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

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 (PPARγ2) mRNA and protein were specifically increased in apoB/BATless mice, as were hepatic mRNA levels of two targets of PPARγ, CD36 and aP2. Treatment of apoB/BATless mice for 4 weeks with intraperitoneal injections of a PPARγ antisense oligonucleotide resulted in dramatic reductions of both PPARγ1 and PPARγ2 mRNA, PPARγ2 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 PPARγ2. The mechanism whereby hepatic Pparγ2 gene expression is increased and how PPARγ2 stimulates lipogenesis is under investigation.