MITF-dependent phenotypic plasticity regulates proton-coupled transport and controls lineage resistance to acidotic stress

Thesis

The lineage addiction oncogene, Microphthalmia-Associated Transcription Factor (MITF), mediates melanoma phenotypic plasticity – a pathophysiological process that underlies intratumoural heterogeneity, where melanoma cells interchangeably alter their phenotypes and transcriptomic gene signatures depending on the expression levels of MITF. This thesis characterised the presence and nature of MITF-dependent phenotypic plasticity in relation to melanoma resistance to acidotic stress. We show that the acid-resistant phenotypes in MITF-High and MITF-Low cells consisted of distinctly different mechanisms. Analysis of the melanoma proton transporter transcriptome, with experimental verification, showed that while MITF-High melanomas asymmetrically expressed ATP hydrolysis-coupled proton pumps, MITF-Low melanomas favoured the expression of solute carrier proton transporters. This molecular dichotomy was epitomised by the increased expression of Monocarboxylate Transporter 1 (MCT1) in cells following experimental MITF knockdown, which was associated with increased proton-coupled transport activities for monocarboxylic pyruvate, L-lactate, and β-hydroxybutyrate. Interestingly, acidotic resistance following stable MITF-knockdown could be selectively abrogated by reducing ambient pyruvate concentration, or by treatment with the small molecule MCT1 inhibitor, AR-C155858, leading to apoptosis in the acidotic cells with MITF-knockdown. Furthermore, combining AR-C155858 treatment with the BRAF^V600E inhibitor, vemurafenib, showed additive effects on the induction of apoptosis in acidotic MITF-knockdown cells. This study thus identified a novel approach to achieve cytotoxicity in the subset of MITF-Low melanoma cells by exploiting their dependency on monocarboxylate transport for acidotic survival.

Authors: Yang, O

• DOI: 10.5287/ora-bp7bxvzvy
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Acidosis; Proton Transporters; Phenotypic Plasticity; Melanoma; MITF
• Subjects: Oncology; Molecular Genetics; Cancer Biology; Molecular Biology