The epigenetic regulator I-BET151 induces BIM-dependent apoptosis and cell cycle arrest of human melanoma cells

Epigenetic changes are widespread in melanoma and contribute to the pathogenic biology of this disease. In the present study, we show that I-BET151, which belongs to a new class of drugs that target the BET family of epigenetic "reader" proteins, inhibits melanoma growth in vivo and induced variable degrees of apoptosis in a panel of melanoma cells. Apoptosis was caspase dependent and associated with G1 cell cycle arrest. All melanoma cells tested had increased levels of the BH3 proapoptotic protein BIM, which appeared to be regulated by the BRD2 BET protein and to some extent by BRD3. In contrast, knockdown experiments indicated that inhibition of BRD4 was associated with decreased levels of BIM. Apoptosis was dependent on BIM in some but not all cell lines, indicating that other factors were determinants of apoptosis, such as downregulation of antiapoptotic proteins revealed in gene expression arrays. G1 cell cycle arrest appeared to be mediated by p21 and resulted from inhibition of the BRD4 protein. The activity of BET protein inhibitors appears independent of the BRAF and NRAS mutational status of melanoma, and further studies to assess their therapeutic role in melanoma are warranted.

Resistance to selective BRAF inhibition can be mediated by modest upstream pathway activation

A high percentage of patients with BRAF(V600E) mutant melanomas respond to the selective RAF inhibitor vemurafenib (RG7204, PLX4032) but resistance eventually emerges. To better understand the mechanisms of resistance, we used chronic selection to establish BRAF(V600E) melanoma clones with acquired resistance to vemurafenib. These clones retained the V600E mutation and no second-site mutations were identified in the BRAF coding sequence. Further characterization showed that vemurafenib was not able to inhibit extracellular signal-regulated kinase phosphorylation, suggesting pathway reactivation. Importantly, resistance also correlated with increased levels of RAS-GTP, and sequencing of RAS genes revealed a rare activating mutation in KRAS, resulting in a K117N change in the KRAS protein. Elevated levels of CRAF and phosphorylated AKT were also observed. In addition, combination treatment with vemurafenib and either a MAP/ERK kinase (MEK) inhibitor or an AKT inhibitor synergistically inhibited proliferation of resistant cells. These findings suggest that resistance to BRAF(V600E) inhibition could occur through several mechanisms, including elevated RAS-GTP levels and increased levels of AKT phosphorylation. Together, our data implicate reactivation of the RAS/RAF pathway by upstream signaling activation as a key mechanism of acquired resistance to vemurafenib, in support of clinical studies in which combination therapy with other targeted agents are being strategized to combat resistance.