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

Abstract

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 BRAFV600E-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.

MITF—the first 25 years

“…Invasion: Recently MITF was shown to control expression of guanosine monophosphate reductase (GMPR) and, consequently, depletion of MITF leads to increased intracellular GTP levels (Bianchi-Smiraglia et al 2017). This is important since increased GTP levels lead to elevated levels of active (GTP-bound) RAC1, RHO-A, and RHO-C, key regulators of the actin cytoskeleton.…”

Potential therapeutic targets of epithelial–mesenchymal transition in melanoma

“…Other evidence suggests that cells exhibiting low MITF expression have greater potential for invasion and that decreasing expression of MITF in vitro promotes greater melanoma invasion [75]. Recently, it was shown that MITF suppresses invasion by reducing intracellular GTP pools by inducing guanosine monophosphate reductase (GMPR); decreased GTP availability results in downregulation of RAC1, RHO-A, and RHO-C [76]. These data have led to a general observation that MITF influences melanoma in a concentration-dependent fashion: high levels of expression are associated with survival and proliferation and low levels of expression with invasion [25].…”

“…Fibroblasts from aged-tumor bearing mice were found to secrete Wnt-antagnoist sFRP2 which was associated with greater vemurafenib resistance and melanoma metastasis [110]. In vemurafenib resistant melanoma cell lines, loss of MITF downregulated expression of guanosine monophosphate reductase resulting in greater activation of RAC1, RHO-A, and RHO-C; these changes conferred greater invasive potential to resistant cell lines [76]. Another study found that inhibition of BRAF caused activation of NRAS, increased expression of MMP1, urokinase, and other proteases, and ultimately increased melanoma invasion and metastasis in vivo [111].…”

Melanoma Suppressor Functions of the Carcinoma Oncogene FOXQ1

“…In melanoma cells, lineage-specific microphthalmia associated transcription factor (MITF), the master regulator of melanocytic differentiation and development, has also been characterized as a major suppressor of the EMT-like process and invasion (Bianchi-Smiraglia et al, 2016; Carreira et al, 2006b; Hartman and Czyz, 2015; Hoek et al, 2008). Although the MITF gene has been reported to undergo amplification in ~15% of melanomas (Garraway et al, 2005), a significant body of literature demonstrates that strong downregulation of MITF, sometimes even below detection, correlates with poor prognosis and disease progression in patients (Carreira et al, 2006a; Salti et al, 2000; Selzer et al, 2002; Wellbrock and Marais, 2005).…”