The interplay of matrix metalloproteinases, morphogens and growth factors is necessary for branching of mammary epithelial cells

The mammary gland develops its adult form by a process referred to as branc hing morphogenesis. Many factors have been reported to affect this process. We have used cultured primary mammary epithelial organoids and mammary epi thelial cell lines in three-dimensional collagen gels to elucidate which gr owth factors, matrix metalloproteinases (MMPs) and mammary morphogens inter act in branching morphogenesis. Branching stimulated by stromal fibroblasts , epidermal growth factor, fibroblast growth factor 7, fibroblast growth fa ctor 2 and hepatocyte growth factor was strongly reduced by inhibitors of M MPs, indicating the requirement of MMPs for three-dimensional growth involv ed in morphogenesis. Recombinant stromelysin 1/MMP3 alone was sufficient to drive branching in the absence of growth factors in the organoids. Plasmin also stimulated branching; however, plasmin-dependent branching was abolis hed by both inhibitors of plasmin and MMPs, suggesting that plasmin activat es MMPs. To differentiate between signals for proliferation and morphogenes is, we used a cloned mammary epithelial cell line that lacks epimorphin, an essential mammary morphogen. Both epimorphin and MMPs were required for mo rphogenesis, but neither was required for epithelial cell proliferation. Th ese results provide direct evidence for a crucial role of MMPs in branching in mammary epithelium and suggest that, in addition to epimorphin, MMP act ivity is a minimum requirement for branching morphogenesis in the mammary g land.