The Scap/SREBP Pathway Is Essential for Developing Diabetic Fatty Liver and Carbohydrate-Induced Hypertriglyceridemia in Animals

Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX 75390-9046, USA.
Cell metabolism (Impact Factor: 17.57). 02/2012; 15(2):240-6. DOI: 10.1016/j.cmet.2011.12.017
Source: PubMed


Insulin resistance leads to hypertriglyceridemia and hepatic steatosis and is associated with increased SREBP-1c, a transcription factor that activates fatty acid synthesis. Here, we show that steatosis in insulin-resistant ob/ob mice was abolished by deletion of Scap, an escort protein necessary for generating nuclear isoforms of all three SREBPs. Scap deletion reduced lipid synthesis and prevented fatty livers despite persistent obesity, hyperinsulinemia, and hyperglycemia. Scap deficiency also prevented steatosis in mice fed high-fat diets. Steatosis was also prevented when siRNAs were used to silence Scap in livers of sucrose-fed hamsters, a model of diet-induced steatosis and hypertriglyceridemia. This silencing reduced all three nuclear SREBPs, decreasing lipid biosynthesis and abolishing sucrose-induced hypertriglyceridemia. These results demonstrate that SREBP activation is essential for development of diabetic hepatic steatosis and carbohydrate-induced hypertriglyceridemia, but not insulin resistance. Inhibition of SREBP activation has therapeutic potential for treatment of hypertriglyceridemia and fatty liver disease.

Download full-text


Available from: Kevin Fitzgerald, Feb 11, 2015
  • Source
    • "For example, a bifurcation point at mTORC1 in the insulin signaling pathway separated the insulin-induced Srebp-1c expression and the insulin-suppressed Pck1 expression [23]. The knockout of SREBP cleaving-activating protein gene in ob/ob mice resulted in the amelioration of hepatic lipogenesis without the improvement of hepatic gluconeogenesis [24]. Additionally in primary rat hepatocytes treated with insulin, the Gck expression reached the peak level 6 hours earlier than did the Srebp-1c expression [10]. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Unattended hepatic insulin resistance predisposes individuals to dyslipidemia, type 2 diabetes and many other metabolic complications. The mechanism of hepatic insulin resistance at the gene expression level remains unrevealed. To examine the effects of vitamin A (VA), total energy intake and feeding conditions on the insulin-regulated gene expression in primary hepatocytes of Zucker lean (ZL) and fatty (ZF) rats, we analyze the expression levels of hepatic model genes in response to the treatments of insulin and retinoic acid (RA). We report that the insulin- and RA-regulated glucokinase, sterol regulatory element-binding protein-1c and cytosolic form of phosphoenolpyruvate carboxykinase expressions are impaired in hepatocytes of ZF rats fed chow or a VA sufficient (VAS) diet ad libitum. The impairments are partially corrected when ZF rats are fed a VA deficient (VAD) diet ad libitum or pair-fed a VAS diet to the intake of their VAD counterparts in non-fasting conditions. Interestingly in the pair-fed ZL and ZF rats, transient overeating on the last day of pair-feeding regimen changes the expression levels of some VA catabolic genes, and impairs the insulin- and RA-regulated gene expression in hepatocytes. These results demonstrate that VA and feeding statuses modulate the hepatic insulin sensitivity at the gene expression level.
    PLoS ONE 08/2014; 9(8):e100868. DOI:10.1371/journal.pone.0100868 · 3.23 Impact Factor
  • Source
    • "Nonetheless, this observation is of particular interest in light of the potentially causal role that hepatic SREBP-1c activation has been proposed to play in metabolic disease in humans. Others have shown that genetic or pharmacological inhibition of SREBP maturation improves hepatic and whole-body metabolism (Moon et al., 2012; Tang et al., 2011). We therefore propose that SREBP-1c activation by PASK is an important Figure 6. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Elevated hepatic synthesis of fatty acids and triglycerides, driven by hyperactivation of the SREBP-1c transcription factor, has been implicated as a causal feature of metabolic syndrome. SREBP-1c activation requires the proteolytic maturation of the endoplasmic-reticulum-bound precursor to the active, nuclear transcription factor, which is stimulated by feeding and insulin signaling. Here, we show that feeding and insulin stimulate the hepatic expression of PASK. We also demonstrate, using genetic and pharmacological approaches, that PASK is required for the proteolytic maturation of SREBP-1c in cultured cells and in the mouse and rat liver. Inhibition of PASK improves lipid and glucose metabolism in dietary animal models of obesity and dyslipidemia. Administration of a PASK inhibitor decreases hepatic expression of lipogenic SREBP-1c target genes, decreases serum triglycerides, and partially reverses insulin resistance. While the signaling network that controls SREBP-1c activation is complex, we propose that PASK is an important component with therapeutic potential.
    Cell Reports 07/2014; 8(1). DOI:10.1016/j.celrep.2014.06.006 · 8.36 Impact Factor
  • Source
    • "Hepatic IR is characterized by its selectivity: insulin fails to suppress glucose production, whereas fatty acid synthesis or lipogenesis is thought to remain intact or to be even hyperstimulated (Brown and Goldstein 2008). Thus, in the face of hyper insulinemia in insulin-resistant conditions, the liver continues to produce glucose but also synthesizes large amounts of fatty acids and triglycerides, which accumulate in the liver, producing the pathological condition known as hepatic steatosis (Moon et al. 2012). Excess triglycerides are secreted via very low density lipoproteins, augmenting the levels of triglycerides in blood. "
    [Show abstract] [Hide abstract]
    ABSTRACT: The metabolic disruptor hypothesis postulates that environmental pollutants may be risk factors for metabolic diseases. Because insulin resistance is involved in most metabolic diseases and current health care prevention programs predominantly target insulin resistance or risk factors thereof, a critical analysis of the role of pollutants in insulin resistance might be important for future management of metabolic diseases. We aim at critically reviewing the available information linking pollutant exposure to insulin resistance and intend to open the discussion on future perspectives for metabolic disruptor identification and prioritization strategies. PubMed and Web of Science were searched for experimental studies reporting on linkages between environmental pollutants and insulin resistance. A total of 23 studies were identified as the prime literature. Recent studies specifically designed to investigate the effect of pollutants on insulin sensitivity show a potential causation of insulin resistance. Based on these studies, a table of viable test systems and endpoints can be composed which allows to gain insight into what is missing and what is needed to create a standardized insulin resistance toxicity testing strategy. It is clear that current research predominantly relies on top-down identification of insulin resistance-inducing metabolic disruptors and that one of the major future research needs is the development of dedicated in vitro or ex vivo screens to allow animal sparing and time- and cost-effective bottom-up screening.
    Environmental Health Perspectives 09/2013; 121(11-12). DOI:10.1289/ehp.1307082 · 7.98 Impact Factor
Show more