CHOLESTEROL IS POORLY AVAILABLE FOR FREE APOLIPOPROTEIN-MEDIATED CELLULAR LIPID EFFLUX FROM SMOOTH-MUSCLE CELLS

To study the mechanism for resistance of smooth muscle cells (SMC) to cholesterol efflux caused by lipid-free apolipoproteins [Komaba, A., et al. (1992) J. Biol. Chem. 267, 17560-17566], the efflux of phospholipids and cholesterol was induced from mouse peritoneal macrophages (MP) and rat aortic SMC by phospholipid/triglyceride microemulsion, by human plasma high- and low-density lipoproteins (HDLs and LDLs), and by lipid-free human apolipoprotein (apo) A-I. The efflux of both lipids by the lipid microemulsion showed essentially the same kinetic profile for these two types of cells except that the rate of phospholipid efflux was 5-6 times slower by weight than cholesterol in both cases. The same ratio of cholesterol to phospholipid was also found in the efflux to LDLs. Lipid-free apoA-I mediated cellular cholesterol efflux, but the rate was much slower from SMC than from MP. However, the rate of apoA-I-mediated phospholipid efflux was similar between these two cells generating HDL-like particles, resulting in a high phospholipid:cholesterol ratio, (4-5):1 by weight, in the lipid efflux from SMC, in contrast with (0.8-1):1 in the lipid efflux from MP. When standardized for the cellular free cholesterol, the V(max) of cholesterol efflux induced by lipid-free apoA-1 was 10 times slower from SMC than from MP, but only by at most 2-fold slower when lipid microemulsion was the acceptor. Thus, free cholesterol of SMC is less available than that of MP for free apolipoprotein-mediated generation of HDLs with cellular lipids. ApoA-I enhanced both cholesterol and phospholipid efflux from SMC to the lipid microemulsions, being consistent with the model that apoA-I in the aqueous phase mediates the enhancement [Hara, H., & Yokoyama, S. (1992) Biochemistry 31, 2040-20461. The phospholipid:cholesterol ratio increased as apoA-I enhanced the lipid efflux, suggesting that the increase of the phospholipid:cholesterol ratio may indicate the contribution of the lipid-free apolipoprotein-mediated reaction. This ratio was higher in cellular lipid efflux from SMC to HDL than that to microemulsion and also than that from MP to HDL.