p53-dependent regulation of Mcl-1 contributes to synergistic cell death by ionizing radiation and the Bcl-2/Bcl-XL inhibitor ABT-737

Treatment with the Bcl-2/Bcl-XL inhibitor ABT-737 is a promising novel strategy to therapeutically induce apoptotic cell death in malignant tumors such as glioblastomas. Although many studies have demonstrated that ABT-737 acts synergistically with chemotherapeutic drugs, the possibility of a combined treatment with ionizing radiation (IR) and ABT-737 has not yet been thoroughly investigated. Similarly, the relationship between p53 function and the pro-apoptotic effects of ABT-737 are still obscure. Here, we demonstrate that IR and ABT-737 synergistically induce apoptosis in glioblastoma cells. The sensitivity to ABT-737-mediated cell death is significantly increased by the IR-dependent accumulation of cells in the G2/M cell cycle phase. Wild type p53 function inhibits the efficacy of a combined IR and ABT-737 treatment via a p21-dependent G1 cell cycle arrest. Moreover, mutant as well as wild type p53 counteract the pro-apoptotic activity of ABT-737 by maintaining the expression levels of the Mcl-1 protein. Thus, p53 regulates the sensitivity to ABT-737 of glioblastoma cells. Our results warrant a further evaluation of a novel combination therapy using IR and ABT-737. The efficacy of such a therapy could be substantially enhanced by Mcl-1-lowering strategies.     

Endogenous Noxa Determines the Strong Proapoptotic Synergism of the BH3-Mimetic ABT-737 with Chemotherapeutic Agents in Human Melanoma Cells

Human melanoma cells are very resistant to treatment with chemotherapeutic agents, and melanoma shows poor response to chemotherapeutic therapy. We describe a strong synergistic proapoptotic effect of the Bcl-2 family inhibitor ABT-737 and the standard antimelanoma drugs, namely, dacarbazine and fotemustine, and the experimental agent, imiquimod. Experiments with human melanoma cells, keratinocytes, and embryonic fibroblasts showed that all three agents activated the mitochondrial apoptosis pathway. ABT-737 on its own was ineffective in melanoma cells unless Mcl-1 was experimentally downregulated. However, ABT-737 strongly enhanced the proapoptotic activity of the chemotherapeutic drugs. Whereas cell death induction by all three agents involved the activity of both BH3-only proteins, Bim and Noxa, the combination with ABT-737 overcame the requirement for Bim. However, the synergism between ABT-737 and imiquimod or dacarbazine required endogenous Noxa, as demonstrated by experiments with Noxa-specific RNAi. Surprisingly, although Bim was activated, it was unable to replace Noxa. Studies of mitochondrial cytochrome c release using BH3 peptides confirmed that a main effect of dacarbazine, fotemustine, and imiquimod was to neutralize Mcl-1, thereby sensitizing mitochondria to the inhibition of other Bcl-2 family members through ABT-737. ABT-737 is thus a promising agent for combination therapy for human melanoma. Importantly, the efficacy of this therapy depends on endogenous Noxa, and the ability of chemotherapeutic drugs to activate Noxa may be a valuable predictor of their synergism with Bcl-2-targeting drugs.     

The combination of BH3-mimetic ABT-737 with the alkylating agent temozolomide induces strong synergistic killing of melanoma cells independent of p53

Metastatic melanoma has poor prognosis and is refractory to most conventional chemotherapies. The alkylating agent temozolomide (TMZ) is commonly used in treating melanoma but has a disappointing response rate. Agents that can act cooperatively with TMZ and improve its efficacy are thus highly sought after. The BH3 mimetic ABT-737, which can induce apoptosis by targeting pro-survival Bcl-2 family members, has been found to enhance the efficacy of many conventional chemotherapeutic agents in multiple cancers. We found that combining TMZ and ABT-737 induced strong synergistic apoptosis in multiple human melanoma cell lines. When the drugs were used in combination in a mouse xenograft model, they drastically reduced tumor growth at concentrations where each individual drug had no significant effect. We found that TMZ treatment elevated p53 levels, and that the pro-apoptotic protein Noxa was elevated in TMZ/ABT-737 treated cells. Experiments with shRNA demonstrated that the synergistic effect of TMZ and ABT-737 was largely dependent on Noxa. Experiments with nutlin-3, a p53 inducer, demonstrated that p53 induction was sufficient for synergistic cell death with ABT-737 in a Noxa-dependent fashion. However, p53 was not necessary for TMZ/ABT-737 synergy as demonstrated by a p53-null line, indicating that TMZ and ABT-737 together induce Noxa in a p53-independent fashion. These results demonstrate that targeting anti-apoptotic Bcl-2 members is a promising method for treating metastatic melanoma, and that clinical trials with TMZ and Bcl-2 inhibitors are warranted.     

