Synergy of the microRNA Ratio as a Promising Diagnosis Biomarker for Mucinous Borderline and Malignant Ovarian Tumors

Epithelial ovarian cancers (EOCs) are a heterogeneous collection of malignancies, each with their own developmental origin, clinical behavior and molecular profile. With less than 5% of EOC cases, mucinous ovarian carcinoma is a rare form with a poor prognosis and a 5-year survival of 11% for advanced stages (III/IV). At the early stages, these malignant forms are clinically difficult to distinguish from borderline (15%) and benign (80%) forms with a better prognosis due to the large size and heterogeneity of mucinous tumors. Improving their diagnosis is therefore a challenge with regard to the risk of under-treating a malignant form or of unnecessarily undertaking radical surgical excision. The involvement of microRNAs (miRNAs) in tumor progression and their potential as biomarkers of diagnosis are becoming increasingly recognized. In this study, the comparison of miRNA microarray expression profiles between malignant and borderline tumor FFPE samples identified 10 down-regulated and 5 up-regulated malignant miRNAs, which were validated by individual RT-qPCR. To overcome normalization issues and to improve the accuracy of the results, a ratio analysis combining paired up-regulated and down-regulated miRNAs was performed. Although 21/50 miRNA expression ratios were significantly different between malignant and borderline tumor samples, any ratio could perfectly discriminate the two groups. However, a combination of 14 pairs of miRNA ratios (double ratio) showed high discriminatory potential, with 100% of accuracy in distinguishing malignant and borderline ovarian tumors, which suggests that miRNAs may hold significant clinical potential as a diagnostic tool. In summary, these ratio miRNA-based signatures may help to improve the precision of histological diagnosis, likely to provide a preoperative diagnosis in order to adapt surgical procedures.

Comparative analysis of response to treatments and molecular features of tumor-derived organoids versus cell lines and PDX derived from the same ovarian clear cell carcinoma

BACKGROUND

In the era of personalized medicine, the establishment of preclinical models of cancer that faithfully recapitulate original tumors is essential to potentially guide clinical decisions.

METHODS

We established 7 models [4 cell lines, 2 Patient-Derived Tumor Organoids (PDTO) and 1 Patient-Derived Xenograft (PDX)], all derived from the same Ovarian Clear Cell Carcinoma (OCCC). To determine the relevance of each of these models, comprehensive characterization was performed based on morphological, histological, and transcriptomic analyses as well as on the evaluation of their response to the treatments received by the patient. These results were compared to the clinical data.

RESULTS

Only the PDX and PDTO models derived from the patient tumor were able to recapitulate the patient tumor heterogeneity. The patient was refractory to carboplatin, doxorubicin and gemcitabine, while tumor cell lines were sensitive to these treatments. In contrast, PDX and PDTO models displayed resistance to the 3 drugs. The transcriptomic analysis was consistent with these results since the models recapitulating faithfully the clinical response grouped together away from the other classical 2D cell culture models. We next investigated the potential of drugs that have not been used in the patient clinical management and we identified the HDAC inhibitor belinostat as a potential effective treatment based on PDTO response.

CONCLUSIONS

PDX and PDTO appear to be the most relevant models, but only PDTO seem to present all the necessary prerequisites for predictive purposes and could constitute relevant tools for therapeutic decision support in the context of these particularly aggressive cancers refractory to conventional treatments.

Impact of automated methods for quantitative evaluation of immunostaining: Towards digital pathology

Introduction

We sought to develop a novel method for a fully automated, robust quantification of protein biomarker expression within the epithelial component of high-grade serous ovarian tumors (HGSOC). Rather than defining thresholds for a given biomarker, the objective of this study in a small cohort of patients was to develop a method applicable to the many clinical situations in which immunomarkers need to be quantified. We aimed to quantify biomarker expression by correlating it with the heterogeneity of staining, using a non-subjective choice of scoring thresholds based on classical mathematical approaches. This could lead to a universal method for quantifying other immunohistochemical markers to guide pathologists in therapeutic decision-making.

