Homocysteine-induced endoplasmic reticulum stress causes dysregulation of the cholesterol and triglyceride biosynthetic pathways
Geoff H. Werstuck, … , M. Rene Malinow, Richard C. Austin
Geoff H. Werstuck, … , M. Rene Malinow, Richard C. Austin
Published May 15, 2001
Citation Information: J Clin Invest. 2001;107(10):1263-1273. https://doi.org/10.1172/JCI11596.
View: Text | PDF
Article

Homocysteine-induced endoplasmic reticulum stress causes dysregulation of the cholesterol and triglyceride biosynthetic pathways

Abstract

Hepatic steatosis is common in patients having severe hyperhomocysteinemia due to deficiency for cystathionine β-synthase. However, the mechanism by which homocysteine promotes the development and progression of hepatic steatosis is unknown. We report here that homocysteine-induced endoplasmic reticulum (ER) stress activates both the unfolded protein response and the sterol regulatory element–binding proteins (SREBPs) in cultured human hepatocytes as well as vascular endothelial and aortic smooth muscle cells. Activation of the SREBPs is associated with increased expression of genes responsible for cholesterol/triglyceride biosynthesis and uptake and with intracellular accumulation of cholesterol. Homocysteine-induced gene expression was inhibited by overexpression of the ER chaperone, GRP78/BiP, thus demonstrating a direct role of ER stress in the activation of cholesterol/triglyceride biosynthesis. Consistent with these in vitro findings, cholesterol and triglycerides were significantly elevated in the livers, but not plasmas, of mice having diet-induced hyperhomocysteinemia. This effect was not due to impaired hepatic export of lipids because secretion of VLDL-triglyceride was increased in hyperhomocysteinemic mice. These findings suggest a mechanism by which homocysteine-induced ER stress causes dysregulation of the endogenous sterol response pathway, leading to increased hepatic biosynthesis and uptake of cholesterol and triglycerides. Furthermore, this mechanism likely explains the development and progression of hepatic steatosis and possibly atherosclerotic lesions observed in hyperhomocysteinemia.

Authors

Geoff H. Werstuck, Steven R. Lentz, Sanjana Dayal, Gazi S. Hossain, Sudesh K. Sood, Yuan Y. Shi, Ji Zhou, Nobuyo Maeda, Skaidrite K. Krisans, M. Rene Malinow, Richard C. Austin

×

Figure 6

Options: View larger image (or click on image) Download as PowerPoint
Overexpression of GRP78/BiP prevents the increase in steady-state mRNA l...
Overexpression of GRP78/BiP prevents the increase in steady-state mRNA levels of SREBP-1 and IPP isomerase by homocysteine. (a) Immunoblot analysis of GRP78/BiP and GRP94 in wild-type (T24/83), vector-transfected (T24/83-pcDNA), or GRP78/BiP–overexpressing (T24/83-GRP78) cells. Total protein lysates (40 μg/lane) were separated on 10% SDS-polyacrylamide gels under reducing conditions and either stained with Coomassie blue (upper panel) or immunostained with an anti-KDEL mAb. (b) Immunolocalization of GRP78/BiP in the ER of wild-type or GRP78/BiP–overexpressing T24/83 cells. Cells grown on glass coverslips were fixed, permeabilized, and immunostained with an anti-GRP78/BiP polyclonal Ab. ×800. (c) Northern blot analysis of the steady-state mRNA levels of SREBP-1 and IPP isomerase in vector-transfected (T24/83-pcDNA) or GRP78/BiP–overexpressing (T24/83-GRP78) cells cultured in the absence or presence of 1 mM homocysteine for the indicated time periods. Total RNA (10 μg/lane) was size fractionated by agarose-gel electrophoresis, transferred to nylon membranes, and subjected to blot hybridization using radiolabeled cDNA probes encoding human SREBP-1 or IPP isomerase. Control for equivalent RNA loading was assessed using a radiolabeled GAPDH cDNA probe.