Branching morphogenesis of human mammary epithelial cells in collagen gels

Abstract

To study the morphogenesis of human epithelial cells in vitro we have used a three-dimensional collagen matrix and a newly developed mammary epithelial cell line, 1-7 HB2. In standard medium 1-7 HB2 cells formed compact balls/spheres inside collagen type I gels, while cocultivation with various fibroblast cell lines or growth in fibroblastconditioned media resulted in the appearance of branching structures. At least two different soluble factors secreted by fibroblasts were found to be implicated in the branching morphogenesis. Firstly, hepatocyte growth factor/scatter factor could induce branching in a concentrationdependent manner. Moreover, a polyclonal serum against hepatocyte growth factor/scatter factor completely inhibited the branching morphogenesis induced by medium conditioned by MRC-5 fibroblast cells. In contrast, a morphogenetic activity secreted by human foreskin fibroblasts was identified that appears to be different from hepatocyte growth factor/scatter factor and from a number of other well-characterized growth factors or cytokines. This model system has been used to examine the role of integrins in mammary morphogenesis. The expression of the α2β1, α3β1 and α6β4 integrins was decreased when cells were plated on collagen gels. The addition of specific blocking monoclonal antibodies directed to the α2and β1-integrin subunits to growth media impaired cell-cell interactions and interfered with the formation of compact structures inside collagen gels, suggesting that the α2β1 integrin can control intercellular adhesion in mammary morphogenesis. In contrast one of the blocking monoclonal antibodies against the α3-integrin subunit (P1B5) mimicked the effect of soluble ‘morphogens’. Our results suggest that the modulation of α3β1 activity may represent an important event in the induction of branching morphogenesis of human mammary epithelial cells.