A short introduction about HMG-CoA reductase
HMG-CoA reductase or 3-hydroxy-3-methylglutaryl coenzyme-A reductase is an endoplasmic reticulum membrane-bound enzyme. It catalyzes the conversion of HMG-CoA to mevalonate. This step, the formation of the mevalonate, is the rate-limiting step of cholesterol biosynthesis as well as the synthesis of nonsterol isoprenoids such as dolichol and farnesyl pyrophosphate. Therefore, HMG-CoA reductase is the first regulatory point of cholesterol biosynthesis.
There are many types of regulatory mechanisms that control the activity of the HMG-CoA reductase and thereby limit the formation of mevalonate. Among these mechanisms, the accelerated degradation of the HMG-CoA reductase from the endoplasmic reticulum membrane is the main one. Accelerated degradation of this enzyme is a type of feedback control initiated by the intracellular signals.
Intracellular accumulated steroids bind to the enzyme and the steroid-enzyme complex ultimately binds to certain types of proteins located in the membrane of the endoplasmic reticulum (Insig-1 and Insig-2).
Regulatory mechanism of the HMG-CoA reductase
Binding of the steroid-enzyme complex to the Insig proteins is purely mediated by the membrane domain of the enzyme HMG-CoA reductase which contains eight transmembrane helices. After binding of the steroid-enzyme complex to the Insig proteins, Insig-associated ubiquitin ligase enzymes are activated that facilitate the ubiquitination of the lysine residue located in the membrane domain of the enzyme.
The lysine residue of the membrane domain is exposed to the cytosol and the ubiquitination of this lysine residue is a mark that signals the dislocation of the enzyme from the endoplasmic reticulum to the cytosol. The HMG-CoA reductase translocated into the cytoplasm is ultimately degraded via the proteasome.
Another feedback mechanism involved in the HMG-CoA reductase is the steroid-induced binding of the Insig proteins to Scap which is another membrane protein of the endoplasmic reticulum. Scrap is associated with the membrane-bound sterol regulatory element-binding proteins (SREBPs) which modulate the transcription of the genes encoding HMG-CoA reductase and other cholesterol biosynthetic enzymes.
When cholesterol is depleted inside the cell, Scap facilitates the transport of SREBPs from the endoplasmic reticulum to the Golgi complex where active fragments of the SREBPs are released from the membrane and migrate to the nucleus where they activate the target genes.
Excess of the intracellular sterols promotes Insig proteins to bind with the Scap proteins. Scap-bound Insig protein complex inhibits the transport of the Scap-SREBP complex from the endoplasmic reticulum to the Golgi complex. In the absence of transport, proteolytic activation of the SREBPs doesn’t occur that lead to the expression of SREBP target genes leading to the decline of the cholesterol.
To clarify the accelerated degradative feedback inhibition of the HMG-CoA reductase, researchers took two lines of mice; 1) transgenic mice capable of expressing membrane domain of HMG-CoA reductase in the liver (which is necessary for the Insig-mediated and sterol-accelerated degradation) and 2) knock-in mice bearing mutations in the endogenous HMG-CoA reductase gene that causes a change in the lysine residue at 89 and 248 positions to argentine. These mutations prevent the sterol-induced ubiquitination and subsequent degradation of the HMG-CoA reductase in the cultured cells.
Molecular biology and biochemical approach
Using molecular biological approach and biochemical procedures the researchers analyze the subcellular fractions to study the protein concentration of nuclear extracts and membrane fractions as well as lipid analysis. They found that the expression of the HMG-CoA reductase gene was highest in the liver of transgenic mice while no significant differences were observed in the plasma and hepatic levels of the cholesterol, free fatty acids, and triglycerides between the transgenic and wild-type mice.
Immunoblotting reveals that the liver of hmgcr(wt/ki) and hmgcr(Ki/Ki) mice fed with ad libitum showed a marked increase in the expression of HMGCR proteins 6-14 times as compared to that of the liver of wide type. In the same way, the expression of the HMGCR enzyme also increases in other tissues of transgenic mice.
These results indicate that mutation of the HMGCR gene at 89 and 248 positions by replacing lysine with argentine blocks the ubiquitination and subsequent degradation of the liver HMGCR that leads to the accumulation of the enzyme.
Researchers also performed comparative studies on various components of the Scap-SREBP pathway in the wild type and knockin mice and the result shows that the precursor and nuclear forms of SREBP-2 were reduced (20-70%) in the livers of hmgcr(WT/Ki) and hmgcr(Ki/Ki) mice.
The reduction of the SREBP-2 was associated with the elevated level of the sterol in the liver of these mice. However, in contrast to this, nuclear SREBP-1 was found to be increased in the livers of these knockin mice that were related to the sterol-mediated activation of the liver X-receptors that mediates the expression of the SREBP-1 proteins.
Isoforms of SREBP-1c are involved in the activation of target genes responsible for the fatty acid synthesis, stearoyl coenzyme A desaturase-1, glycerol-3-phosphate acetyltransferase, and these enzymes were found to be increased in the liver of hmgcr gene knockin mice.
Researchers performed diet feeding studies during which, mice were fed with a cholesterol-free diet (chow diet) or chow diet supplemented with 0.05, 0.2, or 2 % cholesterol. The diet feeding continued for 5 days before the actual study. However, for the lovastatin feeding study, mice were fed with a Teklad diet or an identical diet supplemented with lovastatin (0.02, 0.06, or 0.2 %).
For the fasting and refeeding study, mice were divided into two-three groups as follows; nonfasted fasted, and refed. During this study, nonfasted mice were fed with ad libitum, fasted mice were fasted for 12 hours and refed mice were refed to the high-carbohydrate and low-fat diet for 12 hours prior to the study.
Dietary supplementation of the cholesterol promoted the reduction of the endogenous HMGCR protein as well as SREBP-2 in the livers of the wild type of mice and transgenic mice. Dietary supplementation of the HMGCR inhibitor lovastatin revealed an increased amount of the HMGCR protein in the transgenic mice but it didn’t alter the expression of the hunger gene.
This was because lovastatin causes an increase in the nuclear content of the SERBP-1 in the lovers and enhances the expression of its target genes. In addition to that, lovastatin also lowered the amount of precursor and nuclear types of SREBP-1 which can lead to the loss of endogenous sterol ligands for the LXR.
All these studies reveal that cholesterol feeding promotes the protein to disappear from the liver membranes while supplementation of lovastatin promotes the accumulation of HMGCR in the liver membrane to deplete the cholesterol. However, gene encoding HMGCR remains unchanged regardless of the type of dietary feeding assays. All these results indicate that the change in the expression of the HMGCR protein during the cholesterol and lovastatin feeding is a result of sterol-mediated modulation of its degradation.
Reference: The Journal of Biological Chemistry (Contribution of accelerated degradation to feedback regulation of 3-hydroxy-3-methylglutaryl coenzyme a reductase and cholesterol metabolism in the liver)
Article doi: 10.1074/jbc.M116.728469