Defects In Apoptosis Signaling Research Articles

Heightened JNK Activation and Reduced XIAP Levels Promote TRAIL and Sunitinib-Mediated Apoptosis in Colon Cancer Models.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a potent inducer of apoptosis that may be a promising agent in cancer therapy due to its selectivity toward tumor cells. However, many cancer cells are resistant to TRAIL due to defects in apoptosis signaling or activation of survival pathways. We hypothesized that a disruption of pro-survival signaling cascades with the multi-tyrosine kinase inhibitor sunitinib would be an effective strategy to enhance TRAIL-mediated apoptosis. Here we demonstrate that sunitinib significantly augments the anticancer activity of TRAIL in models of colon cancer. The therapeutic benefit of the TRAIL/sunitinib combination was associated with increased apoptosis marked by enhanced caspase-3 cleavage and DNA fragmentation. Overexpression of the anti-apoptotic factor B-cell lymphoma 2 (BCL-2) in HCT116 cells reduced TRAIL/sunitinib-mediated apoptosis, further supporting that sunitinib enhances the anticancer activity of TRAIL via augmented apoptosis. Analysis of pro-survival factors identified that the combination of TRAIL and sunitinib significantly downregulated the anti-apoptotic protein X-linked inhibitor of apoptosis protein (XIAP) through a c-Jun N-terminal kinase (JNK)-mediated mechanism. Short hairpin RNA (shRNA)-mediated knockdown of JNK confirmed its key role in the regulation of sensitivity to this combination as cells with suppressed JNK expression exhibited significantly reduced TRAIL/sunitinib-mediated apoptosis. Importantly, the therapeutic benefit of the TRAIL/sunitinib combination was validated in the HCT116-Luc and HCT15 colon cancer xenograft models, which both demonstrated significant anti-tumor activity in response to combination treatment. Collectively, our data demonstrate that sunitinib enhances TRAIL-mediated apoptosis by heightened JNK activation, diminished XIAP levels, and augmented apoptosis.

Fibroblast Growth Factor-1 Inhibition Enhances FAS-Induced Apoptosis in Mantle Cell Lymphoma Cells By Accelerated BIRC2/3 Degradation

Fibroblast growth factor-1 Inhibition Enhances FAS-Induced Apoptosis in Mantle Cell Lymphoma Cells by Accelerated BIRC2/3 Degradation.Neeraj Jain1, Tamer Khashab1, Natalie Willingham2, Felipe Samaniego1 and Lalit Sehgal21 Department of Lymphoma/Myeloma, UT MD Anderson Cancer center, Houston, TX 770542 Division of Hematology, Comprehensive Cancer Center, The Ohio State University, Columbus, OH, USAIntroduction Mantle cell lymphoma (MCL) is an aggressive form of non-Hodgkin lymphoma that is characterized by the t(11:14)(q13:p32) translocation. MCL cells have altered cyclinD1 levels, impaired cell cycle regulation, DNA damage response, and likely defects in apoptosis signaling. Furthermore, up-regulated anti-apoptotic mediators such as the target of NF-κB BIRC2 and BIRC3 were correlated with decreased apoptosis signaling. Also many cancer cells and malignant tumors show a prevalent resistance to apoptosis induction by FAS. Thus, by understanding the underpinnings of apoptosis resistance, we will be in a better position to develop strategies that improve Fas-induced killing of lymphoma cells.Methods More detailed insight into MCL pathogenesis has been delayed until the recent development of a tissue culture system, using human mesenchymal stromal cells (hMSC), suitable for propagating primary MCL cells. Isolates of primary MCL cells were co-cultured with human mesenchymal stem cells (hMSCs) and the content of MCL-ICs was analyzed by flow-cytometry based on marker expression profile; CD34-CD3-CD45+CD19-. Cytokine array was used to identify the soluble factors enriched in the co-cultures and the expression of these factors was confirmed by RT-PCR analysis. The signaling pathways employed by the newly-identified factors were blocked in 3 MCL cell lines (JVM2, Mino, Z138) to confirm their essential role in survival of MCL cells and, more importantly, for MCL-ICs.Results Co-cultures of primary MCL isolates with hMSCs supported the growth of MCL cells for over 4 weeks with continued presence of MCL-ICs (CD34-CD3-CD45+CD19-) representing about 1% of MCL cells. We found that IL-6 produced by hMSCs triggered an FGF/FGFR autocrine loop in MCL-ICs. The extent of FGFR expression correlated tightly with expression of SOX11, a pathology related negative prognostic marker in MCL. MCL cell survival and growth was regulated via the FGFR-1 mediated BIRC2/3 axis. Blocking of this signaling pathway with FGFR-1 inhibitors consistently induced early degradation in BIRC2/3 levels and subsequently MCL cell death.Conclusion We established that propagation of primary MCL in co-cultures with hMSCs depends on activation of FGF/FGFR-1 autocrine loop that enhances BIRC2/3 protein expression and thus, supports survival of MCL cells. We identified the factors essential for survival of MCL and MCL-ICs that present new targets for improved MCL treatment strategies. This study reveals that inhibition of FGFR-1 signaling by specific inhibitor has a profound positive impact on extrinsic cell death signaling; it enhances FAS sensitivity by promoting processing of caspases through enhanced BIRC2/3 degradation. The capacity of FGFR-1 inhibition to target stability of BIRC2/3, underscores its potential for enhancing efficacy of conventional cancer therapies. DisclosuresSamaniego:ADC Therapeutics: Research Funding.

