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Citation: Konstantinou, E.; Longange,
E.; Kaya, G. Mechanisms of
Senescence and Anti-Senescence
Strategies in the Skin. Biology 2024,13,
647. https://doi.org/10.3390/
biology13090647
Academic Editor: Yu Sun
Received: 2 July 2024
Revised: 13 August 2024
Accepted: 20 August 2024
Published: 23 August 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
biology
Review
Mechanisms of Senescence and Anti-Senescence Strategies in
the Skin
Evangelia Konstantinou 1, Eliane Longange 1and Gürkan Kaya 1, 2, *
1Department of Medicine, University of Geneva, Rue Michel-Servet 1, CH-1206 Geneva, Switzerland;
evangelia.konstantinou@unige.ch (E.K.); eliane.longange@unige.ch (E.L.)
2Departments of Dermatology and Clinical Pathology, Geneva University Hospitals, Rue Gabrielle
Perret-Gentil 4, CH-1205 Geneva, Switzerland
*Correspondence: gkaya@hug.ch
Simple Summary: The skin is the outermost barrier of the human body and consists of different
layers and cell types. Several environmental and genetic factors can induce skin aging and age-
related diseases. One of the main problems in skin aging is that senescent cells are accumulated and
secrete factors, which can induce senescence in other tissues. Many researchers are trying to identify
treatment modalities (known as senotherapies) to eliminate the senescent cells and reverse the aging
process for chronic age-related diseases. The aim of this study is to address the mechanisms that
induce senescence and the molecules with potential HAFi effects that are currently investigated for
skin aging. Further studies should be conducted to elucidate all the effects of current senotherapies
on the skin and other organs. Current data suggest that ongoing research projects in the field may
lead to the discovery of new effective anti-senescence strategies in the skin.
Abstract: The skin is the layer of tissue that covers the largest part of the body in vertebrates,
and its main function is to act as a protective barrier against external environmental factors, such as
microorganisms, ultraviolet light and mechanical damage. Due to its important function, investigating
the factors that lead to skin aging and age-related diseases, as well as understanding the biology of
this process, is of high importance. Indeed, it has been reported that several external and internal
stressors contribute to skin aging, similar to the aging of other tissues. Moreover, during aging,
senescent cells accumulate in the skin and express senescence-associated factors, which act in a
paracrine manner on neighboring healthy cells and tissues. In this review, we will present the factors
that lead to skin aging and cellular senescence, as well as ways to study senescence
in vitro
and
in vivo
. We will further discuss the adverse effects of the accumulation of chronic senescent cells and
therapeutic agents and tools to selectively target and eliminate them.
Keywords: skin aging; cellular senescence; SASP; p16INK4a; senolytics; senomorphics; senotherapy
1. Introduction
1.1. Skin Morphology
The skin is an organ consisting of a variety of cells and layers that form a single struc-
ture to protect against, and communicate with, the external environment. This dynamic
organ is involved in many vital processes, crucial to the health of vertebrates, such as the
regulation of body temperature, fluid balance, synthesis of vitamins and hormones and
monitoring of immunological responses. The skin is organized into three distinct layers:
epidermis, dermis and hypodermis. These will be described in more detail below [1].
1.1.1. Epidermis
The epidermis is the outermost layer of the skin, which forms a barrier against toxins,
pathogens and dehydration. Morphologically, the epidermis is a stratified squamous
Biology 2024,13, 647. https://doi.org/10.3390/biology13090647 https://www.mdpi.com/journal/biology
Biology 2024,13, 647 2 of 23
epithelium with constant renewal capability, mainly composed of epidermal cells called
keratinocytes (95%). Other cell types present in the epidermis are melanocytes (3%),
Langerhans (2%) and Merkel cells (0.5%). There are up to five morphologically distinct sub-
layers (strata) of the epidermis, from the innermost to the outermost layer: stratum basale,
stratum spinosum, stratum granulosum, stratum lucidum and stratum corneum [2,3].
Cell Populations of the Epidermis
The majority of epidermal cells are keratinocytes, which undergo cellular differenti-
ation to form different epidermal strata. During their maturation process, keratinocytes
undergo changes in their appearance and in the cytoskeletal organization. Keratinocytes
play an important supporting role in epidermal structure, contributing to cell viability and
signaling pathways such as protein synthesis, cell and epithelial activity, etc. [1,2].
Melanocytes are cells that produce melanin and are located in the stratum basale
sub-layer. Melanin plays a role in protection against the harmful effects of UV rays by
absorbing them while also determining skin color. In more detail, melanin production is
regulated by the melanocyte-stimulating hormone and takes place in melanosomes, which
are synthesized from dendritic melanocytes. After its production, melanin is transported
via dendrites to keratinocytes and protects the nucleus from UV light, acting as a filter that
absorbs the majority of UV [1,2].
Langerhans cells are immune sentinels and are most prominent in the stratum spinosum.
These cells can also adopt a dendritic-like phenotype, migrate, and interact with naive T-cells,
activating an immune response [2].
Merkel cells are located in the basal layer of the epidermis and in the epithelial sheath
of hair follicles. They are connected with the nerve endings and function as sensory
receptors [1,2].
1.1.2. Dermal–Epidermal Junction
The epidermis and dermis are closely linked by the dermal–epidermal junction. This
basal membrane is formed by the keratinocytes of the basal layer and the fibroblasts
of the dermis. The dermal–epidermal junction functions as a semi-permeable barrier,
controlling the exchange of oxygen, nutrients and waste molecules between the dermis and
epidermis [
1
,
2
]. Furthermore, the dermal–epidermal junction has an undulating pattern
due to the rete ridges, which are epithelial projections that extend the epidermis more
deeply into the dermis and enhance their connectivity. These structures increase the surface
of the dermal–epidermal junction, improve the mechanical properties of the skin, maintain
skin homeostasis and act as protective niches for keratinocyte stem cells. The dimensions
of rete ridges are different according to the body site and age and are increased during
inflammatory skin diseases [4–6].
1.1.3. Dermis
The dermis is a resistant yet flexible layer that supports the epidermis and links it with
the hypodermis. It also plays a role in thermoregulation, oxygen supply to the epidermis
and elimination of epidermal waste. The dermis is composed of two distinct layers. The
upper layer, named papillary dermis, is located below the dermal–epidermal junction and
consists mainly of loosely packed collagen and elastin fibers forming a spongy structure.
The lower layer, the reticular dermis, is a thicker layer, with denser collagen and elastic
fibers [
1
]. The major cell type in the dermis is dermal fibroblasts, which secrete proteins of
the extracellular matrix, such as collagens, elastin and proteoglycans. The main collagen
types found in the dermis are as follows: type I and type III. On the other hand, elastin and
fibrillin microfibrils are the main types of elastic fibers present in the dermis. Additionally,
two more subtypes of elastic fibers can be found in the dermis: elaunin fibers, which are
located near the junction of papillary and reticular dermis, and oxytalan fibers, which are in
the papillary dermis [
7
]. The dermis also contains other appendages, such as hair follicles,
sweat glands, sebaceous glands, lymphatic vessels, nerves and blood vessels [1,2].
