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

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Abstract
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 μ/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.