Regulation of mammary differentiation by the extracellular matrix.

In multicellular organisms cell growth and differentiation are influenced by soluble factors, cell-cell interactions and cell-extracellular matrix interactions. We have used the rat mammary gland as a model system to study the role of extracellular matrix components in the regulation of milk protein gene expression. Since mammary epithelial cells differentiate on a basement membrane in vivo, we investigated the effects of basement membrane components on the expression of the milk protein genes, alpha-casein, alpha-lactalbumin, and transferrin. We have demonstrated that a basement membrane gel, as well as its major basement membrane component, laminin, induced alpha-casein and alpha-lactalbumin expression as much as 160-fold compared to tissue culture plastic. We demonstrate that laminin affects mRNA stability as well as having an effect on protein stability and secretion. Laminin interacts with mammary epithelial cells via an 68 kD cell surface receptor which is capable of interacting with the cellular cytoskeleton. In order to provide evidence that laminin affects on mammary differentiation are mediated through this receptor via the cytoskeleton, we examined the effects of cytoskeletal disrupting agents on milk protein gene expression. We demonstrate that cytochalasin D or colchicine selectively block laminin-mediated milk protein gene expression by affecting mRNA stability. Based on these experiments, we propose a model in which laminin affects mammary gene expression through interaction with cell surface receptors which interact with the cytoskeleton resulting in stabilization of mRNAs for milk protein genes.