Protein phosphatase 5 is a negative regulator of estrogen receptor-mediated transcription

Abstract

Estrogen receptors (ERs) are transcription factors that can be modulated by both estrogen-dependent and growth factor-dependent phosphorylation. A yeast two-hybrid screening identified a serine/threonine protein phosphatase (PPS) as an interactant of ERβ (1-481), a dominant negative ERβ mutant. Glutathione S-transferase pull-down assays, mammalian two-hybrid assays, and immunoprecipitation studies showed that PP5 directly binds to both ERα and ERβ via its tetratricopeptide repeat domain. E domains of ERα and ERβ, without containing activation domain core regions in transcription activation function 2, were required for the binding to PP5. In ERα-positive breast cancer MCF7 cells, estrogen- and epidermal growth factor-dependent phosphorylation of ERα on serine residue 118, a major phosphorylation site of the receptor, was reduced by expressing PP5 but enhanced by PP5 antisense oligonucleotide. Estrogen-induced transcriptional activities of both ERα and ERβ and mRNA expression of estrogen-responsive genes, including pS2, c-myc, and cyclin D1, were suppressed by PP5 but enhanced by PP5 antisense oligonucleotide. A truncated PP5 mutant consisting only of its tetratricopeptide repeat domain acted as a dominant negative PP5 that enhanced serine residue 118 phosphorylation of ERα and transactivations by ERα and ERβ. We present the first evidence that PP5 functions as an inhibitory regulator of ER phosphorylation and transcriptional activation in vivo.