Effect of liposome-adipocyte interaction on hexose uptake and insulin action

The effect of unilammelar lipid vesicles composed of defined acyl chain phosphatidylcholines or binary vesicles comprised of dioleoyl phosphatidylcholine (DOPC) and a variety of other lipid species on insulin action in isolated rat adipocytes was studied. Cells were treated with vesicles and subsequently analyzed for basal and insulin-stimulated hexose uptake. Of the different vesicles tested, only those containing DOPC and phosphatidylserine (PS) markedly inhibited the effect of insulin. The inhibition was a function of PS concentration in the binary vesicle. Basal uptake, on the other hand, was stimulated by PS-containing vesicles. The changes in insulin-stimulated hexose uptake following interaction of adipocytes with DOPC-PS vesicles were accompanied by alterations in the Vmax of the uptake process and in affinity of insulin binding, although the similar ED50 values of the control and vesicle-treated groups suggest that the observed effects on insulin sensitivity may be mediated by an uncoupling of the insulin receptor from transport activation. Several lines of evidence are consistent with vesicle-cell fusion as the pathway of DOPC-PS uptake under these conditions. Fluorescence microscopic analysis of fat cells following their interaction with vesicles filled with fluorescent dye indicated that the internal contents of the adipocytes became generally labeled. Biochemical studies of cell fractionation after the interaction of adipocytes with radiolabeled vesicles and effects of metabolic inhibitors and cell fixatives on vesicle uptake also support the fusion pathway.