Microphthalmia-associated transcription factor suppresses invasion by reducing intracellular GTP pools

Melanoma progression is associated with increased invasion and, often, decreased levels of microphthalmia-associated transcription factor (MITF). Accordingly, downregulation of MITF induces invasion in melanoma cells; however, little is known about the underlying mechanisms. Here, we report for the first time that depletion of MITF results in elevation of intracellular GTP levels and increased amounts of active (GTP-bound) RAC1, RHO-A and RHO-C. Concomitantly, MITF-depleted cells display larger number of invadopodia and increased invasion. We further demonstrate that the gene for guanosine monophosphate reductase (GMPR) is a direct MITF target, and that the partial repression of GMPR accounts mostly for the above phenotypes in MITF-depleted cells. Reciprocally, transactivation of GMPR is required for MITF-dependent suppression of melanoma cell invasion, tumorigenicity and lung colonization. Moreover, loss of GMPR accompanies downregulation of MITF in vemurafenib-resistant BRAF^V600E-melanoma cells and underlies the increased invasion in these cells. Our data uncover novel mechanisms linking MITF-dependent inhibition of invasion to suppression of guanylate metabolism.