Phosphoinositide 3-kinase/AKT/mTORC1/2 signaling determines sensitivity of Burkitt's lymphoma cells to BH3 mimetics

Burkitt's lymphoma (BL), driven by translocation and overexpression of the c-MYC gene, is an aggressive, highly proliferative lymphoma, and novel therapeutic strategies are required to overcome drug resistance following conventional treatments. The importance of the prosurvival BCL-2 family member BCL-X(L) in BL cell survival suggests that antagonistic BH3-mimetic compounds may have therapeutic potential. Here, we show that treatment of BL cell lines with ABT-737 induces caspase-3/7 activation and apoptosis with varying potency. Using selective inhibitors, we identify phosphoinositide 3-kinase (PI3K) as a proproliferative/survival pathway in BL cells and investigate the potential of combined pharmacologic inhibition of both the BCL-2 family and PI3K signaling pathway. PI3K/AKT inhibition and ABT-737 treatment induced synergistic caspase activation, augmented BL cell apoptosis, and rendered chemoresistant cells sensitive. Targeting mTORC1/2 with PP242 was also effective, either as a monotherapy or, more generally, in combination with ABT-737. The combined use of a dual specificity PI3K/mTOR inhibitor (PI 103) with ABT-737 proved highly efficacious. PI 103 treatment of BL cells was associated with an increase in BIM/MCL-1 expression ratios and loss of c-MYC expression. Furthermore, blocking c-MYC function using the inhibitor 10058-F4 also induced apoptosis synergistically with ABT-737, suggesting that maintenance of expression of BCL-2 family members and/or c-MYC by the PI3K/AKT/mTOR pathway could contribute to BL cell survival and resistance to ABT-737. The combined use of BH3 mimetics and selective mTORC1/2 inhibitors may therefore be a useful novel therapeutic approach for the treatment of B-cell malignancy, including chemoresistant lymphomas.     

OBATOCLAX and ABT-737 Induce ER Stress Responses in Human Melanoma Cells that Limit Induction of Apoptosis

Anti-apoptotic Bcl-2 family proteins, in particular, Mcl-1, are known to play a critical role in resistance of human melanoma cells to induction of apoptosis by endoplasmic reticulum stress and other agents. The present study examined whether the BH3 mimetics, Obatoclax and ABT-737, which inhibit multiple anti-apoptotic Bcl-2 family proteins, would overcome resistance to apoptosis. We report that both agents induced a strong unfolded protein response (UPR) and that RNAi knockdown of UPR signalling proteins ATF6, IRE1α and XBP-1 inhibited Mcl-1 upregulation and increased sensitivity to the agents. These results demonstrate that inhibition of anti-apoptotic Bcl-2 proteins by Obatoclax and ABT-737 appears to elicit a protective feedback response in melanoma cells, by upregulation of Mcl-1 via induction of the UPR. We also report that Obatoclax, but not ABT-737, strongly induces autophagy, which appears to play a role in determining melanoma sensitivity to the agents.     

Dual role of the p38 MAPK/cPLA2 pathway in the regulation of platelet apoptosis induced by ABT-737 and strong platelet agonists

p38 Mitogen-activated protein (MAP) kinase is involved in the apoptosis of nucleated cells. Although platelets are anucleated cells, apoptotic proteins have been shown to regulate platelet lifespan. However, the involvement of p38 MAP kinase in platelet apoptosis is not yet clearly defined. Therefore, we investigated the role of p38 MAP kinase in apoptosis induced by a mimetic of BH3-only proteins, ABT-737, and in apoptosis-like events induced by such strong platelet agonists as thrombin in combination with convulxin (Thr/Cvx), both of which result in p38 MAP kinase phosphorylation and activation. A p38 inhibitor (SB202190) inhibited the apoptotic events induced by ABT-737 but did not influence those induced by Thr/Cvx. The inhibitor also reduced the phosphorylation of cytosolic phospholipase A₂ (cPLA2), an established p38 substrate, induced by ABT-737 or Thr/Cvx. ABT-737, but not Thr/Cvx, induced the caspase 3-dependent cleavage and inactivation of cPLA2. Thus, p38 MAPK promotes ABT-737-induced apoptosis by inhibiting the cPLA2/arachidonate pathway. We also show that arachidonic acid (AA) itself and in combination with Thr/Cvx or ABT-737 at low concentrations prevented apoptotic events, whereas at high concentrations it enhanced such events. Our data support the hypothesis that the p38 MAPK-triggered arachidonate pathway serves as a defense mechanism against apoptosis under physiological conditions.     