Methods

We studied a cohort of 25 cases of HGSOC for which three biomarkers predictive of the response observed ex vivo to the BH3 mimetic molecule ABT-737 had been previously validated by a pathologist. We calibrated our algorithms using Stereology analyses performed by two experts to detect immunohistochemical staining and epithelial/stromal compartments. Immunostaining quantification within Stereology grids of hexagons was then performed for each histological slice. To define thresholds from the staining distribution histograms and to classify staining within each hexagon as low, medium, or high, we used the Gaussian Mixture Model (GMM).

Results

Stereology analysis of this calibration process produced a good correlation between the experts for both epithelium and immunostaining detection. There was also a good correlation between the experts and image processing. Image processing clearly revealed the respective proportions of low, medium, and high areas in a single tumor and showed that this parameter of heterogeneity could be included in a composite score, thus decreasing the level of discrepancy. Therefore, agreement with the pathologist was increased by taking heterogeneity into account.

Conclusion and discussion

This simple, robust, calibrated method using basic tools and known parameters can be used to quantify and characterize the expression of protein biomarkers within the different tumor compartments. It is based on known mathematical thresholds and takes the intratumoral heterogeneity of staining into account. Although some discrepancies need to be diminished, correlation with the pathologist’s classification was satisfactory. The method is replicable and can be used to analyze other biological and medical issues. This non-subjective technique for assessing protein biomarker expression uses a fully automated choice of thresholds (GMM) and defined composite scores that take the intra-tumor heterogeneity of immunostaining into account. It could help to avoid the misclassification of patients and its subsequent negative impact on therapeutic care.

PSMA Expression in Differentiated Thyroid Cancer: Association With Radioiodine, 18FDG Uptake, and Patient Outcome

CONTEXT

Little is known about prostate-specific membrane antigen (PSMA) expression in patients with cervical involvement of differentiated thyroid cancer (DTC).

OBJECTIVE

We investigated PSMA expression in neck persistent/recurrent disease (PRD) using immunohistochemistry and the association with radioiodine (RAI) or 18-fluorodeoxyglucose (18FDG) uptake, and patient outcome.

DESIGN, SETTING, AND PATIENTS

Data from 44 consecutive DTC patients who underwent neck reoperation from 2006 to 2018 in a comprehensive cancer center.

MAIN OUTCOME MEASURE(S)

Immunostaining was performed with vascular endothelial marker CD31 and PSMA. PSMA expression was quantified using the immunoreactive score (IRS). RAI and 18FDG uptake were assessed before surgery using posttherapeutic RAI scintigraphy and 18FDG positron emission tomography with computed tomography. Mean follow-up after reintervention was 6.5 ± 3.7 years.

RESULTS

Thirty patients (68%) showed at least 1 PSMA-positive lesion (IRS ≥ 2) with similar proportions in RAI-positive and RAI-negative patients (75% vs 66%). In RAI-negative patients, however, the proportion of PSMA-positive disease (79% vs 25%, P < 0.01) and the mean IRS (4.0 vs 1.0, P = 0.01) were higher in 18FDG-positive than in 18FDG-negative patients. Furthermore, mean IRS was higher in patients ≥ 55 years, large primary tumors (>40 mm) or aggressive subtypes, and was correlated with structural disease at last follow-up. Strong PSMA expression (IRS ≥ 9) was associated with shorter progression-free survival (PFS).

CONCLUSIONS

Our findings show that PSMA expression was present in two-thirds of patients with neck PRD, that it was related to poor prognostic factors and that very high expression was associated with poorer PFS. This preliminary study may offer new perspectives for the management of RAI-refractory DTC.

