[Show abstract][Hide abstract] ABSTRACT: Checkpoint kinase Chk1 is constitutively active in many cancer cell types and new generation Chk1 inhibitors show marked antitumor activity as single agents. Here we present a hitherto unrecognized mechanism that contributes to the response of cancer cells to Chk1 targeted therapy. Inhibiting chronic Chk1 activity in cancer cells induced the tumor suppressor activity of protein phosphatase PP2A, which by dephosphorylating MYC serine 62, inhibited MYC activity and impaired cancer cell survival. Mechanistic investigations revealed that Chk1 inhibition activated PP2A by decreasing the transcription of CIP2A, a chief inhibitor of PP2A activity. Inhibition of cancer cell clonogenicity by Chk1 inhibition could be rescued in vitro either by exogenous expression of CIP2A or by blocking the CIP2A-regulated PP2A complex. Chk1-mediated CIP2A regulation was extended in tumor models dependent on either Chk1 or CIP2A. The clinical relevance of CIP2A as a Chk1 effector protein was validated in several human cancer types, including neuroblastoma where CIP2A was identified as a NMYC-independent prognostic factor. Since the Chk1-CIP2A-PP2A pathway is driven by DNA-PK activity, functioning regardless of p53 or ATM/ATR status, our results offer explanative power for understand how Chk1 inhibitors mediate single-agent anticancer efficacy. Further, they define CIP2A-PP2A status in cancer cells as a pharmacodynamic marker for their response to Chk1-targeted therapy.
[Show abstract][Hide abstract] ABSTRACT: Epithelial architecture is formed in tissues and organs when groups of epithelial cells are organized into polarized structures. The epithelial function and integrity as well as signaling across the epithelial layer is orchestrated by apical junctional complexes (AJCs), which are landmarks for PAR/CRUMBS and lateral SCRIB polarity modules and by dynamic interactions of the cells with underlying basement membrane (BM). These highly organized epithelial architectures are demolished in cancer. In all advanced epithelial cancers, malignant cells have lost polarity and connections to the basement membrane and they have become proliferative, motile, and invasive. Clearly, loss of epithelial integrity associates with tumor progression but does it contribute to tumor development? Evidence from studies in Drosophila and recently also in vertebrate models have suggested that even the oncogene-driven enforced cell proliferation can be conditional, dependant on the influence of cell-cell or cell-microenvironment contacts. Therefore, loss of epithelial integrity may not only be an obligate consequence of unscheduled proliferation of malignant cells but instead, malignant epithelial cells may need to acquire capacity to break free from the constraints of integrity to freely and autonomously proliferate. We discuss how epithelial polarity complexes form and regulate epithelial integrity, highlighting the roles of enzymes Rho GTPases, aPKCs, PI3K, and type II transmembrane serine proteases (TTSPs). We also discuss relevance of these pathways to cancer in light of genetic alterations found in human cancers and review molecular pathways and potential pharmacological strategies to revert or selectively eradicate disorganized tumor epithelium.
Full-text · Article · Dec 2011 · Advances in Cancer Research
[Show abstract][Hide abstract] ABSTRACT: Machiavelli wrote, in his famous political treatise Il Principe, about disrupting organization by planting seeds of dissension or by eliminating necessary support elements. Tumor cells do exactly that by disrupting the organized architecture of epithelial cell layers during progression from contained benign tumor to full-blown invasive cancer. However, it is still unclear whether tumor cells primarily break free by activating oncogenes powerful enough to cause chaos or by eliminating tumor suppressor genes guarding the order of the epithelial organization. Studies in Drosophila have exposed genes that encode key regulators of the epithelial apicobasal polarity and which, upon inactivation, cause disorganization of the epithelial layers and promote unscheduled cell proliferation. These polarity regulator/tumor suppressor proteins, which include products of neoplastic tumor suppressor genes (nTSGs), are carefully positioned in polarized epithelial cells to maintain the order of epithelial structures and to impose a restraint on cell proliferation. In this review, we have explored the presence and prevalence of somatic mutations in the human counterparts of Drosophila polarity regulator/tumor suppressor genes across the human cancers. The screen points out LKB1, which is a causal genetic lesion in Peutz-Jeghers cancer syndrome, a gene mutated in certain sporadic cancers and a human homologue of the fly polarity gene par-4. We review the evidence linking Lkb1 protein to polarity regulation in the scope of our recent results suggesting a coupled role for Lkb1 as an architect of organized acinar structures and a suppressor of oncogenic c-Myc. We finally present models to explain how Lkb1-dependent formation of epithelial architecture is coupled to suppression of normal and oncogene-induced proliferation.
