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 autocrineloop that promotes the development of many tumors. Todetermine the importance of this pathway, we examinedthe role that IL-6 plays in the biology of 228 and RKOcolorectal tumor cells. IL-6 induced prominent tyrosinephosphorylation of the transcription factor STAT1 inboth cell types. Furthermore, IL-6 exerts functional effectsin these cells in that it inhibited apoptosis induced by Fasligation, and up-regulated Bcl-xl, a STAT target gene,which can promote cell survival. Butyrate, a compoundformed in the intestines of people who consume a high-fiber diet, may confer protection against the developmentof colorectal cancer. Given the potential importance ofIL-6 in the pathogenesis of colorectal tumors, we tested thehypothesis that butyrate acts by inhibiting IL-6-inducedsignaling events in colorectal carcinoma cells. Followingtreatment with butyrate, the activation of STAT1 inresponse to IL-6, but not interferon-g, was completelylost. Butyrate induced a prominent decrease of mRNAand cell surface expression of the IL-6 receptor a (IL-6Ra) chain. Introduction of a soluble form of the IL-6Rachain restored IL-6-induced STAT1 activation and resist-ance to apoptosis of butyrate treated cells. These experi-ments indicate that IL-6 may play an important role in thepathogenesis of colorectal cancers, and that butyrate mayexert its protective effect by specifically blocking IL-6-induced signaling events.IntroductionCytokinesarekeyfactorsinregulatingthegrowthandsurvivalof hematopoietic and epithelial cells, and can also promote theproliferationofmalignantcells.IL-6isofparticularinterest,asit is part of a family of cytokines, which have broad actions onepithelial,hematopoieticandneuronalcells(1).AlthoughbothIL-6 and the IL-6 receptor (IL-6R) are expressed to a smallextent on normal colonic epithelium, both of these proteins areexpressed to a much greater degree in colonic carcinomas (2).This suggests that an IL-6 autocrine loop may be important tothe genesis and/or maintenance of colorectal carcinomas. IL-6exerts its effects by interacting with a cell surface receptor,which is comprised of an 80-kDa subunit necessary for ligandbinding (gp80), and asignal transducing subunit termed gp130(3). gp130, which is involved in the transduction of signalsfrom other cytokines such as ciliary neurotrophic factor, leuk-emia inhibitory factor and oncostatin M, associates withmembers of the Jak family of tyrosine kinases. When IL-6 bindsto its receptor, it induces activation of the Jaks and the sub-sequenttyrosine phosphorylation oflatent transcriptionfactorstermed STATs. Once phosphorylated, STATs dimerize, trans-locate to thenucleus andbind tospecific DNAelementswherethey can modulate transcription of target genes (4,5). STATsare activated inappropriately in a variety of human tumors(6,7). The over-expression of IL-6 and the IL-6R in colorectalcarcinoma suggested that ectopic STAT activation might playa role in the pathogenesis of colorectal cancer.The observation that the incidence of colorectal carcinomawas much greater in industrial societies compared with agri-cultural societies suggested that dietary factors might modu-late the generation and progression of such tumors (8).Although it is controversial (9), it has been suggested thatindividuals who have a greater intake of dietary fiber have adecreased incidence of colorectal neoplasms (10-- 14). Whilethere may be several mechanisms by which fiber might exert aprotective effect, diets high in fiber lead to notable changes inthe microbial flora of the bowel. Butyrate is a non-toxic short-chain fatty acid that is produced naturally during the microbialfermentation of dietary fiber in the colon (15). Butyrate hasbeen reported to cause a G