Biology 2024,13, 647 3 of 23
1.1.4. Hypodermis
The hypodermis, or subcutaneous tissue, is the deepest layer of the skin, anchoring
the dermis to the underlying muscles. This tissue consists mainly of adipocytes (fat cells),
connective tissue (interlobular septa) and larger nerves and blood vessels. The hypodermis
insulates the body against cold and heat, provides physical protection and serves as an
energy reserve [1,2].
1.2. Skin Aging
Skin aging is a complex and multifaceted process induced by intrinsic and extrinsic
factors that promote biochemical and structural changes to the skin and surrounding
tissues. The combination of internal and external factors that induce aging, as well as
the human body’s response to these factors, is known as the “skin aging exposome” [
8
].
Intrinsic aging is induced by chronological aging and is mainly associated with cellular
senescence caused by endogenous damage and genetic alterations, while extrinsic aging is
driven by environmental factors. Given the impact that skin aging may have on people’s
social lifestyle and health, extensive research is required to obtain mechanistic insights into
the aging process and to develop anti-aging treatments. In this review, we will provide
an overview of the mechanisms of skin aging and tools to combat senescent cells in the
skin [9].
1.2.1. Intrinsic Aging
Chronologically dependent skin aging, known as intrinsic aging, is mainly charac-
terized by skin thinning, fine lines and the inability of the skin to repair itself [
10
,
11
].
Intrinsic aging is a slow process, and variations between different genders, ethnicities and
geographical populations may exist [
12
]. The main characteristic of intrinsic aging is the
alteration of several dermal components, most prominently a reduction in the extracellular
matrix (ECM) elements, such as collagens, elastin, glycosaminoglycans, etc., which results
in a decline in skin thickness [
13
,
14
]. Furthermore, over a lifetime, the flexibility of the skin
decreases, and changes that occur in the connective tissue can result in reduced strength
and elasticity of the skin, a phenomenon known as elastosis. Changes in skin elasticity can
begin early in life and progress during aging [9,10].
Moreover, the continuous production of reactive oxygen species (ROS), either by mito-
chondrial leakage or inflammations, results in endogenous damage to cellular components
such as membranes and enzymes and accelerates telomere shortening, thereby promoting
intrinsic aging [
13
,
15
,
16
]. It is well reported that telomeres are repetitive sequences of DNA
at the ends of linear chromosomes; their length is tissue-specific and shortens between
germline and somatic tissues. Studies have shown that telomeres shorten as normal human
fibroblasts grow and also in skin samples from older people, compared to younger [
17
].
During aging, the lack of telomerase activity and the inability of DNA polymerases to
replicate the telomere C-rich strand lagging result in shorter telomeres. As a result, they are
recognized as DNA damage, which triggers a DNA damage response (DDR) and cellular
senescence [
18
,
19
]. The p53-p21 pathway plays an important role in telomere-induced
senescence, as DDR is induced by p53, which in turn positively regulates p21 [20].
During aging, a decrease in the levels of different growth factors, signaling molecules,
hormones and their receptors occurs, resulting in impairment of several skin functions
[21–23]
.
An important example is impaired wound healing. As people age, the proliferative rate of
keratinocytes, extracellular matrix synthesis and angiogenesis decrease compared to younger,
healthy individuals [
23
]. On the other hand, the expression levels of some signaling molecules
are elevated during aging. For example, a study on the IMR-90 cell line revealed that the
levels of transforming growth factor beta 1 start to increase after incubation with H
2
O
2
, which
is a method for the induction of cellular senescence in vitro, and this results in an increase in
senescence-associated
β
-galactosidase (SA-
β
-GAL) activity and in the expression levels of
some senescence-associated genes [
24
]. Another signaling molecule that is elevated during
Biology 2024,13, 647 4 of 23
aging, according to many studies, is AMPK, which is responsible for cellular and organismal
metabolism [25].
The clinical features of aged skin, apart from skin thickness and wrinkles, include
xerosis, laxity and the occurrence of benign neoplasms. These features are accompanied
by changes in the histology of the skin. Briefly, in the epidermis, there is a decrease
in the number of melanocytes and Langerhans cells. Moreover, there is a reduction in
the dermal–epidermal junction, which prevents the exchange of factors between these
two compartments [
26
]. In the dermis, a decline in the dermal volume and in the number of
blood vessels has been reported during skin aging [
27
]. Additionally, the levels of fat tissue
start to run out and accumulate in pockets [
28
,
29
]. Furthermore, with age, there is a loss in
bone mass, resulting in wrinkles and sagging [
30
]. These are some of the characteristics of
skin aging, but they can also appear during the extrinsic aging process.
1.2.2. Extrinsic Aging
Extrinsic skin aging is linked to lifestyle and results from exposure to several external
factors, such as stress, ionizing irradiation, alcohol, environmental pollution, tobacco
smoke and UV radiation. Due to its constant contact with the environment, the skin is more
susceptible to extrinsic aging [
8
,
31
–
33
]. Among these factors, the main cause of extrinsic
aging is UV radiation (also known as photoaging) [
34
,
35
]. Although UVC (100–280 nm)
is mainly absorbed by the ozone layer, UVA (320–400 nm) and UVB (280–320 nm) are
both responsible for alterations in the skin [
35
]. UVA is absorbed by both the epidermis
and dermis and induces damage in the connective tissue of the dermis. Moreover, UVA
increases the levels of ROS, which results in the activation of cell surface receptors for
epidermal growth factor (EGF), insulin or keratinocyte growth factor (KGF); damages the
lipids of the membranes; and causes DNA mutagenesis. Also, it is known that UVA is
the main cause of photoaging [
35
–
38
]. On the other hand, UVB penetrates only the layer
of the epidermis and can cause sunburn, tanning and photocarcinogenesis [
8
,
35
]. UVB
induces DNA damage in keratinocytes and melanocytes of the epidermis and promotes
the formation of thymidine dimers. The inability to resolve or repair thymidine dimers
may lead, over time, to the accumulation of mutations [
38
]. Furthermore, exposure to
UVB leads to the production of cyclobutane pyrimidine dimers (CPDs), which result in
inflammatory responses, suppression of the immune system, induction of mutations or
skin cancer [
39
,
40
]. Some clinical signs of photoaged skin include wrinkles, dryness, loss of
elasticity, impaired wound healing, telangiectasia and formation of purpura [41].