Concomitant Inhibition of MDM2 and Bcl-2 Protein Function Synergistically Induce Mitochondrial Apoptosis in AML

Disruption of Mdm2-p53 interaction activates p53 signaling, disrupts the balance ofantiapoptotic and proapoptotic Bcl-2 family proteins and induces apoptosis in acutemyeloid leukemia (AML). Overexpression of Bcl-2 may inhibit this effect. Thus,functional inactivation of antiapoptotic Bcl-2 proteins may enhance apoptogenic effects ofMdm2 inhibition. We here investigate the potential therapeutic utility of combinedtargeting of Mdm2 by Nutlin-3a and Bcl-2 by ABT-737, recently developed inhibitors ofprotein-protein interactions. Nutlin-3a and ABT-737 induced Bax conformational changeand mitochondrial apoptosis in AML cells in a strikingly synergistic fashion. Nutlin-3ainduced p53-mediated apoptosis predominantly in S and G₂/M cells, while cells in G₁ were protected through induction of p21. In contrast, ABT-737 induced apoptosis predominantly in G₁ , the cell cycle phase with the lowest Bcl-2 protein levels and Bcl-2/Bax ratios. In addition, Bcl-2 phosphorylation on Ser⁷⁰ was absent in G₁ but detectable in G₂/M, thus lower Bcl-2 levels and absence of Bcl-2 phosphorylation appeared to facilitate ABT-737-induced apoptosis of G₁ cells. The complementary effects of Nutlin-3a and ABT-737 in different cell cycle phases could, in part, account for their synergistic activity. Our data suggest that combined targeting of Mdm2 and Bcl-2 proteins could offer considerable therapeutic promise in AML.     

RNA Silencing of Mcl-1 Enhances ABT-737-Mediated Apoptosis in Melanoma: Role for a Caspase-8-Dependent Pathway

Background

Malignant melanoma is resistant to almost all conventional forms of chemotherapy. Recent evidence suggests that anti-apoptotic proteins of the Bcl-2 family are overexpressed in melanoma and may contribute to melanoma''s striking resistance to apoptosis. ABT-737, a small-molecule inhibitor of Bcl-2, Bcl-xl and Bcl-w, has demonstrated efficacy in several forms of leukemia, lymphoma as well as solid tumors. However, overexpression of Mcl-1, a frequent observance in melanoma, is known to confer ABT-737 resistance.

Methodology/Principal Findings

Here we report that knockdown of Mcl-1 greatly reduces cell viability in combination with ABT-737 in six different melanoma cell lines. We demonstrate that the cytotoxic effect of this combination treatment is due to apoptotic cell death involving not only caspase-9 activation but also activation of caspase-8, caspase-10 and Bid, which are normally associated with the extrinsic pathway of apoptosis. Caspase-8 (and caspase-10) activation is abrogated by inhibition of caspase-9 but not by inhibitors of the death receptor pathways. Furthermore, while caspase-8/-10 activity is required for the full induction of cell death with treatment, the death receptor pathways are not. Finally, we demonstrate that basal levels of caspase-8 and Bid correlate with treatment sensitivity.

Conclusions/Significance

Our findings suggest that the combination of ABT-737 and Mcl-1 knockdown represents a promising, new treatment strategy for malignant melanoma. We also report a death receptor-independent role for extrinsic pathway proteins in treatment response and suggest that caspase-8 and Bid may represent potential markers of treatment sensitivity.     

