LIPID-DEPLETED DIET PERTURBS MEMBRANE-COMPOSITION AND INTRACELLULAR-TRANSPORT IN LACTATING MAMMARY CELLS

When rats were fed a control or a lipid-depleted diet for five generat ions, reproduction was not disturbed but pup growth was affected. The membrane organization and the secretory activity of mammary epithelial cells from these lactating rats were investigated. This diet induced a large decrease in the level of polyunsaturated fatty acids of membra ne phospholipids (26.6% versus 44.0%). The level of 20:4 (n-6) was str ongly decreased, mainly in phosphatidylethanolamine. Annexin VI, which interacts preferentially with this phospholipid, accumulated at the p eriphery of the cell and was largely associated to the hydrophobic reg ion of the bilayer as compared to control membranes. Casein synthesis and casein secretion measured in incubated explants, after pulse-chase metabolic labeling, were both reduced by about 60% in lipid-deprived cells. The secretory ratio (radioactive secreted caseins in %) was not modified, suggesting that the mechanism of basal secretion was not ma inly affected. On the contrary, the secretagogue effect of prolactin d isappeared. The intracellular transport of the hormone was considerabl y slowed down by the diet and prolactin did not reach the lumen of the acini after 1 h of chase, in contrast to what occurred in control cel ls. Addition of 20:4 (n-6), in vitro, to mammary fragments from lipid- deprived rats restored the localization of annexin VI, increased synth esis and secretion of caseins as well as intracellular transport of PR L. Together, these data underline the importance of the level of 20:4 (n-6) in membrane phospholipids for exocytic and endocytic transport i n lactating mammary epithelial cells.