Autocrine Regulation of Protein Secretion in Mouse Mammary Epithelial Cells

Milk secretion is under autocrine control by an inhibitory milk protein, named FIL (feedback inhibitor of lactation). Lactating mammary acini and epithelial cells cultured on reconstituted basement membrane (EHS matrix) with lactogenic hormones were used to study the characteristics of autocrine inhibition. FIL inhibited milk protein secretion in lactating acini, but not in epithelial cells on EHS matrix. The latter's insensitivity to FIL was due to formation of multicellular structures termed mammospheres, in which cells surrounded a central luminal space. Cell polarization, and the formation of tight intercellular junctions prevented FIL access to the apical cell surface, which faced the mammosphere lumina. When apical access was permitted either by incomplete mammosphere formation or EGTA treatment, FIL inhibited mammosphere protein secretion to the same extent as in lactating acini. The study shows that autocrine inhibition by FIL occurs specifically through interaction with the apical surface of the mammary epithelial cell, and suggests the presence of a FIL receptor on this, but not the basolateral cell membrane.