Tumor suppressing pathways in renal cancer

Cincinnati Va Medical Center Research, Cincinnati OH

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Abstract

Clear cell renal cell carcinoma (ccRCC) is a serious health concern for military personnel, particularly males beyond 40 years of age, including military veterans. According to The Defense Medical Epidemiology Database for 1995-2004 the incidence of RCC specifically for military members after the 4th decade of life is dramatically increased to 8.5 as compared to 1.5 cases per 100,000 person-years of the overall incidence. This proposal investigates direct mechanistic connection between selective autophagy and glucose metabolism in the context of renal cancer cells. Autophagy is a tightly regulated process of self-digestion. Formation of an autophagosome requires lipidation and insertion of microtubule associated protein 1 light chains A, B and C (MAP1LC3A, B, C, referred to as LC3A, LC3B, and LC3C) into the autophagosomal membrane. We established that LC3C autophagy is tumor suppressing and functions downstream form VHL, tumor suppressor lost in clear cell renal cell carcinoma. LC3C is an evolutionary late gene, present only in higher primates and humans, that contains a unique and conserved C-terminal 20 amino acid peptide that is cleaved during initiation of LC3C autophagy. The C-terminal peptide of LC3C has a proline hydroxylation motif similar to the canonical motifs in HIFαs, where prolines are hydroxylated by 2-oxyglutarate (2OG)-dependent EGLN proline hydroxylases. Our preliminary data indicate that P133 within the LC3C peptide undergoes hydroxylation by EGLN3 proline hydroxylase in an autophagy- dependent manner. Recently we discovered that LC3C autophagy requires glucose metabolic flux. In turn, loss of LC3C increases the steady-state levels of glycolytic and pentose phosphate metabolites, representing hallmarks of oncogenic form of metabolism (Warburg effect) particularly relevant in ccRCC. We determined that LC3C co-immunoprecipitates and targets for autolysosomal degradation malate/2-oxyglutarate(2OG) and aspartate /glutamate antiporters, SCL25A11 and SLC25A13, respectively that are part of the mitochondrial malate-aspartate shuttle (MAS). MAS transfers reducing equivalents between mitochondria and cytoplasm, yielding mitochondrial NADH for ATP synthesis while generating cytosolic NAD to sustain glycolysis. Additionally, the shuttle exchanges glutamate and aspartate that contributes to biosynthetic potential. We propose a novel, metabolism-coupled mechanism of tumor suppressing LC3C activity: LC3C autophagy targets MAS proteins for lysosomal degradation in the process of mitophagy. This acts as a checkpoint for glycolysis by regulating cytosolic NAD/NADH ratio, as well as for SLC25A13-mediated export of aspartate from mitochondria. That indicates that LC3C metabolically partners with transcriptional effects of VHL inhibiting glycolysis. Moreover, we hypothesize that selective activation of LC3C autophagy in the proximity to mitochondrial carriers is caused by 2OG derived from glucose through the TCA cycle and transported through the SLC25A11 which activates EGLN3, leading to P133 hydroxylation. Here we will mechanistically investigate metabolic inputs and functional output of LC3C autophagy in regard to MAS in the context of renal cancer. Aim 1 will determine role of LC3C structural elements in the autophagic degradation of SLC25A11/13 and in functional consequences for MAS activity. We hypothesize that LC3C C-terminal peptide and P133 are necessary and potentially sufficient for this activity. Aim 2 will identify metabolic effects of LC3C tumor suppressing activity. We hypothesize that LC3C will (i) inhibit NAD/NADH regeneration and glycolytic activity and (ii) suppress export of aspartate from mitochondria and its availability for biosynthetic pathways. Aim 3 will determine molecular mechanism by which glucose regulates LC3C activity. We propose that LC3C autophagy is activated by sensing mitochondrial 2OG levels through EGLN3-dependent hydroxylation of P133 in the C-terminal peptide. 2OG utilized by EGLN3-LC3C is derived from glucose and exits mitochondria through SLC25A11, indicating that glucose oxidation is critical for LC3C autophagic activity.