Acidosis Promotes Bcl-2 Family-mediated Evasion of Apoptosis: INVOLVEMENT OF ACID-SENSING G PROTEIN-COUPLED RECEPTOR GPR65 SIGNALING TO MEK/ERK

Acidosis arises in solid and lymphoid malignancies secondary to altered nutrient supply and utilization. Tumor acidosis correlates with therapeutic resistance, although the mechanism behind this effect is not fully understood. Here we show that incubation of lymphoma cell lines in acidic conditions (pH 6.5) blocks apoptosis induced by multiple cytotoxic metabolic stresses, including deprivation of glucose or glutamine and treatment with dexamethasone. We sought to examine the role of the Bcl-2 family of apoptosis regulators in this process. Interestingly, we found that acidic culture causes elevation of both Bcl-2 and Bcl-xL, while also attenuating glutamine starvation-induced elevation of p53-up-regulated modulator of apoptosis (PUMA) and Bim. We confirmed with knockdown studies that these shifts direct survival decisions during starvation and acidosis. Importantly, the promotion of a high anti- to pro-apoptotic Bcl-2 family member ratio by acidosis renders cells exquisitely sensitive to the Bcl-2/Bcl-xL antagonist ABT-737, suggesting that acidosis causes Bcl-2 family dependence. This dependence appears to be mediated, in part, by the acid-sensing G protein-coupled receptor, GPR65, via a MEK/ERK pathway.     

ABT-737 Induces Bim Expression via JNK Signaling Pathway and Its Effect on the Radiation Sensitivity of HeLa Cells

ABT-737 is a BH3 mimetic small molecule inhibitor that can effectively inhibit the activity of antiapoptotic Bcl-2 family proteins including Bcl2, Bcl-xL and Bcl-w, and further enhances the effect of apoptosis by activating the proapoptotic proteins (t-Bid, Bad, Bim). In this study, we demonstrate that ABT-737 improved the radiation sensitivity of cervical cancer HeLa cells and thereby provoked cell apoptosis. Our results show that ABT-737 inhibited HeLa cell proliferation and activated JNK and its downstream target c-Jun, which caused the up-regulation of Bim expression. Blockade of JNK/c-Jun signaling pathway resulted in significant down-regulation of ABT-737-induced Bim mRNA and protein expression level. Also, ABT-737 could evoke the Bim promoter activity, and enhance the radiation sensitivity of HeLa cells via JNK/c-Jun and Bim signaling pathway. Our data imply that combination of ABT-737 and conventional radiation therapy might represent a highly effective therapeutic approach for future treatment of cervical cancer.     

The Stress Protein BAG3 Stabilizes Mcl-1 Protein and Promotes Survival of Cancer Cells and Resistance to Antagonist ABT-737

Members of the Bcl-2 family of proteins are important inhibitors of apoptosis in human cancer and are targets for novel anticancer agents such as the Bcl-2 antagonists, ABT-263 (Navitoclax), and its analog ABT-737. Unlike Bcl-2, Mcl-1 is not antagonized by ABT-263 or ABT-737 and is considered to be a major factor in resistance. Also, Mcl-1 exhibits differential regulation when compared with other Bcl-2 family members and is a target for anticancer drug discovery. Here, we demonstrate that BAG3, an Hsp70 co-chaperone, protects Mcl-1 from proteasomal degradation, thereby promoting its antiapoptotic activity. Using neuroblastoma cell lines, with a defined Bcl-2 family dependence, we found that BAG3 expression correlated with Mcl-1 dependence and ABT-737 resistance. RNA silencing of BAG3 led to a marked reduction in Mcl-1 protein levels and overcame ABT-737 resistance in Mcl-1-dependent cells. In ABT-737-resistant cells, Mcl-1 co-immunoprecipitated with BAG3, and loss of Mcl-1 after BAG3 silencing was prevented by proteasome inhibition. BAG3 and Mcl-1 were co-expressed in a panel of diverse cancer cell lines resistant to ABT-737. Silencing BAG3 reduced Mcl-1 protein levels and overcame ABT-737 resistance in several of the cell lines, including triple-negative breast cancer (MDA-MB231) and androgen receptor-negative prostate cancer (PC3) cells. These studies identify BAG3-mediated Mcl-1 stabilization as a potential target for cancer drug discovery.     

Bcl-2 is a better ABT-737 target than Bcl-xL or Bcl-w and only Noxa overcomes resistance mediated by Mcl-1, Bfl-1, or Bcl-B

