In this proposal we investigate mechanism of selective autophagy in a novel, noncanonical pathway which has tumor suppressing activity in renal cancer. Autophagy is a tightly regulated process of self-digestion, which in cancer can have both tumor suppressing and oncogenic activities. Formation of an autophagosome requires microtubule associated protein 1 light chains A, B and C (MAP1LC3A, B, C referred to as LC3A, B and C). LC3s bind with cargo receptors through the LC3C-interacting regions (LIR) motifs on the receptors. Clear cell renal cell carcinoma (ccRCC) is the most frequent renal cancer characterized by the loss the von Hippel-Lindau gene (VHL). Loss of VHL function leads to activation of Hypoxia Inducible Transcription Factors (HIF) and changes in angiogenesis and metabolism. Our laboratory discovered that VHL regulates autophagy. VHL inhibits LC3B autophagy, which is oncogenic in ccRCC. In contrast, VHL induces tumor suppressing, LC3C autophagy in a mechanism that involves removal of transcription repression by HIF. LC3C is an evolutionary late gene, present only in higher primates and humans that evolved into multifaceted autophagic regulator, more complicated as compared to LC3B/A. It maintains binding site for the canonical LIRs, similar to other LC3s, but it gained a new binding site for LC3C-specific LIR, CLIR, and a highly conserved C-terminal, 20 amino acid peptide, cleaved in the process of glycine lipidation. Our preliminary results show that LC3C autophagy requires novel non-canonical pre- and -initiation complexes, which include ULK3, BECN1, UVRAG and PIK3C2A. We identified two direct targets selectively degraded by LC3C autophagy: (i) Postdivision Midbody Rings (PDMBs), remnants of midbody structures created during cytokinesis. PDMB accumulation in cancer cells promotes stem-like state and cancer progression. (ii) Caveolin 1(CAV1), a lipid raft cholesterol-binding protein. We found that LC3C-dependent degradation of the cargo requires C-terminal peptide on LC3C and a protein complex that includes adapter proteins, TSG101 and CHMP2B. Surprisingly, VHL interacts with the core autophagy apparatus in an LC3C- dependent manner, an indication that it directly serves in the formation of LC3C autophagosomes.
In Aim 1, we will determine activity of the LC3C preinitiation, initiation, and adapter complexes. We hypothesize that LC3C autophagy is regulated by distinctive and selective mediators, as compared to LC3B, of the pre- and initiation complexes and TSG101 anchors LC3C to the selective cargo.
In Aim 2 we will determine the direct role of VHL in regulation of LC3C autophagy. We propose that VHL anchors at the LC3C regulatory complex through the hydroxylated proline in the C-terminal peptide and is necessary for the association with the core autophagic proteins.
In Aim 3 we will determine the role of unique structures of LC3C in the selective degradation of cargo by the non-canonical pathway and in tumor suppressing activity of LC3C. We hypothesize that this process requires the evolutionary recent C-terminal peptide.
The project investigates a novel, selective, human-specific autophagy downstream from the von Hippel Lindau tumor suppressor gene. This pathway is regulated by several non-canonical regulators and targets for degradation oncogenic cargo.
