A novel SREBP-1 splice variant: tissue abundance and transactivation potency.

Sterol regulatory element binding proteins (SREBPs) belong to the family of basic helix-loop-helix-leucine zipper transcription factors. The SREBP-1 gene encodes two different isoforms, SREBP-1a and -1c, that are expressed at varying levels in different tissues and cultured cells and exhibit common and distinct functions. We identified an additional SREBP-1 isoform, termed SREBP-1ac, and determined its mRNA abundance in different human tissues and cell lines. SREBP-1ac mRNA was detectable in all tissues studied, although at lower levels than the major SREBP-1a and -1c isoforms. Transcription of the novel SREBP isoform was not induced by insulin or cholesterol depletion. SREBP-1ac did not transactivate the human LDLR and UCP2 promoters but robustly attenuated the transactivation capacity of SREBP-1a, -1c and -2 in cotransfection experiments.