Mechanistic insights into the cholesterol-lowering activity of γ-oryzanol components

gamma-Oryzanol, a mixture of phytosteryl ferulates present in rice bran, has gained attention for its cholesterol-lowering properties, likely due to its interaction with the pharmacological target enzyme 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGCR). Our study explores the individual contributions of the four major gamma-oryzanol constituents, namely cycloartenyl ferulate, campesteryl ferulate, beta-sitosteryl ferulate, and 24-methylenecycloartenyl ferulate, to cholesterol synthesis and efflux/excretion. First, according to a concerted approach involving molecular docking, surface plasmon resonance binding studies, and enzyme inhibition assays, we determined the binding affinities and inhibitory activities of each constituent toward HMGCR. Additionally, we evaluated their impact on cholesterol metabolism in HepG2 cells by assessing the cellular cholesterol levels and the expression of selected biomarkers of cholesterol metabolism pathways under non-cytotoxic conditions. The major components of gamma-oryzanol, cycloartenyl ferulate (CAF) and 24-methylenecycloartenyl ferulate (24MCAF) significantly affected cholesterol metabolism and reduced cellular cholesterol levels via distinct mechanisms, despite their highly conserved chemical structures. Specifically, CAF reduced the total and free cholesterol levels, consequently triggering a compensatory activation of the SREBP-2 pathway, marked by increased HMGCR and LDL receptor expression, whereas 24MCAF moderately lowered cholesterol and enhanced its clearance primarily by upregulating the cholesterol efflux transporter ABCG1. Instead, both CAF and 24MCAF comparably affected ABCA1, another cholesterol efflux regulatory protein, and CYP7A1, essential for converting cholesterol into bile acids. Together, these dynamics reveal a dual modulation that supports the lipid-lowering effects of gamma-oryzanol components and highlights their potential as therapeutic agents in cholesterol management.