Lysosomal Cell Death and Apoptosis Crosstalk - Synergistic Role in Bcl-2 Inhibitor (ABT-263) Mediated Cell Death in B-Cell Precursor Acute Lymphoblastic Leukemia

Although improved treatment of pediatric Acute Lymphoblastic Leukemia (ALL) has become increasingly successful with cure rates of up to 90%, leukemia reoccurrence is clearly associated with inferior prognosis. Treatment failure and resistance in leukemias are considered to involve defects in apoptosis signaling, which forms the basis for apoptogenic treatment approaches such as the use of Bcl-2 inhibitors. Identification of alternative cell death programs like lysosomal membrane permeabilization-induced cell death may provide a novel strategy for the treatment of ALL. Previous studies showed that lysosomal activity, as well as their traffic properties are greatly altered during tumorigenesis. Intralysosomal hydrolases such as cathepsin B, L and D have been implicated in cancer progression and high expression levels are generally associated with poor prognosis. Lysosomotropic agents modulating lysosomal integrity may overcome cell death resistance and might therefore also improve the outcome of high risk or ALL relapse patients.In this study, we investigated the efficacy of the lysosomotropic compound B10, a derivative of Betulinic acid, as well as its combination with the Bcl-2 inhibitor ABT-263 in B-cell precursor- (BCP-) ALL.In BCP-ALL cell lines, B10 showed induction of cell death combined with increased DNA fragmentation. Interestingly, additional treatment with the pan-caspase inhibitor zVAD.fmk only partially rescued B10 triggered loss of cell viability, indicating that B10 is not exclusively inducing caspase-dependent apoptosis, but also induces an additional alternative cell death program. B10 permeabilized lysosomes as indicated by the significant decrease of LysotrackerRED positive populations detected by flow cytometry, and treatment with E-64d, a potent inhibitor of thiol proteases and cathepsins, reduced B10-induced cell death, thus emphasizing the cathepsin-dependent effect of B10. To exclude that B10-induced lysosomal permeabilization might represent a secondary effect, a panel of different substances was tested for their activity on lysosomal membrane integrity. While treatment with dexamethasone, chloroquine, vincristine and asparaginase also showed lysosomal permeabilization, co-treatment with E-64d did not reduce cell death indicating a secondary effect of these compounds on lysosomes. In addition to cell lines, we tested the effect of B10 on 15 primary leukemia samples isolated from ALL bearing mice of established patient-derived NOD/SCID/hu BCP-ALL xenografts. In a majority of these individual patient-derived leukemias, B10 induced cell death that could be inhibited by E-64d, thus demonstrating a lysosomal protease-dependent death program also in primary ALL.In addition to its classical role in triggering the intrinsic mitochondria dependent apoptosis pathway, the pro-apoptotic Bcl-2 family member Bax was previously described to be involved in the regulation of lysosomal membrane integrity, pointing to a possible synergistic effect of B10 and BH3-mimetics. Intriguingly, in cell fractionation assays we observed increased Bax recruitment to lysosomal membranes when B10 is present. Interestingly, co-treatment with the Bcl-2 inhibitor ABT-263, which displaces Bcl-2 from its inhibitory binding to Bax, led to increased lysosomal permeabilization, loss of mitochondrial membrane potential, and caspase activation indicating involvement of the intrinsic apoptosis pathway. The importance of lysosomal Bax recruitment for the activity of ABT-263 and its concomitant effect on lysosomal membrane permeabilization was further supported by Bax knockdown experiments, since induction of lysosomal disruption, release of cathepsins and their subsequent effect on cell death activation by B10 was reduced.Taken together, our findings suggest an important role of lysosomal membrane permeabilization-induced cell death for the activity of Bcl-2 inhibitors such as ABT-263. The combination of BH3 mimetics with lysosomotropic compounds may provide the basis for novel molecular directed treatment strategies. DisclosuresNo relevant conflicts of interest to declare.

