Ini, a Small Nuclear Protein that Enhances the Response of the Connexin43 Gene to Estrogen

Abstract

This article describes the structural and functional characterization of Ini (AF495522), a novel highly conserved zinc-finger protein that had been identified by screening an estrogen-induced rat myometrial expression library. Ini localizes to the nucleus of HeLa cells and binds to the proximal connexin43 (cx43) promoter, as demonstrated by EMSA. In addition, transient transfection experiments performed with estrogen receptor alpha (ERalpha) cDNA show that overexpression of Ini enhances, in a dose-dependent fashion, the up-regulation of the cx43 gene by estrogen. On binding to the cx43 promoter, Ini stimulates the transcriptional activating function (AF)-1, but not the AF-2, of the ERalpha. This makes Ini one of the few known coactivators specific for AF-1. Because estrogen up-regulates Ini mRNA in the myometrium, it is likely that Ini's physiological role in this tissue is to modulate the response of the cx43 gene to estrogen. Transfection studies with an Ini antisense construct seem to indicate that Ini plays an additional role in the cellular response to estrogen affecting both AF-1 and AF-2 activities of the ERalpha. This broader effect may be associated with cell cycle progression that in yeast has been shown to require Ini.

A novel RING-finger-like protein Ini1 is essential for cell cycle progression in fission yeast

“…We identified an S. pombe homologue of Ini, a mammalian protein that associates with the cx43 gene promoter and has been implicated in the transcriptional regulation of the cx43 gene (Oltra et al, 2003). Although the precise function of the Ini protein in mammalian cells is currently unknown, our analysis of ini1 + in S. pombe has revealed at least one of its potential activities.…”

“…Ini1 is localized to the nucleus In mammals, Ini has been shown to be a nuclear protein that plays a role in regulating gene expression from the cx43 promoter in response to estrogen (Oltra et al, 2003). To test whether the functional similarities between S. pombe Ini1 and human Ini detected in the complementation experiment are accompanied by a similar nuclear localization of S. pombe Ini1, we constructed a GFP-Ini1 fusion protein and introduced it into the ∆ini1 strain.…”

“…To identify protein factors that are involved in the differential regulation of connexin43 (cx43) gene expression in rat heart and uterus, a uterine cDNA expression library was screened for proteins that interact directly with the cx43 gene promoter. This screen identified a protein called Ini, which binds to a 38-nucleotide region of the cx43 promoter (Oltra et al, 2003;Oltra and Werner, 1998). This sequence had been shown previously to function as a cis-activator in the transcription of the cx43 gene (Chen et al, 1995).…”

Treatment with an inhibitor of catechol-O-methyltransferase activity reduces preterm birth and impedes cervical resistance to stretch in pregnant rats

“…We propose that COMT activity may help to mediate estrogen-related functions in the myometrium during pregnancy at term. These functions include increases in the levels of factors associated with contractility, such as CX 43 (Mackenzie & Garfield 1986, Verhoeff et al 1986, Oltra et al 2003, oxytocin (Fang et al 1996), and the receptors for oxytocin (Fang et al 1996) or PGs (Dong & Yallampalli 2000). Therefore, treatment with the COMT inhibitor may cause a reduction in one or more of these factors, which in turn could result in the attenuation of uterine contractility induced by mifepristone.…”

“…At term pregnancy, estrogen helps alter the biochemical and electrical properties of the myometrium by increasing levels of the proteins that are directly or indirectly involved in muscle contraction such as CX 43 (Mackenzie & Garfield 1986, Oltra et al 2003, and the a1 and b (b1) subunits of the L-type voltagedependent calcium channel (Tezuka et al 1995). In addition, estrogen may facilitate myometrial contractility by increasing the expression levels of oxytocin receptor (OTR; Fuchs et al 1983, Larcher et al 1995, Fang et al 1996 and the FP receptor that binds PGF2a (Dong & Yallampalli 2000).…”