[Show abstract][Hide abstract] ABSTRACT: TRAIL ligand induces selectively apoptosis in tumor cells by binding to two death receptors (DR4 and DR5) and holds promise as a potential therapeutic agent against cancer. While it has been known for long time that TRAIL receptors are commonly expressed in wide variety of normal tissues, it is not well understood why TRAIL kills tumor cells but leaves normal cells unharmed. The prototypic oncogene c-Myc promotes the cell cycle and simultaneously primes activation of the Bcl-2 family controlled mitochondria apoptosis pathway. A striking reflection of the c-Myc-dependent apoptotic sensitization is the dramatic c-Myc-induced vulnerability of cells to TRAIL and other death receptor ligands. Here we summarize the recent findings regarding the death mechanisms of TRAIL/TRAIL receptor system and the connection of c-Myc to the mitochondrial apoptosis pathway, focusing on our work that couples c-Myc via Bak to the TRAIL death receptor pathway. Finally, we present a mitochondria-priming model to explain how c-Myc-Bak interaction amplifies the TRAIL-induced caspase 8-Bid pathway to induce full-blown apoptosis. We discuss the implications of these findings for understanding the selective cytotoxicity of TRAIL and for the therapeutic exploitation of the death receptor pathway.
[Show abstract][Hide abstract] ABSTRACT: Cellular organization into epithelial architecture maintains structural integrity and homeostasis by suppressing cell proliferation and apoptosis. However, it is unclear whether the epithelial organization is sufficient to block induction of cell-autonomous cell cycle progression and apoptotic sensitivity by activated oncogenes. We show that chronic activation of oncogenic c-Myc, starting in the developing 3D organotypic mammary acinar structures, results in hyperproliferation and transformed acinar morphology. Surprisingly, acute c-Myc activation in mature quiescent acini with established epithelial architecture fails to reinitiate the cell cycle or transform these structures. c-Myc does reinitiate the cell cycle in quiescent, but structurally unorganized, acini, which demonstrates that proper epithelial architecture is needed for the proliferation blockade. The capability of c-Myc to reinitiate the cell cycle in acinar structures is also restored by the loss of LKB1, a human homologue of the cell polarity protein PAR4. The epithelial architecture also restrains the apoptotic activity of c-Myc, but coactivation of c-Myc and a complementary TNF-related apoptosis-inducing ligand death receptor pathway can induce a strong Bim and Bid-mediated apoptotic response in the established acini. The results together expose surprising proliferation and apoptosis resistance of organized epithelial structures and identify a role for the polarity regulator LKB1 in the development of c-Myc-resistant cell organization.
Full-text · Article · Oct 2007 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Oncogenic c-Myc renders cells sensitive to TRAIL-induced apoptosis, and existing data suggest that c-Myc sensitizes cells to apoptosis by promoting activation of the mitochondrial apoptosis pathway. However, the molecular mechanisms linking the mitochondrial effects of c-Myc to the c-Myc-dependent sensitization to TRAIL have remained unresolved. Here, we show that TRAIL induces a weak activation of procaspase-8 but fails to activate mitochondrial proapoptotic effectors Bax and Bak, cytochrome c release or downstream effector caspase-3 in non-transformed human fibroblasts or mammary epithelial cells. Our data is consistent with the model that activation of oncogenic c-Myc primes mitochondria through a mechanism involving activation of Bak and this priming enables weak TRAIL-induced caspase-8 signals to activate Bax. This results in cytochrome c release, activation of downstream caspases and postmitochondrial death-inducing signaling complex -independent augmentation of caspase-8-Bid activity. In conclusion, c-Myc-dependent priming of the mitochondrial pathway is critical for the capacity of TRAIL-induced caspase-8 signals to activate effector caspases and for the establishment of lethal caspase feedback amplification loop in human cells.
[Show abstract][Hide abstract] ABSTRACT: Activation of c-Myc sensitizes cells to apoptosis induction by ligand-activated death receptors. Such sensitization to death receptors by oncogenes may well be the mechanism underlying tumor cell sensitivity to tumor necrosis factor (TNF) or TNF-related apoptosis-inducing ligand (TRAIL). The mechanism by which this c-Myc-induced sensitization occurs is unclear but could involve modulation of expression of death receptors or their ligands or potentiation of the sensitivity of mitochondria to release pro-apoptotic effectors such as holocytochrome c. Here, we show that ectopic expression of the death receptor signaling protein RIP (receptor-interactive protein) triggers apoptosis via a FAS-associated death domain protein (FADD) and caspase 8-dependent pathway. Induction of apoptosis by this intracellular activation of the death receptor signaling pathway is significantly augmented by c-Myc expression. Moreover, c-Myc expression strongly promotes the potential of RIP to induce cytochrome c release from mitochondria. This implicates the mitochondrial apoptotic pathway in this synergy, a notion confirmed by the inability of c-Myc to sensitize to RIP killing in cells lacking the obligate mitochondrial apoptotic effectors Bax and Bak. We conclude that the lethality of the RIP-activated cytosolic caspase 8 pathway is augmented by c-Myc priming mitochondria to release cytochrome c. This places the intersection of apoptotic synergy between c-Myc and death receptor signaling downstream of the death receptors.
