Concurrent inhibition of ICMT and RAF/MEK suppresses RAC1P29S-driven MAPK-pathway-inhibitor resistance in BRAFV600E...

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 carboxylmethyltransferase (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 MAPK-pathway-inhibitors 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 inhibitors alone. Mechanistically, we found that the combined treatment impaired the nuclear translocation of TAZ, whose transcriptional activity is shown to account for resistance to MAPK pathway inhibitors 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 resistance to MAPK pathway inhibitors in native cells, phenocopying the effect of RAC1P29S. The novel application of MAPK pathway inhibitors and cysmethynil combination in RAC1P29S-driven MAPK-pathway-inhibitor-resistant melanoma cells extends the potential utility of ICMT inhibitors, and also provides a new mechanism for targeting ICMT in cancer.