The second most important factor that can cause extrinsic aging is cigarette smok-
ing [
42
,
43
]. Smoking is linked to the dysfunction of many signaling pathways, such as the
Tumor Necrosis Factor signaling pathway or Janus kinase signal transducer and activator of
transcription (JAK-STAT) pathway. Moreover, it affects genome stability through telomere
shortening [
44
,
45
]. It has been shown that, in smokers’ skin, the levels of matrix metallopro-
teinase 1 (MMP-1) mRNA are elevated, which leads to extracellular matrix breakdown [
8
].
The characteristics of aged skin, after years of smoking, include facial wrinkling, mainly
around the mouth and eyes, and hyperpigmentation of the oral mucosa [
43
,
46
–
48
]. Aside
from active smoking, passive smoking can also have detrimental effects. A study by Per-
coco et al. demonstrated that exposing human living skin explants to smoke results in
an alteration in the skin surface physicochemistry. This alteration changes how the skin
interacts with the environment. Additionally, the study found that smoke exposure raises
the pH of the skin and disrupts its function as a natural barrier [49].
Particulate matter affects human skin, contributing to skin aging and the formation of
wrinkles. Depending on the dose to which humans are exposed, it can induce cytotoxicity
and increased expression of IL-1a. A study by Patatian et al. demonstrated that when
human skin explants are exposed to a mixture of air pollutants, these pollutants penetrate
the deepest epidermal layers, alter the gene expression profile and significantly increase
the levels of extracellular vesicles in the stratum spinosum, indicating extensive cell com-
munication [
8
,
50
–
53
]. A major air pollutant produced by diesel engines is diesel particulate
Biology 2024,13, 647 5 of 23
matter, which induces ROS production and apoptosis in human dermal keratinocytes and
elevates the expression of MMP-1 and MMP-3 in human dermal fibroblasts [
54
]. More-
over, nutrition plays an important role in the appearance of the skin, since a diet rich
in antioxidants, with a high intake of vitamin C and less alcohol, can delay skin aging
and the appearance of wrinkles [
55
–
57
]. Among the factors that can cause extrinsic skin
aging are stress, which can interrupt skin integrity, sleep deprivation, which affects the
appearance of the skin and elevated temperature of human skin, can increase the expression
of MMP-1 [
58
,
59
]. Some of the extrinsic factors causing skin aging in the context of the skin
aging exposome are represented in Figure 1[8,58,59].
Biology 2024, 13, x FOR PEER REVIEW 5 of 24
the skin interacts with the environment. Additionally, the study found that smoke expo-
sure raises the pH of the skin and disrupts its function as a natural barrier [49].
Particulate maer affects human skin, contributing to skin aging and the formation
of wrinkles. Depending on the dose to which humans are exposed, it can induce cytotox-
icity and increased expression of IL-1a. A study by Patatian et al. demonstrated that when
human skin explants are exposed to a mixture of air pollutants, these pollutants penetrate
the deepest epidermal layers, alter the gene expression profile and significantly increase
the levels of extracellular vesicles in the stratum spinosum, indicating extensive cell com-
munication [8,50–53]. A major air pollutant produced by diesel engines is diesel particu-
late maer, which induces ROS production and apoptosis in human dermal keratinocytes
and elevates the expression of MMP-1 and MMP-3 in human dermal fibroblasts [54].
Moreover, nutrition plays an important role in the appearance of the skin, since a diet rich
in antioxidants, with a high intake of vitamin C and less alcohol, can delay skin aging and
the appearance of wrinkles [55–57]. Among the factors that can cause extrinsic skin aging
are stress, which can interrupt skin integrity, sleep deprivation, which affects the appear-
ance of the skin and elevated temperature of human skin, can increase the expression of
MMP-1 [58,59]. Some of the extrinsic factors causing skin aging in the context of the skin
aging exposome are represented in Figure 1 [8,58,59].
Figure 1. Schematic representation of extrinsic factors that can cause skin aging [8] (created with
BioRender.com).
1.2.3. Clinical Aspects of Skin Aging
As mentioned above, the most important clinical alterations seen in aged skin are the
appearance of wrinkles, decreased skin thickness (atrophy), reduced elasticity and den-
sity, and loss of skin tone accompanied by a sallow color. Elderly skin shows reduced lipid
content and decreased sebaceous and sweat gland secretion. These changes result in dry-
ness of the skin (xerosis), with a tendency for irritation and erythema, contributing to skin
Figure 1. Schematic representation of extrinsic factors that can cause skin aging [
8
] (created with
BioRender.com).
1.2.3. Clinical Aspects of Skin Aging
As mentioned above, the most important clinical alterations seen in aged skin are
the appearance of wrinkles, decreased skin thickness (atrophy), reduced elasticity and
density, and loss of skin tone accompanied by a sallow color. Elderly skin shows reduced
lipid content and decreased sebaceous and sweat gland secretion. These changes result
in dryness of the skin (xerosis), with a tendency for irritation and erythema, contributing
to skin barrier disruption. Capillary degeneration and decreased vascular content lead
to dysfunctions in circulation and thermoregulation. Aged skin is susceptible to envi-
ronmental injury, and, consequently, wound healing is delayed [
60
]. Dermatoporosis is a
particular form of skin aging characterized by skin atrophy, senile purpura and pseudoscars
(see below). There are several age-dependent skin diseases such as bullous pemphigoid,
herpes zoster and erysipelas. The intrinsic and extrinsic skin aging process can also induce
benign hyperproliferative, precancerous or malignant lesions, such as seborrheic keratosis,
senile hemangioma, solar lentigo, actinic keratosis, basal cell carcinoma and squamous
cell carcinoma.
Biology 2024,13, 647 6 of 23
1.2.4. Histopathological and Ultrastructural Features of Intrinsic and Extrinsic Skin Aging
The thickness of the epidermis, during aging, is linked with a decrease in the number
of dendritic cells and melanocytes. Consequently, the lower number of melanocytes affects
the role of the epidermis as a protective barrier against UV radiation [
61
]. Moreover, during
aging, there is a decrease in the number and secretion of sweat glands but not in their
morphology, resulting in impaired thermoregulation in the human body [
61
]. Additionally,
as skin ages, keratinocytes alter their shape and change from actively dividing cells to
non-dividing ones [
62
]. Fragmented collagen fibers are present in aged skin and contribute
to reduced collagen synthesis, possibly due to insufficient mechanical tension needed by
fibroblasts to produce collagen [
63
]. Furthermore, the number of elastic fibers decreases
in elderly people, and the density of hair follicles is reduced, without any change in their
morphology [
64
,
65
]. A biopsy of a wrinkle shows a thinner stratum spinosum at the bottom
of the wrinkle than on the wrinkle flanks, and the wrinkle cavity is often filled by a horny
plug, well characterized by scanning electron microscopy [66].