No preview · Article · Dec 2002 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: Expression of the Kaposi's sarcoma-associated herpesvirus (KSHV) cyclin D homolog, K cyclin, is thought to contribute to viral oncogenesis. We show that K cyclin expression in primary cells sensitizes to apoptosis and induces growth arrest, both of which are dependent on p53 but independent of E2F1 or p19(ARF). DNA synthesis, but not cytokinesis, continues in K cyclin-expressing cells, leading to multinucleation and polyploidy. Such polyploid cells exhibit pronounced centrosome amplification and consequent aneuploidy. Our data suggest that K cyclin expression leads to cytokinesis defects and polyploidy, which activates p53. However, in the absence of p53, such cells survive and expand as an aneuploid population. Corroborating these findings, in vivo Emu; K cyclin expression cooperates with p53 loss in the induction of lymphomas.
[Show abstract][Hide abstract] ABSTRACT: The contact of natural killer (NK) cells with foreign cells and with certain virus-infected or tumor cells triggers the cytolytic machinery of NK cells. This triggering leads to exocytosis of the cytotoxic NK cell granules. The oncoproteins c-Myc and E1A render cells vulnerable to NK cell mediated cytolysis yet the mechanisms of sensitization are not well understood. In a model where foreign cells (rat fibroblasts) were cocultured with human IL-2 activated NK cells, we observed that NK cells were capable of efficiently killing their targets only if the cells overexpressed the oncogene c-Myc or E1A. Both the parental and the oncogene expressing fibroblasts similarly triggered phosphoinositide hydrolysis in the bound NK cells, demonstrating that NK cells were cytolytically activated in contact with both resistant parental and oncogene expressing sensitive target fibroblasts. The cell death was independent of wild-type p53 and was not inhibited by an anti-apoptotic protein EIB19K. These results provided evidence that c-Myc and E1A activated the NK cell induced cytolysis at a post-triggering stage of NK cell-target cell interaction. In consistence, the c-Myc and E1A overexpressing fibroblasts were more sensitive to the cytolytic effects of isolated NK cell-derived granules than parental cells. The data indicate that oncogenes activate the cytotoxicity of NK cell granules. This mechanism can have a role in directing the cytolytic action of NK cells towards the virus-infected and cancer cells.
[Show abstract][Hide abstract] ABSTRACT: Normal fibroblasts are resistant to the cytotoxic activity of tumor necrosis factor-alpha, but are rendered TNF-sensitive upon oncogenic expression of c-Myc. Free radical generation has been implicated in non-cytotoxic TNF-signaling but also as a mediator of TNF-induced cell death. In this study we used Rat1 fibroblasts containing a conditionally active form of oncogenic c-Myc (MycER) to investigate single cell line TNF-induced free oxygen radical formation during the non-cytotoxic TNF-response (inactive c-Myc) and cytotoxic response (active c-Myc). The generation of reactive oxygen species (ROS) was assayed using a fluorescent probe, dichlorodihydrofluorescein (DCFH-DA), and the following cellular injury by measuring the high energy nucleotide (ATP, ADP and AMP) depletion. We found that TNF treatment of Rat1 cells containing c-Myc in an inactive form did not induce a detectable level of ROS generation. In contrast, TNF treatment of Rat1 cells containing activated c-Myc caused fluorescence reaction indicative of ROS generation within 80 min after DCFH-DA exposure of the cells. The nucleotide depletion likely reflected the action of ROS, since the nucleotide depletion caused by TNF or oxidants such as menadione or H2O2 in cells with active c-Myc was partly inhibited by the anti-oxidant N-acetylcysteine.
