Regulation of Apolipoprotein C-III Gene Expression by Nuclear Receptors Hepatocyte Nuclear Factor 4 Alpha and Chicken Ovalbumin Upstream Promoter Transcription Factor II, but not Retinoids in Hepatic Cells

Retinoic acid (RA) treatment induces hyperlipidemia in humans and animals. RA regulates the expression levels of various genes through the induction, repression, or coactivation of nuclear receptors, mediating its effects. RA-induced hyperlipidemia has been attributed to the induction of apolipoprotein CIII (gene, Apoc3), which inhibits lipoprotein lipase activity (LPL). We have shown that vitamin A (VA) status and retinoid treatment regulates hepatic lipogenic gene expression, suggesting that the induction of lipogenic genes may also contribute to hyperlipidemia. To test the hypothesis that retinoids may not affect Apoc3 expression, we analyzed the expression levels of Apoc3 mRNA in response to retinoid treatments or adenovirus- mediated over-expression of nuclear receptors mediating RA responses in primary rat hepatocytes and HL1C rat hepatoma cells using real-time PCR. We report that retinoids did not induce Apoc3 mRNA expression in these cells. The over-expression of hepatocyte nuclear factor 4 alpha (HNF4α [alpha]) or chicken ovalbumin upstream promoter transcription factor 2 (COUP-TFII) significantly induced or inhibited the Apoc3 mRNA level, respectively. Therefore, it is concluded that retinoid treatment could not directly induce Apoc3 mRNA levels in rat hepatocytes. Instead, the hepatic expression of Apoc3 mRNA may be controlled by the expression levels of HNFα and COUP-TFII. Furthermore, the mRNA level of Apoc3 in isolated and cultured hepatocytes of Zucker Lean (ZL) and Zucker Fatty (ZF) was not significantly changed. However, the Rarb and Srebp-1c mRNA levels, two RA-responsive genes, are significantly higher in the liver tissue and isolated hepatocytes from ZF rats thanthat from ZL rats. Therefore, we conclude that RA-induced hyperlipidemia may not be attributed to the direct induction of Apoc3 mRNA level in hepatocytes by RA, but at least in part to the RA-mediated induction of lipogenic genes such as Srebp1-c.