Sulforaphane potentiates growth-inhibiting and apoptosis-promoting activities of cisplatin following oxidative stress and mitochondrial dysfunction in malignant mesothelioma cells

Functional defects in apoptosis signaling have been considered the important mechanism of chemoresistance in malignant mesothelioma (MM). This study was designed to explore the growth-inhibiting and apoptosis-promoting effects of sulforaphane and cisplatin on the human MM cell lines, MSTO-211H and H-2452, and examine the underlying mechanisms. Compared to normal mesothelial Met-5A cells, sulforaphane and cisplatin exhibited synergistic anti-cancer efficacy towards MM cells. By inducing ROS accumulation and mitochondrial dysfunction, sulforaphane potentiated cisplatin-induced cytotoxic effects, as evidenced by the increased nuclear fragmentation and chromatin condensation with the enhanced cleavage of procaspase-3 and PARP, the increased percentage of Annexin V-PE (+) cells, and the appearance of a sub-G0/G1 peak in the flow cytometric analysis. The apoptosis-inducing effects of two compounds were associated with the increased Bax/Bcl-2 ratio due to a decrease in the anti-apoptotic Bcl-2 level. A G2/M phase transition delay in the cell cycle following the combined treatment of sulforaphane and cisplatin was linked to the down-regulation of cyclin B1 and phospho-Cdc2 (Thr161) levels. In addition, the inhibition of ROS with N-acetylcysteine attenuated the apoptosis induced by the combination treatment, and recovered the depletion of mitochondrial membrane potential, revealing the vital role of ROS in the synergism. By targeting redox homeostasis, such a pro-oxidant-based combinational approach will help to enhance the therapeutic efficacy and preferential cytotoxicity on MM cells that conventional therapy would be ineffective.

