Axl Inhibition Enhances Sensitivity of BRAF Mutant Melanoma Cells to Vemurafenib

Abstract

Malignant melanoma is one of the most aggressive types of cancer with poor prognosis after the onset of distant metastases. About 50% malignant melanomas harbor BRAFV600E gene mutation, which is amenable to targeted therapy vemurafenib. Patients with BRAFV600E mutation who respond to vemurafenib show remarkable tumor regression. However, the antitumorigenic effect of vemurafenib is only transient due to acquired drug resistance. Most patients that receive targeted treatment present with tumor recurrence and eventually succumb to their disease. Thus, it seems imperative to understand the underlying mechanisms of melanoma targeted therapy resistance to maximize the clinical success of vemurafenib. The receptor tyrosine kinase Axl, which is often overexpressed in neoplastic settings, promotes metastasis and therapeutic resistance. In melanoma, Axl is suggested to be a major signaling molecule that assists melanoma cells to circumvent the effects of vemurafenib. Preliminary evidence suggests that Axl interacts with the molecules of MAPK and PI3K pathway to promote growth, survival and invasion. Therefore, inhibition of Axl could be a novel strategy to overcome melanoma targeted therapy resistance. Here we investigated the effect of Axl inhibition with BGB324 in BRAF mutant vemurafenib sensitive melanoma cells. Our results suggest that Axl inhibitor enhances the anti-cancer potential of vemurafenib in BRAFV600E mutant melanoma, indicating Axl expression might be involved in desensitizing BRAFV600E mutant melanoma cells to targeted inhibition. Secondly, we show that Axl inhibition influences MITF gene transcription and MiTF protein expression in BRAF mutant and wildtype melanoma cells. We also show that MiTF is inversely correlated with Axl and this correlation can be reversed by inhibiting Axl. Finally, in an attempt to understand the effect of vitamin K dependent γ-carboxylation of Gas6 on Axl signaling, we demonstrate that Axl signaling is dependent on the manner in which Gas6 is presented to the receptor, and it is enhanced by oncogenic processes such as EMT. To conclude, our findings suggest Axl as an attractive candidate for anticancer therapies, given its implication in drug resistance, EMT and invasion.