Estrogen stimulates growth of mammary tumor cells ZR‐75 without activation of S6 kinase and S6 phosphorylation

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Abstract
Growth of the human mammary tumor cell line ZR-75-1 is stimulated by epidermal growth factor (EGF) and .alpha.-type transforming growth factor (.alpha.TGF), as well as estradiol (E2). The role of activation of S6 kinase and S6 phosphorylation in the EGF(.alpha.TGF)-induced and E2-induced growth was investigated. Maximal effects on growth are observed at 10 nM EGF or .alpha.TGF. EGF as well as .alpha.TGF treatment of serum-starved cells leads to rapid activation of S6 kinase; the activity is increased about tenfold after 30 min of EGF treatment and declines with the time reaching about 25% of the maximal activity after 2 h of EGF treatent. Similar to the growth response, S6 kinase is activated at lower doses of EGF than .alpha.TGF and shows a maximal response at 10 nM for both growth factors. In contrast to this finding the incubatoin of serum-starved cells with E2 over a concentration range between 1 pM and 10 nM and times from 30 min to 4 h does not lead to increased S6 kinase activity. On investigating whether this lack of response to E2 is due to desensitiation of the system by induction of .alpha.TGF it was found that preincubation of cells with .alpha.TGF for 2-6 h desensitizes them to reactivation of S6 kinase by .alpha.TGF, whereas preincubation weith E2 does not. When S6 phoshorylation is monitored over times from 1 h to 6 h, it is observed that EGF leads to increased S6 phosphorylation, whereas E2 does not. The rate of onset of protein synthesis in the first 2 h of stimulation, when EGF-induced S6 phosphorylation is maximal, is more rapid with EGF than with E2. The results suggest that different pathways are involved in E2-induced and EGF(.alpha.TGF)-induced proliferation.

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