Multiple phosphorylation sites on the human estrogen receptor (hER)α were identified and shown to influence mammary carcinogenesis. In contrast, functional phosphorylation sites of hERβ have yet to be experimentally identified and validated. Here, using mass spectrometry, we uncovered three serines (S75, S87, and S105) in the N-terminus of hERβ as targets of ERK1/2 and p38 kinases. We raised a specific antibody against phosphorylated S105 (pS105) and demonstrated that this site was endogenously phosphorylated in MDA-MB-231 and BT-474 cells. A phospho-mimetic mutant generated from hERβ1 was found to exhibit higher transactivation activity than hERβ1. Ectopic expression of this mutant inhibited cell migration and invasion, but did not affect cell growth and cell-cycle progression in these cell models. In breast cancer specimens, pS105-hERβ immunoreactivity was detected with a higher prevalence and intensity than that of hERβ1. These results underscore the functional importance of the first experimentally identified hERβ-phosphorylation site in breast cancer.
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