[Show abstract][Hide abstract] ABSTRACT: Normal fibroblasts are resistant to the cytotoxic action of tumor necrosis factor (TNF), but are rendered TNF-sensitive upon deregulation of c-Myc. To assess if oncoproteins induce the cytotoxic TNF activity by modulating TNF signaling, we investigated the TNF-elicited signaling responses in fibroblasts containing a conditionally active c-Myc protein. In association with cell death, c-Myc impaired TNF-induced activation of phospholipase A2, JNK protein kinase and cell survival-signaling-associated NF-kappaB transcription factor complex. The TNF-induced death of mouse primary fibroblasts expressing deregulated c-Myc was inhibited by transient overexpression of the p65 subunit of NF-kappaB, which increased NF-kappaB activity in the cells. Unlike other TNF-induced signals, TNF-induced accumulation of the wild-type p53 mRNA and protein was not inhibited by c-Myc. TNF, with c-Myc, induced apoptosis in mouse primary fibroblasts but only weakly in p53-deficient primary fibroblasts. The C-terminal domain of p53, which is a transacting dominant inhibitor of wild-type p53, failed to inhibit apoptosis by c-Myc and TNF, suggesting that the cell death was not dependent on the transcription-activating function of p53. Taken together, the present findings show that the cytotoxic activity of TNF towards oncoprotein-expressing cells involves p53 and an impaired signaling for survival in such cells.
[Show abstract][Hide abstract] ABSTRACT: Gene activation by translocation between an oncogene and an immunoglobulin heavy-chain gene, which leads to increased expression of the oncoprotein, is a well-known mechanism in the genesis of B-cell lymphomas. In contrast, the role of gene amplification in activation of oncogenes in non-Hodgkin's lymphomas is poorly characterized. To study the BCL2 amplification we performed comparative genomic hybridization (CGH), Southern blot hybridization, Western analysis, immunohistochemistry, metaphase fluorescence in situ hybridization, and chromosome analysis on 26 cases of diffuse large B-cell lymphoma (large noncleaved cell lymphoma). The gain or high-level amplification of 18q was found in eight tumors (31%) by CGH, and Southern analysis revealed BCL2 amplification in these cases, but not in the cases with normal chromosome 18 or t(14;18)(q32;q21). Western immunoblot analysis and immunohistochemistry revealed a high-level expression of BCL2 protein in the cases with BCL2 amplification and t(14;18)(q32;q21). However, translocation (14;18)(q32;q21) was not detected in any of the cases with BCL2 amplification. Therefore, our results suggest that amplification of the BCL2 gene is an important mechanism for BCL2 protein overexpression in diffuse large B-cell lymphoma.
[Show abstract][Hide abstract] ABSTRACT: The vascular endothelial growth factor (VEGF) family has recently been expanded by the isolation of two additional growth factors, VEGF-B and VEGF-C. Here we compare the regulation of steady-state levels of VEGF, VEGF-B and VEGF-C mRNAs in cultured cells by a variety of stimuli implicated in angiogenesis and endothelial cell physiology. Hypoxia, Ras oncoprotein and mutant p53 tumor suppressor, which are potent inducers of VEGF mRNA did not increase VEGF-B or VEGF-C mRNA levels. Serum and its component growth factors, platelet-derived growth factor (PDGF) and epidermal growth factor (EGF) as well as transforming growth factor-beta (TGF-beta) and the tumor promoter phorbol myristate 12,13-acetate (PMA) stimulated VEGF-C, but not VEGF-B mRNA expression. Interestingly, these growth factors and hypoxia simultaneously downregulated the mRNA of another endothelial cell specific ligand, angiopoietin-1. Serum induction of VEGF-C mRNA occurred independently of protein synthesis; with an increase of the mRNA half-life from 3.5 h to 5.5-6 h, whereas VEGF-B mRNA was very stable (T 1/2>8 h). Our results reveal that the three VEGF genes are regulated in a strikingly different manner, suggesting that they serve distinct, although perhaps overlapping functions in vivo.
[Show abstract][Hide abstract] ABSTRACT: Tumor necrosis factor-alpha (TNF) is a multifunctional cytokine which is cytotoxic for some tumor cells and transformed cells. The molecular mechanisms which render transformed and tumor cells sensitive to the cytotoxic action of TNF are unclear. We show here that an increased expression of the c-Myc oncoprotein strongly increases cellular sensitivity to TNF cytotoxicity. In Rat1A fibroblasts, which are resistant to TNF, the addition of TNF with a concomitant activation of a hormone-inducible c-Myc-estrogen receptor chimera (MycER) resulted in apoptotic cell death. Similarly, c-Myc overexpression enhanced the sensitivity of NIH3T3 fibroblasts to TNF-induced death. The c-Myc and TNF-induced apoptosis was inhibited by ectopic expression of the Bcl2 oncoprotein and by the free oxygen radical scavenging enzyme Mn superoxide dismutase. Furthermore, in highly TNF-sensitive fibrosarcoma cells, antisense c-myc oligodeoxynucleotides caused a specific inhibition of TNF cytotoxicity. Our results suggest that the deregulation of c-Myc, which is common in human tumors and tumor cell lines is one reason why these cells are TNF sensitive.
