Phosphorylation of the human estrogen receptor on tyrosine 537 in vivo and by src family tyrosine kinases in vitro.

Its reactivity to the antiphosphotyrosine 4G10 monoclonal antibody by Western blot analysis demonstrated that the human estrogen receptor (hER) from human MCF-7 cells and the recombinant hER expressed in Sf9 insect cells were phosphorylated on tyrosine(s). Reverse phase-HPLC separation of a tryptic digest of the 32P-labeled purified hER from Sf9 and MCF-7 cells followed by amino acid and radiolabel sequencing revealed that tyrosine-537 was phosphorylated. The phosphorylation on tyrosine-537 was independent of estradiol treatment of MCF-7 cells, indicating that tyrosine-537 is a basal phosphorylation site. Two src family tyrosine kinases, p60c-src and p56lck, phosphorylated the purified recombinant hER on tyrosine-537 in vitro. In addition, two tyrosine phosphatases, protein tyrosine phosphatase-1B and src homology-2 protein tyrosine phosphatase-1, dephosphorylated phosphotyrosine-537 of the hER in vitro. These data suggest that tyrosine phosphorylation of the hER is regulated by potentially oncogenic tyrosine kinases and phosphatases that may modulate the function of ER in normal and/or abnormal cell growth.