Independent Regulation of Cholesterol Incorporation into Free Apolipoprotein-mediated Cellular Lipid Efflux in Rat Vascular Smooth Muscle Cells

Abstract

Cholesterol was poorly available to free apolipoprotein (apo)A-I-mediated cellular lipid efflux from cholesterol-loaded rat vascular smooth muscle cells generating cholesterol-poorer pre-␤-HDL particles than those generated from macrophages by the same reaction (Li, Q., Komaba, A., and Yokoyama, S. (1993) Biochemistry 32, 4597-4603). The factors known to induce transformation of the smooth muscle cells into a macrophage-like stage were used in order to modulate this reaction, such as human platelet-derived growth factor, macrophage colony-stimulating factor, and phorbol 12-myristate-13-acetate (PMA). When the cells were stimulated by PMA following the pretreatment with platelet-derived growth factor plus macrophage colony-stimulating factor, cholesterol efflux mediated by free apoA-I increased 3-fold without changing phospholipid efflux, resulting in generation of pre-␤-HDL particles more rich in cholesterol. This treatment had only a little or no effect on apparent cellular cholesterol efflux to HDL or lipid microemulsion, respectively. Overall cellular free cholesterol pool size was unaffected by the treatment, and probing by extracellular cholesterol oxidase did not detect gross change in the cellular surface cholesterol. This specific enrichment of cholesterol in the apoA-Imediated cellular lipid efflux was reversed by protein kinase C inhibitors. Measurement of intracellular cholesterol esterification suggested that PMA induced translocation of intracellular cholesterol to a specific pool for apoA-I-mediated efflux, and a protein kinase C inhibitor reversed this effect.

Apolipoprotein A-I Activates Protein Kinase Cα Signaling to Phosphorylate and Stabilize ATP Binding Cassette Transporter A1 for the High Density Lipoprotein Assembly

“…To test the hypothesis that PKC␣-related signal transduction is involved in the apoA-I-induced cellular events (22)(23)(24)(25), the effect of PKC modulators on this reaction was examined in WI-38 human fibroblasts (Fig. 3).…”

“…We have shown that PKC is involved in apolipoprotein-mediated cholesterol release by modulating its incorporation to the HDL (24,25). More recently, we demonstrated that the apoA-I-mediated removal of cellular SPM causes its replenishment by transfer of phosphorylcholine from PC to ceramide (18).…”

ABCA1 and Biogenesis of HDL

“…When the cells were incubated with whole HDL particles, the activation of protein kinase C was observed (69). Stimulation and inhibition of protein kinase C modulated cholesterol content in the HDL generated by apolipoprotein as mentioned above, and at the same time the intracellular cholesterol compartment available for the esterification by acylCoA, cholesterol acyltransferase (ACAT), was changed reciprocally to HDL cholesterol content (65,66). The involvement of protein kinase C was thus implicated in the process of cholesterol mobilization by apolipoprotein.…”

“…Rat vascular smooth muscle cells generated cholesterol-poor HDL by apolipoprotein-cell interaction (26), and treatment with cytokines and protein kinase C activators increased the cholesterol incorporation into HDL (65). In contrast, the treatment of macrophages and the aboveconditioned vascular smooth muscle cells with protein kinase C inhibitors decreased the cholesterol content in HDL generated by apolipoprotein (65,66).…”

“…In contrast, the treatment of macrophages and the aboveconditioned vascular smooth muscle cells with protein kinase C inhibitors decreased the cholesterol content in HDL generated by apolipoprotein (65,66). Cellular cholesterol levels may also contribute to the regulation of cholesterol content in the HDL produced (67,68).…”