Clear cell renal cell carcinoma (ccRCC) is characterized by the constitutive up-regulation of the hypoxia inducible factor-1. One of its target enzymes, pyruvate dehydrogenase (PDH) kinase 1 (PDHK1) showed increased protein expression in tumor as compared to patient-matched normal tissues. PDHK1 phosphorylated and inhibited PDH whose enzymatic activity was severely diminished, depriving the TCA cycle of acetylCoA. We and others have shown a decrease in the protein expressions of all respiratory complexes alluding to a compromise in oxidative phosphorylation (OXPHOS). On the contrary, we found that key parameters of OXPHOS, namely ATP biosynthesis and membrane potential were consistently measurable in mitochondria isolated from ccRCC tumor tissues. Interestingly, an endogenous mitochondrial membrane potential (MMP) was evident when ADP was added to mitochondria isolated from ccRCC but not in normal tissues. In addition, the MMP elicited in the presence of ADP by respiratory substrates namely malate/glutamate, succinate, α-ketoglutarate and isocitrate was invariably higher in ccRCC. Two additional hallmarks of ccRCC include a loss of uncoupling protein (UCP)-2 and an increase in UCP-3. Based on our data, we proposed that inhibition of UCP3 by ADP could contribute to the endogenous MMP observed in ccRCC and other cancer cells.
Keywords: Clear cell renal cell carcinoma (ccRCC); Mitochondrial membrane potential (MMP); Oxidative phosphorylation (OXPHOS); Uncoupling protein 2 (UCP2); Uncoupling protein 3 (UCP3); Warburg effect.
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