Histone 3 Methyltransferase (EZH2) Inhibition Enhances TRAIL-Induced Apoptosis In Mantle Cell Lymphoma Cells By Accelerated cFLIP Degradation

Introduction Mantle cell lymphoma (MCL) is an aggressive form of non-Hodgkin lymphoma that is characterized by the t(11:14)(q13:p32) translocation. MCL cells have altered cyclinD1 levels, impaired cell cycle regulation, DNA damage response, and likely defects in apoptosis signaling. Furthermore, up-regulated anti-apoptotic mediators such as the target of NF-κB c-FLIP were correlated with decreased apoptosis signaling. Also many cancer cells and malignant tumors show a prevalent resistance to apoptosis induction by TRAIL. Thus, by understanding the underpinnings of apoptosis resistance, we will be in a better position to develop strategies that improve TRAIL-induced killing of lymphoma cells. Methods/Results MCL cell lines (Mino, JeKo-1, JVM-2 and Z-138) were treated with DZNep (3-Deazaneplanocin A; 0.2-5µM) for 24 h followed by incubation with TRAIL (10-20ng/ml, 6-16h). Cell death, DNA fragmentation, and mitochondrial membrane potential (Δψm) were determined by calcein staining, subG1 analysis, and TMRM staining, respectively. Neither DZnep alone nor in combination with TRAIL showed a significant induction of necrosis as determined by LDH-release levels, but DZNep alone showed strong antiproliferative properties at higher concentrations (Promega CellTiter 96 assay). Activation of the caspase signaling cascade (caspase-8, -9, -3, Bid, and PARP cleavage) was analyzed by Western blotting. TRAIL-induced signaling was significantly increased and caspase-8 processing enhanced in DZNep pretreated cells indicating a regulation at the TRAIL/DISC. Although a reduced expression of DR5 in total cell lysates of DZNep treated cell was observed, the surface receptor levels were not altered. Interestingly, downregulation of the well-known caspase inhibitor, cFLIP, correlated with the DZNep-induced increased TRAIL sensitivity in MCL cell lines. However, it appears that cFLIP levels are not reduced due to blocked NF-kB signaling but rather by an accelerated ubiquitin-mediated degradation. Conclusions This study reveals that inhibition of histone methyltransferase (EZH2) activity by DZNep has a profound positive impact on TRAIL signaling; it enhances TRAIL sensitivity by promoting processing of caspase-8 through enhanced cFLIP degradation. The capacity of DZNep to target stability of cFLIP, which represents a center piece in DISC regulation underscores its potential for enhancing efficacy of TRAIL-based cancer therapies. Disclosures: No relevant conflicts of interest to declare.

Abstract 669: Sunitinib enhances TRAIL-induced apoptosis in preclinical colon cancer models

Abstract Tumor necrosis factor related apoptosis-inducing ligand (TRAIL) is a potent inducer of apoptosis that may have promise as an anticancer agent due to its selectivity toward tumor cells. However, resistance to TRAIL occurs frequently and is primarily due to defects in apoptosis signaling or activation of survival pathways. Sunitinib is an inhibitor of multiple receptor tyrosine kinases that promote cell survival and drug resistance. We hypothesized that disrupting pro-survival signaling cascades with suntinib would circumvent this mechanism of resistance and strongly enhance TRAIL-mediated apoptosis. We tested this hypothesis by investigating whether inhibition of survival signaling pathways with sunitinib could augment the anticancer activity of TRAIL in preclinical models of colon cancer. Sunitinib significantly enhanced the anticancer activity of TRAIL in a panel of colon cancer cell lines as evidenced by increased caspase cleavage and DNA fragmentation. The TRAIL/sunitinib combination also promoted enhanced JNK activation and stimulated a decrease in the expression of XIAP, c-IAP2 and Bcl-xL, which correlated with apoptosis. siRNA-mediated knockdown of JNK reduced TRAIL/sunitinib-mediated apoptosis indicating that it is an important mediator of efficacy of this combination. We further investigated this combination regimen in HCT116 and HCT15 xenograft models treated with vehicle, TRAIL, sunitinib, or the combination. The TRAIL/sunitinib combination was very well tolerated and significantly reduced tumor burden in both models compared to either single agent treatment. The reduction in tumor volume correlated with increased apoptosis as measured by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay and was associated with decreases in XIAP, c-IAP2 and Bcl-xL expression. Our collective results suggest that sunitinib enhances TRAIL-mediated apoptosis by increasing JNK activation and downregulating multiple anti-apoptotic proteins. These findings are the first report of the therapeutic potential of the combination of sunitinib and TRAIL in preclinical models of colon cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 669.