[Show abstract][Hide abstract] ABSTRACT: The frequencies of major breakpoint region (MBR) and minor cluster region (MCR) breakpoint sites of t(14;18) were examined by polymerase chain reaction and Southern blotting in 50 non-Hodgkin's lymphomas with cytogenetic evidence of t(14;18). A translocation breakpoint was detected in 41 cases (82%). The MBR was involved in 66%, and the MCR in 16% of the cases. Most cases in the present series were lymphomas with a follicular or diffuse growth pattern, 38 being of the centroblastic/centrocytic type and eight of the centroblastic type. The series also included four lymphomas of probable non-follicular center cell origin. MBR and/or MCR breakpoints were found in all studied lymphoma subtypes and in the majority of these most of the breakpoints were in the MBR and a minor portion of the breakpoints in the MCR. Our results suggest that a breakpoint site is not related to growth pattern or neoplastic cell type in follicular center cell lymphomas with t(14;18).
No preview · Article · May 1994 · Leukemia Research
[Show abstract][Hide abstract] ABSTRACT: One of the first oncogenes identified from human tumors was c-myc, which is frequently activated in Burkitt's lymphomas due to chromosomal translocations. Subsequently, members of the myc oncogene family were found to be amplified in neuroblastoma and small-cell lung cancer. In normal cells, Myc activity has been shown to be both necessary and sufficient for resting cells to enter the cell cycle. Interestingly, it appears that Myc not only drives the cell cycle, but also induces cell death by apoptosis in certain situations. Myc contains a transcriptional activation domain and a basic helix-loop-helix-leucine zipper DNA-binding and dimerization domain. As a heterodimer with a structurally related protein, Max, Myc can bind DNA in a sequence-specific manner. These results suggest that the Myc/Max heterodimer functions as a transcriptional activator of genes that are critical for the regulation of cell growth.
No preview · Article · Feb 1994 · Critical reviews in oncogenesis
[Show abstract][Hide abstract] ABSTRACT: TNF-alpha is a multifunctional cytokine which is cytotoxic for some cell lines. In order to characterize the early genomic response to TNF-alpha, we have analyzed the induction of a sub-set of serum-inducible immediate early genes in WEHI-S and L929 fibrosarcoma cell lines, which are sensitive to TNF-alpha, and in the 3T3-LI pre-adipocytic cell line, which is resistant to TNF-alpha cytotoxicity. Among 77 immediate early mRNAs screened by dot blot and/or Northern blot analyses, the expression of 23 mRNAs was found to be induced by TNF-alpha. Ten of these mRNAs encode proteins known to function as pro-inflammatory cytokines or transcription factors, while 13 others have as yet uncharacterized activities. The magnitude of c-fos induction by TNF-alpha inversely correlated with cell-type-specific cytotoxicity. Rapid and transient mRNA responses were observed in the TNF-alpha-resistant cells, whereas a slower and more persistent response was characteristic for TNF-alpha-sensitive cells. The prolonged induction of immediate early mRNAs may contribute to TNF-alpha-induced cellular cytotoxic responses.
No preview · Article · Nov 1993 · International Journal of Cancer
[Show abstract][Hide abstract] ABSTRACT: TNF-α is a multifunctional cytokine which is cytotoxic for some cell lines. In order to characterize the early genomic response to TNF-α, we have analyzed the induction of a sub-set of serum-inducible immediate early genes in WEHI-S and L929 fibrosarcoma cell lines, which are sensitive to TNF-α, and in the 3T3-L1 pre-adipocytic cell line, which is resistant to TNF-α cytotoxicity. Among 77 immediate early mRNAs screened by dot blot and/or Northern blot analyses, the expression of 23 mRNAs was found to be induced by TNF-α. Ten of these mRNAs encode proteins known to function as pro-inflammatory cytokines or transcription factors, while 13 others have as yet uncharacterized activities. The magnitude of c-fos induction by TNF-α inversely correlated with cell-type-specific cytotoxicity. Rapid and transient mRNA responses were observed in the TNF-α-resistant cells, whereas a slower and more persistent response was characteristic for TNF-α-sensitive cells. The prolonged induction of immediate early mRNAs may contribute to TNF-α-induced cellular cytotoxic responses.
No preview · Article · Oct 1993 · International Journal of Cancer