Shirya Rashid

Molecular Mechanisms of Hepatic Steatosis and Insulin Resistance in the AGPAT2-Deficient Mouse Model of Congenital Generalized Lipodystrophy

Mutations in 1-acylglycerol-3-phosphate-O-acyltransferase 2 (AGPAT2) cause congenital generalized lipodystrophy. To understand the molecular mechanisms underlying the metabolic complications associated with AGPAT2 deficiency, Agpat2 null mice were generated. Agpat2(-/-) mice develop severe lipodystrophy affecting both white and brown adipose tissue, extreme insulin resistance, diabetes, and hepatic steatosis. The expression of lipogenic genes and rates of de novo fatty acid biosynthesis were increased approximately 4-fold in Agpat2(-/-) mouse livers. The mRNA and protein levels of monoacylglycerol acyltransferase isoform 1 were markedly increased in the livers of Agpat2(-/-) mice, suggesting that the alternative monoacylglycerol pathway for triglyceride biosynthesis is activated in the absence of AGPAT2. Feeding a fat-free diet reduced liver triglycerides by approximately 50% in Agpat2(-/-) mice. These observations suggest that both dietary fat and hepatic triglyceride biosynthesis via a monoacylglycerol pathway may contribute to hepatic steatosis in Agpat2(-/-) mice.

Expression of Human Hepatic Lipase in the Rabbit Model Preferentially Enhances the Clearance of Triglyceride-Enriched Versus Native High-Density Lipoprotein Apolipoprotein A-I

Background We have shown previously that triglyceride (TG) enrichment of HDL, as occurs in hypertriglyceridemic states, contributes to HDL lowering in humans by enhancing the clearance of HDL apolipoprotein (apo) A‐I from the circulation. In the New Zealand White rabbit, an animal naturally deficient in hepatic lipase (HL), we demonstrated that TG enrichment of HDL per se is not sufficient to enhance HDL clearance in the absence of ex vivo lipolysis by HL. Here, we examined in the rabbit the interaction between in vivo HL lipolytic action and HDL TG enrichment on the subsequent metabolic clearance of HDL apoA‐I. Methods and Results The clearance of HDL, TG‐enriched with human VLDL (12% mass TG), was compared with a simultaneously injected native rabbit HDL tracer (8% TG) 5 to 7 days after injection of recombinant (r) adenovirus expressing either the human HL or lacZ transgene (n=6 animals each). In rHL‐Adv rabbits, HL activity levels were 2‐ to 7‐fold higher (versus rlacZ‐Adv controls; P<0.01), and there were significant (P<0.05) reductions in HDL TG (−18%), cholesterol (−21%), cholesteryl ester (−24%), and phospholipid (−14%). Moreover, the clearance of TG‐enriched versus native HDL was significantly greater (by 50%; 0.122±0.022 versus 0.081±0.015 pools/h; P<0.01) in rHL‐Adv rabbits but not in controls. Conclusions These studies have shown that TG enrichment of HDL in the presence but not in the absence of in vivo expression of moderate levels of lipolytically active HL results in enhanced HDL clearance, demonstrating the important interaction between TG enrichment and HL action in the pathogenesis of HDL lowering in hypertriglyceridemic states. (Circulation. 2003;107:3066‐3072.)

Human Resistin Stimulates Hepatic Overproduction of Atherogenic ApoB-Containing Lipoprotein Particles by Enhancing ApoB Stability and Impairing Intracellular Insulin Signaling

Rationale: Obese individuals are at high risk for developing atherosclerosis primarily attributable to elevated plasma concentrations of apolipoprotein (apo)B-containing particles, including very-low-density lipoprotein (VLDL). Plasma levels of the adipose tissue adipokine resistin are increased in human obesity, and resistin expression is positively correlated with coronary atherosclerosis and VLDL levels. Objective: We sought to determine for the first time whether resistin directly stimulates human hepatocyte production of apoB-containing particles and to elucidate the mechanisms responsible. Methods and Results: Treatment of human hepatocytes with resistin at levels observed in human obesity stimulated apoB secretion up to 10-fold, because of increased microsomal triglyceride transfer protein (MTP) activity and decreased expression/phosphorylation of proteins in the insulin signaling pathways (insulin receptor substrate-2, Akt, and extracellular signal-regulated kinase). Resistin also increased hepatocyte lipid content by stimulating de novo lipogenesis via the SREBP1 and SREBP2 pathways. Furthermore, obese serum with elevated resistin levels induced greater hepatocyte stimulation of apoB secretion than lean human serum, an effect that was ameliorated by antibody immunoprecipitation removal of serum resistin. Conclusions: Resistin has a direct deleterious impact on human hepatic lipid and lipoprotein regulation. Resistin greatly increased hepatocyte VLDL apoB and lipid secretion because of MTP activation and induction of hepatocyte insulin resistance. Conversely, antibody removal of serum resistin ameliorated human serum stimulation of apoB secretion. Increased hepatic cellular lipids mediated by resistin reflects the fatty liver/steatosis observed with elevated resistin in humans. Thus, human resistin is a novel therapeutic target for mitigating common hepatic pathophysiological processes associated with human obesity, dyslipidemia and atherosclerosis.

