Abstract
Ubiquitin-conjugating enzyme 9 (Ubc9) is required for sumoylation and inhibits apoptosis via Bcl-2 by unknown mechanism. Methionine adenosyltransferase 2A (MAT2A) encodes for MATα2, the catalytic subunit of the MATII isoenzyme that synthesizes S-adenosylmethionine (SAMe). Ubc9, Bcl-2 and MAT2A expression are up-regulated in several malignancies. Exogenous SAMe decreases Ubc9 and MAT2A expression and is pro-apoptotic in liver and colon cancer cells. Here we investigated whether there is interplay between Ubc9, MAT2A and Bcl-2. We used human colon and liver cancer cell lines RKO and HepG2, respectively, and confirmed key finding in colon cancer specimens. We found MATα2 can regulate Bcl-2 expression at multiple levels. MATα2 binds to Bcl-2 promoter to activate its transcription. This effect is independent of SAMe as MATα2 catalytic mutant was also effective. MATα2 also directly interacts with Bcl-2 to enhance its protein stability. MATα2's effect on Bcl-2 requires Ubc9 as MATα2's stability is influenced by sumoylation at K340, K372 and K394. Overexpressing wild type (but not less stable MATα2 sumoylation mutants) protected from 5-fluorouracil-induced apoptosis in both colon and liver cancer cells. Colon cancer have higher levels of sumoylated MATα2, total MATα2, Ubc9 and Bcl-2 and higher MATα2 binding to the Bcl-2 P2 promoter. Taken together, Ubc9's protective effect on apoptosis may be mediated at least in part by sumoylating and stabilizing MATα2 protein, which in turn positively maintains Bcl-2 expression. These interactions feed forward to further enhance growth and survival of the cancer cell.
Highlights
ubiquitin-related modifier (SUMO) is a small ubiquitin-like protein that can be covalently attached to proteins through the formation of isopeptide bonds with specific lysine residues of target proteins [1]
Since SAMe lowers the expression of both Ubiquitin-conjugating enzyme 9 (Ubc9) and Methionine adenosyltransferase 2A (MAT2A) and knockdown of Ubc9 and MAT2A leads to apoptosis, we examined whether there might be interplay between MAT2A and B-cell lymphoma 2 (Bcl-2) that is regulated by sumoylation
We examined whether Ubc9 regulate apoptosis and Bcl-2 expression in HepG2 and RKO cells
Summary
SUMO is a small ubiquitin-like protein that can be covalently attached to proteins through the formation of isopeptide bonds with specific lysine residues of target proteins [1]. SUMO-1 regulates protein stability and activity with crucial implications for many cellular pathways [3]. Protein conjugates with heterologous SUMO-2/3ubiquitin chains are preferentially targeted for proteasome degradation and the function of SUMO-4 is unknown [4,5,6]. Ubiquitin conjugating enzyme 9 (Ubc9) is the only E2 conjugating enzyme and a key regulator of the sumoylation machinery, transferring the activated SUMO to protein substrates [7]. Sumoylation is involved in many vital processes including transcriptional regulation, signal transduction, protein degradation, cell cycle regulation, chromatin organization, and nuclear transport [8]. Dysregulated sumoylation contributes to carcinogenesis by affecting post-transcriptional modification of key proteins [9], including those involved in cancer metastasis [10,11,12]
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