Apolipoprotein C-I binds free fatty acids and reduces their intracellular esterification.

Mice that overexpress human apolipoprotein C-I (apoC-I) homozygously (APOC1(+/+) mice) are protected against obesity and show cutaneous abnormalities. Although these effects can result from our previous observation that apoC-I inhibits FFA generation by LPL, we have also found that apoC-I impairs the uptake of a FFA analog in adipose tissue. In this study, we tested the hypothesis that apoC-I interferes with cellular FFA uptake independent of LPL activity. The cutaneous abnormalities of APOC1(+/+) mice were not affected after transplantation to wild-type mice, indicating that locally produced apoC-I prevents lipid entry into the skin. Subsequent in vitro studies with apoC-I-deficient versus wild-type macrophages revealed that apoC-I reduced the cell association and subsequent esterification of [(3)H]oleic acid by approximately 35% (P < 0.05). We speculated that apoC-I binds FFA extracellularly, thereby preventing cell association of FFA. We showed that apoC-I was indeed able to mediate the binding of oleic acid to otherwise protein-free VLDL-like emulsion particles involving electrostatic interaction. We conclude that apoC-I binds FFA in the circulation, thereby reducing the availability of FFA for uptake by cells. This mechanism can serve as an additional mechanism behind the resistance to obesity and the cutaneous abnormalities of APOC1(+/+) mice.