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To determine how estrogens are involved in the growth of endometrial cancer with varying degrees of differentiation, we investigated the status of p72, a novel specific coactivator for estrogen receptor α (ERα) activation function-1 (AF-1), AIB1, a steroid receptor coactivator amplified in breast cancer 1, erbB-2, a receptor tyrosine kinase, and ERα in endometrial cancer. Gene expression of ERα, p72, AIB1 and erbB-2 was measured in 26 samples of primary endometrial cancers by real-time RT-PCR, and their in vivo cellular effects on the transactivation function of ERα were examined by a transient expression assay. The mRNA levels of erbB-2 increased and those of ERα, p72 and AIB1 decreased with the loss of histological differentiation. Transient expression of p72, AIB1 and erbB-2 in human embryonic kidney 293T cells led to a synergistic promotion of the transactivation function of ERα in the presence of 17α-estradiol or 4-hydroxytamoxifen, an ERα AF-1 agonist/AF-2 antagonist, as a ligand. In conclusion, estrogen action through ERα AF-1 might be exerted by the increased expression of the coactivators p72 and AIB1, together with cross talk between erbB-2 and p72, to accelerate the transactivation of ERα AF-1 in endometrial cancer. These findings also suggest that the cooperative transactivation of ERα AF-1 by the overexpression of p72, AIB1 and erbB-2 might be involved in tamoxifen-stimulated growth of endometrial cancer. Introduction Estrogen binds to estrogen receptors (ERs) which belong to the nuclear receptor superfamily and function as ligandinducible transcriptional factors to control transcription of target genes (1-3). The N-terminal A/B domain and the C-terminal E/F domain provide transactivation functions of ER. The autonomous activation function-1 (AF-1) in the A/B domain is constitutively active while AF-2 in the E/F domain is dependent on ligand binding (4). A ligand-bound ER forms a large complex, thought to contain basic transcriptional machinery and transcriptional cofactors, to initiate transcription (5). CBP/p300 and SRC-1 family proteins (SRC-1/TIF2/ AIB1) are known as cofactors which bind to ERα AF-2 in a ligand-dependent manner to promote transcription (6-9). SRA is an RNA coactivator selective for ERα AF-1 (10). In particular, AIB1 possesses a configuration that is phosphorylated by mitogen-activated protein kinase (MAPK) (11). Recently, we found that two DEAD-box proteins, p72 and p68, form a complex with SRC-1 family proteins and SRA, functioning as specific coactivators for ERα AF-1 by directly binding to the ERα A/B domain (12-14). The interaction of p72/68 with the ERα A/B domain was potentiated by phosphorylation of the Ser118 residue in the ERα A/B domain by MAPK, leading to the enhancement of ERα AF-1 activity (13-15). Endometrial cancer is the most common female genital tract malignancy in Western countries. Histologically more differentiated cases with high expression levels of sex steroid receptors respond best to hormone treatment (16-18). EGF is known to play a regulatory role in the proliferation of endometrial cancer cells (19,20). Overexpression of erbB-2, a receptor tyrosine kinase similar to the EGF receptor in structure, has been reported to be associated with poor survival in patients with endometrial cancer (21,22). ErbB-2 initiates its intracellular signal transduction by tyrosine-phosphorylating its intracellular domain, and provides docking sites for signaling molecules (23). The intracellular signaling induced by the phosphorylation of erbB-2 activates the MAPK cascade, which MOLECULAR MEDICINE REPORTS 1: 387-390, 2008 387 Analysis of the status of the novel estrogen receptor α (ERα) coactivator p72 in endometrial cancer and its cross talk with erbB-2 in the transactivation of ERα LIN ZHAO1*, MICHIKO WATANABE1*, TETSU YANO1, JUNN YANAGISAWA2, SHUNSUKE NAKAGAWA1, HAJIME OISHI1, OSAMU WADA-HIRAIKE1, KATSUTOSHI ODA1, TAKEO MINAGUCHI1, TOSHIHARU YASUGI1, SHIGEAKI KATO3 and YUJI TAKETANI1 1Department of Obstetrics and Gynecology, Faculty of Medicine, University of Tokyo, Tokyo 113-8655; 2Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba Science City, Ibaraki 305-8572; 3Institute of Molecular and Cellular Biosciences, University of Tokyo, Tokyo 113-0032 Japan Received December 27, 2007; Accepted February 19, 2008 _________________________________________ Correspondence to: Dr Tetsu Yano, Department of Obstetrics and Gynecology, Faculty of Medicine, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8655, Japan E-mail: [email protected] *Contributed equally