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, triacylglycerol 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-1a mRNA contains an IRES (internal ribosome entry site). Transfection experiments with the SREBP-1a 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-1 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-1a 5'-UTR allows translation to be maintained under conditions that are inhibitory to cap-dependent translation.
Damiano
Fabrizio
F
Department of Biological and Environmental Science and Technologies, University of Salento, Lecce, Italy.
Alemanno
Simone
S
Gnoni
Gabriele V
GV
Siculella
Luisa
L
D020121:5' Untranslated Regions; D001483:Base Sequence; D002460:Cell Line; D016895:Culture Media, Serum-Free; D017931:DNA Primers; D017930:Genes, Reporter; D006801:Humans; D014176:Protein Biosynthesis; D012333:RNA, Messenger / Q000235:genetics*; D012270:Ribosomes / Q000378:metabolism*; D051780:Sterol Regulatory Element Binding Protein 1 / Q000235:genetics*
D020121:5' Untranslated Regions; D016895:Culture Media, Serum-Free; D017931:DNA Primers; D012333:RNA, Messenger; C497387:SREBF1 protein, human; D051780:Sterol Regulatory Element Binding Protein 1