Free fatty acid transport across adipocytes is mediated by an unknown membrane protein pump

The role of cell membranes in regulating the flux of long chain free fatty acids (FFA) into and out of adipocytes is intensely debated. Four different membrane proteins including, FABPpm, CD36/FAT, caveolin-1, and FATP have been identified as facilitating FFA transport. Moreover, CD36 and caveolin-1 are also reported to mediate transport in conjunction with lipid rafts. The principal evidence for these findings is a correlation of the level of FFA uptake with the expression level of these proteins and with the integrity of lipid rafts. The 3T3-L1 and 3T3-F442A cell lines in their preadipocyte states reveal little or no expression of these proteins and correspondingly low levels of uptake. Here we have microinjected the adipocyte and preadipocyte cell lines with ADIFAB, the fluorescent indicator of FFA. The ADIFAB fluorescence allowed us to monitor the intracellular unbound FFA concentration during FFA influx and efflux. We show that these measurements of transport, in contrast to FFA uptake measurements, correlate neither with expression of these proteins nor with lipid raft integrity in preadipocytes and adipocytes. Transport characteristics, including the generation of an ATP-dependent FFA concentration gradient, are virtually identical in adipocytes and preadipocytes. We suggest that the origin of the discrepancy between uptake and our measurements is that most of the FFA transported into the cells is lost during the uptake but not in the transport protocols. We conclude that long chain fatty acid transport in adipocytes is very likely mediated by an as-yet-unidentified membrane protein pump.