Low mRNA expression of the apoptosis‐related genes CASP3, CASP8, and FAS is associated with low induction treatment response in childhood acute lymphoblastic leukemia (ALL)

Defects in apoptosis signaling have been considered to be responsible for treatment failure in many types of cancer, although with controversial results. The objective of the present study was to assess the expression profile of key apoptosis-related genes in terms of clinical and biological variables and of the survival of children with acute lymphoblastic leukemia (ALL). The levels of mRNA expression of the apoptosis-related genes CASP3, CASP8, CASP9, FAS, and BCL2 were analyzed by quantitative real-time PCR in consecutive samples from 139 consecutive children with ALL at diagnosis treated by the Brazilian protocol (GBTLI-ALL 99). Gene expression levels and clinical and biological features were compared by the Mann-Whitney test. Event-free survival (EFS) was calculated by Kaplan-Meier plots and log-rank test. A significant correlation was detected between CASP3, CASP8, CASP9, and FAS expression levels (P < 0.01) in ALL samples. Higher levels of BCL2 were significantly associated with white blood cell (WBC) count <50,000/mm(3) at diagnosis (P = 0.01) and low risk group classification (P = 0.008). Lower expression levels of CASP3, CASP8 and FAS gene were associated with a poor response at day 7 according the GBTLI-ALL 99 protocol (P = 0.03, P = 0.02 and P = 0.008, respectively). There was a relationship between FAS gene expression lower than the 75th percentile and lower 5-year EFS (P = 0.02). These findings suggest an association between lower expression levels of the pro-apoptotic genes and a poor response to induction therapy at day 7 and prognosis in childhood ALL.

Suppression of Mcl-1 via RNA interference sensitizes human hepatocellular carcinoma cells towards apoptosis induction

BackgroundHepatocelluar carcinoma (HCC) is one of the most common cancers worldwide and a major cause of cancer-related mortality. HCC is highly resistant to currently available chemotherapeutic drugs. Defects in apoptosis signaling contribute to this resistance. Myeloid cell leukemia-1 (Mcl-1) is an anti-apoptotic member of the Bcl-2 protein family which interferes with mitochondrial activation. In a previous study we have shown that Mcl-1 is highly expressed in tissues of human HCC. In this study, we manipulated expression of the Mcl-1 protein in HCC cells by RNA interference and analyzed its impact on apoptosis sensitivity of HCC cells in vitro.MethodsRNA interference was performed by transfecting siRNA to specifically knock down Mcl-1 expression in HCC cells. Mcl-1 expression was measured by quantitative real-time PCR and Western blot. Induction of apoptosis and caspase activity after treatment with chemotherapeutic drugs and different targeted therapies were measured by flow cytometry and fluorometric analysis, respectively.ResultsHere we demonstrate that Mcl-1 expressing HCC cell lines show low sensitivity towards treatment with a panel of chemotherapeutic drugs. However, treatment with the anthracycline derivative epirubicin resulted in comparatively high apoptosis rates in HCC cells. Inhibition of the kinase PI3K significantly increased apoptosis induction by chemotherapy. RNA interference efficiently downregulated Mcl-1 expression in HCC cells. Mcl-1 downregulation sensitized HCC cells to different chemotherapeutic agents. Sensitization was accompanied by profound activation of caspase-3 and -9. In addition, Mcl-1 downregulation also increased apoptosis rates after treatment with PI3K inhibitors and, to a lower extent, after treatment with mTOR, Raf I and VEGF/PDGF kinase inhibitors. TRAIL-induced apoptosis did not markedly respond to Mcl-1 knockdown. Additionally, knockdown of Mcl-1 efficiently enhanced apoptosis sensitivity towards combined treatment modalities: Mcl-1 knockdown significantly augmented apoptosis sensitivity of HCC cells towards chemotherapy combined with PI3K inhibition.ConclusionOur data suggest that specific downregulation of Mcl-1 by RNA interference is a promising approach to sensitize HCC cells towards chemotherapy and molecularly targeted therapies.

