Serine protease Omi/HtrA2 targets WARTS kinase to control cell proliferation

The serine protease Omi/HtrA2 was initially regarded as a proapoptotic molecule that proteolyses several proteins to induce cell death. Recent studies, however, indicate that loss of Omi protease activity increases susceptibility to stress-induced cell death. These complicated findings suggest that the protease activity of Omi is involved not only in apoptosis but also in cellular homeostasis. However, the targets which Omi uses to mediate this novel process are unknown. Previously, we showed that WARTS (WTS)/large tumor-suppressor 1 mitotic kinase interacts with the protein/discs-large protein/zonula (PDZ) domain of Omi and promotes its protease activity. We now report that WTS is a substrate for Omi protease activity, thus it is not only a regulator but also a downstream target of this protease. Interaction with Omi PDZ domain is required for WTS to be proteolysed. When caspase-9-deficient mouse embryonic fibroblasts (MEFs) were treated with staurosporine, WTS was proteolysed by activated endogenous Omi without induction of cell death. Therefore, protease activity of Omi and proteolysis of WTS are not necessarily required for cell death. We found that depletion of Omi from HeLa cells results in accelerated cell proliferation despite no significant change in the duration of mitosis. The depletion of WTS showed the same effect on S phase progression. Therefore, WTS proteolytic fragment(s) generated by Omi may act as an inhibitor of G1/S progression. Our data reveal a role for Omi-mediated processing of WTS in negative regulation of cell cycle progression at interphase, suggesting a novel function of Omi other than apoptosis.     

A candidate tumor suppressor HtrA1 is downregulated in ovarian cancer

We report here that HtrA1, a candidate tumor suppressor, is downregulated in ovarian cancer. Expression of HtrA1 is downregulated in five of seven ovarian cancer cell lines. In total, 59% of primary ovarian tumors have either a complete absence or markedly reduced levels of HtrA1 expression compared to the brushings of ovarian surface epithelium. Primary ovarian tumors show high frequencies of loss of an allele at microsatellite markers near htrA1 locus on 10q26. Downregulation of HtrA1 in SKOV3 by antisense transfection promotes anchorage-independent growth, while exogenous expression of HtrA1 in OV202 induces cell death. HtrA1-induced cell death is not inhibited by the broad caspase inhibitor, zVAD(OMe)fmk, but instead reflects serine protease activity associated with HtrA1. These observations raise the possibility of HtrA1 as a candidate tumor suppressor involved in promoting serine-protease-mediated cell death and that downregulation of HtrA1 in ovarian cancer may contribute to malignant phenotype.     

Participation of Omi Htra2 serine-protease activity in the apoptosis induced by cisplatin on SW480 colon cancer cells

Apoptosis is triggered by two interconnected pathways, extrinsic and intrinsic. The intrinsic pathway is activated by genomic stress promoting mitochondrial release of apoptotic proteins. One of these proteins is Omi/Htra2, a serine protease which inactivates Inhibitor of Apoptosis Proteins (IAPs). In the present work, we assessed the participation of Omi/Htra2 in the cell death induced by the chemotherapeutic drugs 5-fluorouracil (5-FU) and cisplatin (CDDP) in SW480 colon cancer cells. CDDP and 5-FU induced apoptosis mediated by the intrinsic pathway in colon cancer cells, as demonstrated by morphological analyses, mitochondrial cytochrome c release and cleavage of caspase 3. Omi/Htra2 was also released from mitochondria of cells exposed to these drugs, as demonstrated by immunofluorescence and western blot assays of subcellular fractions. Exposure of cells to the Omi/Htra2 serine protease inhibitor UCF-101 prevented death p<0.0001 and partially suppressed reproductive cell death of cells exposed to cisplatin p<0.05, but not to 5-FU p=0.49. From these experiments we show that Omi/Htra2 serine protease activity participates in the cell death induced by CDDP but not of 5-FU in colon cancer cells.     

