Acquired and intrinsic BRAF inhibitor resistance in BRAF V600E mutant melanoma

Abstract

The discovery of activating BRAF V600E mutations in 50% of all cutaneous melanomas has revolutionized the understanding of melanoma biology and provided new strategies for the therapeutic management of this deadly disease. Highly potent small molecule inhibitors of BRAF are now showing great promise as a novel therapeutic strategy for melanomas harboring activating BRAF V600E mutations and are associated with high levels of response. This commentary article discusses the latest data on the role of mutated BRAF in the development and progression of melanoma as the basis for understanding the mechanism of action of BRAF inhibitors in the preclinical and clinical settings. We further address the issue of BRAF inhibitor resistance and outline the latest insights into the mechanisms of therapeutic escape as well as describing approaches to prevent and abrogate the onset of both intrinsic and acquired drug resistance. It is likely that our evolving understanding of melanoma genetics and signaling will allow for the further personalization of melanoma therapy with the goal of improving clinical responses.

Figure 1. Role of mutant BRAF in preventing apoptosis in melanoma cells

The inhibition of BRAF decreases the phosphorylation of BIM through the MEK/ERK pathway, preventing its proteasomal degradation. Once stabilized, BIM antagonizes the anti-apoptotic proteins Bcl-2 and Bcl-XL and leads to apoptosis induction. In PTEN-null melanoma cells, BRAF inhibition leads to the increased PI3K/AKT-mediated phosphorylation of FOXO3a resulting in reduced BIM transcription. Inhibition of BRAF in PTEN null melanoma cells also impairs apoptosis through the AKT-mediated phosphorylation and inactivation of BAD. The phosphorylation of BAD prevents its binding to Bax and relieves the antagonism of Bax on Bcl-2 and Bcl-XL. Intrinsic BRAF inhibitor resistance in the PTEN-null cells can be overcome through dual inhibition of BRAF and PI3K.

Figure 2. Mutated BRAF drives the invasion of melanoma cells

Mutant BRAF regulates melanoma cell invasion through MEK/ERK-mediated signaling to RND3 and Rho/ROCK/LIM-mediated Cofilin phosphorylation. It is also known that ERK can upregulate BRN2 expression leading to the downregulation of PDE5A and cytoplasmic accumulation of cGMP and Ca²⁺. Increased intracellular calcium in turn leads to increased MLC2 phosphorylation, contractility, and invasion.

Figure 3. Mechanisms of acquired resistance to BRAF inhibitors

Multiple mechanisms of acquired BRAF inhibitor resistance have been identified. BRAF V600E melanoma cells chronically treated with BRAF inhibitors acquire drug resistance via switching between the three isoforms of RAF (ARAF, BRAF, CRAF) to activate the MAPK pathway. Increased IGFR1 and PDGFR signaling may also allow for resistance by activating PI3K/AKT signaling as well as other pathways. Resistance can also arise following the reactivation of the MAPK pathway, this can occur following the acquisition of activating mutations in NRAS (Q61K) and MEK (C121S) and the increased expression of the MAP3K8 (COT).