Mcl-1 is an anti-apoptotic factor for human hepatocellular carcinoma

Defects in apoptosis signaling in hepatocytes contribute to tumorigenesis in hepatocellular carcinoma (HCC). In addition, treatment with chemotherapeutic drugs is often ineffective in HCC patients due to the apoptosis resistance of cancer cells. Anti-apoptotic members of the Bcl-2 family, including myeloid cell leukemia-1 (Mcl-1), which regulate intrinsic apoptosis induction at the mito-chondrial level, are often overexpressed in human cancer, and are implicated with disease grade and prognosis. Yet, little is known about the role of Mcl-1 in HCC. In this study, we analyzed the relevance of Mcl-1 expression for the apop-tosis resistance of human HCC. Mcl-1 protein expression was considerably enhanced in human HCC tissue compared to adjacent non-tumor tissue. In addition, Mcl-1 was prominently expressed in various HCC cell lines. Mcl-1 basal expression is dependent on a functional phosphatidylinositol-3 kinase (PI3K)/Akt signaling pathway; treatment of the cells with a specific PI3 kinase inhibitor led to both decreased Mcl-1 expression and a sensitization towards chemotherapeutic drug-induced apoptosis. Furthermore, the hepatocyte growth factor and epidermal growth factor induced Mcl-1 expression in an Akt- and ERK-dependent manner. Finally, specific upregulation of Mcl-1 in HCC cells inhibited chemotherapeutic drug-induced apoptosis. Our data suggest that Mcl-1 is an important factor for the apoptosis resistance of human HCC, and constitutes an interesting target for HCC therapy.

Inhibition of Nuclear Translocation of Nuclear Factor-κB Despite Lack of Functional IκBα Protein Overcomes Multiple Defects in Apoptosis Signaling in Human B-Cell Malignancies

Defective apoptosis signaling is a typical feature of classic Hodgkin's lymphoma, multiple myeloma, and activated B-cell-like diffuse large B-cell lymphoma. In these malignancies, the transcription factor nuclear factor-kappaB (NF-kappaB) is a critical mediator of apoptosis resistance and oncogenic growth, making it an attractive therapeutic target. Here, we sought to determine how to overcome apoptosis resistance experimentally in these malignancies by targeting NF-kappaB. We investigated the effect of different inhibitors of NF-kappaB on classic Hodgkin's lymphoma, multiple myeloma, and activated B-cell-like diffuse large B-cell lymphoma cell lines harboring different molecular defects in apoptosis signaling both quantitatively and qualitatively. The cyclopentenone prostaglandin, 15-deoxy-Delta12,14-prostaglandin J(2), a known inhibitor of NF-kappaB, induced caspase-dependent apoptosis; it restored mitochondrial apoptotic signaling by down-regulation of X-linked inhibitor of apoptosis protein and heat shock protein 27 and led to breakdown of the mitochondrial membrane potential and, finally, cleavage of caspase-3 irrespective of IkappaBalpha mutational status. Surprisingly, 15-deoxy-Delta12,14-prostaglandin J(2) and the IkappaB kinase inhibitor curcumin both reduced nuclear levels of p65 in cell lines lacking IkappaBalpha, suggesting that inhibition of nuclear translocation of NF-kappaB can occur in the absence of IkappaBalpha. Finally, a synthetic peptide that specifically abrogates the assembly of the IkappaB kinase complex killed IkappaBalpha-defective cells by induction of apoptosis, paralleled by reduction of nuclear NF-kappaB. These results show that molecular defects in apoptotic signaling, such as IkappaBalpha mutations, can be circumvented by targeting NF-kappaB through inhibition of the IkappaB kinase complex followed by induction of apoptosis in classic Hodgkin's lymphoma, multiple myeloma, and activated B-cell-like diffuse large B-cell lymphoma. Thus, targeting IkappaB kinases may represent an attractive therapeutic approach against these malignancies regardless of the mutational status of IkappaBalpha.

