武内 謙憲

Sterol regulatory element-binding protein (SREBP)-1c is now well established as a key transcription factor for the regulation of lipogenic enzyme genes such as FAS in hepatocytes. Meanwhile, the mechanisms of lipogenic gene regulation in adipocytes remain unclear. Here, we demonstrate that those in adipocytes are independent of SREBP-1c. In adipocytes, unlike in hepatocytes, the stimulation of SREBP-1c expression by liver X receptor agonist does not accompany lipogenic gene upregulation, although nuclear SREBP-1c protein is concomitantly increased, indicating that the activation process of SREBP-1c by the cleavage system is intact in adipocytes. Supportively, transcriptional activity of the mature form of SREBP-1c for the FAS promoter was negligible when measured by reporter analysis. As an underlying mechanism, accessibility of SREBP-1c to the functional elements was involved, because chromatin immunoprecipitation assays revealed that SREBP-1c does not bind to the functional SRE/E-box site on the FAS promoter in adipocytes. Moreover, genetic disruption of SREBP-1 did not cause any changes in lipogenic gene expression in adipose tissue. In summary, in adipocytes, unlike in hepatocytes, increments in nuclear SREBP-1c are not accompanied by transactivation of lipogenic genes; thus, SREBP-1c is not committed to the regulation of lipogenesis.

Sterol regulatory element-binding proteins (SREBPs) are transcription factors that are predominately involved in the regulation of lipogenic and cholesterogenic enzyme gene expression. To identify unknown proteins that interact with SREBP, we screened nuclear extract proteins with S-35-labeled SREBP-1 bait in Far Western blotting analysis. Using this approach, high mobility group protein-B1 (HMGB1), a chromosomal protein, was identified as a novel SREBP interacting protein. In vitro glutathione S-transferase pull-down and in vivo coimmunoprecipitation studies confirmed an interaction between HMGB1 and both SREBP-1 and -2. The protein-protein interaction was mediated through the helix-loop-helix domain of SREBP-1, residues 309-344, and the A box of HMGB1. Furthermore, an electrophoretic mobility shift assay demonstrated that HMGB1 enhances SREBPs binding to their cognate DNA sequences. Moreover, luciferase reporter analyses, including RNA interference technique showed that HMGB1 potentiates the transcriptional activities of SREBP in cultured cells. These findings raise the intriguing possibility that HMGB1 is potentially involved in the regulation of lipogenic and cholesterogenic gene transcription.

Obesity is a major health problem in industrialized societies, and fatty liver disease (hepatic steatosis) is common in obese individuals. Oxidative stress originating from increased intracellular levels of fatty acids has been implicated as a cause of hepatocellular injury in steatosis, although the precise mechanisms remain to be elucidated. p53, widely known as a tumor suppressor, has been shown often to be activated in stressed cells, inducing cell cycle arrest or death. Here we demonstrate that p53 is involved in the molecular mechanisms of hepatocellular injury associated with steatosis. We found that p53 in the nucleus is induced in the liver from two mouse models of fatty liver disease, ob/ob and a transgenic mouse model that overexpresses an active form of sterol regulatory element-binding protein-1 in the liver (TgSREBP-1), the one with obesity and the other without obesity. This activation of the p53 pathway leads to the elevation of p21 mRNA expression, which can be considered an indicator of p53 activity, because ob/ob mice lacking p53 generated by targeting gene disruption exhibited the complete restoration of the p21 elevation to wild type levels. Consistent with these results, the amelioration of hepatic steatosis caused by Srebp-1 gene disruption in ob/ob mice lowered the p21 expression in a triglyceride content-dependent manner. Moreover, p53 deficiency in ob/ob mice resulted in a marked improvement of plasma alanine aminotransferase levels, demonstrating that p53 is involved in the mechanisms of hepatocellular injury. In conclusion, we revealed that p53 plays an important role in the pathogenesis of fatty liver disease.