A PSMA-targeted theranostic approach is unlikely to be efficient in serous ovarian cancers

PURPOSE

Until now, results evaluating the expression of PSMA in ovarian cancer were sparse and contradictory. The aim was to reinvestigate the feasibility of a PSMA targeted theranostic approach in epithelial ovarian cancers with data from the tumour bank of a referring cancer centre.

MATERIALS AND METHODS

The OvaRessources Biological Resources Center database was screened from January 2004 to December 2017 to seek patients referred for the initial management of a serous epithelial ovarian cancer and for whom peritoneal histological samples were available in the tumour bank. Immunodetection of PSMA was performed to assess its cellular and neovascular expression. Slides were controlled by a certified pathologist, recorded as tiled tiff images and processed to compute the proportion of DAB stained surface.

RESULTS

Of the 51 patients identified by the database screening, 32 patients were included resulting in 57 samples (32 pre-chemotherapy and 25 post-chemotherapy histological samples). Nine patients were chemo-sensitive, 10 were partially chemo-sensitive and 13 were chemo-resistant/refractory. In the entire dataset, the expression of PSMA was quasi-inexistent: %DAB_PSMA = 0.04 (± 0.12) %. There was no significant difference in the %DAB_PSMA of sensitive, partially sensitive and resistant/refractory patients. There was also no significant difference in %DAB_PSMA in tumours before and after chemotherapy in the 25 patients for whom both samples were available.

CONCLUSION

The present work demonstrates that PSMA expression is negligible and a fortiori non-sufficient to ensure its usefulness as a prognosticator or a target for a theranostic strategy in ovarian cancers.

Functional miRNA Screening Identifies Wide-ranging Antitumor Properties of miR-3622b-5p and Reveals a New Therapeutic Combination Strategy in Ovarian Tumor Organoids

Novel therapeutic strategies are urgently required for the clinical management of chemoresistant ovarian carcinoma, which is the most lethal of the gynecologic malignancies. miRNAs hold promise because they play a critical role in determining the cell phenotype by regulating several hundreds of targets, which could constitute vulnerabilities of cancer cells. A combination of gain-of-function miRNA screening and real-time continuous cell monitoring allows the identification of miRNAs with robust cytotoxic effects in chemoresistant ovarian cancer cells. Focusing on miR-3622b-5p, we show that it induces apoptosis in several ovarian cancer cell lines by both directly targeting Bcl-xL and EGFR-mediating BIM upregulation. miR-3622b-5p also sensitizes cells to cisplatin by inhibiting Bcl-xL in ovarian cancer cell lines escaping BIM induction. miR-3622b-5p also exerts antimigratory capacities by targeting both LIMK1 and NOTCH1. These wide-ranging antitumor properties of miR-3622b-5p in ovarian cancer cells are mimicked by the associations of pharmacologic inhibitors targeting these proteins. The combination of an EGFR inhibitor together with a BH3-mimetic molecule induced a large decrease in cell viability in a panel of ovarian cancer cell lines and several ovarian patient-derived tumor organoids, suggesting the value of pursuing such a combination therapy in ovarian carcinoma. Altogether, our work highlights the potential of phenotype-based miRNA screening approaches to identify lethal interactions which might lead to new drug combinations and clinically applicable strategies.

Control of cell death/survival balance by the MET dependence receptor

Control of cell death/survival balance is an important feature to maintain tissue homeostasis. Dependence receptors are able to induce either survival or cell death in presence or absence of their ligand, respectively. However, their precise mechanism of action and their physiological importance are still elusive for most of them including the MET receptor. We evidence that pro-apoptotic fragment generated by caspase cleavage of MET localizes to the mitochondria-associated membrane region. This fragment triggers a calcium transfer from endoplasmic reticulum to mitochondria, which is instrumental for the apoptotic action of the receptor. Knock-in mice bearing a mutation of MET caspase cleavage site highlighted that p40MET production is important for FAS-driven hepatocyte apoptosis, and demonstrate that MET acts as a dependence receptor in vivo. Our data shed light on new signaling mechanisms for dependence receptors’ control of cell survival/death balance, which may offer new clues for the pathophysiology of epithelial structures.

