Altered MAP kinase (ERK1,2) regulation in primary cultures of mammary tumor cells: elevated basal activity and sustained response to EGF.

An elevation in total MAP kinase activity and expression has been observed in breast cancer tissue. However, the mechanisms underlying these changes in kinase activity and regulation by growth factors are not well characterized. In these studies, the effect of the potent mammary mitogen, epidermal growth factor (EGF), on the activation of the mitogen-activated protein kinases, ERK1 and ERK2 (extracellular regulated protein kinases 1 and 2, respectively), was compared in primary cultures of normal mouse mammary epithelial cells and in a hormone-responsive mouse mammary tumor. In normal epithelium, EGF stimulated an early rise in ERK activity at 4 min followed by a rapid decline, whereas a sustained (1 h) elevation of ERK activity was observed in the tumor cells. The time course of ERK activity in both cell types coincided with the phosphorylation state of the EGF receptor, suggesting that altered regulation of EGF receptor phosphorylation or EGF receptor turnover produces an enhanced ERK response to EGF in tumor cells. The MEK inhibitor, PD 098059 inhibited EGF-stimulated proliferation and ERK activity in a parallel, dose-dependent manner showing that ERK activation is at least permissive for the proliferative response to EGF. In addition, tumor cells showed a 4-fold elevation in basal (or ligand-independent) activity over normal cells without an increase in total enzyme level, and a preferential activation of ERK1 by EGF. These EGF-dependent and -independent changes in ERK regulation in the hormone-responsive mammary tumor underscore how multiple alterations in the regulation of this pathway may play a role in mammary tumorigenesis.