Maintenance of normal rat mammary epithelial cells by insulin and insulin-like growth factor 1.

Normal rat mammary epithelial cells were cultured within a rat tail collagen gel matrix formed under improved conditions for controlling pH and osmolarity. Under these conditions, growth can be maintained for up to 3 weeks with a 10- to 15-fold increase in cell number. The cells grow in response to prolactin, progesterone, epidermal growth factor, and cholera toxin, in a medium of DME: Ham's F12 supplemented with BSA and insulin at 10 micrograms/ml. When the insulin concentration was reduced to more physiological levels (10 ng/ml) the cells did not grow. However, at these more physiological concentrations it could be shown that insulin had a concentration-dependent effect on the maintenance of the cells with an optimum concentration around 25 ng/ml. The cells could be maintained in hormone-supplemented medium with low levels of insulin in a quiescent state for up to 14 days. The high levels of insulin needed for optimal growth could be replaced by insulin-like growth factor 1 (IGF-1) at much lower concentrations (25-50 ng/ml). The superphysiological level of insulin required for optimum growth is probably due to its acting weakly through an IGF-1-mediated growth-promoting mechanism. Insulin's effect on cell maintenance occurs at physiological levels and may better reflect its role in mammary cell growth.