Figure 6.

Assays of β-casein expression, in the absence of added laminin, in primary cell cultures, clonal epithelial cells, and cocultures of clonal epithelial cells and fibroblasts. (A) Primary murine mammary epithelial cell cultures, SCp2 clonal epithelial cells, and cocultures of SCp2 cells and NIH3T3 fibroblasts (10:1) were assayed for β-casein expression in both flat cells (F) and rounded cells (R) (suspension culture) exposed to prolactin in the absence of added laminin. Cell rounding in suspension cultures permitted the induction of β-casein in primary cultures but not in the clonal SCp2 cell line. Coculture of SCp2 cells with a mesenchymal component (NIH3T3 fibroblasts) permitted the induction of β-casein. The same immunoblot filters were also probed for E-cadherin to demonstrate normalization for equal cell number. (B) SCp2 cells and NIH3T3 fibroblasts were cocultured in suspension (prerounded) and exposed to prolactin in the presence of function-blocking antibodies against the β1, α6, α1, α5, and αv integrin subunits, without the addition of laminin. β-Casein expression induced by endogenous basement membrane formation was inhibited by the β1 integrin-blocking antibody but was not inhibited effectively by the α6 integrin-blocking antibody GoH3.