Evaluation of the potential of a new ribavirin analog impairing the dissemination of ovarian cancer cells

Epithelial ovarian cancers are insidious pathologies that give a poor prognosis due to their late discovery and the increasing emergence of chemoresistance. Development of small pharmacological anticancer molecules remains a major challenge. Ribavirin, usually used in the treatment of hepatitis C virus infections and more recently few cancers, has been a suggestion. However, Ribavirin has many side-effects, suggesting that the synthesis of analogs might be more appropriate.

We have investigated the effect of a Ribavirin analog, SRO-91, on cancer cell behavioral characteristics considered as some of the hallmarks of cancer. Two human ovarian adenocarcinoma cell lines (SKOV3 and IGROV1) and normal cells (mesothelial and fibroblasts) have been used to compare the effects of SRO-91 with those of Ribavirin on cell behavior underlying tumor cell dissemination. SRO-91, like Ribavirin, inhibits proliferation, migration, clonogenicity and spheroids formation of cancer cells. Unlike Ribavirin, SRO-91 is preferentially toxic to cancer compared with normal cells. An in vitro physiologically relevant model showed that SRO-91, like Ribavirin or cisplatin, inhibits cancer cell implantation onto peritoneal mesothelium. In conclusion, SRO-91 analog effects on tumor dissemination and its safety regarding non-cancerous (normal) cells are encouraging findings a promising drug for the treatment of ovarian cancer.

Inhibition of store-operated channels by carboxyamidotriazole sensitizes ovarian carcinoma cells to anti-BclxL strategies through Mcl-1 down-regulation

The anti-apoptotic proteins Bcl-xL and Mcl-1 have been identified to play a pivotal role in apoptosis resistance in ovarian cancer and constitute key targets for innovative therapeutic strategies. Although BH3-mimetics (i.e. ABT-737) potently inhibit Bcl-xL activity, targeting Mcl-1 remains a hurdle to the success of these strategies. Calcium signaling is profoundly remodeled during carcinogenesis and was reported to activate the signaling pathway controlling Mcl-1 expression. In this context, we investigated the effect of carboxyamidotriazole (CAI), a calcium channel inhibitor used in clinical trials, on Mcl-1 expression. CAI had an anti-proliferative effect on ovarian carcinoma cell lines and strongly down-regulated Mcl-1 expression. It inhibited store-operated calcium entry (SOCE) and Mcl-1 translation through mTORC1 deactivation. Moreover, it sensitized ovarian carcinoma cells to anti-Bcl-xL strategies as their combination elicited massive apoptosis. Its effect on mTORC1 and Mcl-1 was mimicked by the potent SOCE inhibitor, YM58483, which also triggered apoptosis when combined with ABT-737. As a whole, this study suggests that CAI sensitizes to anti-Bcl-xL strategies via its action on Mcl-1 translation and that modulation of SOCE could extend the therapeutic arsenal for treatment of ovarian carcinoma.

Atelocollagen-mediated in vivo siRNA transfection in ovarian carcinoma is influenced by tumor site, siRNA target and administration route

