Metabolic Reprogramming and Epigenetic Regulation in Renal Cell Carcinoma

Abstract

Renal cell carcinoma comprises approximately 2-3% of all malignancies, with the majority, approximately 70%, being clear cell renal cell carcinoma (ccRCC). ccRCC features many cancer hallmarks such as suppressed mitochondrial glucose oxidation, apoptosis-resistance, angiogenesis, immune evasion, uninhibited proliferation, and invasion and metastasis, thought to be driven by sporadic mutations or epigenetic silencing of the tumor suppressor gene, von Hippel Lindau (VHL). VHL loss leads to up-regulation of its target protein, hypoxia inducible factor (HIF), even in normoxia. HIF controls many downstream targets in order to prepare the cell for hypoxia, with the net effect being suppression of mitochondrial glucose oxidation, up-regulation of glycolysis, angiogenesis and apoptosis-resistance. Unchecked, as in VHL-deficiency, this becomes a critical step in tumorigenesis. We show that reversing the suppression of mitochondrial glucose oxidation in ccRCC using dichloroacetate, an inhibitor of the HIF target gene mitochondrial pyruvate dehydrogenase kinase (PDK), which inhibits a major producer of acetyl-CoA, the Pyruvate Dehydrogenase Complex (PDC), results in increased production of mitochondrial acetyl-CoA via PDC, reduced proliferation and angiogenesis, and induces apoptosis in animal models. We also show that mitochondrial PDC dynamically translocates to the nucleus in multiple types of cancer cells, including ccRCC, in response to growth factor signaling, to produce acetyl-CoA in the nucleus. This local production of acetyl-CoA by nuclear PDC is used to acetylate core histones involved in S-phase progression and cellular proliferation. Next, we show that VHL, which has been described as a multipurpose adapter protein, directly binds the tumor suppressor p53, preventing its activation, promoter binding and expression of its target genes. This process is independent of the VHL-HIF axis and results in an attenuation of p53 inducing therapies. Finally, we study Myocyte Enhancer Factor 2A (Mef2A), a transcription factor with ties to multiple hallmarks of cancer, like mitochondrial suppression, invasion and immune evasion. We show that Mef2A expression is up-regulated in patient tumors compared to adjacent normal kidney parenchyma and that nuclear Mef2A levels correlate with larger tumor size. Mef2A activity is increased in VHL-deficient ccRCC cells due to HIF-mediated growth factor signaling and it induces expression of the pro-tumor chemokine, CCL20. Inhibition of Mef2A results in reduced ccRCC tumor growth in vivo and may provide mechanistic insight into immune evasion as immune checkpoint inhibitors develop into the first line therapy in metastatic ccRCC.