Implantation of the developing blastocyst is regulated by multiple effectors, 
such as steroid hormones, growth factors, and cytokines. To understand how these 
diverse signaling pathways interact to modulate uterine gene expression, we 
employed a gene expression screen technique to identify the molecules that are 
induced in the periimplantation rat uterus. Here we report the isolation of a 
complementary DNA representing a novel gene, interferon-regulated gene 1 (IRG1). 
This gene exhibits significant homology to interferon (IFN)-alpha/beta-inducible 
human genes p27 and 6-16, indicating that these genes may belong to the same 
family. Consistent with this finding, expression of IRG1 messenger RNA (mRNA) in 
rat uterus increased about 20-fold in response to IFNalpha. Uterine expression 
of IRG1 was also stimulated by estrogen and was partially inhibited by an 
antiestrogen, ICI 182,780. In pregnant rats, IRG1 expression was high on day 1, 
but declined on days 2 and 3. The level of IRG1 mRNA again rose transiently on 
day 4 immediately preceding implantation. In situ hybridization analysis 
localized the IRG1 mRNA expression in the endometrial epithelium and the 
surrounding stroma. Interestingly, the expression of p27, which shows high 
homology to IRG1, was strongly enhanced in human endometrium during the 
midsecretory phase of the menstrual cycle, overlapping the putative window of 
implantation. Both IRG1 and p27 mRNAs are therefore induced in the endometrium 
in an implantation stage-specific manner. We also observed a synergistic 
interaction between IFNalpha and estrogen receptor signaling pathways that led 
to maximal induction of p27 mRNA in Ishikawa cells. Although the functional 
roles of IRG1 and p27 remain unclear, we describe for the first time, 
identification of a gene family regulated by IFNalpha in both rodent and human 
uteri. More importantly, our studies reveal that a complex interplay between the 
steroid hormone and IFN pathways regulates the expression of these genes in the 
endometrium at the time of implantation.