Inhibition of ABCA1 Protein Expression and Cholesterol Efflux by TNF [alpha] in MLO-Y4 Osteocytes

Hip fracture and myocardial infarction cause significant morbidity and mortality. In vivo studies raising serum cholesterol levels as well as pro-inflammatory cytokines such as TNF [alpha] manifest bone loss and atherosclerotic vascular disease, suggesting that abnormalities of cholesterol transport may contribute to osteoporosis. We used the mouse osteocyte cell line (MLO-Y4) to investigate the effects of TNF [alpha] on the expression of cholesterol acceptor proteins such as apolipoprotein A-I (apo A-I) and apolipoprotein E (apo E), as well as on the cholesterol transporters ATP-binding cassette-1 (ABCA1), scavenger receptor class B type 1 (SRB1), and cluster of differentiation 36 (CD36). MLO-Y4 cells do not express apo A-I or apo E; however, they do express all three cholesterol transporters (ABCA1, SRB1, and CD36). Treatment of MLO-Y4 cells with TNF [alpha] had no effect on SRB1, CD36, and osteocalcin levels; however, TNF [alpha] reduced ABCA1 protein levels in a dose-dependent manner and cholesterol efflux to apo A-I. Interestingly, TNF [alpha] treatment increased ABCA1 promoter activity and ABCA1 mRNA levels, and increased liver X receptor [alpha] protein expression, but had no effect on retinoid X receptor [alpha] and retinoic acid receptor [alpha] levels. Pharmacological inhibition of p38 mitogen-activated protein (MAP) kinase, but not c-jun-N-terminal kinase 1 or mitogen-activated protein kinase (MEK), restored ABCA1 protein levels in TNF [alpha]-treated cells. These results suggest that pro-inflammatory cytokines regulate cholesterol metabolism in osteocytes in part by suppressing ABCA1 levels post-translationally in a p38 MAP kinase-dependent manner.