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.

Bcl-XL and MCL-1 constitute pertinent targets in ovarian carcinoma and their concomitant inhibition is sufficient to induce apoptosis.

In ovarian carcinomas, recurrence and acquired chemoresistance are the first leading causes of therapeutic failure and are responsible for the poor overall survival rate. Cisplatin exposure of sensitive cells has been previously associated with a down-regulation of Bcl-X(L) expression and apoptosis, whereas recurrence was systematically observed when Bcl-X(L) expression was maintained. Bcl-X(L) down-regulation could thus constitute an interesting chemosensitizing strategy. We showed that a Bcl-X(L)targeted RNA interference strategy efficiently sensitized chemoresistant ovarian carcinoma cells to cisplatin, but some of them were still able to re-proliferate. Considering the possible cooperation between Bcl-X(L)and MCL-1, we investigated the possibility to avoid recurrence in vitro using a multi-targeted RNAi strategy directed against these two anti-apoptotic proteins. We showed that their concomitant inhibition lead to massive apoptosis in absence of cisplatin, this multi-targeted RNAi approach being much more efficient than conventional chemotherapy. We thus demonstrated that Bcl-X(L) and MCL-1 cooperate to constitute together a strong molecular "bolt", which elimination could be sufficient to allow chemoresistant ovarian carcinoma cells apoptosis. Moreover, we demonstrated that in presence of a low concentration of cisplatin, the concomitant down-regulation of Bcl-X(L) and MCL-1 allowed a complete annihilation of tumour cells population thus avoiding subsequent recurrence in vitro in cell lines highly refractory to any type of conventional chemotherapy. Therefore, Bcl-X(L) and MCL-1 targeted strategies could constitute an efficient therapeutic tool for the treatment of chemoresistant ovarian carcinoma, in association with conventional chemotherapy.

Mcl-1 is an important determinant of the apoptotic response to the BH3-mimetic molecule HA14-1 in cisplatin-resistant ovarian carcinoma cells.

Chemoresistance of ovarian carcinoma has been associated previously to the absence of Bcl-x(L) expression downregulation in response to cisplatin. Among BH3-mimetic molecules constituting promising anticancer agents able to inhibit the activity of antiapoptotic Bcl-2 family proteins, we evaluated the effect of one of them, HA14-1, on various ovarian carcinoma cell lines. In response to HA14-1, the cisplatin-resistant IGROV1-R10 cell line underwent massive cell death, whereas other cell lines presented a partial response (IGROV1, SKOV3, and A2780) or did not respond to this molecule (OAW42 and OAW42-R). However, the expression of HA14-1 targets (Bcl-2 and Bcl-x(L)) did not correlate to these different responses. In contrast, cell death was associated with the disappearance of Mcl-1 after exposure to HA14-1. We showed that, in the HA14-1 nonresponsive cell lines (SKOV3 and OAW42), small interfering RNA-mediated Mcl-1 downregulation allowed HA14-1-induced massive apoptosis in the absence of chemotherapy. Furthermore, cisplatin-induced Mcl-1 downregulation was also able to sensitize highly chemoresistant SKOV3 cells to HA14-1. Taken together, these results show that Bcl-x(L) and Mcl-1 are able to cooperate to protect ovarian carcinoma cells against oncogenic stress or chemotherapy-induced apoptosis and suggest that the development of multitargeted strategies directed against these two antiapoptotic proteins may constitute a major challenge for the therapeutic care of chemoresistant ovarian carcinomas. BH3-mimetic compounds represent promising tools for this purpose either on their own (direct or indirect pan-inhibitors) or in combination with new drugs aiming to inactivate Mcl-1.