The novel anticancer drug ABT-737 is a Bcl-2 Homology 3 (BH3)-mimetic that induces apoptosis by inhibiting pro-survival Bcl-2 proteins. ABT-737 binds with equal affinity to Bcl-2, Bcl-xL and Bcl-w in vitro and is expected to overrule apoptosis resistance mediated by these Bcl-2 proteins in equal measure. We have profiled ABT-737 specificity for all six pro-survival Bcl-2 proteins, in p53 wild-type or p53-mutant human T-leukemic cells. Bcl-B was untargeted, like Bfl-1 and Mcl-1, in accord with their low affinity for ABT-737 in vitro. However, Bcl-2 proved a better ABT-737 target than Bcl-xL and Bcl-w. This was reflected in differential apoptosis-sensitivity to ABT-737 alone, or combined with etoposide. ABT-737 was not equally effective in displacing BH3-only proteins or Bax from Bcl-2, as compared with Bcl-xL or Bcl-w, offering an explanation for the differential ABT-737 sensitivity of tumor cells overexpressing these proteins. Inducible expression demonstrated that BH3-only proteins Noxa, but not Bim, Puma or truncated Bid could overrule ABT-737 resistance conferred by Bcl-B, Bfl-1 or Mcl-1. These data identify Bcl-B, Bfl-1 and Mcl-1, but also Bcl-xL and Bcl-w as potential mediators of ABT-737 resistance and indicate that target proteins can be differentially sensitive to BH3-mimetics, depending on the pro-apoptotic Bcl-2 proteins they are complexed with.     

Immunosensitization with a Bcl-2 small molecule inhibitor

Several tumor immunotherapy approaches result in a low percentage of durable responses in selected cancers. We hypothesized that the insensitivity of cancer cells to immunotherapy may be related to an anti-apoptotic cancer cell milieu, which could be pharmacologically reverted through the inhibition of antiapoptotic Bcl-2 family proteins in cancer cells. ABT-737, a small molecule inhibitor of the antiapoptotic proteins Bcl-2, Bcl-w and Bcl-x_L, was tested for the ability to increase antitumor immune responses in two tumor immunotherapy animal models. The addition of systemic therapy with ABT-737 to the immunization of BALB/c mice with tumor antigen peptide-pulsed dendritic cells (DC) resulted in a significant delay in CT26 murine colon carcinoma tumor growth and improvement in survival. However, the addition of ABT-737 to either a vaccine strategy involving priming with TRP-2 melanoma antigen peptide-pulsed DC and boosting with recombinant Listeria monocytogenes expressing the same melanoma antigen, or the adoptive transfer of TCR transgenic cells, did not result in superior antitumor activity against B16 murine melanoma. In vitro studies failed to demonstrate increased cytotoxic lytic activity when testing the combination of ABT-737 with lymphokine activated killer (LAK) cells, or the death receptor agonists Fas, TRAIL-ligand or TNF-alpha against the CT26 and B16 cell lines. In conclusion, the Bcl-2 inhibitor ABT-737 sensitized cancer cells to the antitumor effect of antigen-specific immunotherapy in a vaccine model for the CT26 colon carcinoma in vivo but not in two immunotherapy strategies against B16 melanoma.     

Synergistic Bcl-2 inhibition by ABT-737 and cyclosporine A

Survival of lymphocytes and melanocyte stem cells critically depends on B cell lymphoma 2 (Bcl-2). In T lymphocytes, a basal calcineurin activity maintains Bcl-2 expression in naïve cells, and the activation of the calcineurin pathway orchestrates the regulation of the intrinsic apoptosis pathway after antigen recognition. Therefore, calcineurin inhibitors might potentiate the pro-apoptotic effect of pharmacological Bcl-2 inhibitors on lymphatic cells. In vitro, a reduced Bcl-2 expression in lymphocytes exposed to calcineurin inhibitors increased their sensitivity to the small molecule Bcl-2 inhibitor ABT-737. This correlated with an augmented pro-apoptotic activity of ABT-737 on lymphocytes in combination with cyclosporine A in naïve mice in vivo. Interestingly, similar processes were observed in melanocytes. ABT-737 induced a fur depigmentation at the site of injection, and this effect was expanded to a generalized depigmentation in combination with cyclosporine A. Thus, inhibiting calcineurin increases the pro-apoptotic potency of ABT-737 in cells depending on Bcl-2 for survival. The increased efficacy of Bcl-2 inhibitors in combination with cyclosporine A might be relevant to exploit their anti-neoplastic and immuno-modulatory properties.     

