Niacin is an effective agent for raising HDL, but its cellular target sites are largely unknown. We examined effects of niacin on the surface expression of ATP synthase b chain, a newly described HDL/apolipoprotein A-I (apoA-I) receptor for HDL endocytosis, in HepG2 cells. A significant amount of immunodetectable b chain was observed on the surface of HepG2 cells, which was competitively displaced by apoA-I. Niacin treatment reduced the surface expression of b chain in HepG2 cells by ?27%, and decreased 125 I-labeled HDL uptake up to ?35%. However, nicotinamide, a niacin metabolite that does not have clinical lipid effects, exhibited weaker inhibition on the b chain cell surface expression, and failed to show inhibitory action on 125 I-labeled HDL uptake. Furthermore, anti-b chain antibody significantly reduced 125 I-labeled HDL uptake and abolished the inhibitory effect of niacin. Niacin did not change b chain mRNA expression. These data suggest that niacin inhibits cell surface expression of the ATP synthase b chain, leading to reduced hepatic removal of HDL protein, thus implicating a potential cellular target for niacin action to raise HDL.-Zhang, L-H., V. S. Kamanna, M. C. Zhang, and M. L. Kashyap. Niacin inhibits surface expression of ATP synthase b chain in HepG2 cells: implications for raising HDL.