Effect of Atorvastatin on High-Density Lipoprotein Apolipoprotein A-I Production and Clearance in the New Zealand White Rabbit

Background—HMG-CoA reductase inhibitors reduce the incidence of cardiovascular disease predominantly by their LDL-lowering effect. Recently, there has been great interest in the pleiotropic effects of statins, which appear to differ among the various agents in this class. Unlike other statins, atorvastatin exhibits a decline in its HDL-raising effect at higher doses in humans. Whether atorvastatin-mediated alterations in HDL turnover in vivo contribute to this effect has not previously been investigated. We therefore studied the effect of atorvastatin on HDL apolipoprotein (apo) A-I production and clearance in normolipidemic male New Zealand White rabbits. Methods and Results—Kinetic studies of HDL-apoA-I radiolabeled with 131I were performed in chow-fed rabbits after 3 weeks of atorvastatin treatment of 5 mg · kg−1 · d−1 (n=7) versus placebo-treated rabbits (n=7). Our results showed a significantly (P <0.001) more rapid clearance (≈2-fold) of HDL apoA-I in atorvastatin-treated animals compared with the control group (0.121±0.012 versus 0.061±0.004 pools/h, respectively), accompanied by a lesser 48% increase in the apoA-I production rate (3.84±0.38 versus 2.59±0.41 mg · kg−1 · h−1, P =0.06). Accordingly, plasma apoA-I levels in atorvastatin-treated animals declined significantly (P <0.05, n=8 animals) after 3 weeks of treatment (173.5±1.8 mg/dL) from baseline values. Conclusions—These data suggest that the effect on apoA-I levels observed with atorvastatin at higher drug doses in humans may be caused at least in part by enhanced HDL apoA-I catabolism, which is not entirely offset by a concomitant increase in apoA-I production. Whether this finding results from an effect of atorvastatin on HDL particle composition or on receptors involved in circulating HDL holoparticle clearance will require further study.

Thematic review series: patient-oriented research. What have we learned about HDL metabolism from kinetics studies in humans?

Plasma measurements of lipids, lipoproteins, and apolipoproteins provide information on the static levels of these fractions without providing key information on the dynamic fluxes of lipoproteins in the circulation. Kinetics studies, in contrast, provide additional information on the production and clearance rates of lipoproteins and the flow of lipids and apolipoproteins through lipoprotein fractions. This information is crucial in accurately delineating the metabolism of HDL in plasma, because plasma concentrations of HDL are the net result of the de novo production and catabolism of HDL as well as the recycling of HDL particles and the contribution to HDL from components of other lipoproteins. Studies aimed at measuring the metabolism of HDL particles have shown that HDL metabolism in vivo is complex and consists of multiple components. Kinetics studies provide a window into the metabolism of HDL, allowing us to better understand the mechanisms of HDL decrease in human conditions and the functionality of HDL particles. Here, we review the progress in our understanding of HDL metabolism derived from in vivo kinetics studies, focusing primarily on studies in humans but also reviewing key studies in animal models.

Expression of SREBP-1c requires SREBP-2-mediated generation of a sterol ligand for LXR in livers of mice

The synthesis of cholesterol and fatty acids (FA) in the liver is independently regulated by SREBP-2 and SREBP-1c, respectively. Here, we genetically deleted Srebf-2 from hepatocytes and confirmed that SREBP-2 regulates all genes involved in cholesterol biosynthesis, the LDL receptor, and PCSK9; a secreted protein that degrades LDL receptors in the liver. Surprisingly, we found that elimination of Srebf-2 in hepatocytes of mice also markedly reduced SREBP-1c and the expression of all genes involved in FA and triglyceride synthesis that are normally regulated by SREBP-1c. The nuclear receptor LXR is necessary for Srebf-1c transcription. The deletion of Srebf-2 and subsequent lower sterol synthesis in hepatocytes eliminated the production of an endogenous sterol ligand required for LXR activity and SREBP-1c expression. These studies demonstrate that cholesterol and FA synthesis in hepatocytes are coupled and that flux through the cholesterol biosynthetic pathway is required for the maximal SREBP-1c expression and high rates of FA synthesis. DOI: http://dx.doi.org/10.7554/eLife.25015.001