Concurrent inhibition of ICMT and RAF/MEK suppresses RAC1P29S-driven MAPKi resistance in BRAFV600E melanoma by regulating TAZ activity

The RAC1 GTPase hotspot mutation P29S (RAC1P29S) is among the top driver oncogenes of cutaneous melanoma, which is known to develop resistance to MAPK pathway inhibitors including those targeting BRAF and MEK. Isoprenylcysteine carboxyl-methyltransferase (ICMT) is the enzyme catalyzing the last step of post-translational prenylation of RAC1, which is among its substrates. We demonstrate that RAC1P29S/C189S, which lacks C-terminal prenylation site, has lost the ability to induce resistance toward vemurafenib or trametinib in BRAFV600E melanoma cells. Furthermore, the combination of vemurafenib with cysmethynil, a proof-of-concept ICMT inhibitor, showed efficacy in combating RAC1P29S-driven resistance of BRAFV600E melanoma cells in both in vitro and in vivo settings. Concurrent treatment with cysmethynil and the MAPK pathway inhibitors efficiently inhibited proliferation and tumor formation of RAC1P29S cells that are resistant to MAPK pathway inhibitor alone. Mechanistically, we found that the combined treatment impaired the nuclear translocation of TAZ, whose transcriptional activity is shown to account for MAPKi resistance in RAC1P29S melanoma. We further validated the role of TAZ in RAC1P29S-driven resistance by demonstrating that introducing a constitutively-active TAZ mutant enhanced the MAPKi resistance in native cells, phenocopying the effect of RAC1P29S. The novel application of MAPKi and cysmethynil combination in RAC1P29S-driven MAPKi-resistant melanoma cells extends the potential utility of ICMT inhibitors, and also provides a new mechanism for targeting ICMT in cancer.