Regulation of milk protein and basement membrane gene expression: the influence of the extracellular matrix.

Synthesis and secretion of milk proteins (alpha-casein, beta-casein, gamma-casein, and transferrin) by cultured primary mouse mammary epithelial cells is modulated by the extracellular matrix. In cells grown on released or "floating" type I collagen gels, mRNA for beta-casein and transferrin is increased as much as 30-fold over cells grown on plastic. Induction of beta-casein expression depends strongly on the presence of lactogenic hormones, especially prolactin, in the culture. When cells are plated onto partially purified reconstituted basement membrane, dramatic changes in morphology and milk protein gene expression are observed. Cells cultured on the matrix for 6 to 8 d in the presence of prolactin, insulin, and hydrocortisone form hollow spheres and duct-like structures that are completely surrounded by matrix. The cells lining these spheres appear actively secretory and are oriented with their apices facing the lumen. Hybridization experiments indicate that mRNA for beta-casein can be increased as much as 70-fold in these cultures. Because greater than 90% of the cultured cells synthesize immunoreactive beta-casein, as compared with only 40% of cells in the late pregnant gland, the matrix appears to be able to induce protein expression in previously silent cells. Synthesis of laminin and assembly of a mammary-specific basal lamina by cells cultured on different extracellular matrices also appears to depend on the presence of lactogenic hormones. These studies provide support for the concept of "dynamic reciprocity" in which complex interactions between extracellular matrix and the cellular cytoskeleton contribute to the induction and maintenance of tissue-specific gene expression in the mammary gland.