Mechanisms of Hepatic Very Low Density Lipoprotein Overproduction in Insulin Resistance

144

110

Insulin-resistant apoB/BATless mice have hypertriglyceridemia because of increased assembly and secretion of very low density apolipoprotein B (apoB) and triglycerides compared with mice expressing only apoB (Siri, P., Candela, N., Ko, C., Zhang, Y., Eusufzai, S., Ginsberg, H. N., and Huang, L. S. (2001) J. Biol. Chem. 276, 46064 -46072). Despite increased very low density lipoprotein secretion, apoB/BATless mice have fatty livers. We found that hepatic mRNA levels of key lipogenic enzymes, acetyl-CoA carboxylase, fatty-acid synthase, and stearoyl-CoA desaturase-1 were increased in apoB/BATless mice compared with levels in apoB mice, suggesting increased lipogenesis in apoB/BATless mice. This was confirmed by determining incorporation of tritiated water into fatty acids. Neither the hepatic mRNA of the lipogenic transcription factor, SREBP-1c (sterol-response element-binding protein 1c), nor the nuclear levels of the mature form of SREBP-1 protein were elevated in apoB/BATless mice. By contrast, hepatic levels of peroxisomal proliferator-activated receptor 2 (PPAR␥2) mRNA and protein were specifically increased in apoB/BATless mice, as were hepatic mRNA levels of two targets of PPAR␥, CD36 and aP2. Treatment of apoB/BATless mice for 4 weeks with intraperitoneal injections of a PPAR␥ antisense oligonucleotide resulted in dramatic reductions of both PPAR␥1 and PPAR␥2 mRNA, PPAR␥2 protein, and mRNA levels of fatty-acid synthase and acetyl-CoA carboxylase. These changes were associated with decreased hepatic de novo lipogenesis and hepatic triglyceride concentrations. We conclude that hepatic steatosis in apoB/ BATless mice is associated with elevated rates of hepatic lipogenesis that are linked directly to increased hepatic expression of PPAR␥2. The mechanism whereby hepatic Ppar␥2 gene expression is increased and how PPAR␥2 stimulates lipogenesis is under investigation.We reported previously that apoB/BATless mice, a model generated by crossing mice expressing human apolipoprotein B (apoB) 3 (1) with mice lacking brown adipose tissue (BATless) (2), have hypertriglyceridemia and hypercholesterolemia because of a 2-3-fold increase in the secretion of very low density lipoprotein (VLDL) apoB and triglycerides (TG) relative to mice expressing apoB only (3). Similar levels of apoB mRNA in the livers of apoB and apoB/BATless mice indicated that the differences in apoB secretion resulted from differences in posttranscriptional regulation of VLDL assembly and secretion (4). There were no differences in hepatic levels of microsomal triglyceride transfer protein mRNA (3), a critical factor in the early co-and post-translational regulation of apoB-containing lipoprotein secretion (5). Low density lipoprotein receptor mRNA levels in liver were higher in apoB/BATless compared with apoB mice (3), indicating that increased apoB secretion in apoB/BATless mice did not result from reduced interactions of the low density lipoprotein receptor with nascent apoB lipoproteins (6).Associated with a rising prevalence of obesity and insuli...

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Aberrant Hepatic Expression of PPARγ2 Stimulates Hepatic Lipogenesis in a Mouse Model of Obesity, Insulin Resistance, Dyslipidemia, and Hepatic Steatosis

Zhang

Hernandez‐Ono

Siri

et al. 2006

144

110

“…Lack of expression of the Mtp gene in mice is associated with an absence of lipid droplets in the ER lumen (13) and defective secretion of apoB (13)(14). On the other hand, increases in MTP protein and activity are associated with increased assembly and secretion of apoB lipoproteins in mice (15) and hamsters (16). ApoB100 (4536 amino acids) is predicted to be comprised of an amino-terminal globular domain followed by three amphipathic ␣-helical domains alternating with two hydrophobic ␤ sheet domains (17).…”

Section: Microsomal Triglyceride Transfer Protein (Mtp)mentioning

confidence: 99%

Microsomal Triglyceride Transfer Protein Binding and Lipid Transfer Activities Are Independent of Each Other, but Both Are Required for Secretion of Apolipoprotein B Lipoproteins from Liver Cells

