Forkhead Homologue in Rhabdomyosarcoma Functions as a Bifunctional Nuclear Receptor-interacting Protein with Both Coactivator and Corepressor Functions

Abstract

In a search for novel transcriptional intermediary factors for the estrogen receptor (ER), we used the ligandbinding domain and hinge region of ER as bait in a yeast two-hybrid screen of a cDNA library derived from tamoxifen-resistant MCF-7 human breast tumors from an in vivo athymic nude mouse model. Here we report the isolation and characterization of the forkhead homologue in rhabdomyosarcoma (FKHR), a recently described member of the hepatocyte nuclear factor 3/forkhead homeotic gene family, as a nuclear hormone receptor (NR) intermediary protein. FKHR interacts with both steroid and nonsteroid NRs, although the effect of ligand on this interaction varies by receptor type. The interaction of FKHR with ER is enhanced by estrogen, whereas its interaction with thyroid hormone receptor and retinoic acid receptor is ligand-independent. In addition, FKHR differentially regulates the transactivation mediated by different NRs. Transient transfection of FKHR into mammalian cells dramatically represses transcription mediated by the ER, glucocorticoid receptor, and progesterone receptor. In contrast, FKHR stimulates rather than represses retinoic acid receptor-and thyroid hormone receptor-mediated transactivation. Most intriguingly, overexpression of FKHR dramatically inhibits the proliferation of ER-dependent MCF-7 breast cancer cells. Therefore, FKHR represents a bifunctional NR intermediary protein that can act as either a coactivator or corepressor, depending on the receptor type.The nuclear hormone receptors (NRs) 1 play an important role in a variety of physiological functions such as cell growth, development, differentiation, and homeostasis (1, 2). The NR superfamily is often divided into steroid and nonsteroid receptor subfamilies, which show different features in DNA binding and dimerization and a different effect on the basal transcriptional activity of the target (2, 3). The estrogen receptor (ER), a member of the steroid receptor family, is critical for the development and progression of breast cancer, and it is a useful diagnostic and therapeutic target (4 -8). Like other NRs, ER contains two distinct transactivation function domains (AFs): the ligand-independent (AF-1) and ligand-dependent (AF-2) activation domains (4,8). A large number of ER-interacting proteins have been identified that modify ER activity. Several coactivators have been characterized recently including SRC-1, Grip1/TIF2, RIP140, Trip1, CBP/P300, SPA/L7, and AIB1/ ACTR/RAC3/p/CIP (9 -12). In addition, several corepressors have also been identified including N-CoR and SMRT (13). The relative expression and/or activity of coactivators and corepressors in a particular environment may modulate the agonistic/ antagonistic activities of the partial ER antagonist, tamoxifen (Tam) (14 -17). Most recently, two bifunctional NR intermediary proteins, TIF1 and NSD1, have been described that can regulate transcription either positively or negatively, depending on both the promoter context and the cell type (18,19).To identify novel transcriptional...

FOXO-binding partners: it takes two to tango

“…In support of this it has recently been demonstrated that in human keratinocytes, FOXOs are essential for 11 of 115 immediate gene activation responses to TGFb (Gomis et al, 2006a). Taken together, these results suggest that (Nechamen et al, 2007) Hepatic nuclear factor-4 (HNF4) (Hirota et al, 2003) HOXA5 (Foucher et al, 2002 (Zhao et al, 2001) FOXO-binding partners KE van der Vos and PJ Coffer formation of a FOXO and Smad transcription factor complex is critical in the control of cell growth and proliferation, and that perturbation of the association or function of this complex could contribute to neoplasia.…”

“…Ablation of the ERa gene delays the onset of tumour development in mouse models, indicating that oestrogen receptor-mediated signalling indeed plays an important role in breast cancer (Bocchinfuso and Korach, 1997). FOXO1 was found to interact with ERa in a ligand-dependent manner, however there are conflicting reports as to the effect of this interaction on ER transcriptional activity (Schuur et al, 2001;Zhao et al, 2001). Schuur et al have also reported that ERa reciprocally repressed FOXO1-mediated promoter transactivation, while cell cycle arrest induced by FOXO1 in MCF7 cells was abrogated by estradiol (Schuur et al, 2001).…”

Loss of expression and nuclear/cytoplasmic localization of the FOXP1 forkhead transcription factor are common events in early endometrial cancer: relationship with estrogen receptors and HIF-1α expression

“…Nevertheless, other members of the FOX family, such as FOXO3a/FKHRL-1, do regulate ER-a gene transcription and interact with ER-a in the presence of beta estradiol. [25][26][27] The biological importance of an association with nuclear FOXP1 expression and ER-a positivity is that the nuclear receptor box in the FOXP1 protein may also enable it to act as a coregulator of ER-a.…”

Many forks in the path: cycling with FoxO

“…However, the interaction between FoxM and FoxO remains an intriguing prospect, and the equilibrium between proliferation and cell cycle arrest may be maintained in part by cooperation and interaction of these factors. For example, in addition to FoxObinding oestrogen receptor in breast cancer cells (Schuur et al, 2001;Zhao et al, 2001), which is critical in regulating the proliferation and differentiation of breast tissue, evidence suggests that FoxM1 and FoxO3a may cooperate to regulate ERa gene transcription .…”