Cyclic AMP stimulates efflux of intracellular sterol from cholesterol-loaded cells

The interaction of high density lipoprotein with its putative receptor stimulates translocation and efflux of intracellular sterols by a process involving activation of protein kinase C. This study shows that activation of cAMP-dependent protein kinase also stimulates efflux of intracellular sterols. When intracellular sterol pools of cholesterol-loaded cultured human skin fibroblasts and bovine aortic endothelial cells were radiolabeled with the biosynthetic precursor [3H]mevalonolactone, high density lipoprotein3 (HDL3)-mediated 3H-sterol efflux was enhanced by addition of the adenylylcyclase activator forskolin, the phosphodiesterase inhibitors theophylline and 3-isobutyl-1-methylxanthine, and the cAMP analogues N6-benzoyl-cAMP (N6-cAMP) and 8-thiomethyl-cAMP. The effect of N6-cAMP was abolished by an inhibitor of cAMP-dependent protein kinase (H8). The enhanced sterol efflux was independent of receptor binding of HDL3, as similar effects were observed in the presence of tetranitromethane-modified HDL3, which lacks receptor binding activity. N6-cAMP stimulated efflux of several subspecies of newly synthesized sterols, including cholesterol. Elevation of cAMP levels increased the proportion of radiosterols that were accessible to treatment of cells with the enzyme cholesterol oxidase, suggesting that activation of cAMP-dependent protein kinase stimulates translocation of sterols from intracellular compartments to the plasma membrane where they desorb from the cell surface. Thus, at least two distinct protein kinase signalling pathways modulate transport of intracellular sterols in cholesterol-loaded cells.