Abstract 3401: Vemurafenib-induced RhoB expression leads to melanoma cell resistance.

Abstract

Abstract Most of melanoma harbour mutations leading to the constitutive activation of the Ras/Raf/MEK/ERK mitogen-activated protein kinase (MAPK) pathway (i.e: activating mutations of BRAF, NRAS, c-KIT). Addiction of melanoma to the MAPK pathways led to the development of inhibitors targeting either mutant BRAF, such as Vemurafenib (PLX4032), or MEK. Melanoma treatment recently met a breakthrough with the clinical evaluation of Vemurafenib that demonstrated an unprecedented 80% response rate among patients with BRAF V600E melanomas treated with Vemurafenib, but rapidly limited by drug resistance, which still requires to be elucidated. The MAPK pathway regulates the expression of many genes which can mediate therapeutic response but also resistance, such as RhoGTPases. We analysed the mRNA expression of the RhoGTPases in two BRAF mutated melanoma cell lines, WM266-4 and A375, in response to inhibition of the MAPK pathway by PLX4032. We observed that the mRNA of RhoB was increased by PLX4032 treatment. This result was confirmed in many other BRAF mutated melanoma cell lines by inhibition of the MAPK pathway with BRAF inhibitors (PLX4032 or SB-590885) as well as MEK inhibitors (AZD6244 or AS703026). MEK inhibitors (AZD6244 or AS703026) upregulate RhoB in BRAF wild-type melanoma cells harbouring N-Ras, K-Ras or c-KIT mutation suggesting that inhibiting MAPK pathway is critical for RhoB expression modulation. It is noteworthy that this modulation is cell type dependent, while it was not found in BRAF mutated non-melanoma cells as colon carcinoma cells (Colo205 and RKO). To investigate the role of RhoB induction in cellular responses to BRAF and MEK inhibitors, the RhoB induction was suppressed using RNA interference. The cells treated with siRNA targeting RhoB, WM266-4 and A375 cells, were more sensitive to PLX4032 and AZD6244. Moreover the blockade of RhoB induction led to a significant increase of PLX4032-induced apoptosis as shown by the increase of the subG1 population and PARP cleavage. Our findings show that RhoB upregulation is involved in PLX4032 response and both inhibitions of MAPK pathway and RhoB expression trigger cell death and should prompt a search for agents. Indeed co-targeting MAPK and RhoB pathways, could lead to a synthetic lethality and prevent resistance to either BRAF or MEK inhibitors. Ongoing work focuses on confirming this role in tumour-xenografted mouse models. Citation Format: Audrey Delmas, Julia Cherier, Claire Medale-Giamarchi, Anne Pradines, Gilles Favre. Vemurafenib-induced RhoB expression leads to melanoma cell resistance. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3401. doi:10.1158/1538-7445.AM2013-3401