cis-unsaturated fatty acids induce both lipogenesis and calcium binding in adipocytes

The addition of 0.4-3 mM of cis-unsaturated fatty acids such as oleic acid (18:1) or linoleic acid (18:2) to intact rat adipocytes stimulated lipogenesis at 37°C. Saturated or trans-unsaturated fatty acids were ineffective. Fluorescence photobleaching recovery studies performed under similar conditions indicated that the cis-unsaturated fatty acids do not alter lateral mobility of either a lipid probe or a general protein marker in the plasma membrane. A high concentration (7 mM) of Ca²⁺, which by itself has some stimulatory effect on lipogenesis, significantly potentiated the effect of oleic acid on this insulin-like activity. Measurement of ⁴⁵Ca²⁺ binding by fat cells has indicated that cis-unsaturated (but not saturated) fatty acids increased 12- to 20-fold the amount of Ca²⁺ associated with the cells. The dependence of this effect on the fatty acid concentration correlates well with the effect of the fatty acid on the induction of lipogenesis. Our results suggest that cis-unsaturated fatty acids affect membrane organization in a manner which induces a significant increase in membrane associated or intracellular Ca²⁺. This increase may be responsible for inducing exocytotic-like processes which facilitate translocation of glucose transport activity from storage sites to the plasma membrane and thus produce an insulin-like effect.