Methylseleninic acid potentiates multiple types of cancer cells to ABT-737-induced apoptosis by targeting Mcl-1 and Bad

ABT-737, a novel small molecule inhibitor of Bcl-2 family proteins, holds great promise to complement current cancer therapies. However many types of solid cancer cells are resistant to ABT-737. One practical approach to improve its therapeutic efficacy is to combine with the agents that can overcome such resistance to restore the sensitivity. In the present study, a second-generation selenium compound methylseleninic acid (MSeA) synergistically sensitized MDA-MB-231 human breast cancer cells, HT-29 human colon cancer cells and DU145 human prostate cancer cells to apoptosis induction by ABT-737, as evidenced by greater than additive enhancement of Annexin V/FITC positive (apoptotic) cells and activation of multiple caspases and PARP cleavage. Mechanistic investigation demonstrated that MSeA significantly decreased basal Mcl-1 expression and ABT-737-induced Mcl-1 expression. Knocking down of Mcl-1 with RNAi approach supported the functional significance of this molecular target. More importantly, we identified inactivation of Bad by phosphorylation on ser-136 and ser-112 as a novel mechanism involved in ABT-737 resistance, which can be overcome by combining with MSeA. In addition, we found that expression of Bax was required for the efficient execution of synergistic sensitization. Our findings, for the first time, provide a strong mechanistic rationale for developing MSeA as a novel sensitizing agent of ABT-737.     

Flavonoids inhibit cell growth and induce apoptosis in B16 melanoma 4A5 cells

We investigated the growth inhibitory activity of several flavonoids, including apigenin, luteolin, kaempherol, quercetin, butein, isoliquiritigenin, naringenin, genistein, and daizein against B16 mouse melanoma 4A5 cells. Isoliquiritigenin and butein, belonging to the chalcone group, markedly suppressed the growth of B16 melanoma cells and induced cell death. The other flavonoids tested showed little growth inhibitory activity and scarcely caused cell death. In cells treated with isoliquiritigenin or butein, condensation of nuclei and fragmentation of nuclear DNA, which are typical phenomena of apoptosis, were observed by Hoechst 33258 staining and by agarose gel electrophoresis of DNA. Flowcytometric analysis showed that isoliquiritigenin and butein increased the proportion of hypodiploid cells in the population of B16 melanoma cells. These results demonstrate that isoliquiritigenin and butein inhibit cell proliferation and induce apoptosis in B16 melanoma cells. Extracellular glucose decreased the proportion of hypodiploid cells that appeared as a result of isoliquiritigenin treatment. p53 was not detected in cells treated with either of these chalcones, however, protein of the Bcl-2 family were detected. The level of expression of Bax in cells treated with either of these chalcones was markedly elevated and the level of Bcl-XL decreased slightly. Isoliquiritigenin did not affect Bcl-2 expression, but butein down-regulated Bcl-2 expression. From these results, it seems that the pathway by which the chalcones induce apoptosis may be independent of p53 and dependent on proteins of the Bcl-2 family. It was supposed that isoliquiritigenin induces apoptosis in B16 cells by a mechanism involving inhibition of glucose transmembrane transport and promotion of Bax expression. On the other hand, it was suggested that butein induces apoptosis via down-regulation of Bcl-2 expression and promotion of Bax expression. This mechanism differs from the isoliquiritigenin induction pathway.     

p38 Mitogen-activated protein kinases is required for counteraction of 2-methoxyestradiol to estradiol-stimulated cell proliferation and induction of apoptosis in ovarian carcinoma cells via phosphorylation Bcl-2

INTRODUCTION: 2-Methoxyestradiol (2ME2), a natural endogenous product of estradiol (E2) metabolism, has been shown to be a selective apoptotic agent for cancer cells but not for normal cells. In this study, we determined that 2ME2 counteracts E2-stimulated cell growth and induces apoptosis in ovarian carcinoma cells. In addition, we demonstrate that 2ME2 induces apoptosis via p38 and phospho-Bcl2 pathway. METHODS: 2ME2 and/or E2 were administered to the OVCAR-3 (human ovarian cancer) cell line. Cell growth inhibition was analyzed by [3H] Thymidine incorporation assay and DNA fluorometric assay. Cell apoptosis was tested by DNA fragmentation analysis and FACS. The signaling pathway was determined by a series of biochemical assays. RESULTS: 2ME2 inhibited estradiol-stimulated cell growth and induced apoptosis in an ovarian carcinoma cell line. MAPK and p38, but not JNK, were found to be critical mediators in this process. Expression of a dominant negative mutant of p38 kinase or p38 specific inhibitor, SB 203580, almost completely blocked the process. Furthermore, Bcl-2 phosphorylation was required for 2ME2-induced effects. CONCLUSION: Our data suggest that 2ME2 inhibits E2-stimulated proliferation and induces apoptosis in ovarian carcinoma cells. Furthermore, activation of p38 and phosphorylation of Bcl-2 plays a critical role in the mechanism. 2ME2 therefore, may have a clinical application for the treatment of ovarian cancer.