Participation of Omi Htra2 Serine-Protease Activity in the Apoptosis Induced by Cisplatin on SW480 Colon Cancer Cells

Abstract

Apoptosis is triggered by two interconnected pathways, extrinsic and intrinsic. The intrinsic pathway is activated by genomic stress promoting mitochondrial release of apoptotic proteins. One of these proteins is Omi/Htra2, a serine protease which inactivates Inhibitor of Apoptosis Proteins (IAPs). In the present work, we assessed the participation of Omi/Htra2 in the cell death induced by the chemotherapeutic drugs 5-fluorouracil (5-FU) and cisplatin (CDDP) in SW480 colon cancer cells. CDDP and 5-FU induced apoptosis mediated by the intrinsic pathway in colon cancer cells, as demonstrated by morphological analyses, mitochondrial cytochrome c release and cleavage of caspase 3. Omi/Htra2 was also released from mitochondria of cells exposed to these drugs, as demonstrated by immunofluorescence and western blot assays of subcellular fractions. Exposure of cells to the Omi/Htra2 serine protease inhibitor UCF-101 prevented death p<0.0001 and partially suppressed reproductive cell death of cells exposed to cisplatin p<0.05, but not to 5-FU p=0.49. From these experiments we show that Omi/Htra2 serine protease activity participates in the cell death induced by CDDP but not of 5-FU in colon cancer cells.     

Omi/HtrA2对前列腺癌细胞株PED/PEA-15表达及PC-3细胞凋亡的影响

背景与目的：促、抑凋亡因子间的相互作用与肿瘤的发生、发展密切相关。Omi／HtrA2是新近发现的一种凋亡调节因子,PED／PEA—15是一种广泛表达的抗凋亡蛋白。本研究旨在探讨Omi／HtrA2对PED／PEA—15表达和前列腺癌细胞PC-3凋亡的影响。方法：构建Omi／HtrA2的表达载体和siRNA载体。并用脂质体法分别将两载体转染至PC-3细胞中,Westernblot和ELISA法检测Omi／HtrA2对PED／PEA—15表达和细胞凋亡的影响;Caspase-8检测试剂盒检测PED／PEA-15对Caspase-8活性的影响;Westernblot、RT—PCR法检测Omi／HtrA2特异siRNA序列对其转录、翻译的影响,流式细胞仪检测siRNA导致Omi／HtrA2基因沉默后PC一3细胞凋亡的变化。结果：酶切和DNA测序证实Omi／HtrA2的表达载体和siRNA载体构建成功。通过转染Omi／HtrA2表达载体高表达Omi／HtrA2可抑制PED／PEA-15表达,并增加肿瘤细胞的凋亡率;抑制PED／PEA—15的表达可提高Caspase-8活性。siRNA沉默Omi／HtrA2基凶后PC-3细胞对顺铂的敏感性降低。结论：Omi／HtrA2可通过抑制抗凋亡蛋白PED／PEA—15表达而在PC-3细胞凋亡中发挥重要作用。     

喉癌中Omi/HtrA2的表达及作用初探

目的:检测Omi/HtrA2在喉癌组织中的表达,初步探讨其在喉癌发生、发展中的意义。方法:采用RT-PCR与免疫组织化学方法,检测20例喉鳞状细胞癌及12例声带息肉手术切除标本中Omi/HtrA2mRNA与蛋白的表达情况。结果:癌旁正常组织中未检测到Omi/HtrA2蛋白表达,而在喉癌组织与声带息肉组织中均有表达。Omi/HtrA2基因在高分化喉鳞癌组织中的表达水平高于低分化型喉鳞癌组织、声带息肉以及癌旁的正常组织(P<0.01)。结论:Omi/HtrA2在喉鳞状细胞癌中有表达,表明Omi/HtrA2的促凋亡作用参与了喉癌的发生发展过程,为癌症治疗提供了新的理论基础。     

GRIM-19 associates with the serine protease HtrA2 for promoting cell death

We have isolated a novel interferon (IFN)-retinoid regulated cell death regulatory protein genes associated with retinoid-IFN-induced mortality (GRIM)-19 earlier. To understand its mechanism of action, we have employed a yeast-two-hybrid screen and identified serine protease HtrA2 as its binding partner. GRIM-19 physically interacts with HtrA2 and augments cell death in an IFN/all-trans retinoic acid (RA)-dependent manner. In the presence of GRIM-19, the HtrA2-driven destruction of the antiapoptotic protein X-linked inhibitor of apoptosis (XIAP) is augmented. These interactions were disrupted by an human herpes virus-8 (HHV-8)-coded oncoprotein, vIRF1, and conferred resistance to IFN/RA-induced cell death. These data show a critical role of HtrA2 in a cytokine-induced cell death response for the first time and its inhibition by a viral protein.     