On the other hand, extrinsic aging has the most dramatic effects on the skin. In more
detail, photoaging induces an increase in the number of mast cells, histiocytes and fibrob-
lasts. In addition to their increased number, these cells also exhibit abnormal morphology;
for example, fibroblasts appear elongated and collapsed [
64
]. Furthermore, during extrinsic
aging, there is an increase in the expression of specific proteases in the dermis, which affects
the expression of collagen and elastin. Consequently, photoaged skin shows the accumu-
lation of abnormal elastic tissue called elastosis. Ultrastructural analysis by transmission
electron microscopy reveals the fragmentation of collagen fibrils, which are replaced by
an amorphous elastotic material in aged and photoaged skin [
67
]. Additionally, specific
kinases are upregulated, leading to the transcription of molecules such MMPs, which
degrade the ECM [
64
,
67
]. Air pollution and cigarette smoke, which are two factors that
promote extrinsic aging, induce the expression of MMPs and proinflammatory cytokines,
which are responsible for the remodeling of the skin [64].
It should be noted that, irrespective of the cause of skin aging, there are some al-
terations in the dermal–epidermal junction. The dermal–epidermal junction, composed
of a network of extracellular matrix macromolecules, plays a crucial role in maintaining
structural integrity and regulating the cellular microenvironment [
5
,
68
]. Aging signifi-
cantly impacts the structure and function of this junction, leading to alterations in skin
physiology. In aged skin, the dermal–epidermal junction becomes flattened, resulting in
the loss of epidermal ridges and dermal papillary projections. This reduced contact surface
between the epidermis and dermis contributes to skin fragility and hampers the exchange
of nutrients and oxygen. Furthermore, aging is associated with decreased expression levels
of key components of the dermal–epidermal junction, including laminin-332, collagen IV,
collagen VII and collagen XVII [5].
1.2.5. Dermatoporosis: A Particular Form of Skin Aging
During aging, a gradual loss of hyaluronate (HA), the major component of the ECM
that functions as a viscoelastic system between the epidermis and dermis, can be observed.
This leads to extreme fragility of the skin and many complications such as lacerations,
nonhealing ulcers and dissecting hematomas. We proposed, in 2007, the term “dermato-
porosis” to cover different characteristics of a chronic cutaneous insufficiency syndrome.
This new dimension of skin aging extends beyond cosmetics and appearance, aiming to
understand its molecular mechanisms and develop preventive or therapeutic strategies for
what turned out to be a prevalent skin condition recognized by the European Academy of
Dermatology and Venereology [
69
–
75
]. Morphological markers of dermatoporosis include
skin atrophy, characterized by wrinkles, senile purpura and pseudoscars. The first clinical
signs of dermatoporosis are seen after the age of 40 and as wrinkles and appearance modi-
fications; however, classical morphological markers of skin fragility develop after 60 years.
Dermatoporosis usually starts with skin atrophy. Other signs, such as senile purpura,
pseudoscars and superficial excoriations, may follow as the condition advances [
2
,
17
]. The
Biology 2024,13, 647 7 of 23
topography of dermatoporosis indicates the role of ultraviolet irradiation in its etiology:
the posterior side of forearms, dorsum of hands, presternal area, scalp and pretibial zones.
Dermatoporosis has been proposed to have two forms: primary dermatoporosis, the most
common type, resulting from chronological aging and long-term unprotected sun exposure,
and secondary dermatoporosis, due to the chronic use of topical and/or systemic corti-
costeroids. Four stages of dermatoporosis have been proposed [
76
]: Stage I: in this stage,
we find the above-mentioned morphological markers of dermatoporosis; this is the most
common stage of dermatoporosis. Stage IIa: localized and small superficial lacerations
(<3 cm) due to skin fragility. Stage IIb: larger lacerations (>3 cm). Stage IIIa: superficial
hematomas. Stage IIIb: deep dissecting hematomas without skin necrosis. Stage IV: large
areas of skin necrosis with potential lethal complications. Dermatoporosis is frequent
among the elderly population. Resulting in skin tears and deep dissecting hematoma, it
can lead to significant morbidity and mortality. Better knowledge and treatment of this
pathology should be spread among healthcare professionals to better prevent and treat the
resulting lesions.
Many different factors are responsible for the fragility of skin during dermatoporosis,
such as alterations in the viscoelasticity of the skin or the upregulation of MMPs in combi-
nation with the downregulation of their inhibitors, which degrade the ECM. In addition,
studies have shown that a reduction in the expression of HA and its receptor CD44 can
lead to epidermal atrophy [
69
]. CD44 is the main receptor of HA, and it regulates the
proliferation of keratinocytes. As a result, defects in HA and CD44 play an important role
in skin atrophy and the development of dermatoporosis [
77
–
80
]. This decrease in HA and
CD44 levels can also be observed due to UVA or UVB exposure or as a result of the topical
use of corticosteroids [81].
1.3. Cellular Senescence
During aging, senescent cells accumulate in the skin, disrupting its normal function
and structure [
82
–
84
]. Cellular senescence was first characterized as a hallmark of aging
by Hayflick and Moorhead (1961). They reported that cellular senescence contributes to a
decline in tissue functionality by impeding tissue repair and regeneration [
83
–
87
]. Several
stressors can cause senescence in cells, such as telomere shortening, mitogenic signals,
oncogenic activation, irradiation, stress, mitochondrial dysfunction, inflammation, nutrient
deprivation, etc. Given that cellular senescence is a heterogenous process, various markers
can be used to characterize these cells [88–91].
Despite multiple molecular, cellular and phenotypic changes (such as enlarged and
flattened morphology) occurring in senescent cells, these cells remain metabolically active,
show resistance to apoptosis and are characterized by an irreversible cell-cycle arrest,
mediated by p21 and p16INK4a. Senescent cells express higher levels of DNA damage
markers, such as
γ
-H2AX as well as cell cycle regulators such as p53, p21 and p16INK4a,
as opposed to normal cells [
34
,
92
,
93
]. Moreover, senescent cells exhibit a senescence-
associated secretory phenotype (SASP), which includes the secretion of high levels of
proinflammatory cytokines, chemokines, growth modulators, angiogenic factors and matrix
metalloproteinases [
84
,
94
]. Table 1summarizes the main SASP factors, which are released
by senescent cells (cellular senescence) and the types of cells that express them. These factors
can promote senescence in neighboring normal cells in a paracrine manner. In addition,
SASPs may activate immune responses, contribute to chronic inflammation (inflammaging)
and have been reported to promote tumorigenesis [
84
,
85
,
87
,
95
–
101
]. Recently, it has been
shown that circulating SASPs are an indicator of age and are a medical risk in humans [
102
].
The expression of SASPs is regulated by NF-
κ
B and p38 MAPK signaling pathways, while
their recycling occurs in an autocrine way [103,104].
Biology 2024,13, 647 8 of 23
Table 1. Summary of some of the SASP factors that are expressed during aging and the type of cells
that express each SASP factor. Data are mainly based on Zorina et al., 2022 [88].
