TGFb-Induced Deptor Suppression Recruits mTORC1 and Not mTORC2 to Enhance Collagen I (a2) Gene Expression

Enhanced TGFb activity contributes to the accumulation of matrix proteins including collagen I (a2) by proximal tubular epithelial cells in progressive kidney disease. Although TGFb rapidly activates its canonical Smad signaling pathway, it also recruits noncanonical pathway involving mTOR kinase to regulate renal matrix expansion. The mechanism by which chronic TGFb treatment maintains increased mTOR activity to induce the matrix protein collagen I (a2) expression is not known. Deptor is an mTOR interacting protein that suppresses mTOR activity in both mTORC1 and mTORC2. In proximal tubular epithelial cells, TGFb reduced deptor levels in a time-dependent manner with concomitant increase in both mTORC1 and mTORC2 activities. Expression of deptor abrogated activity of mTORC1 and mTORC2, resulting in inhibition of collagen I (a2) mRNA and protein expression via transcriptional mechanism. In contrast, neutralization of endogenous deptor by shRNAs increased activity of both mTOR complexes and expression of collagen I (a2) similar to TGFb treatment. Importantly, downregulation of deptor by TGFb increased the expression of Hif1a by increasing translation of its mRNA. TGFb-induced deptor downregulation promotes Hif1a binding to its cognate hypoxia responsive element in the collagen I (a2) gene to control its protein expression via direct transcriptional mechanism. Interestingly, knockdown of raptor to specifically block mTORC1 activity significantly inhibited expression of collagen I (a2) and Hif1a while inhibition of rictor to prevent selectively mTORC2 activation did not have any effect. Critically, our data provide evidence for the requirement of TGFb-activated mTORC1 only by deptor downregulation, which dominates upon the bystander mTORC2 activity for enhanced expression of collagen I (a2). Our results also suggest the presence of a safeguard mechanism involving deptor-mediated suppression of mTORC1 activity against developing TGFb-induced renal fibrosis. Citation: Das F, Bera A, Ghosh-Choudhury N, Abboud HE, Kasinath BS, et al. (2014) TGFb-Induced Deptor Suppression Recruits mTORC1 and Not mTORC2 to Enhance Collagen I (a2) Gene Expression. PLoS ONE 9(10): e109608. doi:10.1371/journal.pone.0109608 Editor: Soumitro Pal, Children’s Hospital Boston & Harvard Medical School, United States of America Received June 10, 2014; Accepted September 2, 2014; Published October 15, 2014 This is an open-access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication. Data Availability: The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are within the paper and its Supporting Information files. Funding: This work was supported by National Institutes of Health RO1 DK50190 and Veterans Administration Research Service Merit Review 5I01BX000926 grants to GGC. GGC is a recipient of Veterans Administration Senior Research Career Scientist Award. HEA is supported by National Institutes of Health. NGC and BSK are supported by VA Merit Review grants. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * Email: [email protected]