Random mutagenesis of PDZ(Omi) domain and selection of mutants that specifically bind the Myc proto-oncogene and induce apoptosis

Omi is a mammalian serine protease that is localized in the mitochondria and released to the cytoplasm in response to apoptotic stimuli. Omi induces cell death in a caspase-dependent manner by interacting with the X-chromosome linked inhibitor of apoptosis protein, as well as in a caspase-independent way that relies on its proteolytic activity. Omi is synthesized as a precursor polypeptide and is processed to an active serine protease with a unique PDZ domain. PDZ domains recognize the extreme carboxyl terminus of target proteins. Internal peptides that are able to fold into a beta-finger are also reported to bind some PDZ domains. Using a modified yeast two-hybrid system, PDZ(Omi) mutants were isolated by their ability to bind the carboxyl terminus of human Myc oncoprotein in yeast as well as in mammalian cells. One such PDZ(m) domain (PDZ-M1), when transfected into mammalian cells, was able to bind to endogenous Myc protein and induce cell death. PDZ-M1-induced apoptosis was entirely dependent on the presence of Myc protein and was not observed when c-myc null fibroblasts were used. Our studies indicate that the PDZ domain of Omi can provide a prototype that could easily be exploited to target specifically and inactivate oncogenes by binding to their unique carboxyl terminus.     

Catechin, green tea component, causes caspase-independent necrosis-like cell death in chronic myelogenous leukemia

Management strategies of chronic phase chronic myelogenous leukemia (CML) have been revolutionized due to the discovery of a selective tyrosine kinase inhibitor, imatinib (Gleevec, STI571), which is substantially improving median survival. However, emergence of imatinib-resistance has put up a serious problem that requires novel treatment methods. Catechins, polyphenolic compounds in green tea, are gathering much attention due to their potential antitumor effects. So far (–)-epigallocatechin-3-gallate (EGCG), the most abundant component of catechin, has been shown to cause typical apoptosis in several tumor cell lines in most cases through activation of caspases. In this study, we showed that EGCG predominantly caused necrosis-like cell death via a caspase-independent mechanism in CML cells, K562 and C2F8, whereas imatinib induced the typical apoptotic cell death. Moreover, this caspase-independent cell death partially mediated the release of apoptosis-inducing factor, AIF, and serine protease, HtrA2/Omi, from the mitochondria to cytosol. In addition, EGCG enhanced the imatinib-induced cell death ( P <  0.01) resulting in additive cell death in K562 cells and EGCG alone, effectively reduced the viability of imatinib-resistant K562 cells ( P <  0.01). Catechin is a possible candidate for an antitumor agent that causes cell death in CML cells via a caspase-independent mechanism. ( Cancer Sci 2009; 100: 349–356)     

Omi/HtrA2在腺样囊性癌的表达及其临床意义

目的 探讨Omi/HtrA2在不同分型腺样囊性癌中的表达差异,并评价其对腺样囊性癌的临床意义.方法 纳入确诊为腺样囊性癌的患者100例,应用免疫组织化学染色方法,检测Omi/HtrA2在不同分型腺样囊性癌中的表达;并检测腺样囊性癌转移组和非转移组患者中Omi/HtrA2表达情况.结果 Omi/HtrA2表达主要位于细胞胞质中,在腺样囊性癌组织中其阳性率为83％;腺样囊性癌低级组与高级组间表达情况具有统计学差异(P＜0.05),但不同分型的腺样囊性癌组织中其表达差异无统计学意义(P＞ 0.05);Omi/HtrA2在转移组腺样囊性癌中阳性率为93.1％,在非转移组中其阳性率为46.2％,两组比较具有统计学差异(P＜0.05).结论 Omi/HtrA2与腺样囊性癌的发病密切相关,可作为临床判断腺样囊性癌的辅助性指标,对临床的诊治及预后的判断具有一定的指导价值.     