Category SASP Factors
Cell Types (That Mainly Express These SASP)
Interleukins IL-1, IL-1b, IL-6, IL-7, IL-13, IL-15 Fibroblasts, Keratinocytes & Melanocytes
Chemokines IL-8, GRO-a, GRO-b, GRO-g, MCP-2, MCP-4,
MIP-1a, MIP-3a, HCC-4, eotaxin-1, eotaxin-3 Fibroblasts, Keratinocytes and Melanocytes
Insoluble factors Fibronectin, collagens, laminin Fibroblasts & Keratinocytes
Inflammatory molecules TGFβ, GM-CSE, G-CSE, IFN-g, BCL, MIF
Keratinocytes.
TGFβand G-CSE are also secreted from
fibroblasts, GM-CSE from macrophages, IFN-g
from T-cells, BCL from lymphocytes and MIF
from macrophages
Non-protein molecules ROS, NOS, miRNAs
Keratinocytes and Fibroblasts.
ROS are also secreted from melanocytes, NOS
from endothelial cells and miRNAs
from melanocytes
Growth factors and regulators
EGF, bFGF, HGF, KGF, VEGF, SCF, SDF-1,
PIGF, NGF Fibroblasts and Keratinocytes
Receptors and ligands ICAM-1, ICAM-3, OPG, sTNFRI, sTNFRII,
TRAIL-R3, Fas, uPAR, EGFR Fibroblasts and Keratinocytes
Proteases and regulators MMP1, MMP3, MMP10, MMP12, MMP13,
MMP14, TIMP1, TIMP2, PAI1, PAI2 Fibroblasts, Keratinocytes and Macrophages
In addition to the aforementioned SASP factors, extracellular vesicles (EVs) are also
associated with senescence. Specifically, EVs are cell-derived nanoparticles released by
almost all cells, carrying bioactive materials such as nucleic acids, lipids and proteins
to other cells. They are involved in regulating intercellular communication and various
processes, including immune regulation, cell growth and differentiation [
105
,
106
]. Evidence
suggests that EVs significantly influence aging, with senescent cells releasing more EVs
that can induce senescence in neighboring cells [
106
,
107
]. Beyond their role in promoting
senescence, EVs hold potential as therapeutic agents; they can deliver anti-aging drugs or
serve as biomarkers, with their removal potentially offering protection against age-related
diseases [107,108].
Furthermore, senescent cells are characterized by strong SA-
β
-GAL activity, a marker
that distinguishes senescent cells from quiescent and postmitotic cells [
109
]. It should be
mentioned that, in contrast to cellular senescence, quiescence is a temporary arrest of the
cell cycle, which can be reversed under certain circumstances. However, some studies have
reported that senescent cells, mainly in tumors, can re-enter the cell cycle [110–112].
It is important to note that aging and cellular senescence are not synonymous terms.
Senescence can occur at any stage of life, with beneficial effects such as the maintenance
of tissue homeostasis and wound healing [
113
,
114
]. In addition, during embryogenesis,
senescent cells contribute to tissue development, and in later life stages, they can play
an important role in tissue repair and tumor suppression [
91
,
114
,
115
]. Controlled and
regulated senescence is favorable for tissue homeostasis, in contrast to senescent cells
expressing SASPs, which may inhibit tissue repair and regeneration, thereby contributing
to aging [
83
,
84
]. Moreover, cellular senescence can result in cell-cycle arrest and prevent the
carcinogenic mutations to pass through other cells and induce the clearance from the im-
mune system. Nevertheless, there are some pre-clinical and clinical data that show a strong
positive correlation between a reduction in senescent cells and functional improvements in
the skin [116].
Biology 2024,13, 647 9 of 23
1.3.1. Senescence in Skin Cells
Fibroblasts are the most abundant cell type in the dermis, and their senescence is a
primary driver of skin aging [
35
]. Many intrinsic and extrinsic factors can promote the
shortening of telomeres, mitochondrial dysfunction and cell-cycle arrest in fibroblasts, lead-
ing to their senescence. Senescence in fibroblasts results in the expression of SASP factors
such as interleukins, MMPs and other chemokines, accompanied by an increase in the
levels of p16INK4a, p21, p53 and SA-
β
-gal [
109
,
117
,
118
]. Elevated MMP expression leads
to a reduction in collagen and the formation of wrinkles [
119
–
121
]. Mechanistically, during
chronological aging, the expression levels of TGF
β
RII are reduced, thereby compromising
TGF
β
signaling [
122
]. Consequently, MMPs, which are normally downregulated through
the TGF-
β
signaling pathway, become upregulated and lead to the degradation of collagen,
promoting a reduction in skin thickness [123].
During aging, keratinocytes from the basal layer of the epidermis exhibit upregulated
expression of MMPs (such as MMP-1), which induces downregulated expression of ECM
components (such as collagen) and reduced proliferation rates [
124
,
125
]. Furthermore,
UV exposure can induce the generation of ROS in keratinocytes and trigger the ERK, p38
and JNK signaling pathways, leading to activation of the transcription factor AP-1 and
enhancing the expression of MMPs. Apart from MMPs, proinflammatory cytokines are
highly expressed in senescent keratinocytes [
126
]. Previous studies have indicated that
UVB exposure increases the levels of p16INK4a, p21 and p53 in keratinocytes, along with
enhanced SA-
β
-GAL activity. On the other hand, samples from photoprotected skin areas
revealed that melanocytes are predominantly p16-positive [
126
,
127
]. Moreover, an increase
in the number of p16-positive cells has also been observed in the epidermis of age-related
pathologies, such as dermatoporosis [128].
The populations of melanocytes decline during skin aging, leading to reduced melanin
production and decreased protection from the harmful effects of UVR [
41
]. Senescent
melanocytes are subjected to morphological changes such as increased size and expression
of inflammaging markers. There is also a change in the size of melanosomes and an
elongation of their dendrites [
129
]. Moreover, it is proven that melanocytes are the primary
epidermal cells expressing p16INK4a during aging, especially in parts of the skin which
are photoprotected. Their SASP induces in the neighboring cells, such as keratinocytes, a
telomere dysfunction and prevents their proliferation [127,130].
The skin provides a defense for our body, through several types of immune cells, such
as Langerhans cells, dendritic cells, macrophages, monocytes and T cells [
131
]. During
aging, the functionality of the immune cells of the skin is decreased, and this results
in an increased risk of skin infections and cancer [
132
]. For example, Langerhans cells
show decreased proliferation and migration rates, combined with reduced expression of
antimicrobial peptides, avoiding the protective barrier of the skin against microorganisms.
Moreover, there is a reduction in the levels of IL-1
β
, which is linked to age-related changes
in immune function, due to either overall reduced transcription or a decreased number of
Langerhans cells, as they are, especially in mice, the main source of IL-1
β
. In addition to
the reduction in the number of immune cells in the skin, an increase in specific populations
may have adverse effects. Previous studies have reported that samples from photoaged
skin show increased numbers of monocytes and macrophages, which express MMPs and
ROS, leading to degradation of the ECM and chronic inflammation [133,134].
