Rice protein regulates HDL metabolism-related gene expression and enzyme activity in adult rats

Abstract

To elucidate whether rice protein possess a vital function in regulating high density lipoprotein (HDL) metabolism, the effects of rice protein (RP) on HDL metabolism-related gene expressions and enzyme activities were investigated in adult rats. Compared with casein, lipid contents in plasma and liver were effectively reduced by RP-feeding, accompanying significant increased ratio of HDL to cholesterol or triglyceride. RP increased mRNA levels of lecithin-cholesterol acyltransferase, adenosine triphosphate-binding cassette transporter A1, scavenger receptor class B type I and liver X receptor α, as compared to casein. Hepatic activities of lipoprotein lipase and hepatic lipase were significantly stimulated by RP, whereas no significant difference of plasma lecithin-cholesterol acyltransferase activity was found in RP with respect to casein. Results demonstrated that rice protein could effectively regulate HDL metabolism through modifying the HDL metabolism-related gene expression levels and lipolytic enzyme activities, which involved with a hypocholesterolemic action during the adult period.

Introduction

High density lipoprotein (HDL) can exert an antiatherogenic function by promoting the reverse cholesterol transport (RCT), which involves the HDL-mediated transports of cholesterol efflux from non-hepatic cells and of hepatic cholesterol uptake for metabolic transformation into bile acid (Hothblat and Phillips, 2010, Lewis and Rader, 2005). As a consequence, it is well established that high levels of HDL cholesterol (HDL-C) in the plasma are associated with a decreased risk of cardiovascular disease.

Diet plays an important role in regulating lipid metabolism (Santhosha et al., 2013, Singh et al., 2013, Suburu et al., 2013, Sundari et al., 2013). Rice is a staple cereal and widely consumed in the world (Kishine et al., 2008, Ohtsubo and Nakamura, 2007, Worasith and Goodman, 2013, Yang et al., 2012). Some studies have demonstrated that rice protein exerts the hypocholesterolemic action in growing and adult rats, mainly reflected by decreased concentrations of plasma non-HDL-C. In contrast, the plasma HDL-C level did not change in rice protein group, resulting in the ratio of HDL-C to total cholesterol was increased by rice protein-feeding (Yang et al., 2012, Yang et al., 2013). Furthermore, (Yang and Kadowaki, 2009, Yang and Kadowaki, 2011, Yang et al., 2013) reported that rice protein stimulated biliary secretion of cholesterol in growing and adult rats, implying rice protein might exert a potential function involved in “reverse cholesterol transport”, in which HDL might promote the transfer of peripheral free cholesterol to the liver for metabolism and excretion in the bile. However, the mechanism as to how rice protein regulates HDL metabolism is unknown.

It has been demonstrated that HDL metabolism is complex. The concentration of HDL-C in circulating blood is the net result of HDL biogenesis, remodeling and catabolism (Zannis, Chroni, & Krieger, 2006). Several factors are implicated to exert the beneficial effects in modifying plasma HDL-C levels. Adenosine triphosphate-binding cassette transporter A1 (ABCA1) is an essential regulator for HDL metabolism, which can facilitate cholesterol efflux and play a critical role in HDL initial formation and a rate-controlling function for RCT (Oram & Vaughan, 2006). Scavenger receptor class B type I (SR-BI) is a physiologically relevant HDL receptor, which controls the structure, composition and concentration of plasma HDL and plays a role in the HDL-mediated delivery of cholesterol from the circulation to the liver and steroidogenic organs (Trigatti, Krieger, & Rigotti, 2003). Lecithin-cholesterol acyltransferase (LCAT) is essential for optimal cholesterol uptake by HDL through converting free cholesterol into cholesteryl ester bound to HDL, showing a role in making the newly synthesized HDL (Tanigawa et al., 2009). In addition, lipoprotein lipase (LPL), hepatic lipase (HL) and liver X receptor α (LXRα) are suggested to participate in HDL metabolism (Hong et al., 2012; Borggreve et al., 2003). Thus, to elucidate the HDL metabolism exerted by rice protein, in addition to ABCA1, SR-BI and LCAT, the influences of rice protein on several lipid metabolism-related enzymes and transcription factors should be also taken into an account.

Aging is one of major risk factors for developing hypercholesterolemia (Choi, Goto, Ikeda, & Sugano, 1989). However, Yang et al. (2007) reported that a stronger cholesterol-lowering effect of rice protein was observed in adult rats than that in young rats, suggesting that adult rats seem to be an appropriate animal model for the study of dietary protein on cholesterol metabolism. Thus, in the present study, adult rats were used to investigate how rice protein modifies the HDL metabolism under cholesterol-free dietary condition.