Ovarian cancer is the leading cause of death from gynecological malignancies worldwide, and innate or acquired chemoresistance of ovarian cancer cells is the major cause of therapeutic failure. It has been demonstrated that the concomitant inhibition of Bcl-xL and Mcl-1 anti-apoptotic activities is able to trigger apoptosis in chemoresistant ovarian cancer cells. In this context, siRNA-mediated Bcl‑xL and Mcl-1 inhibition constitutes an appealing strategy by which to eliminate chemoresistant cancer cells. However, the safest and most efficient way to vectorize siRNAs in vivo is still under debate. In the present study, using in vivo bioluminescence imaging, we evaluated the interest of atelocollagen to vectorize siRNAs by intraperitoneal (i.p.) or intravenous (i.v.) administration in 2 xenografted ovarian cancer models (peritoneal carcinomatosis and subcutaneous tumors in nude mice). Whereas i.p. administration of atelocollagen-vectorized siRNA in the peritoneal carcinomatosis model did not induce any gene downregulation, a 70% transient downregulation of luciferase expression was achieved after i.v. injection of atelocollagen-vectorized siRNA in the subcutaneous (s.c.) model. However, the use of siRNA targeting Bcl-xL or Mcl-1 did not induce target-specific downregulation in vivo in nude mice. Our results therefore show that atelocollagen complex formulation, the administration route, tumor site and the identity of the siRNA target influence the efficiency of atelocollagen‑mediated siRNA delivery.

Calcium signals inhibition sensitizes ovarian carcinoma cells to anti-Bcl-xL strategies through Mcl-1 down-regulation

Ovarian carcinoma is the leading cause of death from gynecologic cancer in the developed world and is characterized by acquired chemoresistance leading to an overall 5-year survival rate of about 30 %. We previously showed that Bcl-xL and Mcl-1 cooperatively protect platinum-resistant ovarian cancer cells from apoptosis. Despite BH3-mimetics represent promising drugs to target Bcl-xL, anti-Mcl-1 strategies are still in pre-clinical studies and required new investigations. Calcium is a universal second messenger and dysregulation of calcium signal is often observed during carcinogenesis. As change in cytosolic free calcium concentration [Ca(2+)]i is known to control the fate of the cell by regulating Bcl-2 family members, we wonder if calcium signal could impact on Mcl-1 expression and if its pharmacological inhibition could be useful to sensitize ovarian carcinoma cells to anti-Bcl-xL strategies. We therefore studied the effect of different calcium signals inhibitors in ovarian carcinoma cell lines SKOV3 and IGROV1-R10 and analysed their effects on proliferation and Mcl-1 expression. We also exposed these cells to these inhibitors in combination with anti-Bcl-xL strategies (siRNA or BH3-mimetic: ABT-737). We found that calcium signaling regulates Mcl-1 through translational events and a calmodulin-mediated pathway. BAPTA-AM and calmodulin inhibitor combination with ABT-737 leads to apoptosis, a process that is reversed by Mcl-1 enforced expression. As Mcl-1 represents a crucial hurdle to the success of chemotherapy, these results could open to new area of investigation using calcium modulators to directly or indirectly target Mcl-1 and thus efficiently sensitize ovarian carcinoma cells to anti-Bcl-xL strategies.

Identification of predictive factors of response to the BH3-mimetic molecule ABT-737: an ex vivo experiment in human serous ovarian carcinoma

Ovarian cancers are addicted to Bcl-xL and Mcl-1, antiapoptotic members of the Bcl-2 family. Bcl-xL can be inhibited by the BH3-mimetic ABT-737. In vitro, ABT-737 can induce apoptosis of cancer cells, and its activity is potentiated by Mcl-1 inactivation. Herein, we assessed the sensitivity of human ovarian tumor nodes to ABT-737 when combined with carboplatin, which can indirectly inhibit Mcl-1. Fresh samples from 25 patients with high-grade serous ovarian cancer (HGSOC) who were chemo-naïve and had undergone surgery were prospectively exposed ex vivo to ABT-737 ± carboplatin. The treatment effect was studied on sliced tumor nodes by assessment of cleaved-caspase 3 immunostaining. We also studied the association between baseline Bcl-2 family protein expression (via immunohistochemistry) and the response of nodes to treatment. ABT-737 induced apoptosis as a single agent but its efficacy was not improved by the addition of carboplatin. Bim was frequently expressed (20/25) and its absence or low expression was associated with the absence of response to ABT-737, p value = 0.019 by Fisher's test and sensitivity = 93%, (95% confidence interval, 66-100). Moreover, we observed that in tumors in which Bim was expressed, a low expression of phospho-Erk1/2 or Mcl-1 improved the proportion of responses. This pilot study showed that ABT-737 has promise as monotherapy for HGSOC in a specific subgroup of tumors. Bim, Mcl-1, and phospho-Erk1/2 appeared to be relevant biomarkers that could be used for the selection of patients in the design of clinical trials using Navitoclax (an orally available compound related to ABT-737).

