The RAF inhibitor PLX4032 inhibits ERK signaling and tumor cell proliferation in a V600E BRAF-selective manner

Abstract

Tumors with mutant BRAF and some with mutant RAS are dependent upon ERK signaling for proliferation, and their growth is suppressed by MAPK/ERK kinase (MEK) inhibitors. In contrast, tumor cells with human EGF receptor (HER) kinase activation proliferate in a MEK-independent manner. These findings have led to the development of RAF and MEK inhibitors as anticancer agents. Like MEK inhibitors, the RAF inhibitor PLX4032 inhibits the proliferation of BRAF V600E tumor cells but not that of HER kinase-dependent tumors. However, tumors with RAS mutation that are sensitive to MEK inhibition are insensitive to PLX4032. MEK inhibitors inhibit ERK phosphorylation in all normal and tumor cells, whereas PLX4032 inhibits ERK signaling only in tumor cells expressing BRAF V600E . In contrast, the drug activates MEK and ERK phosphorylation in cells with wildtype BRAF. In BRAF V600E tumor cells, MEK and RAF inhibitors affect the expression of a common set of genes. PLX4032 inhibits ERK signaling output in mutant BRAF cells, whereas it transiently activates the expression of these genes in tumor cells with wild-type RAF. Thus, PLX4032 inhibits ERK signaling output in a mutant BRAFselective manner. These data explain why the drug selectively inhibits the growth of mutant BRAF tumors and suggest that it will not cause toxicity resulting from the inhibition of ERK signaling in normal cells. This selectivity may lead to a broader therapeutic index and help explain the greater antitumor activity observed with this drug than with MEK inhibitors.

An Integrated Model of RAF Inhibitor Action Predicts Inhibitor Activity against Oncogenic BRAF Signaling

“…The current clinical RAF inhibitors suppress RAF activity selectively in cells expressing BRAF V600E (Halaban et al, 2010;Hatzivassiliou et al, 2010;Heidorn et al, 2010;Joseph et al, 2010;Poulikakos et al, 2010), and they do not inhibit wild-type BRAF because it signals as dimer Yao et al, 2015). This is the basis of the high therapeutic index of these drugs, but RAF dimerization is also predicted to result in drug resistance Samatar and Poulikakos, 2014).…”

“…They bind and inhibit all RAF isoforms in in vitro kinase assays, but they suppress RAF activity and downstream ERK signaling selectively in cells expressing BRAF V600E (Halaban et al, 2010;Hatzivassiliou et al, 2010;Heidorn et al, 2010;Joseph et al, 2010;Poulikakos et al, 2010). In cells with BRAF WT they paradoxically activate RAF and ERK signaling, via a RASdependent mechanism that remains incompletely understood.…”

EGFR inhibition evokes innate drug resistance in lung cancer cells by preventing Akt activity and thus inactivating Ets-1 function

“…This insight reveals a previously unidentified point of convergence of MAPK and Akt signaling and may provide a strategy to treat cancers in which MAPK and Akt activities are abnormal. For example, B-Raf-mutated tumors are sensitive to MEK and B-Raf inhibitors, which attenuate cyclin D1 expression and cause G1 arrest (35,36). Inhibition of both MEK and Akt or both B-Raf and Akt may result in more complete and lasting inhibition of Ets-1 target genes, including cyclin D1.…”

N-(3-Ethynyl-2,4-difluorophenyl)sulfonamide Derivatives as Selective Raf Inhibitors

“…17,18 It was shown that 3s displayed similar antiproliferative effects against B-Raf V600E mutated cancer cells to that of vemurafenib, whereas compound 3o is moderately more potent (Table 2). In contrast, none of the compounds showed obvious growth inhibition against cell lines expressing B-Raf WT .…”