical tests were two-sided. Differences were considered statistically significant when the P value was less than .05. Results: The IFNAR1 level was statistically significantly (P < .001) lower in BRAF(V600E) primary melanoma tumors than in BRAF wild-type tumors. IFNAR1 downregulation was reversed by BRAF-I treatment in the three melanoma cell lines (P ≤ .02) and in three out of four metastases. The IFNAR1 level in the melanoma tumors analyzed was increased as early as 10 to 14 days following the beginning of the treatment. These changes were associated with: 1) an increased susceptibility in vitro of melanoma cells to the antiproliferative (P ≤ .04), pro-apoptotic (P ≤ .009) and immunomodulatory activity, including upregulation of HLA class I antigen APM component (P ≤ .04) and MA expression as well as recognition by cognate T-cells (P < .001), of BRAF-I and IFNα combination and 2) an increased survival (P < .001) and inhibition of tumor growth of melanoma cells (P < .001) in vivo by BRAF-I and IFNα combination. Conclusions: The described results provide a strong rationale for the clinical trials implemented in BRAF(V600E) melanoma patients with BRAF-I and IFNα combination.
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The MAPK pathway is frequently activated in many human cancers, particularly melanomas. A single nucleotide mutation in BRAF resulting in the substitution of glutamic acid for valine (V600E) causes constitutive activation of the downstream MAPK pathway. Selective BRAF and MEK inhibitor therapies have demonstrated remarkable anti-tumor responses in BRAFV600E mutant melanoma patients. However, initial tumor shrinkage is transient and the vast majority of patients develop resistance. We previously reported that SCH772984, an ERK 1/2 inhibitor, effectively suppressed MAPK pathway signaling and cell proliferation in BRAF, MEK and concurrent BRAF/MEK inhibitor resistant tumor models. ERK inhibitors are currently being evaluated in clinical trials and in anticipation of the likelihood of clinical resistance, we sought to prospectively model acquired resistance to SCH772984. Our data show that long term exposure of cells to SCH772984 leads to acquired resistance, attributable to a mutation of glycine to aspartic acid (G186D) in the DFG motif of ERK1. Structural and biophysical studies demonstrated specific defects in SCH772984 binding to mutant ERK. Taken together, these studies describe the interaction of SCH772984 with ERK and identify a novel mechanism of ERK inhibitor resistance through mutation of a single residue within the DFG motif.
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Human HRAS, KRAS, and NRAS genes encode four isoforms of Ras, a p21 GTPase. Mutations in KRAS account for the majority of RAS-driven cancers. The KRAS splice variants, K-Ras4A and K-Ras4B, differ in four catalytic domain residues (G151R/D153E/K165Q/H166Y) and in their disordered C-terminal hypervariable region (HVR). In K-Ras4A, the HVR is not as strongly positively charged as in K-Ras4B (+6e vs +9e). Here, we performed all-atom molecular dynamics simulations to elucidate isoform-specific differences between the two splice variants. We observe that the catalytic domain of GDP-bound K-Ras4A has a more exposed nucleotide binding pocket than K-Ras4B, and the dynamic fluctuations in Switch I and II regions also differ; both factors may influence guanine-nucleotide exchange. We further observe that like K-Kas4B, full-length K-Ras4A exhibits nucleotide-dependent HVR fluctuations; however, these fluctuations differ between the GDP-bound forms of K-Ras4A and K-Ras4B. Unlike K-Ras4B where the HVR tends to cover the effector binding region, in K-Ras4A, autoinhibited states are unstable. With lesser charge, the K-Ras4A HVR collapses on itself, making it less available for binding the catalytic domain. Since the HVRs of N- and H-Ras are weakly charged (+1e and +2e respectively), autoinhibition may be a unique feature of K-Ras4B.
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Ras genes are the most common targets for somatic gain-of-function mutations in human cancer. Recently, germline mutations that affect components of the Ras-Raf-mitogen-activated and extracellular-signal regulated kinase kinase (MEK)-extracellular signal-regulated kinase (ERK) pathway were shown to cause several developmental disorders, including Noonan, Costello and cardio-facio-cutaneous syndromes. Many of these mutant alleles encode proteins with aberrant biochemical and functional properties. Here we will discuss the implications of germline mutations in the Ras-Raf-MEK-ERK pathway for understanding normal developmental processes and cancer pathogenesis.