Russell A. DeBose-Boyd

Protein sensors for membrane sterols.

Cholesterol is an essential component of animal cell membranes, and its concentration is tightly controlled by a feedback system that operates at transcriptional and posttranscriptional levels. Here, we discuss recent advances that explain how cells employ an ensemble of membrane-embedded proteins to monitor sterol concentrations and adjust sterol synthesis and uptake.

Unsaturated fatty acids inhibit transcription of the sterol regulatory element-binding protein-1c (SREBP-1c) gene by antagonizing ligand-dependent activation of the LXR

Sterol regulatory element-binding protein-1c (SREBP-1c) enhances transcription of genes encoding enzymes of unsaturated fatty acid biosynthesis in liver. SREBP-1c mRNA is known to increase when cells are treated with agonists of liver X receptor (LXR), a nuclear hormone receptor, and to decrease when cells are treated with unsaturated fatty acids, the end products of SREBP-1c action. Here we show that unsaturated fatty acids lower SREBP-1c mRNA levels in part by antagonizing the actions of LXR. In cultured rat hepatoma cells, arachidonic acid and other fatty acids competitively inhibited activation of the endogenous SREBP-1c gene by an LXR ligand. Arachidonate also blocked the activation of a synthetic LXR-dependent promoter in transfected human embryonic kidney-293 cells. In vitro, arachidonate and other unsaturated fatty acids competitively blocked activation of LXR, as reflected by a fluorescence polarization assay that measures ligand-dependent binding of LXR to a peptide derived from a coactivator. These data offer a potential mechanism that partially explains the long-known ability of dietary unsaturated fatty acids to decrease the synthesis and secretion of fatty acids and triglycerides in livers of humans and other animals.

Accelerated degradation of HMG CoA reductase mediated by binding of insig-1 to its sterol-sensing domain.

Sterols accelerate degradation of the ER enzyme 3-hydroxy-3-methylglutaryl CoA reductase (HMG CoA reductase), which catalyzes a rate-controlling step in cholesterol biosynthesis. This degradation contributes to feedback inhibition of synthesis of cholesterol and nonsterol isoprenoids. Here, we show that degradation of HMG CoA reductase is accelerated by the sterol-induced binding of its sterol-sensing domain to the ER protein insig-1. Accelerated degradation is inhibited by overexpression of the sterol-sensing domain of SREBP cleavage-activating protein (SCAP), suggesting that both proteins bind to the same site on insig-1. Whereas insig-1 binding to SCAP leads to ER retention, insig-1 binding to HMG CoA reductase leads to accelerated degradation that is blocked by proteasome inhibitors. Insig-1 appears to play an essential role in the sterol-mediated trafficking of two proteins with sterol-sensing domains, HMG CoA reductase and SCAP.

Transport-dependent proteolysis of SREBP: Relocation of Site-1 protease from Golgi to ER obviates the need for SREBP transport to Golgi

Cholesterol homeostasis in animal cells is achieved by regulated cleavage of membrane-bound transcription factors, designated SREBPs. Proteolytic release of the active domains of SREBPs from membranes requires a sterol-sensing protein, SCAP, which forms a complex with SREBPs. In sterol-depleted cells, SCAP escorts SREBPs from ER to Golgi, where SREBPs are cleaved by Site-1 protease (S1P). Sterols block this transport and abolish cleavage. Relocating active S1P from Golgi to ER by treating cells with brefeldin A or by fusing the ER retention signal KDEL to S1P obviates the SCAP requirement and renders cleavage insensitive to sterols. Transport-dependent proteolysis may be a common mechanism to regulate the processing of membrane proteins.

Insig-dependent Ubiquitination and Degradation of Mammalian 3-Hydroxy-3-methylglutaryl-CoA Reductase Stimulated by Sterols and Geranylgeraniol

The endoplasmic reticulum enzyme 3-hydroxy-3-methylglutaryl-CoA reductase produces mevalonate, which is converted to sterols and to other products, including geranylgeraniol groups attached to proteins. The enzyme is known to be ubiquitinated and rapidly degraded when sterols and nonsterol end products of mevalonate metabolism accumulate in cells. Here, we use RNA interference to show that sterol-accelerated ubiquitination of reductase requires Insig-1 and Insig-2, membrane-bound proteins of the endoplasmic reticulum that were shown previously to accelerate degradation of reductase when overexpressed by transfection. Alanine substitution experiments reveal that binding of reductase to Insigs and subsequent ubiquitination require the tetrapeptide sequence YIYF in the second membrane-spanning helix of reductase. The YIYF peptide is also found in the sterol-sensing domain of SCAP, another protein that binds to Insigs in a sterol-stimulated fashion. When lysine 248 of reductase is substituted with arginine, Insig binding persists, but the reductase is no longer ubiquitinated and degradation is markedly slowed. Lysine 248 is predicted to lie immediately adjacent to a membrane-spanning helix, suggesting that a membrane-bound ubiquitin transferase is responsible. Finally, we show that Insig-dependent, sterol-stimulated degradation of reductase is further accelerated when cells are also supplied with the 20-carbon isoprenoid geranylgeraniol, but not the 15-carbon farnesol, raising the possibility that the nonsterol potentiator of reductase regulation is a geranylgeranylated protein.

