STAT1 expression is not required for polyp formation in Min mice.

Recent studies have suggested the importance of interleukin (IL)-6 signaling in the development of colon cancer. Expression of IL-6 and the IL-6 receptor have been found to be elevated in colorectal carcinoma tissue, and IL-6 has been found to be critical for tumor formation in mouse models of colon cancer. IL-6 mediated activation of the transcription factor STAT1 has been shown to be important in protection of colorectal carcinoma cells from apoptotic signals. To test the hypothesis that the IL-6-STAT1 axis plays a role in early stages of colon cancer development, we examined the role of this pathway in the mouse multiple intestinal neoplasia (Min) model of intestinal tumorigenesis. Due to low fecundity, we were unable to generate Min mice lacking expression of IL-6. We then focused on the role of STAT1 in intestinal polyp formation in these animals. Min mice lacking STAT1 or heterozygous for STAT1 developed polyps in similar numbers as those expressing STAT1. Furthermore, the anatomic distribution and histological characteristics of these polyps did not vary among these populations. These results indicate that STAT1 does not play a role in the pathogenesis of the Min model for colon cancer. However, they do not rule out the possibility that STAT1 plays a role in other stages of colon cancer development.

Tyrosine phosphorylation is required for functional activation of disulfide-containing constitutively active STAT mutants.

Aberrant activation of STAT transcription factors has been implicated in a variety of cancers. Constitutively active forms of STAT1 and STAT3 (STAT1C and STAT3C) have been developed to determine the effects of STAT activation in isolation from other cytokine-stimulated signaling pathways. These mutants were created by engineering cysteine residues into the carboxy terminus of each STAT molecule, allowing a hypothesized disulfide bond to form between two unphosphorylated monomers. To determine whether the presence of cysteine residues is sufficient to allow for functional activation in the absence of tyrosine phosphorylation, we developed STAT1C and STAT3C mutants that are unable to be phosphorylated on the critical tyrosine residue. Without the tyrosine residue, cysteine containing constitutive STAT mutants failed to transactivate STAT target genes. Furthermore, transfection of STAT dominant negative mutants prevented the activation of STAT1C and STAT3C. Cytokine-induced activation of STAT1C and STAT3C was dramatically prolonged when compared to wild-type proteins and led to extended STAT-dependent gene activation. These data show that tyrosine phosphorylation is required for activation of STAT1C and STAT3C. Additionally, these findings suggest the existence of basal phosphorylation that is a dynamic process that involves both phosphorylation and dephosphorylation. The constitutive STAT mutants likely show heightened activity because of the cysteine residues stabilizing these dimers and preventing dephosphorylation, resulting in the accumulation of trancriptionally active STAT dimer complexes.

IL-6-induced survival of colorectal carcinoma cells is inhibited by butyrate through down-regulation of the IL-6 receptor.

Colorectal carcinoma cells are characterized by over-expression of IL-6 and the IL-6 receptor, an autocrine loop that promotes the development of many tumors. To determine the importance of this pathway, we examined the role that IL-6 plays in the biology of 228 and RKO colorectal tumor cells. IL-6 induced prominent tyrosine phosphorylation of the transcription factor STAT1 in both cell types. Furthermore, IL-6 exerts functional effects in these cells in that it inhibited apoptosis induced by Fas ligation, and up-regulated Bcl-xl, a STAT target gene, which can promote cell survival. Butyrate, a compound formed in the intestines of people who consume a high-fiber diet, may confer protection against the development of colorectal cancer. Given the potential importance of IL-6 in the pathogenesis of colorectal tumors, we tested the hypothesis that butyrate acts by inhibiting IL-6-induced signaling events in colorectal carcinoma cells. Following treatment with butyrate, the activation of STAT1 in response to IL-6, but not interferon-gamma, was completely lost. Butyrate induced a prominent decrease of mRNA and cell surface expression of the IL-6 receptor alpha (IL-6Ralpha) chain. Introduction of a soluble form of the IL-6Ralpha chain restored IL-6-induced STAT1 activation and resistance to apoptosis of butyrate treated cells. These experiments indicate that IL-6 may play an important role in the pathogenesis of colorectal cancers, and that butyrate may exert its protective effect by specifically blocking IL-6-induced signaling events.