Stimulation and inhibition of human mammary epithelial cell duct morphogenesis in vitro

We previously determined that normal human mammary epithelial cells (HMECs) placed on the basement membrane-like substance Matrigel form structures, whereas malignant breast cells do not (1). In the present study, we determined that the structures formed by normal cells on Matrigel resembled breast ducts in vivo by electron microscopy, and the process of their formation recapitulated what is known of duct formation in vivo. We therefore used this model to study less well-understood aspects of breast morphogenesis. Two priming signals appeared necessary for initiation of morphogenesis: one provided by the Matrigel and one by the cells in an autocrine fashion. Evidence for this included diminished duct formation by cells plated low-concentration Matrigel or at low cell densities, and the reversal of the latter by conditioned medium from high-density cells on Matrigel. Antibodies to bFGF inhibited morphogenesis, suggesting a stimulatory autocrine role for this factor, and antibodies to TGF-beta 1 stimulated duct formation, suggesting an inhibitory autocrine role. Added TGF-beta 1 abolished morphogenesis and stimulated normal cells to wander through Matrigel as do malignant cells. Conditioned medium from normal cells did not stimulate malignant cells to form ducts, but conditioned medium from tumor cells diminished normal morphogenesis, suggesting that malignant cells secrete an inhibitor of morphogenesis.