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Molecular and Cellular Biology, September 2006, p. 6748-6761, Vol. 26, No. 18
0270-7306/06/$08.00+0     doi:10.1128/MCB.00560-06

Phosphatidylinositol 3-Kinase/Protein Kinase C-Induced Phosphorylation of Sp1 and p107 Repressor Release Have a Critical Role in Histone Deacetylase Inhibitor-Mediated Depression of Transcription of the Luteinizing Hormone Receptor Gene

Section on Molecular Endocrinology, Endocrinology and Reproduction Research Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892

Received 30 March 2006/ Returned for modification 28 April 2006/ Accepted 5 July 2006

We have demonstrated that silencing of luteinizing hormone receptor (LHR) gene transcription is mediated via a proximal Sp1 site at its promoter. Trichostatin A (TSA) induced histone acetylation and gene activation in JAR cells that prevailed in the absence of changes in Sp1/Sp3 expression, their binding activity, disassociation of the histone deacetylase/mSin3A complex from the Sp1 site, or demethylation of the promoter. This indicated a different mechanism involved in TSA-induced derepression. The present studies have revealed that phosphatidylinositol 3-kinase/protein kinase C (PI3K/PKC)-mediated Sp1 phosphorylation accounts for Sp1 site-dependent LHR gene activation. TSA caused marked phosphorylation of Sp1 at serine 641 in JAR and MCF-7 cells. Blockade of PI3K or PKC activity by specific inhibitors, kinase-deficient mutants, or small interfering RNA abolished the effect of TSA on the LHR gene and Sp1 phosphorylation. PKC was shown to associate with Sp1, and this association was enhanced by TSA. Sp1 phosphorylation at serine 641 was required for the release of the pRb homologue p107 from the LHR gene promoter, while p107 acted as a repressor of the LHR gene. Inhibition of PKC activity blocked the dissociation of p107 from the LHR gene promoter and markedly reduced Sp1 phosphorylation and transcription. These results have demonstrated that phosphorylation of Sp1 by PI3K/PKC is critical for TSA-activated LHR gene expression. These studies have revealed a novel mechanism of TSA action through derecruitment of a repressor from the LHR gene promoter in a PI3K/PKC-induced Sp1 phosphorylation-dependent manner.

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