PI3K/mTOR dual inhibitor NVP-BEZ235 decreases Mcl-1 expression and sensitizes ovarian carcinoma cells to Bcl-xL-targeting strategies, provided that Bim expression is induced

We previously showed that Bcl-xL and Mcl-1 cooperatively protect platinum-resistant ovarian cancer cells from apoptosis. Here we assessed the anticancer potential of combining ABT-737-induced inhibition of Bcl-xL with Mcl-1 inhibition via PI3K/Akt/mTOR pathway disruption using NVP-BEZ235. NVP-BEZ235 inhibited cell proliferation without inducing apoptosis. It strongly repressed Mcl-1 expression and induced Puma expression in both cell lines tested while differentially modulating Bim between the two. Interestingly, NVP-BEZ235 efficiently sensitized ovarian carcinoma cells to ABT-737, provided that Bim expression was induced. Moreover, inhibiting the ERK1/2 pathway restored Bim expression and sensitized low Bim-expressing cancer cells to the BEZ235/ABT-737 treatment.

Platinum compounds sensitize ovarian carcinoma cells to ABT-737 by modulation of the Mcl-1/Noxa axis

Ovarian cancer is the leading cause of death from gynecological cancer. The anti-apoptotic protein Bcl-x(L) is frequently overexpressed in ovarian carcinoma which correlates with chemotherapy resistance. It has been demonstrated that Bcl-x(L) cooperates with another anti-apoptotic protein, Mcl-1, to protect ovarian cancer cells against apoptosis, and that their concomitant inhibition induces massive cell death. Here, we examined the interest of ABT-737, a potent BH3-mimetic molecule targeting Bcl-x(L), both alone and in combination with Mcl-1 modulators, in ovarian cancer cell lines. As a single agent, ABT-737 was ineffective at promoting cell death in the four cell lines we tested in vitro. However, the specific inhibition of Mcl-1 by siRNA dramatically increased the sensitivity of chemoresistant cells to ABT-737. Platinum compounds also sensitize to ABT-737 by dose-dependently decreasing Mcl-1 expression or by increasing the expression of pro-apoptotic BH3-only proteins Noxa and, to a lower extent, Bim. Furthermore, we demonstrated that Noxa accumulation was involved in apoptosis occurring in response to the combination of ABT-737 and platinum compounds, since cells were protected from apoptosis by its silencing. Moreover, the combination was also highly cytotoxic ex vivo in sliced SKOV3 tumor nodes. However we observed in these slices a strong basal expression of Noxa and apoptotic cell death in response to ABT-737 alone. Therefore, we have revealed that the modulation of the Mcl-1/Noxa axis by platinum compounds results in a strong sensitization of chemoresistant ovarian carcinoma cells to ABT-737, which could constitute a promising therapeutic in these cancers.

Rapid and soft formulation of folate-functionalized nanoparticles for the targeted delivery of tripentone in ovarian carcinoma

