Independent regulation of sterol regulatory element-binding proteins 1 and 2 in hamster liver.

Abstract

Two sterol regulatory element-binding proteins (SREBPs, designated SREBP-1 and SREBP-2), each -1150 amino acids in length, are attached to membranes of the endoplasmic reticulum and nuclear envelope in human and hamster tissue culture cells. In the absence of sterols, soluble fragments of -470 amino acids are released from both proteins by proteolytic cleavage. The soluble fragments enter the nucleus, where they bind to sterol regulatory elements in the promoters of genes encoding the low density lipoprotein receptor and 3-hydroxy-3-methylglutaryl CoA synthase, thereby activating transcription. Proteolytic processing of both SREBPs is blocked coordinately by sterol overloading and enhanced coordinately when sterols are depleted by treatment with an inhibitor of cholesterol synthesis. In contrast to these findings in cultured cells, the current data show that SREBP-1 and -2 are not coordinately regulated in hamster liver. In untreated animals the soluble fragment of SREBP-1, but not of SREBP-2, was detected by immunoblotting of a liver nuclear extract. Depletion of sterols by treatment with a bile acid-binding resin (colestipol) and a cholesterol synthesis inhibitor (mevinolin) led to a marked increase in the nuclear form of SREBP-2 and a reciprocal decline in the nuclear form of SREBP-1. These findings suggest that SREBP-1 is responsible for basal transcription of the low density lipoprotein receptor and 3-hydroxy-3-methylglutaryl CoA synthase genes in hamster liver and that SREBP-2 is responsible for the increased transcription that follows sterol depletion with a bile acid-binding resin and a cholesterol synthesis inhibitor.Two sterol regulatory element-binding proteins (SREBPs) have been proposed to control the transcription of the genes for the low density lipoprotein (LDL) receptor, 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) synthase, and other genes whose control regions contain sterol regulatory elements (1-4). The two SREBPs, designated SREBP-1 and SREBP-2, have a similar domain structure and are believed to operate through a similar mechanism.The SREBPs are synthesized as long polypeptide chains of 1150 amino acids, which are designated as precursor forms. Both precursors migrate on SDS/polyacrylamide gels with apparent molecular masses of '125 kDa (1-5). They are attached to membranes of the endoplasmic reticulum and nuclear envelope by means of hydrophobic sequences located between residues 470 and 570. Within the NH2-terminal 470 residues, each protein contains a basic-helix-loop-helixleucine zipper motif that allows the protein to form homodimers and to bind to sterol regulatory elements. The NH2-terminal domains of both proteins also contain an acidic region that acts as a transcriptional activator (4). When human and hamster cultured cells are grown in the absence of sterols, a protease cleaves the SREBPs between the leucine zipper and the membrane attachment domain, releasing the mature NH2-terminal portion of the protein from the membrane (3, 4). Although the NH2-terminal portions a...

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