Methods for Induction of Cellular Senescence
Despite the scientific knowledge gained over the past few years in the field of aging,
the mechanisms of cellular senescence and their implication in age-related diseases are
not fully understood. The development of assays to study senescence
in vitro
are of major
importance, in order to bridge the gap in knowledge regarding the molecular pathways
that are implicated in senescence [135].
Regarding cellular models for studying senescence, the use of different cell lines is
quite common. One of the most widely used cell lines is the normal human epidermal ker-
Biology 2024,13, 647 10 of 23
atinocytes (NHEKs), which display the same biochemical properties as skin keratinocytes.
However, one limitation is that primary keratinocytes have a high heterogenous popu-
lation, as they composed of stem cells and differentiated cells [
136
]. Another cell type
that is extensively used is the HaCaT cell line, which can divide infinitely. It is easy to
handle and resembles the characteristics of human keratinocytes [
137
,
138
]. A limitation of
using HaCaT cells is their poor response to Ca
2+
compared to NHEKs and their abnormal
expression of proteins like filaggrin and loricrin [
139
,
140
]. Other than these cell types,
fibroblasts and melanocytes can also be used for the study of cellular senescence
in vitro
.
Except from 2D cell-line-based assays, 3D
in vitro
skin models containing all viable and
nonviable cell layers and different cell types are commonly used now. Cultivation is easy,
with the probability of multiplexing, while it may limit or even replace the use of animal
models. Despite their advantages, 3D skin models still have some limitations, as they
are composed of only one cell type, and there are no collagens, fibroblasts, blood vessels,
melanocytes, Langerhans cells and leukocytes [141].
One of the most well-studied methods to induce senescence
in vitro
is the use of
ionizing or ultraviolet irradiation to induce DNA damage. Apart from different types of
irradiation, chemotherapeutic drugs and crosslinking agents, such as doxorubicin, can
also promote cellular senescence
in vitro
through the transcription and replication of DNA
damage [
142
–
144
]. In addition, the induction of oxidative stress through exposure to
hydrogen peroxide (H
2
O
2
) leads the cells to a non-proliferative state, which is known as
stress-induced premature senescence. Moreover, histone deacetylase inhibitors and DNA
demethylating agents can be used
in vitro
in order to promote senescence in normal cells,
since the expression of genes implicated in the induction of senescence is controlled by
histone acetylation and DNA methylation [
145
,
146
]. Several methods and assays have
been developed to monitor cellular senescence, such as the detection of
β
-galactosidase
(
β
-GAL) activity or immunodetection of the phosphorylated form of the histone variant
H2AX at serine 139, a sensitive marker for DNA double-strand breaks. Immunoblotting
for the senescence-associated proteins, such as p16, p21, p53 or the immuno-detection of
SASPs through ELISA, could be applied [147].
1.3.2. Investigation of Aging In Vivo
Apart from the cell-based assays previously described, transgenic mice have been
developed in order to study the aging process and the role of p16INK4a protein
in vivo
.
Many different mouse models are used to investigate the role of senescent cells and the
impact of their removal in aging and age-related diseases [
148
,
149
]. The p16-3MR mice
express a trimodality reporter fusion protein, which contains domains of Renilla luciferase
(rLUC), monomeric red fluorescent protein (mRFP) and herpes simplex virus thymidine
kinase (HSV-TK) under the control of an artificial p16 promoter. This mouse model allows
for the elimination of senescent cells by using ganciclovir, which is converted into a toxic
DNA chain terminator by HSV-TK [
114
]. In the INK-ATTAC mouse model, an FKBP-
Caspase 8 fusion protein is expressed, fused with an enhanced green fluorescent protein
(eGFP) under the control of a p16 promoter. The drug AP20187 induces the dimerization
of FKBP-Caspase 8 fusion protein that is transcriptionally active in senescent cells [
150
].
Another mouse model that is used to study aging is BubR1
H/H
. It is known that a reduction
in BubR1 protein causes chromosome aneuploidy and senescence [
151
,
152
]. BubR1
H/H
mice express less BubR1 protein than wild-type mice and exhibit a variety of age-related
phenotypes. During aging, p16 protein is accumulated in many tissues, and its elimination
can delay many defects that are linked with age. By crossing BubR1 with INK-ATTAC mice,
a progeroid mouse model is created. In this mouse model, the elimination of p16+ cells
can be achieved by using the drug AP20187 [
150
–
152
]. Sod1
−/−
is also a progeroid mouse
model, which exhibits increased oxidative DNA damage, elevated p16 and p21 expression
and many changes that also appear during human aging, such as hearing loss, cataracts,
epidermal thinning, etc. [
153
–
155
]. These mouse models are widely used in studies for
Biology 2024,13, 647 11 of 23
aging, but it is not still clear if the elimination of senescent cells or their SASP is responsible
for the positive effects in different age-related diseases [87].
1.4. Senescent Cells: A Novel Therapeutic Target for Skin Aging
Many efforts are invested in discovering therapeutics that will directly target senes-
cent cells or eliminate the SASP, which induce senescence in a paracrine way, in order to
prevent age-related diseases and increase the healthspan [
156
]. The clearance of p16Ink4a-
expressing cells in BubR1-hypomorphic progeroid mice delays aging-associated disor-
ders [
150
,
157
], and the results of other studies suggested that elimination or weakening
of the function of senescent cells may be a promising approach for the modulation of
fundamental aging processes. These strategies are collectively named ‘senotherapies’ [
87
].
There are two kinds of senotherapeutics: senolytics, which induce senolysis in senescent
cells by facilitating apoptosis due to their own SASP, and senomorphics, which attenuate
their pathological proinflammatory secretory phenotype to cause senostasis [
158
]. Each
senotherapeutic modality has various advantages and disadvantages. Many senolytic
agents including synthetic small molecules and peptides have been developed for the
in vitro
and
in vivo
elimination of senescent cells [
159
,
160
]. There is also some amount of
pre-clinical and clinical data showing a strong positive correlation between a reduction
in senescent cells and functional improvements in the skin [
116
]. Moreover, the potential
senotherapeutic effect of different compounds in skin aging has been suggested in different
studies. For example, combining Dasatinib and Quercetin reduces the expression of p16
and p21 in the human epidermis, whereas rapamycin has been found to reduce senescent
cells in human skin [161–165].
Senescent cells avoid apoptosis through the induction of prosurvival pathways. For
this reason, inhibitors of prosurvival pathways may specifically target senescent cells [
166
].
The first senolytic drugs identified, which act through inhibition of these prosurvival
pathways, were Dasatinib and Quercetin [
101
,
167
]. Dasatinib is an inhibitor of multiple
tyrosine kinases and is used as an anticancer drug, whereas Quercetin is a flavonol [
101
,
159
].
