ABSTRACT
It is estimated that ~50% of patients with melanoma harbour BRaf (BRAF)V600 driver mutations, with the most common of these being BRAFV600E, which leads to the activation of mitogenactivated protein kinase proliferative and survival pathways. BRAF inhibitors are used extensively to treat BRAFmutated metastatic melanoma; however, acquired resistance occurs in the majority of patients. The effects of longterm treatment with PLX4032 (BRAFV600 inhibitor) were studied in vitro on sensitive V600E BRAFmutated melanoma cell lines. After several weeks of treatment with PLX4032, the majority of the melanoma cells died; however, a proportion of cells remained viable and quiescent, presenting senescent cancer stem celllike characteristics. This surviving population was termed SUR cells, as discontinuing treatment allowed the population to regrow while retaining equal drug sensitivity to that of parental cells. RNA sequencing analysis revealed that SUR cells exhibit changes in the expression of 1,415 genes (P<0.05) compared with parental cells. Changes in the expression levels of a number of epigenetic regulators were also observed. These changes and the reversible nature of the senescence state were consistent with epigenetic regulation; thus, it was investigated as to whether the senescent state could be reversed by epigenetic inhibitors. It was found that both parental and SUR cells were sensitive to different histone deacetylase (HDAC) inhibitors, such as SAHA and MGCD0103, and to the cyclindependent kinase (CDK)9 inhibitor, CDKI73, which induced apoptosis and reduced proliferation both in the parental and SUR populations. The results suggested that the combination of PLX4032 with HDAC and CDK9 inhibitors may achieve complete elimination of SUR cells that persist after BRAF inhibitor treatment, and reduce the development of resistance to BRAF inhibitors.