Tumor suppressor WARTS ensures genomic integrity by regulating both mitotic progression and G1 tetraploidy checkpoint function

Defects in chromosomes or mitotic spindles activate the spindle checkpoint, resulting in cell cycle arrest at prometaphase. The prolonged activation of spindle checkpoint generally leads to mitotic exit without segregation after a transient mitotic arrest and the consequent formation of tetraploid G(1) cells. These tetraploid cells are usually blocked to enter the subsequent S phase by the activation of p53/pRb pathway, which is referred to as the G(1) tetraploidy checkpoint. A human homologue of the Drosophila warts tumor suppressor, WARTS, is an evolutionarily conserved serine-threonine kinase and implicated in development of human tumors. We previously showed that WARTS plays a crucial role in controlling mitotic progression by forming a regulatory complex with zyxin, a regulator of actin filament assembly, on mitotic apparatus. However, when WARTS is activated during cell cycle and how the loss of WARTS function leads to tumorigenesis have not been elucidated. Here we show that WARTS is activated during mitosis in mammalian cells, and that overexpression of a kinase-inactive WARTS in Rat1 fibroblasts significantly induced mitotic delay. This delay resulted from prolonged activation of the spindle assembly checkpoint and was frequently followed by mitotic slippage and the development of tetraploidy. The resulting tetraploid cells then abrogated the G(1) tetraploidy checkpoint and entered S phase to achieve a DNA content of 8N. This impairment of G(1) tetraploidy checkpoint was caused as a consequence of failure to induce p53 expression by expressing a kinase-inactive WARTS. WARTS thus plays a critical role in maintenance of ploidy through its actions in both mitotic progression and the G(1) tetraploidy checkpoint.     

FAPalpha, a surface peptidase expressed during wound healing, is a tumor suppressor

Fibroblast activation protein-alpha (FAP) is a cell surface serine protease expressed at sites of tissue remodeling in embryonic development. FAP is not expressed by mature somatic tissues except activated melanocytes and fibroblasts in wound healing or tumor stroma. FAP expression is specifically silenced in proliferating melanocytic cells during malignant transformation. To study the role of FAP as a tumor suppressor, the gene for mouse fap was cloned and mutated at the catalytic domain (FAP serine mutant, FSM). We found that expression of FAP or FSM at physiologic levels in mouse melanoma cells abrogated tumorigenicity. Remarkably, the mutant form FSM lacking specific serine protease activity was a more potent tumor suppressor. Tumor rejection was not due to adaptive immune responses because RAG1-/- mice challenged with melanoma cells expressing either FAP or FSM were not tumorigenic. In in vitro assays, FAP or FSM expression restored contact inhibition, led to cell cycle arrest at G0/G1 phase, and increased susceptibility to stress-induced apoptosis. Cell death in FAP+ or FSM+ melanoma cells was readily triggered by depletion of survival factors from the media, leading to subsequent activation of caspases via the intrinsic pathway. These results show that expression of FAP is a tumor suppressor that abrogates tumorigenicity through regulation of cell growth and survival.     

A new function for the serine protease HtrA2 in controlling radiation‐induced senescence in cancer cells

Radiation therapy can induce cellular senescence in cancer cells, leading to short‐term tumor growth arrest but increased long‐term recurrence. To better understand the molecular mechanisms involved, we developed a model of radiation‐induced senescence in cultured cancer cells. The irradiated cells exhibited a typical senescent phenotype, including upregulation of p53 and its main target, p21, followed by a sustained reduction in cellular proliferation, changes in cell size and cytoskeleton organization, and senescence‐associated beta‐galactosidase activity. Mass spectrometry‐based proteomic profiling of the senescent cells indicated downregulation of proteins involved in cell cycle progression and DNA repair, and upregulation of proteins associated with malignancy. A functional siRNA screen using a cell death‐related library identified mitochondrial serine protease HtrA2 as being necessary for sustained growth arrest of the senescent cells. In search of direct HtrA2 substrates following radiation, we determined that HtrA2 cleaves the intermediate filament protein vimentin, affecting its cytoplasmic organization. Ectopic expression of active cytosolic HtrA2 resulted in similar changes to vimentin filament assembly. Thus, HtrA2 is involved in the cytoskeletal reorganization that accompanies radiation‐induced senescence and the continuous maintenance of proliferation arrest.     