Treatment with these drugs in a murine model showed a reduction in the viability of
senescent cells, with the combination of both drugs being more effective, as opposed to
single treatments. In more detail, in aged mice, there was a decrease in the number of
senescent mouse embryonic fibroblasts (MEFs) and in the p16 expression in fat tissue and
liver. Furthermore, the expression of anti-apoptotic regulator PAI-2 was reduced, and the
prosurvival networks of senescent cells were eliminated. Moreover, the combined treatment
decreased the expression of p16 in muscle cells and the number of cells, which were positive
for SA-
β
-GAL following irradiation-induced senescence [
101
]. Importantly, the combined
administration of Dasatinib and Quercetin reduced the expression of p16 and SA-
β
-gal in
a phase I clinical trial in patients with diabetic kidney disease and idiopathic pulmonary
disease [
167
,
168
]. Several other studies have shown that the Dasatinib and Quercetin
combination is highly specific in targeting senescent cells, increasing the survival of mice
and improving their healthspan [
167
,
169
]. Moreover, systemic treatment of patients with
systemic sclerosis with Dasatinib for 169 days has shown a decrease in the expression of
SASP factors and other age-related genes that belong to pathways, such as hypoxia, the TNF-
alpha pathway, the p53 pathway and the inflammatory response signaling pathway, when
comparing clinical improvers with non-improvers [
170
]. While a reduction in senescence
markers in skin after systemic treatment with the senolytic combination Dasatinib and
Quercetin has been demonstrated in one clinical study [
167
], clinical studies showing
improvements in skin function after senolytic intervention are still missing. Similarly, there
is only a single preclinical study showing relief of UV damage in mouse skin following
topical senolytic (fisetin) application [171].
Senescent cells develop resistance to apoptosis through the upregulation of negative
modulators, such as members of the B-cell lymphoma 2 (BCL-2). For this reason, inhibitors
of the BCL-2 family of proteins can be used as senolytic drugs [
172
,
173
]. ABT-263 (known as
navitoclax) and ABT-737 are the most frequently used BCL-2 inhibitors
in vivo
, which foster
Biology 2024,13, 647 12 of 23
senescent cells to initiate apoptosis. However, these drugs did not show high potency, and
further studies should be performed in the future in order to assess them [
174
]. Additionally,
during senescence, forkhead box protein O4 (FOXO4) binds to p53. The administration of
FOXO4 inhibitors can interfere with FOXO4 binding to p53 and lead to the locomotion of
p53 and subsequent release of cytochrome C from the mitochondria and the apoptosis of
senescent cells [
175
]. Ginsenoside Rb1, an ingredient of traditional Chinese medicine, is
known to have several positive effects. A study from Yu et al. showed that ginsenoside
prevents the aging process in mice by regulating cell-cycle progression but has no effect on
the expression of SASP factors [176].
Another senolytic molecule known for its physiological functions, including sleep,
circadian rhythms and neuroendocrine actions, is melatonin. The levels of melatonin
decrease with age, but both
in vivo
and
in vitro
studies have shown anti-aging properties.
In more detail, it increases the activity of DNA repair enzymes; it prevents apoptosis,
protects the DNA from UV irradiation and protects from inflammaging [
177
–
179
]. A recent
study showed that melatonin also has senolytic effects ex vivo, as it can downregulate
the mTORC1 pathway and the expression of MMP-1 and upregulates the expression of
molecules which are important for skin rejuvenation [180].
As mentioned before, CD44 is the main cell surface receptor for HA and, in combi-
nation with other molecules, forms the hyalurosome. Suppression of CD44 leads to skin
atrophy [
80
], but a combination of retinaldehyde (RAL) with HA fragments (HAFi) induces
skin hyperplasia [
181
]. Topical application of RAL and HAFi for 1 month significantly
reduced the number of p16Ink4a-positive cells in the epidermis and dermis in dermato-
porosis patients, which also showed a significant clinical improvement with an increase in
skin thickness, showing a senotherapeutic effect [181,182] (Figure 2).
1
Figure 2. Schematic representation of skin with dermatoporosis, before and after the treatment with
senolytics (created with BioRender.com).
Apart from directly targeting senescent cells, another strategy is to block the produc-
tion and secretion of SASP factors, with agents known as senomorphics [
158
]. It is known
that the mTOR pathway promotes the expression of SASP factors. In more detail, mTOR is
a serine/threonine kinase that regulates cellular growth and metabolism. It is evolutionary
conserved and consists of two different complexes, mTORC1 and mTORC2, which differ
functionally and structurally [
183
,
184
]. As many of the hallmarks of aging are affected by
mTOR, the depletion of mTORC1 in non-vertebrates enhances longevity [
185
]. Rapamycin
Biology 2024,13, 647 13 of 23
is used as an inhibitor of the mTOR pathway and has been reported to delay aging and
extend lifespan in mice through a reduction in p16 protein levels and an increase in the
expression of collagen VII [
186
–
188
]. The disadvantage of this therapy is that its long-term
effects on people’s healthspan are not fully clarified, and the mechanism of mTORC1 down-
regulation is not fully understood [
189
]. Another compound with a senomorphic effect is
metformin, which modulates the NF-
κ
B signaling pathway and reduces the secretion of
SASP [
190
,
191
]. Furthermore, a number of monoclonal antibodies against components of
the SASP or their receptors, such as IL-6, IL-1
α
, IL-1
β
and TNF, have been described to
harbor senomorphic effects [192–195].
Resveratrol is another agent that may have both beneficial and detrimental effects
during the aging process. At low concentrations, it can act as a senomorphic and can
suppress cellular senescence and SASPs, but at higher concentrations, it can promote
senescence and/or cell death. Furthermore,
in vivo
studies have shown that, in mice with
high-fat diets, resveratrol can extend their lifespan, but it does not have the same effect
in mice with standard diets [
196
,
197
]. Additionally, procyanidin C1 (PCC1), a compound
found in grape seed extract flavonoids, has both senolytic and senomorphic effects. At
low concentrations, it can eliminate the SASP factors, whereas at higher concentrations, it
can selectively kill senescent cells through the induction of apoptosis, according to
in vivo
studies [198,199].
Currently, senolytics seem to be a better anti-senescence strategy compared to senomor-
phics, since they can directly target senescent cells, and there is no need for the continuous
administration of inhibitors for the expression of SASP factors. Moreover, by using senolyt-
ics, there is less risk for the generation of mutations in senescent cells, which will favor
tumorigenesis [87].
While current senotherapies appear promising, they are associated with various side
effects, and there are no established guidelines for their timing and dosage. For example,
the systemic administration of senolytics may increase the risk of cirrhosis in patients with
liver fibrosis or hinder wound healing, since senescent cells are crucial for this process.
