Sterol Regulatory Element-binding Proteins (SREBPs): Key Regulators of Nutritional Homeostasis and Insulin Action*

The sterol regulatory element-binding proteins (SREBPs) were first identified by two groups working independently on cholesterol metabolism (1, 2) and fat cell differentiation (3). Subsequent studies have demonstrated there are three major SREBP isoforms encoded by two different genes (4). These unique members of the basic helix-loop-helix leucine zipper (bHLHLZ) family of transcriptional regulatory proteins can be distinguished from other family members by two characteristics. The first is they are synthesized as precursors that are threaded into membranes of the endoplasmic reticulum and nuclear envelope in a hairpin orientation such that the amino and carboxyl tails both face the cytoplasm. The amino-terminal half of the precursor is clipped out of the membrane in two steps responding to regulatory cues that signal the need for increased cellular cholesterol (5). The released amino-terminal fragment, which contains the transcriptional activation and DNA binding domains, is targeted to the nucleus where it activates expression of SREBP target genes. The second distinguishing feature of the SREBPs is that they have a unique dual DNA binding specificity, which is discussed below. Two of the three major isoforms are produced from the SREBP-1 gene, which contains two promoters (6). Transcription from each promoter produces an mRNA with a unique first exon that encodes one of the alternative amino termini referred to as 1a and 1c, respectively (Fig. 1). These alternate exons are attached during mRNA splicing to a common second exon in the same reading frame, and therefore, the remaining protein coding information of both isoforms is identical. There is alternative mRNA splicing at the 39-end as well (7), but this does not appear to be conserved in all mammalian species and its functional significance remains unclear (8). In contrast, mRNAs produced through alternative promoter usage at the 59-end yield proteins with significant differences in their capacity to activate gene expression (9). The longer amino-terminal region in SREBP-1a contains a high percentage of acidic amino acids that make it a potent transcriptional activation domain. The SREBP-1c isoform is a much weaker activator of gene expression because it lacks 29 acidic amino acids present in 1a. Using a nuclease mapping technique to evaluate the relative levels of SREBP-1a and SREBP-1c mRNA, the ratio was shown to vary over an ;50–100-fold range in different tissues of the body. In liver and adipocytes, 1c mRNA is 9and 3-fold, respectively, more abundant than SREBP-1a, whereas in spleen SREBP-1a is 10 times more abundant than 1c (8). Whether these ratios reflect similar differences in the levels of each protein remains to be firmly established. In all cultured cell lines examined, SREBP-1a was expressed at higher levels (8). 1c mRNA is the predominant isoform in adult liver and adipocytes, so it is likely to be the key protein involved in SREBP-1-dependent processes in these tissues. Why different tissues express different ratios of SREBP-1a and -1c is not clear. It is possible that the more active 1a isoform is preferentially expressed when there is a high demand for cholesterol and fatty acids such as when new membrane is required during periods of rapid cell division. SREBP-1a stimulates gene expression in vitro and in cultured cells by interacting with the transcriptional coactivators CBP and P300 (10, 11) (Fig. 1). These are large, ubiquitous transcriptional coactivator proteins that are recruited to specific promoters through binding to activation domains of several DNA binding transcription factors in addition to SREBPs (12). The shorter activation domain of SREBP-1c does not interact efficiently with CBP or P300, and how this isoform activates transcription is not clearly understood. The single SREBP-2 isoform similarly interacts with CBP and P300 to activate transcription (Fig. 1). The amino-terminal domain of SREBP-1a interacts also with a separate multisubunit complex alternately called vitamin D receptor-interacting protein (DRIP) or activator-recruited cofactor (ARC) (13). This heterogeneous complex increases transcription through interacting with activation domains of several other DNA-binding transcriptional regulatory proteins in addition to SREBP-1a. The DRIP/ARC interaction is independent of CBP/P300. As members of the bHLHLZ family of DNA-binding proteins, SREBPs form dimers that recognize the inverted repeat E-box 59-CANNTG-39 (where N represents any base). bHLH subfamilies can be classified according to their preference for specific bases at the middle positions of the E-box (14). SREBPs belong to the same subfamily as Myc/Max and USF, which all prefer the 59-CACGTG-39 E-box. However, SREBPs are further distinguished because they not only bind to this inverted repeat but also to the direct repeat sterol regulatory element (SRE) 59-TCACNCCAC-39 or to related sites (15). This flexibility is because of a unique tyrosine residue in the SREBP basic domain that corresponds to an arginine in all other E-box-binding bHLH proteins. Kim et al. (16) demonstrated the importance of the tyrosine residue by changing it to an arginine to mimic other bHLH proteins. The resulting protein bound only to Eboxes. The reciprocal mutation changing the arginine of the related USF1 protein to a tyrosine converted it into an E-box* This minireview will be reprinted in the 2000 Minireview Compendium, which will be available in December, 2000. This is the third article of three in the “Nutrient Control of Gene Transcription Minireview Series. ” Work in the author’s laboratory is supported in part by National Institutes of Health Grant HL48044 and American Diabetes Association Grant ADA 26825. ‡ To whom correspondence and reprint requests should be addressed. Tel. : 949-824-2979; Fax: 949-824-8551; E-mail: tfosborn@uci.edu. 1 The abbreviations used are: SREBP, sterol regulatory elementbinding protein; SRE, sterol regulatory element; SREBP-1c, sterol regulatory element-binding protein-1c or adipocyte differentiation and determination factor-1; bHLH, basic helix-loop-helix; bHLHLZ, bHLH leucine zipper; LDL, low density lipoprotein; CBP, cAMP-response element-binding protein; DRIP, vitamin D receptor-interacting protein; ARC, activator-recruited cofactor; FAS, fatty acid synthase; PUFA, polyunsaturated fatty acid(s). 2 K. A. Dooley and T. F. Osborne, unpublished data. Minireview THE JOURNAL OF BIOLOGICAL CHEMISTRY Vol. 275, No. 42, Issue of October 20, pp. 32379–32382, 2000 © 2000 by The American Society for Biochemistry and Molecular Biology, Inc. Printed in U.S.A.