Relationship between epidermal growth factor receptor levels, autophosphorylation and mitogenic‐responsiveness in normal mouse mammary epithelial cells in vitro

Mammary epithelial cells were isolated from mid-pregnant BALB/c mice, grown within collagen gels and maintained on DME/F12 (1:1) media containing 10% bovine calf serum and 10 micrograms/ml insulin. Initial time-course and dose-response studies showed that epidermal growth factor (EGF)-induced autophosphorylation of the EGF-receptor (EGF-R) in these cells was maximal 5 min after exposure to 75 ng/ml EGF. Mammary epithelial cells displaying little or no growth during their first 2 days in primary culture cells were found to contain low levels of EGF-R. However, EGF-induced autophosphorylation of the EGF-R in these cells was extremely intense. Subsequent studies demonstrated that during the proliferative and plateau phases of growth, EGF-R levels progressively increased, while conversely EGF-induced autophosphorylation of the EGF-R decreased over time in primary culture. These results demonstrate that EGF-R levels and autophosphorylation do not show a direct correlation with mammary epithelial cell mitogen-responsiveness. Intense EGF-R autophosphorylation appears to be required for initiating growth, but sustained mammary epithelial cell proliferation occurs when EGF-R autophosphorylation is low. This inverse relationship between EGF-R levels and autophosphorylation may reflect changes in receptor affinity and function during the various phases of mammary epithelial cell growth in primary culture.