Molecular and Cellular PharmacologyRegulation mechanism of ABCA1 expression by statins in hepatocytes
Introduction
HMG-CoA reductase inhibitors (statins) reduce low-density lipoprotein (LDL) cholesterol concentration through blockade of the mevalonate pathway and consequent increment of LDL receptor expression in the liver (Goldstein and Brown, 1990). Statins are the most widely used cholesterol-lowering agents for prevention of cardiovascular disease (Havel and Rapaport, 1995). Atherosclerotic diseases including cardiovascular disease are the main cause of death in individuals with metabolic syndrome. The reverse cholesterol transport system including ATP-binding cassette transporter A1 (ABCA1) and high-density lipoprotein (HDL) plays an important role in atherosclerosis (Chinetti et al., 2006). ABCA1 expression in liver was shown to be necessary for HDL formation. Inactivation of ABCA1 gene in mice leads to a severe HDL deficiency (Aiello et al., 2003), and targeted disruption of ABCA1 gene in mouse hepatocytes reduces plasma HDL level by 80% (Lee and Parks, 2005). On the other hand, overexpression of ABCA1 in mouse liver markedly increases plasma HDL level (Wellington et al., 2003). However, the effects of statins on ABCA1 in the liver and the precise mechanisms of their actions have been obscure.
Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily. Three subtypes of PPARs, PPARα, PPARβ/δ and PPARγ, have been identified. PPARα is predominantly expressed in the liver, kidney, heart and skeletal muscle, where it controls fatty acid metabolism. PPARβ/δ is ubiquitously expressed and controls brain lipid metabolism and fatty acid-induced adipogenesis and preadipocyte proliferation. PPARγ, which is highly expressed in brown and white adipose tissue and the intestine, triggers cellular differentiation, promotes lipid storage and modulates the action of insulin (Torra et al., 2001). ABCA1 gene expression is known to be regulated by PPARα (Ogata et al., 2009) and PPARγ (Chawla et al., 2001). Although recent studies have demonstrated that PPARα mRNA level was affected by statins in an in vitro study (Seo et al., 2008), it has remained unclear that PPARα or the other isoforms, PPARβ/δ and PPARγ were affected by statins in vitro and in vivo.
The aims of this study were to determine whether statins (atorvastatin (Ato) and pitavastatin (Pit)) affect hepatic ABCA1 expression and to clarify the mechanisms of their actions.
Section snippets
Chemicals
Ato was kindly donated by Sankyo (Tokyo, Japan). Pit was kindly donated by Kowa (Tokyo, Japan). Clofibrate and GW6471 were obtained from Sigma-Aldrich (St. Louis, MO). All other reagents were of the highest grade available and used without further purification.
Cell culture
HepG2 cells were kept in Dulbecco's modified Eagle's medium (Sigma Aldrich Japan, Tokyo) with 10% fetal bovine serum (ICN Biomedicals, Inc., Aurora, OH) and 1% penicillin-streptomycin at 37 °C under 5% CO2 as described previously (
Effects of statins on ABCA1 expression in hepatocytes
First, we examined the alteration of ABCA1 mRNA level induced by Ato and Pit in HepG2 cells. Pit significantly up-regulated ABCA1 mRNA level. On the other hand, Ato had little effect (Fig. 1A). To investigate whether Pit regulated ABCA1 mRNA level in vivo, Pit were given to rats. Pit significantly increased Abca1 mRNA level in the rat liver, but not Ato (Fig. 1B). The results of in vitro and in vivo studies suggest that Pit increase ABCA1 expression in hepatocytes and that the effects of
Discussion
In the present study, we found that Pit induced ABCA1 expression in both HepG2 cells and the rat liver. As tested in HepG2 cells, this inductive effect of Pit was mediated through a PPARα-dependent pathway. In an in vitro study, Pit increased mRNA level of ABCA1, but Ato had little effect on the mRNA level (Fig. 1). Moreover, Pit significantly increased plasma HDL concentration. On the other hand, Ato had no effect (Table 2). Yokote et al. (2008) reported that a significant increase in HDL-C
References (22)
- et al.
Regulation of macrophage cholesterol efflux through hydroxymethylglutaryl-CoA reductase inhibition: a role for RhoA in ABCA1-mediated cholesterol efflux
J. Biol. Chem.
(2005) - et al.
A PPAR gamma-LXR-ABCA1 pathway in macrophages is involved in cholesterol efflux and atherogenesis
Mol. Cell
(2001) - et al.
Control of the peroxisomal-oxidation pathway by a novel family of nuclear hormone receptor
Cell
(1992) - et al.
Association between risk of myopathy and cholesterol-lowering effect: a comparison of all statins
Life Sci.
(2008) - et al.
Inhibitory effects of statins on human monocarboxylate transporter 4
Int. J. Pharm.
(2006) - et al.
Protein measurement with the folin phenol reagent
J. Biol. Chem.
(1951) - et al.
Phenobarbital suppresses growth and accelerates restoration of differentiation markers of primary culture rat hepatocytes in the chemically defined hepatocyte growth medium containing hepatocyte growth factor and epidermal growth factor
Exp. Cell Res.
(1998) - et al.
On the mechanism for PPAR agonists to enhance ABCA1 gene expression
Atherosclerosis
(2009) - et al.
Sterol regulatory element-binding protein-2- and liver X receptor-driven dual promoter regulation of hepatic ABC transporter A1 gene expression: mechanism underlying the unique response to cellular cholesterol status
J. Biol. Chem.
(2007) - et al.
Alterations of plasma lipids in mice via adenoviral mediated hepatic overexpression of human ABCA1
J. Lipid Res.
(2003)
Cited by (0)
- 1
These authors contributed equally as first author.
- 2
Present address: Department of Pharmacy, Hirosaki University School of Medicine and Hospital, Hirosaki 036-0563, Japan.