Opposing effects of metformin mediated mTORC1 inhibition on IRES possessing anti-apoptotic proteins in breast cancer cell lines

Abstract

Anti-diabetic drug, metformin, has attracted attentions as an anti-cancer agent due to the suppression of mTORC1 as a master switch of cell growth in various cancers. Upon mTORC1 inhibition, translation of proteins possessing IRES elements in their mRNAs still occurs in order to maintain proper cell function. Thus, inhibitor of apoptosis proteins are expected to be upregulated by mTORC1 inhibition due to the possession of IRES elements. Surprisingly, however, inhibition of mTORC1 with metformin in breast cancer cell lines of MDA-MB-231 and MCF-7, caused the downregulation of IRES-element possessing proteins of cIAP1, XIAP and Bcl-2, 24 h post-treatment. Interestingly, a shorter treatment time of 8 h, however, was accompanied by increased expression of these proteins. Importantly, inclusion of the proteasome inhibitor, Bortezomib, caused the upregulation of the tested anti-apoptotic proteins in both cell lines. These observations suggest that mTORC1 inhibition has a bifold effect; first upregulation of IRES-dependent survival proteins to prevent untimely cell death followed by an opposite effect, which is the enhanced proteosomal degradation of these protein in order to maintain a balanced response.