Regulation of the hepatic ACAT2 expression and roles of HNF1alhpa and HNF4alpha in cholesterol metabolism

Abstract

Acyl-Coenzyme A:cholesterol acyltransferases (ACATs) 1 and 2 are integral membrane proteins located in rough endoplasmatic reticulum that catalyzes the formation of cholesteryl esters (CEs) from cholesterol and long-chain fatty acids. ACAT1 is present in most tissues, whereas ACAT2 is confined to enterocytes and hepatocytes. Disparities in tissue expressions, together with animal studies, suggests that ACAT2-derived CEs are incorporated into hepatic and intestinal apoB-containing lipoproteins and secreted into plasma, whereas ACAT1 is involved in esterification of cholesterol in other cells (e.g. macrophages) and thereby prevents apoptosis. Hepatic nuclear factors (HNFs) 1 and 4 are involved in diverse metabolic pathways (e.g. glucose, cholesterol, and fatty acid metabolism) and are highly expressed in liver, pancreas, and kidney.

The overall aim of this thesis was to gain more insight into the molecular mechanisms that participate in the hepatic regulation of ACAT2 and the roles of HNF1alpha and HNF4alpha in cholesterol metabolism.

In Paper I we aimed to investigate a possible transcriptional regulation by cholesterol of the human ACAT2 gene. In addition, we aimed to appraise the use of two human hepatoma cell lines, HuH7 and HepG2, as model systems in studies of ACAT. We showed a dose-dependent increase of ACAT2 mRNA expression, an increased enzymatic activity of ACAT2, and increased esterified cholesterol mass upon cholesterol loading. These results suggested that ACAT2, but not ACAT1, is transcriptionally regulated by cholesterol in humans. Additionally, we showed that cell differentiation affects the mRNA expression of ACAT1 and ACAT2 in HuH7, but not in HepG2 cells. Since HuH7 cells required much lower concentrations of cholesterol to obtain similar results as HepG2 cells, and were more sensitive to cholesterol depletion, HuH7 cells may represent a better system for sterol-studies of ACAT.

In Paper II we aimed to characterize mechanisms that control the liver-specific expression of the human ACAT2 gene. We identified an important HNF1 binding site, located -871 to -866 bp upstream of the transcription start site, which serves as a positive regulator of the ACAT2 gene expression and showed that this site is functionally active both in vitro and in vivo. The transcription factors HNF1alpha and HNF1beta, which binds to this site, play an important part in the regulation of the human ACAT2 promoter.

Paper III: Maturity onset diabetes of the young (MODY) is a group of syndromes characterized by autosomal dominant inheritance, early onset diabetes, and beta-cell dysfunction. Mutations of the genes encoding HNF1alpha and HNF4alpha cause MODY3 and MODY1, respectively. ACAT2 is thought to be responsible for production of CEs in hepatic very low density lipoprotein (VLDL) assembly. We identified HNF1alpha as an important regulator of ACAT2. HNF4alpha is an upstream regulator of HNF1alpha. Thus we hypothesized that MODY3 and possibly MODY1 subjects may have lower VLDL esterified cholesterol. Unexpectedly, we found that MODY1 subjects had lower VLDL and low density lipoprotein (LDL) esterified cholesterol levels, whereas MODY3 subjects had similar lipoprotein composition as controls. Hence, we characterized the role of HNF4alpha in the transcriptional regulation of ACAT2 and identified HNF4alpha as an important regulator of the hepatocyte-specific expression of ACAT2. These studies suggested that the lower levels of esterified cholesterol in VLDL- and LDL-particles in MODY1 subjects may - at least in part - be due to lower ACAT2 activity in these patients.

Paper IV: Niemann-Pick C1 like 1 (NPC1L1) is highly expressed in human liver and intestine. NPC1L1 is a key regulator of intestinal cholesterol absorption but its hepatic function is not well defined. Thus, we aimed to gain more insight into the hepatic expression of the human NPC1L1 gene. Gene expression analyses were performed in liver samples from Chinese patients with or without cholesterol gallstone disease. Strong positive correlations between NPC1L1 and sterol regulatory element binding protein 2 (SREBP2) and between NPC1L1 and HNF4alpha were observed. HNF4alpha is an upstream regulator of HNF1alpha. Thus, we further investigated possible roles of SREBP2, HNF4alpha, and HNF1alpha in the hepatic regulation of NPC1L1. We identified an important HNF1 binding site located -158 to -144 bp upstream of the transcription start site of the human NPC1L1 promoter. Also, we showed that SREBP2 and HNF1alpha are important transcription factors for the hepatic NPC1L1 promoter activity that can bind to and regulate its expression in humans. Moreover, it is possible that HNF4alpha may function by transactivating NPC1L1 via binding to other transcription factors, including HNF1alpha.

Collectively, these studies imply that ACAT2 is under metabolic control and that HNF1alpha and HNF4alpha participates in several important processes in cholesterol metabolism. HNF1alpha may participate in hepatic cholesterol esterification, uptake of free cholesterol (FC) in hepatocytes, and in bile acid synthesis. HNF4alpha may participate in esterification of cholesterol in high density lipoprotein (HDL), affect plasma levels of esterified cholesterol and triglycerides in VLDL- and LDL-particles, and indirectly participate in the regulation of uptake of FC in hepatocytes.