STAT1-independent inhibition of cyclooxygenase-2 expression by IFNγ; a common pathway of IFNγ-mediated gene repression but not gene activation

Cyclooxygenase-2 (COX-2), the rate-limiting enzyme in the synthesis of prostaglandins, promotes the development of colorectal cancer, and is a key molecular target of non-steroidal anti-inflammatory drugs, compounds that reduce the relative risk of developing colon cancer. In this study, we showed that interferon γ (IFN γ ) inhibits the expression of COX-2 protein in intestinal epithelial cells (IECs) through a pathway that requires Janus-activated kinase (JAK) activity. In contrast, we demonstrated that transcriptional inhibition of COX-2 by IFN β or IFN γ occurs in cells with silenced signal transducer and activator of transcription 1 (STAT1) expression and that IFNs retained the ability to inhibit COX-2 transcription in cells with activated RasV12, in which IFN γ failed to induce STAT1. Thus, unlike the activity of JAK, STAT1 is not required for the inhibition of COX-2 expression by IFN γ . In contrast to COX-2, the activation of genes in response to IFN γ , such as interferon regulatory factor-1, was severely impaired by both STAT1 silencing and by constitutive Ras signaling. To determine whether there is a general differential requirement for STAT1 in gene activation and gene repression in response to IFN γ in intestinal cells, we performed genome-wide analysis of IFN γ target genes in an IEC line in which STAT1 expression was silenced by small interfering RNA. The results confirmed that the activation of the majority of genes by IFN γ required STAT1. In contrast, the repression of several genes, as we showed for COX-2 specifically, was largely unaffected in cells with silenced STAT1. Our results therefore demonstrate that in general gene activation by IFN γ is more sensitive to STAT1 deficiency than gene repression, and suggest that IFN γ activates and represses gene expression via distinct pathways that can be distinguished, at least in part, by their requirement for STAT1.