Feedback Regulation of Cholesterol Synthesis: Sterol-Accelerated Ubiquitination and Degradation of HMG CoA Reductase

3-Hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase produces mevalonate, an important intermediate in the synthesis of cholesterol and essential nonsterol isoprenoids. The reductase is subject to an exorbitant amount of feedback control through multiple mechanisms that are mediated by sterol and nonsterol end-products of mevalonate metabolism. Here, I will discuss recent advances that shed light on one mechanism for control of reductase, which involves rapid degradation of the enzyme. Accumulation of certain sterols triggers binding of reductase to endoplasmic reticulum (ER) membrane proteins called Insig-1 and Insig-2. Reductase-Insig binding results in recruitment of a membrane-associated ubiquitin ligase called gp78, which initiates ubiquitination of reductase. This ubiquitination is an obligatory reaction for recognition and degradation of reductase from ER membranes by cytosolic 26S proteasomes. Thus, sterol-accelerated degradation of reductase represents an example of how a general cellular process (ER-associated degradation) is used to control an important metabolic pathway (cholesterol synthesis).

Failure to cleave sterol regulatory element-binding proteins (SREBPs) causes cholesterol auxotrophy in Chinese hamster ovary cells with genetic absence of SREBP cleavage-activating protein

We describe a line of mutant Chinese hamster ovary cells, designated SRD-13A, that cannot cleave sterol regulatory element-binding proteins (SREBPs) at site 1, due to mutations in the gene encoding SREBP cleavage-activating protein (SCAP). The SRD-13A cells were obtained by two rounds of γ-irradiation followed first by selection for a deficiency of low density lipoprotein receptors and second for cholesterol auxotrophy. In the SRD-13A cells, the only detectable SCAP allele encodes a truncated nonfunctional protein. In the absence of SCAP, the site 1 protease fails to cleave SREBPs, and their transcriptionally active NH2-terminal fragments cannot enter the nucleus. As a result, the cells manifest a marked reduction in the synthesis of cholesterol and its uptake from low density lipoproteins. The SRD-13A cells grow only when cholesterol is added to the culture medium. SREBP cleavage is restored and the cholesterol requirement is abolished when SRD-13A cells are transfected with expression vectors encoding SCAP. These results provide formal proof that SCAP is essential for the cleavage of SREBPs at site 1.

Insig-dependent ubiquitination and degradation of 3-hydroxy-3-methylglutaryl coenzyme a reductase stimulated by δ- and γ-tocotrienols

Sterol-regulated ubiquitination marks 3-hydroxy-3-methylglutaryl coenzyme A reductase, a rate-determining enzyme in cholesterol synthesis, for endoplasmic reticulum (ER)-associated degradation by 26 S proteasomes. This degradation, which results from sterol-induced binding of reductase to ER membrane proteins called Insigs, contributes to the complex, multivalent feedback regulation of the enzyme. Degradation of HMG-CoA reductase is also stimulated by various forms of vitamin E, a generic term for α-, β-, δ-, and γ-tocopherols and tocotrienols, which are primarily recognized for their potent antioxidant activity. Here, we show that δ-tocotrienol stimulates ubiquitination and degradation of reductase and blocks processing of sterol regulatory element-binding proteins (SREBPs), another sterol-mediated action of Insigs. The γ-tocotrienol analog is more selective in enhancing reductase ubiquitination and degradation than blocking SREBP processing. Other forms of vitamin E neither accelerate reductase degradation nor block SREBP processing. In vitro assays indicate that γ- and δ-tocotrienol trigger reductase ubiquitination directly and do not require further metabolism for activity. Taken together, these results provide a biochemical mechanism for the hypocholesterolemic effects of vitamin E that have been observed in animals and humans.

Regulation of Cholesterol and Fatty Acid Synthesis

In mammals, intracellular levels of cholesterol and fatty acids are controlled through a feedback regulatory system mediated by a family of transcription factors called sterol regulatory element-binding proteins (SREBPs). SREBPs are synthesized as inactive precursors bound to membranes of the endoplasmic reticulum. When cells are deprived of cholesterol and fatty acids, NH(2)-terminal fragments of SREBPs become proteolytically released from membranes and migrate to the nucleus to activate transcription of genes required for lipid synthesis and uptake. Conversely, lipid repletion inhibits proteolytic processing of SREBPs and thereby suppresses lipid accumulation. We review here studies in cultured cells that reveal the mechanism for regulation of SREBP proteolytic activation, and those in animal models in which SREBP proteolysis has been either activated or inhibited to show the essential role of SREBPs in regulating hepatic lipid homeostasis.

Sterol-regulated Degradation of Insig-1 Mediated by the Membrane-bound Ubiquitin Ligase gp78

Insig-1 and Insig-2, closely related endoplasmic reticulum membrane proteins, mediate transcriptional and post-transcriptional mechanisms that assure cholesterol homeostasis through their sterol-induced binding to Scap (SREBP cleavage-activating protein) and 3-hydroxy-3-methylglutaryl coenzyme A reductase. Recent studies show that Insig-1 (but not Insig-2) is ubiquitinated and rapidly degraded when cells are depleted of sterols. Conversely, ubiquitination of Insig-1 is blocked, and the protein is stabilized when intracellular sterols accumulate. Here, we report that the ubiquitin ligase gp78, which binds with much higher affinity to Insig-1 than Insig-2, is required for ubiquitination and degradation of Insig-1 in sterol-depleted cells. Sterols prevent Insig-1 ubiquitination and degradation by displacing gp78 from Insig-1, an event that results from sterol-induced binding of Scap to Insig-1. In addition to providing a mechanism for sterol-regulated degradation of Insig-1, these results help to explain why Scap is subject to endoplasmic reticulum retention upon Insig-1 binding, whereas 3-hydroxy-3-methylglutaryl coenzyme A reductase is ubiquitinated and degraded.