Apoptosis in cancer—implications for therapy

Resistance towards apoptosis is a key factor for the survival of a malignant cell. Cancer results if there is too little apoptosis and cells grow faster and live longer than normal cells. In addition, defects in apoptosis signaling contribute to drug resistance of tumor cells. Thus, one of the main goals for oncologic treatment is to overcome resistance of tumor cells towards apoptosis. The exciting challenge in oncology is to translate the growing knowledge of apoptotic pathways into clinical applications. In this review we address the role of apoptosis signaling in tumorigenesis and drug resistance of tumor cells and discuss therapeutic approaches interfering with apoptosis pathways.

Recent advances in the development of anticancer agents targeting cell death inhibitors in the Bcl-2 protein family.

Hematopoietic malignancies frequently are characterized by defects in apoptosis signaling. This renders the malignant cells resistant to endogenous apoptotic stimuli, as well as exogenous stimuli, such as chemotherapy drugs and radiation. The defective apoptosis seen in human cancers often results from overexpression of antiapoptotic proteins in the Bcl-2 protein family, particularly Bcl-2 and Bcl-X(L). A great deal of effort is currently aimed at developing novel agents to inhibit the expression or function of these proteins. Antisense agents directed against Bcl-2 mRNA are showing considerable promise in clinical trials. In addition, detailed knowledge of the structures of Bcl-2 and Bcl-X(L), coupled with high-throughput and computer-assisted screening of chemical libraries, has led to the identification of a number of short peptides and small organic molecules capable of inhibiting Bcl-2 and Bcl-X(L) function. These newly described agents hold considerable promise for enhancing the chemo- and radiation sensitivities of Bcl-2- and Bcl-X(L)-overexpressing cancers. This review will highlight recent advances in the development and testing of agents targeting cell death inhibitors in the Bcl-2 protein family.

Altered mechanisms of apoptosis in colon cancer: Fas resistance and counterattack in the tumor-immune conflict.

Fas (CD95/APO-1) is a cell surface "death receptor" that mediates apoptosis upon engagement by its ligand, FasL. Fas-mediated apoptosis of lymphocytes normally serves immunoregulatory roles, including tolerance acquisition, immune response termination, and maintenance of immune privilege in certain organs. Colon tumors can exploit this lymphocyte death program by expressing FasL. This may enable colon tumors to mount a "Fas counterattack" against antitumor lymphocytes, impairing antitumor immune responses. FasL-expressing colon tumor-derived cell lines can trigger Fas-mediated apoptosis of cocultured T cells in vitro. FasL expressed in esophageal cancer has been significantly associated with apoptosis and depletion of tumor-infiltrating lymphocytes (TIL) in vivo. FasL may also facilitate metastatic colonization of Fas-sensitive organs such as the liver, by inducing apoptosis of target organ cells. Normal colonic epithelial cells express Fas and are relatively sensitive to Fas-mediated apoptosis. By contrast, colon tumor-derived cell lines are usually resistant to induction of Fas-mediated apoptosis, and colon cancer cells frequently coexpress Fas and FasL. The mechanisms allowing resistance to Fas-mediated apoptosis are complex, and defects have been identified at several levels of Fas signal transduction. The "Bcl-2 rheostat" may be pitched against apoptosis in colon cancer, inasmuch as overexpression of Bcl-2, downregulation of Bak, and mutation of Bax are common defects in colon tumors. Caspase-1 is also downregulated in colon cancer. The high frequency of p53 mutations in late-stage cancers may also inhibit Fas signaling. Fundamental defects in apoptosis signaling may contribute to both immuno- and chemoresistance in colon cancer and allow expression of FasL to counterattack antitumor lymphocytes.