We report the development of folate-functionalized nanoparticles able to target folate receptors, and to deliver a poorly water soluble cytotoxic agent, a tripentone, in ovarian carcinoma. The stability under incubation of lipid nanoparticles formulated by a low-energy phase inversion temperature method was investigated. Thanks to the presence of Labrasol(®), a macrogolglyceride into the composition of the nanocarriers, the conjugation of different quantities of a folate derivate (folic acid-polyethylene glycol2000-distearylphosphatidylethanolamine) to nanoparticles was possible by a rapid, soft, very simple post-insertion process. As determined by dynamic light scattering, nanoparticles present a monodisperse diameter of about 100 nm, a spherical shape as attested by transmission electron micrographs, a weakly negative surface zeta potential, and are able to encapsulate the tripentone MR22388. The presence of folate receptors on SKOV3 human ovarian cancer cells was identified by fluorescent immunocytochemistry. Cellular uptake studies assessed by flow cytometry indicated that these nanoparticles reached the SKOV3 cells rapidly, and were internalized by a folate-receptor mediated endocytosis pathway. Moreover, nanoparticles allowed the rapid delivery of the antitumor agent tripentone into cells as shown in vitro by real-time cellular activity assay. Such folate-lipid nanoparticles are a potential carrier for targeted delivery of poorly water soluble compounds into ovarian carcinoma.

Sensitization of ovarian carcinoma cells to Bcl-xL-targeting strategies through indirect modulation of Mcl-1 activity by MR22388, a molecule of the tripentone family

Background

Our work has been carried out in the context of the therapeutic failure in ovarian carcinoma, which remains the leading cause of death by gynecologic malignancy. In these tumours, recurrence and subsequent acquired chemoresistance constitute major hurdles to successful therapy. Here we studied the interest of a member of the tripentone chemical family, MR22388, for the treatment of chemoresistant ovarian cancer cells.

Findings

MR22388 activity has been assessed in vitro on cisplatin-resistant (SKOV3 and IGROV1-R10) ovarian cancer cell lines by conventional analysis, alone or combined to a BH3-mimetic molecule, ABT-737. MR22388 exerts its activity on cisplatin resistant cells, and we showed that it induces a decrease of the Mcl-1 anti-apoptotic protein expression. Considering our previous work demonstrating that the efficiency of Bcl-x_L targeting strategies is conditioned to the concomitant inhibition of Mcl-1 we studied the interest of the association of this MR22388 with ABT-737, and showed that this combination was highly cytotoxic in chemoresistant cells.

Conclusions

This work thus opens new perspectives for the use of this promising molecule for the treatment of highly chemoresistant ovarian cancer cells and for sensitization of emerging Bcl-x_L targeting strategies such as the use of BH3-mimetic molecules.

Downregulation of Bcl-xL and Mcl-1 is sufficient to induce cell death in mesothelioma cells highly refractory to conventional chemotherapy

Malignant pleural mesothelioma (MPM) is an aggressive tumor with poor prognosis and limited response to platinum-based chemotherapy. Several lines of evidence support a role for the anti-apoptotic protein Bcl-x(L) in MPM chemoresistance. Since it has been recently suggested that Mcl-1 cooperates with Bcl-x(L) for protection against cell death, we investigated the response of mesothelioma cell lines to the downregulation of Bcl-x(L) (alone or in combination with cisplatin) and the potential interest of its concomitant inhibition with that of Mcl-1. Using RNA interference, we showed that Bcl-x(L) depletion sensitized two highly chemoresistant mesothelioma cell lines to cisplatin and that under this treatment, one cell line, MSTO-211H, displayed an apoptotic type of cell death, whereas the other, NCI-H28, evidenced mainly necrotic-type cell death. Otherwise, the inhibition of Mcl-1 by cisplatin may contribute to this induction of cell death observed after Bcl-x(L) downregulation. Strikingly, we observed that the simultaneous inhibition of Bcl-x(L) and Mcl-1 using small interfering RNA (siRNA) induced a massive cell death in the absence of chemotherapy and was sufficient to avoid escape to treatment in MSTO-211H cells. In NCI-H28, the addition of a low cisplatin concentration allowed to impede the long-term recovery observed after treatment by the siRNA combination. Together, these findings provide a strong molecular basis for the clinical evaluation of therapies targeting both Bcl-x(L) and Mcl-1, alone or in combination with conventional chemotherapy, for the treatment of MPM.