The effect of a protein feedback inhibitor of lactation (FIL) on casein synthesis and secretion was examined using isolated acini from lactating mouse mammary

Exocytotic release of proteins can occur via distinct pathways in numerous secretory cell types (Burgess and Kelly, 1987). Secretory proteins may be stored in vesicles or granules and accumulated in the cytoplasm prior to exocytosis regulated by changes in intracellular second messengers, usually a rise in cytosolic calcium concentration. Alternatively, they may be packaged into constitutive secretory vesicles and released immediately following synthesis. In secretory cells these two pathways usually coexist (Burgess and Kelly, 1987). Copius quantities of milk proteins, mainly caseins, are secreted by lactating mammary epithelial cells (Saacke and Heald, 1974; Linzell and Peaker, 1971) corresponding to up to 80% of the total synthesised protein (Turner et al., 1992). Although the mammary gland has been studied extensively, little is known about the factors that regulate milk protein secretion. It has generally been assumed that exocytotic secretion is a constitutive process in lactating cells, and constitutive secretion of milk proteins has been confirmed in lactating mouse mammary epithelial cells in vitro (Turner et al., 1992). In addition, however, it has been found that a portion of newly synthesised casein is stored and may be secreted by a regulated pathway following elevation of cytosolic Ca2+ concentration (Turner et al., 1992; Rennison et al., 1992). The physiological signal triggering regulated exocytosis has not yet been identified. Secreted milk proteins accumulate in the alveolar lumen of the gland, until they are removed by milking or suckling assisted by the milk ejection reflex. It is this extracellular storage that provides an additional local level of control on the rate of milk secretion. Extensive studies in lactating animals (Wilde et al., 1988; Wilde and Peaker, 1990) indicate that the regulatory mechanism involves a chemical inhibitor of milk secretion, and experiments in tissue culture have identified a small whey protein that fulfils this role (Wilde et al., 1987; C. V. P. Addey et al., unpublished observations). The inhibitory protein, termed FIL (feedback inhibitor of lactation), is synthesised by the secretory epithelial cells of the mammary gland and secreted into the alveolar lumen along with other milk constituents. FIL has been purified and N-terminal sequencing 641 Journal of Cell Science 106, 641-648 (1993) Printed in Great Britain © The Company of Biologists Limited 1993