Consequently, further research is needed to discover new senotherapies or to improve
existing ones to reduce these side effects [
200
,
201
]. These novel therapies would modulate
the senescence secretome by selectively targeting SASPs, such as interleukines, chemokines,
growth factors or MMPs (Table 2). In addition, further studies should be performed in
order to determine whether prolonged treatment with senolytics or senomorphics or their
administration in advanced age stages may show toxicity.
Table 2. Summary of some of the senolytics and senomorphics that have been identified as well as
their method of action.
Compound Target Disease Effect References
Dasatinib + Quercetin Inhibition of prosurvival
pathways
Idiopathic pulmonary
fibrosis (IPF)
Systemic sclerosis (SSc)
Persistent physical
dysfunction
Senolytics
Reduce SASP factors in
humans and the adipose
tissue senescent cell
burden and in mice reduce
senescent cells
[167,169,170]
ABT-263 & ABT-737
Block the interaction of
BCL-2, BCL-XL, BCL-W
with BCL-2 homology
domain containing
proapoptotic proteins
DNA damage in lungs
Aged epidermis
Senolytics
Elimination of senescent
cells in mice
[158,202,203]
FOXO4-DRI Blocks the interaction of
FOXO4 and p53 Aging
Senolytic
Eliminates senescent
human fibroblasts and
chondrocytes in vitro and
senescent cells in mice
[175,204]
Biology 2024,13, 647 14 of 23
Table 2. Cont.
Compound Target Disease Effect References
Fisetin Antioxidant
Progeroid syndrome
Skin damage from UV
exposure
Senolytic
Decreases specific types of
senescent cells in murine
models and in human
adipose tissue explants
[171,205]
Geldanamycin HSP90 inhibitor Aging
Senolytic
In human cells and in
mouse models delayed
age-related co-morbidities
[206,207]
Panobinostat Histone deacetylase
inhibitor
Upper aerodigestive
and lung malignancies
Senolytic
In vitro
kills senescent cells
that accumulate during
chemotherapy
[208]
Metformin Prevention of oxidative
stress
Stress-induced
senescence of adipose
derived stromal cells
Senomorphic
In human cells decreases
senescence and SASPs
[209]
Rapamycin Target mTORC1 Cellular senescence
Senomorphic
Reduces senescence and
SASPs in cell lines from
human, mouse and rat
[158]
SB203580 p38MAPK inhibitor Senescence caused by
irradiation
Senomorphic
Reduces the secretion of
SASP in mice
[210]
Procyanidin C1
Inhibition of SASP or
induction of mitochondrial
dysfunction
Aging and age-related
diseases
Senolytic and senomorhic
At low concentrations
reduce the SASP, whereas
in higher concentrations
promotes the production
of ROS
[198,199]
Rutin Weakens the interaction
between ATM and TRAF6 Cellular senescence
Senomorphic
Reduces the expression
of SASP
[211]
Niacinamide &
Hyaluronic acid
Antioxidant
Anti-inflammatory
Immunomodulatory
Skin aging
Senomorphic
Reduces the expression
of SASP
[212]
Polyphenolic
Flavonoids
Targeting of regulatory
pathways (such as p38
signaling pathway,
PI3K/Akt, mTOR and
JAK/STAT) or the
expression of transcription
factors or directly
SASP factors
Age-related diseases
Senomorphic
Reduces the expression
of SASP
[213]
1.5. Discussion
Skin senescence occurs due to exposure to several environmental factors or with
age onset. Since the skin is a tissue which communicates with many different organs,
SASPs, which are secreted from skin senescent cells, can direct senescence in other tis-
sues/organs [
9
]. Given the nature of senescence and the high heterogeneity of SASPs
in aged tissues, the discovery of selective biomarkers is challenging [
34
]. Therefore, a
multilevel and high-throughput approach is required. Indeed, Gorgoulis et al. proposed a
multi-marker approach consisting of a three-step workflow in order to detect senescent
cells in a given sample. The first step includes screening for SA-
β
-gal activity. The second
Biology 2024,13, 647 15 of 23
step is the verification of senescence with co-staining for frequently observed markers of
senescent cells, like p16INK4a, p21 and some other SASP factors. The last step includes the
identification of markers that are expressed in specific types of senescence [
91
]. Moreover,
single-cell analysis can provide more insight about the presence of senescent cells in specific
tissues and their role in tissue function as well as in the aging process [91,214].
Until now, the selective removal of senescent cells or the modulation of SASP factors
has been assessed by using specific drugs (senolytics or senomorphics), which can prevent
their accumulation and suppress age-related pathologies. It should be noted that these
drugs should exhibit high potency in order to prevent senescent cells from acquiring drug
resistance. This phenomenon may occur through the acquisition of mutations, which
may favor the cell-cycle re-entry of a subpopulation of senescent cells, leading to the
generation of aggressive clones [
91
,
111
,
160
,
215
]. To this end, further research is required
to investigate potential interactions between the pathways that are responsible for SASP
expression. Other future objectives include the identification of the whole repertoire of
SASP factors, the investigation of their role in age-related diseases and the identification
of cell subpopulations that undergo senescence [
216
–
218
]. Studies using the INK-ATTAC
mouse model have shown that the elimination of p16INK4a-positive senescent cells delays
age-related diseases and improves the healthy lifespan, as senescent cells promote tumor
progression and impairment in certain organs, and there were no adverse effects from
their removal [
150
,
157
]. According to studies, the use of senolytic agents is a promising
approach for age-related pathologies, as they eliminate the appearance of senescent cells,
but we should mention that the impact of senolytic-based therapies in humans is still
unknown [182].
In conclusion, skin aging results from various internal and external factors that induce
cellular senescence. This process significantly affects people’s lives, being associated with
different pathologies and promoting senescence in other tissues through the secretion of
SASPs. Recently, senotherapies have been developed to eliminate senescent cells or the
factors they secrete, but it remains unclear whether these therapies have any consequences
for humans. To this end, further research should be performed in order to elucidate the
role of senescent cells and SASP in organismal aging, as it is known that they are not
synonymous and shed light on the impact of their removal in humans as well as the effect
of skin aging in the aging process of other tissues and organs. For this reason, our group is
currently working on the identification of some molecules that can reduce senescent cells
and SASP factors in aged mouse and human skin. The therapeutic removal of p16Ink4a-
positive cells that accumulate during senescence may be an attractive approach to reverse
skin aging and lead to discoveries of novel targeted therapy strategies.
Author Contributions: Conceptualization, E.K. and G.K., writing—original draft preparation, E.K.
and E.L., review, editing and supervision, G.K. All authors have read and agreed to the published
version of the manuscript.
Funding: This work was funded by a grant from the Louis-Jeantet Foundation (funding number
ME11886), Geneva, Switzerland.
Conflicts of Interest: The authors declare no conflicts of interest.
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