BREFELDIN-A DIFFERENTLY AFFECTS BASAL AND PROLACTIN-STIMULATED MILK PROTEIN SECRETION IN LACTATING RABBIT MAMMARY EPITHELIAL-CELLS

When lactating mammary epithelial cells were treated with prolactin in vitro, numerous small vesicles rapidly accumulated in the Golgi area, and secretion of milk proteins increased, The effects of brefeldin A on these intracellular events mere investigated. As observed by electr on microscopy; stacks of the median Golgi were not altered after incub ation in the presence of 50 nM brefeldin A but were dissociated when t he drug concentration was greater than or equal to 500 nM. Small vesic les did not accumulate in the Golgi area when mammary cells were incub ated in medium containing both prolactin and brefeldin A, whatever the concentration of the latter, Immunofluorescence experiments showed th at 50 nM brefeldin A did not modify the localization of the CTR 433 me dian Golgi protein, but it induced redistribution of trans-Golgi netwo rk-associated proteins such as TGN38, AP-1 adaptor and clathrin. These effects occurred in the presence of brefeldin A plus prolactin, Pulse -chase experiments showed that brefeldin A concentrations greater than or equal to 100 nM induced the intracellular accumulation of milk pro teins, provoked the appearance of immature forms of caseins, and inhib ited milli protein secretion, In contrast, concentrations of brefeldin A of less than or equal to 50 nM did not affect basal casein secretio n but inhibited the secretagogue effect of prolactin, These data show not only that several biochemical events in the transport of milk prot eins which are sensitive to different brefeldin A concentrations occur in lactating mammary epithelial cells, but also that it is possible t o inhibit a hormonal stimulus in a selective manner, while the machine ry responsible for basal secretion is still active.