Liang

Ginsberg

2001

Recent studies indicate that microsomal triglyceride transfer protein (MTP) and apolipoprotein B (apoB) interact physically via two specific binding sites located within the amino-terminal globular region of apoB100. The first site is thought to be within the first 5.8% of the amino-terminal sequence, and the second site is between 9 and 16% of the amino-terminal sequence. It is not clear from prior studies whether these sites have unique or overlapping functions. Furthermore, there are no data differentiating between lipid transfer and potential chaperone functions of MTP. In the present study we have attempted to further characterize the physiologic interaction between apoB and MTP and to determine the relationship between the binding and lipid transfer aspects of the interaction. HepG2 cells were transiently transfected with apoB cDNAs, and MTP binding to apoB polypeptides was determined by two-step immunoprecipitation. MTP bound equally well to apoB polypeptides with (apoB13, 16,␤, apoB34, and apoB42) or without (apoB16, apoB13, and 16 or apoB13, 13, and 16) ␤ sheet domains. When proteasomal degradation of newly synthesized apoB polypeptides was blocked, MTP binding to all of the apoB polypeptides was only modestly affected by lipid availability and was independent of MTP-associated lipid transfer. Furthermore, MTP did not bind directly to a portion of the first ␤ sheet domain. We created two apoB constructs (apoB16 del and apoB34 del ) by deleting the first 210 amino acids of apoB16 and apoB34. These apoB polypeptides, therefore, lacked the putative first MTP binding site. MTP binding to apoB16 del and apoB34 del was decreased significantly. However, the secretion of apoB16 del was not different from apoB16, whereas the secretion of apoB34 del was impaired significantly. Our results indicate that the interaction between MTP and apoB involves independent binding and lipid transfer activities but that both activities are required for the secretion of apolipoprotein B from liver cells. Microsomal triglyceride transfer protein (MTP)1 is a heterodimer consisting of protein disulfide isomerase and the 97-kDa-large subunit and is found mainly in the endoplasmic reticulum (ER) lumen of liver and intestinal cells (1-3). The essential role of MTP in the translocation of apolipoprotein B (apoB) across the ER membrane and the assembly of apoB with lipids has been demonstrated clearly by co-expressing apoB and MTP in cells that do not normally express either protein and by the characterization of mice lacking MTP. The expression of apoB alone in cells without MTP resulted in the intracellular degradation of nascent apoB and no secretion; co-expression of apoB with MTP resulted in the secretion of apoB on lipoprotein particles (4 -6). Furthermore, the inhibition of MTP activity in cultured liver cells blocked the assembly and secretion of apoB-containing lipoproteins (7-11). We demonstrated that inhibition of MTP activity was associated with increased ubiquitination of apoB, particularly when proteasomal degradation was ...

“…Unlike livers from rat or mouse, the liver of the golden Syrian hamster secretes only apoB100-containing VLDL, and its lipoprotein metabolism more closely resembles that of humans (10). We have shown that insulin resistance in the fructose-fed golden Syrian hamster is associated with mild hypertriglyceridemia, VLDL-apoB oversecretion, increased intracellular apoB-containing lipoprotein particle stability, and increased expression of microsomal triglyceride transfer protein (MTP) (11). The present studies were conducted to explore the effect of improving insulin sensitivity in this insulin-resistant animal model by treatment with rosiglitazone, a peroxisome proliferator-activated receptor ␥ agonist and insulin sensitizer and to gain further insight into the molecular mechanisms of VLDL oversecretion in insulin resistant states.…”

mentioning

confidence: 99%

Ameliorated Hepatic Insulin Resistance Is Associated with Normalization of Microsomal Triglyceride Transfer Protein Expression and Reduction in Very Low Density Lipoprotein Assembly and Secretion in the Fructose-fed Hamster

Carpentier¹,

Taghibiglou²,

Leung³

et al. 2002

Self Cite

To determine whether reduction of insulin resistance could ameliorate fructose-induced very low density lipoprotein (VLDL) oversecretion and to explore the mechanism of this effect, fructose-fed hamsters received placebo or rosiglitazone for 3 weeks. Rosiglitazone treatment led to normalization of the blunted insulinmediated suppression of the glucose production rate and to a ϳ2-fold increase in whole body insulin-mediated glucose disappearance rate (p < 0.001). Rosiglitazone ameliorated the defect in hepatocyte insulin-stimulated tyrosine phosphorylation of the insulin receptor, IRS-1, and IRS-2 and the reduced protein mass of IRS-1 and IRS-2 induced by fructose feeding. Protein-tyrosine phosphatase 1B levels were increased with fructose feeding and were markedly reduced by rosiglitazone. Rosiglitazone treatment led to a ϳ50% reduction of VLDL secretion rates (p < 0.05) in vivo and ex vivo. VLDL clearance assessed directly in vivo was not significantly different in the FR (fructose-fed ؉ rosiglitazone-treated) versus F (fructose-fed ؉ placebo-treated) hamsters, although there was a trend toward a lower clearance with rosiglitazone. Enhanced stability of nascent apolipoprotein B (apoB) in fructose-fed hepatocytes was evident, and rosiglitazone treatment resulted in a significant reduction in apoB stability. The increase in intracellular mass of microsomal triglyceride transfer protein seen with fructose feeding was reduced by treatment with rosiglitazone. In conclusion, improvement of hepatic insulin signaling with rosiglitazone, a peroxisome proliferator-activated receptor ␥ agonist, is associated with reduced hepatic VLDL assembly and secretion due to reduced intracellular apoB stability.The typical dyslipidemia of insulin-resistant states and Type 2 diabetes consists of hypertriglyceridemia due to VLDL 1 overproduction, low high density lipoprotein cholesterol, and small dense low density lipoprotein particles (1). Elevated plasma free fatty acid (FFA) flux from peripheral and intra-abdominal adipose tissue depots due to resistance to the insulin antilipolytic and esterification effect in adipose tissue is felt to play an important role in driving VLDL assembly and secretion in insulin resistant states (2-4). Nevertheless, previous studies in humans suggest that insulin also has an important direct effect on the liver in controlling VLDL secretion (5-7).Rat and mouse models of insulin resistance and type 2 diabetes have provided important insights into the molecular mechanisms of insulin resistance. These animal models may not, however, be ideal for the study of human lipoprotein disorders because, unlike humans, their livers secrete both apoB48 and apoB100-containing VLDL, and they do not necessarily develop VLDL oversecretion as the basis for their hypertriglyceridemia (8, 9). Unlike livers from rat or mouse, the liver of the golden Syrian hamster secretes only apoB100-containing VLDL, and its lipoprotein metabolism more closely resembles that of humans (10). We have shown that insulin resistance in the fru...