Influence of casein kinase II in tumor necrosis factor-related apoptosis-inducing ligand-induced apoptosis in human rhabdomyosarcoma cells.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces apoptosis via the death receptors DR4 and DR5 in transformed cells in vitro and exhibits potent antitumor activity in vivo with minor side effects. Protein kinase casein kinase II (CK2) is increased in response to diverse growth stimuli and is aberrantly elevated in a variety of human cancers. Rhabdomyosarcoma tumors are the most common soft-tissue sarcoma in childhood. In this investigation, we demonstrate that CK2 is a key survival factor that protects tumor cells from TRAIL-induced apoptosis. We have demonstrated that inhibition of CK2 phosphorylation events by 5,6-dichlorobenzimidazole (DRB) resulted in dramatic sensitization of tumor cells to TRAIL-induced apoptosis. CK2 inhibition also induced rapid cleavage of caspase-8, -9, and -3, as well as the caspase substrate poly(ADP-ribose) polymerase after TRAIL treatment. Overexpression of Bcl-2 protected cells from TRAIL-induced apoptosis in the presence of the CK2 inhibitor. Death signaling by TRAIL in these cells was Fas-associated death domain and caspase dependent because dominant negative Fas-associated death domain or the cowpox interleukin 1beta-converting enzyme inhibitor protein cytokine response modifier A prevented apoptosis in the presence of DRB. Analysis of death-inducing signaling complex (DISC) formation demonstrated that inhibition of CK2 by DRB increased the level of recruitment of procaspase-8 to the DISC and enhanced caspase-8-mediated cleavage of Bid, thereby increasing the release of the proapoptotic factors cytochrome c, HtrA2/Omi, Smac/DIABLO, and apoptosis inducing factor (AIF) from the mitochondria, with subsequent degradation of X-linked inhibitor of apoptosis protein (XIAP). To further interfere with CK2 function, JR1 and Rh30 cells were transfected with either short hairpin RNA targeted to CK2alpha or kinase-inactive CK2alpha (K68M) or CK2alpha' (K69M). Data show that the CK2 kinase activity was abrogated and that TRAIL sensitivity in both cell lines was increased. Silencing of CK2alpha expression with short hairpin RNA was also associated with degradation of XIAP. These findings suggest that CK2 regulates TRAIL signaling in rhabdomyosarcoma by modulating TRAIL-induced DISC formation and XIAP expression.     

Make WARTS, not cancer!

The WARTS gene encodes a kinase that localizes to the mitotic apparatus of a dividing cell. Named WARTS after the growths that develop in the eyes of Drosophila in which the gene is deleted. WARTS is also implicated as a tumor suppressor in mice and humans. In this issue of Oncogene, Iida et al. describe experiments suggesting that, in addition to a role in regulating mitosis, WARTS functions to prevent further rounds of DNA synthesis and mitosis in tetraploid cells. As well as opening up new possibilities of exploring the as yet ill-defined mechanistic basis of the tetraploidy checkpoint, the involvement of a tumor-suppressor gene in this checkpoint supports its importance as a safeguard against the acquisition of genomic instability, a key event in the progression to cancer.     

The HtrA1 serine protease is down-regulated during human melanoma progression and represses growth of metastatic melanoma cells

Differential gene expression of cell lines derived from a malignant melanoma or its autologous lymph node metastasis using cDNA arrays indicated down-regulation of PRSS11, a gene encoding the serine protease HtrA1, a homolog of the Escherichia coli protease HtrA, in the metastatic line. Stable PRSS11 overexpression in the metastatic cell line strongly inhibited proliferation, chemoinvasion and Nm23-H1 protein expression in vitro, as well as cell growth in vivo in nu/nu mice. A polyclonal anti-HtrA1 serum demonstrated a significantly higher expression in primary melanomas when compared to unrelated metastatic lesions in a human melanoma tissue array, and down-modulation of HtrA1 expression in autologous lymph node melanoma metastases in seven out of 11 cases examined. These results suggest that down-regulation of PRSS11 and HtrA1 expression may represent an indicator of melanoma progression.     

