Translational control of the sterol-regulatory transcription factor SREBP-1 mRNA in response to serum starvation or ER stress is mediated by an internal ribosome entry site

SREBPs (sterol-regulatory-element-binding proteins) are a family of transcription factors that modulate the expression of several enzymes implicated in endogenous cholesterol, fatty acid, triacylelycerol and phospholipid synthesis. In the present study, evidence for SREBP-1 regulation at the translational level is reported. Using, several experimental approaches, we have demonstrated that the 5'-UTR (untranslated region) of the SREBP-1 a mRNA contains an IRES (internal ribosome entry site). Transfection experiments with the SREBP-1 a 5'-UTR inserted in a dicistronic reporter vector showed a remarkable increase in the downstream cistron translation, through a cap-independent mechanism. Insertion of the SREBP-1c 5'-UTR in the same vector also stimulated the translation of the downstream cistron, but the observed effect can be ascribed, at least in part, to a cryptic promoter activity. Cellular stress conditions, such as serum starvation, caused an increase in the level of SREBP-1 precursor and mature form in both Hep G2 and HeLa cells, despite the overall reduction in protein synthesis, whereas mRNA levels for SREBP-1 were unaffected by serum starvation. Transfection experiments carried out with a dicistronic construct demonstrated that the cap-dependent translation was affected more than IRES-mediated translation by serum starvation. The thapsigargin-and tunicamycin-induced UPR (unfolded protein response) also increased SREBP-I expression in Hep G2 cells, through the cap-independent translation mediated by IRES. Overall, these findings indicate that the presence of IRES in the SREBP-I a 5'-UTR allows translation to be maintained under conditions that are inhibitory to cap-dependent translation.