Brefeldin A inhibits cholesterol efflux without affecting the rate of cellular uptake and re-secretion of apolipoprotein A-I in adipocytes

A possible role of cellular uptake and re-secretion of apoA-I in the mechanism of cholesterol efflux induced by apoA-I was investigated using a novel experimental approach. Incubation of adipocytes with a recombinant human apoA-I containing a consensus PKA phosphorylation site, pka-ApoA-I, leads to the appearance of phosphorylated protein in the cell culture medium unambiguously proving cellular uptake and re-secretion of pka-ApoA-I. Phosphorylation of apoA-I is abolished by PKA inhibitors and enhanced by PKA activators demonstrating the specific involvement of PKA. Studies on the concentration dependence of pka-apoA-I phosphorylation and competition experiments with human apoA-I suggest that apolipoprotein uptake is a receptor mediated process. A possible role of apoA-I recycling in the mechanism of cholesterol efflux was investigated by determining the rates of apoA-I induced cholesterol efflux and apoA-I recycling in the presence and in the absence of Brefeldin A (BFA). The studies showed that BFA strongly inhibits cholesterol efflux without affecting the rate of apoA-I recycling. Since BFA affects vesicular trafficking of ABCA1, this study suggests that the interaction of apoA-I with ABCA1 does not mediate apolipoprotein uptake and re-secretion. This result suggests that lipidation of apoA-I and apolipoprotein uptake/re-secretion are independent processes.