Caspase-dependent and -independent cell death pathways after DNA damage (Review)

Apoptosis is known to be an important phenomenon in exerting antitumor response to cancer therapy, which is regulated by Bcl-2 family proteins through mitochondrial permeability transition (MPT). Insertion by the activated Bax/Bak in response to DNA damage induces mitochondrial membrane permeabilization (MMP) via an anion channel, VDAC in mitochondrial outer membrane that plays a crucial role in releasing small molecules such as cytochrome c, Smac/DIABLO, Omi/HtrA2, AIF, and endonuclease G leading to cell death. The released small molecules are involved in caspase-dependent and -independent cell death pathway that is inhibited by Bcl-2/xL. Despite the fact that the pancaspase inhibitor, zVAD-fmk inhibited the caspase cascade, cell death mediated by caspase-independent pathway was not blocked. Similarly, although etoposide induced-apoptosis was inhibited in Bax(-/-)/Bak(-/-)mouse embryonic fibroblasts, autophagy was not inhibited, which was regulated by Bcl-xL. It appears that the cross-talk between caspase-dependent and -independent apoptotic cell death including autophagic cell death that was mediated by MPT affects overall tumor response to anticancer treatment. In this review, to assist a comprehensive understanding of MPT-mediated cell death pathway for exploring appropriate targets in cancer therapy, role of the caspase-dependent and -independent cell death pathway in the interaction of these pathways is discussed.     

The c-Myc-interacting adaptor protein Bin1 activates a caspase-independent cell death program

Cell death processes are progressively inactivated during malignant development, in part by loss of tumor suppressors that can promote cell death. The Bin1 gene encodes a nucleocytosolic adaptor protein with tumor suppressor properties, initially identified through its ability to interact with and inhibit malignant transformation by c-Myc and other oncogenes. Bin1 is frequently missing or functionally inactivated in breast and prostate cancers and in melanoma. In this study, we show that Bin1 engages a caspase-independent cell death process similar to type II apoptosis, characterized by cell shrinkage, substratum detachment, vacuolated cytoplasm, and DNA degradation. Cell death induction was relieved by mutation of the BAR domain, a putative effector domain, or by a missplicing event that occurs in melanoma and inactivates suppressor activity. Cells in all phases of the cell cycle were susceptible to death and p53 and Rb were dispensable. Notably, Bin1 did not activate caspases and the broad spectrum caspase inhibitor ZVAD.fmk did not block cell death. Consistent with the lack of caspase involvement, dying cells lacked nucleosomal DNA cleavage and nuclear lamina degradation. Moreover, neither Bcl-2 or dominant inhibition of the Fas pathway had any effect. In previous work, we showed that Bin1 could not suppress cell transformation by SV40 large T antigen. Consistent with this finding, we observed that T antigen suppressed the death program engaged by Bin1. This observation was interesting in light of emerging evidence that T antigen has roles in cell immortalization and human cell transformation beyond Rb and p53 inactivation. In support of a link to c-Myc-induced death processes, AEBSF, a serine protease inhibitor that inhibits apoptosis by c-Myc, potently suppressed DNA degradation by Bin1. Our findings suggest that the tumor suppressor activity of Bin1 reflects engagement of a unique cell death program. We propose that loss of Bin1 may promote malignancy by blunting death penalties associated with oncogene activation.     

BRCA1-associated protein-1 is a tumor suppressor that requires deubiquitinating activity and nuclear localization

BRCA1-associated protein-1 (BAP1), a deubiquitinating enzyme of unknown cellular function, is mutated in breast and lung cancers. In this study, we have shown for the first time that BAP1 has tumor suppressor activity in vivo by showing that BAP1 can suppress tumorigenicity of lung cancer cells in athymic nude mice. We show that BAP1 fulfills another criterion of a genuine tumor suppressor because cancer-associated BAP1 mutants are deficient in deubiquitinating activity. We show for the first time that one of the two predicted nuclear targeting motifs is required for nuclear localization of BAP1 and that a truncation mutant found in a lung cancer cell line results in BAP1 that fails to localize to the nucleus. Furthermore, we show that deubiquitinating activity and nuclear localization are both required for BAP1-mediated tumor suppression in nude mice. We show that BAP1 exerts its tumor suppressor functions by affecting the cell cycle, speeding the progression through the G(1)-S checkpoint, and inducing cell death via a process that has characteristics of both apoptosis and necrosis. Surprisingly, BAP1-mediated growth suppression is independent of wild-type BRCA1. Because deubiquitinating enzymes are components of the ubiquitin proteasome system, this pathway has emerged as an important target for anticancer drugs. The identification of the deubiquitinating enzyme BAP1 as a tumor suppressor may lead to further understanding of how the ubiquitin proteasome system contributes to cancer and aid in the identification of new targets for cancer therapy.