Evidence for a role of human organic anion transporters in the muscular side effects of HMG-CoA reductase inhibitors

Abstract

The purpose of this study was to elucidate the role of human organic anion transporters (human OATs) in the induction of drug-induced skeletal muscle abnormalities. 3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors have been clinically used for lowering plasma cholesterol levels, and are known to induce various forms of skeletal muscle abnormalities including myopathy and rhabdomyolysis. Immunohistochemical analysis revealed that human OAT1 and human OAT3 are localized in the cytoplasmic membrane of the human skeletal muscles. The activities of human OATs were measured using mouse cell lines from renal proximal tubules stably expressing human OATs. Human OAT3, but not human OAT1, mediates the transport of pravastatin. Fluvastatin inhibited organic anion uptake mediated by human OAT1 in a mixture of competitive and noncompetitive manner, whereas simvastatin and fluvastatin noncompetitively inhibited the organic anion uptake mediated by human OAT3. In conclusion, the organic anion transporters OAT1 and OAT3 are localized in the cytoplasmic membrane of human skeletal muscles. Pravastatin, simvasatin, and fluvasatin inhibit human OATs activity. These results suggest that muscle organic anion transporters play a role in the muscular side effects of HMG-CoA reductase inhibitors.

Introduction

The excretion of numerous organic anions, including endogenous metabolites, drugs, and xenobiotics, is an important physiological function of the kidney. Recently, cDNAs encoding the human organic anion transporters (human OATs) have been successively cloned, including human OAT1 Reid et al., 1998, Hosoyamada et al., 1999, human OAT2 (Enomoto et al., 2002), human OAT3 (Cha et al., 2001), and human OAT4 (Cha et al., 2000). We have been elucidating the localization and functional roles of human OATs mainly in the kidney.

3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase is the rate-limiting step in cholesterol biosynthesis from acetate. The inhibition of this enzyme reduces cytoplasmic cholesterol levels in hepatocytes, which respond by increasing the synthesis of low-density lipoprotein (LDL) receptors that are expressed on hepatocyte surface membranes. This in turn increases hepatic LDL uptake from the plasma, reducing the plasma LDL concentration (Ritter et al., 1999). Thus, HMG-CoA reductase inhibitors have been used to lower plasma total and LOL levels in patients with hypercholesterolemia, which prevents the progression of atherosclerosis.

It has been recently shown that pravastatin, a HMG-CoA reductase inhibitor that possesses anionic moieties, is transported by rat OAT1 and rat OAT3 (Hasegawa et al., 2002). In addition to the kidney, mRNAs from human OATs were shown to be expressed in various organs including the brain, liver, skeletal muscles, and placenta Hosoyamada et al., 1999, Cha et al., 2000, Cha et al., 2001. In the human skeletal muscles, HMG-CoA reductase inhibitors, including pravastatin, fluvastatin, and simvastatin (Fig. 1), were shown to induce various forms of skeletal muscle abnormalities ranging from mild myopathy to myositis, and occasionally rhabdomyolysis and even death (Evans and Rees, 2002). These lines of evidence raise the possibility that human OAT1 and human OAT3 localized in the cytoplasmic membrane of the skeletal muscles mediate the uptake of HMG-CoA reductase inhibitors into the skeletal muscles and the induction of skeletal muscle abnormalities. In order to elucidate this hypothesis, firstly, we performed an immunohistochermical analysis of human OAT1 and human OAT3 in the human skeletal muscles. Secondly, we examined whether human OAT1 and human OAT3 mediate the transport of pravastatin. Thirdly, we elucidated the interactions of human OAT1 and human OAT3 with HMG-CoA reductase inhibitors, including fluvastatin and simvastatin. The activities of human OATs were measured using mouse cell lines from renal proximal tubules stably expressing human OATs.