Investigating therapeutic strategies targeting metabolism in NRAS-mutant melanoma

Abstract

In human cancers, RAS mutations are among the most commonly identified mutations; however therapies targeting RAS remain elusive. Recent investigations have demonstrated that mutated RAS can reprogram metabolism in cancer cells. In the field of melanoma, the role of mutant BRAF in the regulation of metabolism has also been elucidated. However, surprisingly little is known about the importance of mutant NRAS in melanoma metabolism. Alongside this, there are limited targeted therapies available for the treatment of patients with NRAS-mutant melanoma. Consequently, this thesis aims to characterise the metabolic response of RAS-mutant cells to targeted therapies and to use this knowledge to develop novel therapeutic strategies targeting glucose metabolism in NRAS-mutant melanoma.

Using human cancer cell lines, the studies in this thesis demonstrate that a number of similarities exist between NRAS and BRAF-mutant melanoma cells with respect to their metabolic responses to MAPK pathway inhibition. MEK inhibition, consistent with prior reports of the effects of BRAF inhibition in BRAF-mutant melanoma, resulted in suppression of glycolysis evidenced by decreased lactate production, glucose uptake and extra-cellular acidification rate. Importantly, known transcriptional regulators of glycolysis in BRAF-mutant melanoma (HIF1α, MYC and MondoA) also play a role in the response of NRAS-mutant melanoma cells to MEK inhibition. Furthermore, in the setting of MEK inhibition NRAS-mutant melanoma cells have increased oxidative metabolism, with increased PGC1α and MITF expression. This adaptation has previously been reported in BRAF-mutant melanoma. The studies in this thesis investigated the relative importance of the MAPK and PI3K effector pathways of RAS, demonstrating that MAPK pathway inhibition had the most consistent and significant effects on glucose metabolism in NRAS-mutant melanoma cells. Finally, a comparison of NRAS and KRAS-mutant cells revealed that NRAS-mutant cells are more sensitive to MEK inhibition, with a more pronounced reduction in parameters relating to glycolysis.

Given these findings, it was hypothesised that combining a MEK inhibitor with an inhibitor of glucose metabolism would be an effective therapeutic strategy in NRAS-mutant melanoma. To this end, the MEK inhibitor trametinib was combined with the mitochondrial inhibitor PENAO. In vitro, PENAO enhanced the anti-proliferative activity of trametinib in NRAS-mutant melanoma cells, with additive effects on glycolysis and mitochondrial metabolism. In vivo, the combination was well tolerated, however the addition of PENAO did not enhance the effect of trametinib on tumour growth. These studies are important in demonstrating the feasibility of a combination targeting two key metabolic processes in vivo, particularly when one process is under the control of an oncogenic aberration. A chemical screen to identify combinations that enhance the suppression of glycolysis achieved by MEK inhibition has been commenced.

In summary, this work has characterised important metabolic adaptations in the context of MEK inhibition in NRAS-mutant melanoma. Although an effective targeted therapy for RAS remains elusive, this research supports the ongoing exploration of strategies that target RAS effector pathways in combination with key metabolic processes, particular in the context of NRAS-mutant melanoma.