Molecular mechanisms underlying the estrogen regulated lipid homeostasis in the liver.

摘要

Estrogen plays an important role in the regulation of hepatic lipid homeostasis. Disruption of estrogen signaling pathways in animal models, either by blocking endogenous estrogen synthesis or genetic deletion of estrogen receptor (ERalpha), leads to the accumulation of liver triglycerides and hepatic steatosis. The specific molecular mechanisms underlying these liver-associated metabolic actions of estrogen are poorly understood. The objective of this dissertation research is to dissect the hepatic estrogen function by identifying estrogen/ERalpha target genes in liver cells. Using rat hepatocyte as a model system, two approaches have been conducted.;In one study, we investigated the possible role of a liver-selective coactivator (PGC-1alpha) in the ERalpha-mediated gene expression. To identify genes that are regulated by the PGC-1alpha/ERalpha complex, we engineered an ERalpha-specific PGC-1alpha. This was accomplished by the identification of an ERalpha-selective interacting LXXLL-containing peptide from an M13 phage display peptide library and defining the specific LXXLL motif within PGC-1alpha that is required for ERalpha interaction. The LXXLL motif was then replaced by the ERalpha-selective peptide, creating an engineered PGC-1alpha selectively coactivates ERalpha. ERalpha/PGC-1alpha target genes were identified in a microarray study using rat hepatocytes infected with the ERalpha-selective PGC-1alpha as well as the wild type PGC-1alpha. Candidate target genes from this analysis indicate that PGC-1alpha-activated ERalpha signaling may be involved in the upregulation of the fatty acid beta-oxidation pathway.;In the second approach, to dissect hepatic estrogen action, we identified small heterodimer partner (SHP) as a primary hepatic estrogen target gene. The gene expression of SREBP-1c in rat hepatocytes has been observed to be reduced by estrogen treatment. Knockdown of the SHP expression by RNA interference confirmed that SHP is the key that mediates the estrogen repression of SREBP-lc gene expression. The estrogen induced SHP expression, in turn, antagonizes LXRalpha transcriptional activity on the SREBP-1c gene promoter. This is consistent with the role of SREBP-lc as a key regulator of hepatic de novo lipogenesis. Reduction of SREBP-1c gene expression by estrogen treatment reduced fatty acid biosynthesis in the lipogenesis measurements.;Integrating our findings from two independent approaches to probe estrogen action in the liver, we proposed a model wherein estrogens regulate hepatic lipid homeostasis by (a) repressing fatty acid production through the repression of SREBP-1c gene expression; and (b) promoting fatty acid consumption through the up-regulation of fatty acid beta-oxidation. The sum effect of these estrogen regulated pathways is a net reduction in hepatic lipid storage.