Carrageenans are highly sulfated polysaccharides that are widely used as food additives in the Western diet, in order to improve the texture of processed foods. Although native and degraded carrageenans induce colonic ulcerations, polyps, and colorectal tumors in animal models, very little is known about the effects of carrageenan on human colonocytes. We evaluated effects of lambda-carrageenan (lambdaCGN) on the normal human colonocyte cell line NCM460, using a concentration of 1 mug/ml, about less than one tenth the average daily exposure to carrageenan in the Western diet. We measured secreted bone morphogenetic protein-4 (BMP4) in spent media and quantified its expression by quantitative RT-PCR. Wnt-related genes were measured by an oligonucleotide array. Cellular beta-catenin was quantified by ELISA. We found a marked decline in secreted BMP4 (P < 0.001) following exposure of NCM460 cells to lambdaCGN for 24 hr. Quantitative RT-PCR for BMP4 transcripts revealed 24% and 45% inhibition of expression on days 2 and 4. cDNA gene expression array of Wnt signaling pathway target genes demonstrated significant changes, including 4.5-fold induction of Wnt 9A and suppression of Dickkopf 3 and RHOU genes. Measurement of beta-catenin by ELISA revealed concomitant accumulation with increases of 67.8%, 61.6%, and 73.9% on days 1, 2, and 4, compared to untreated controls. We conclude that treatment of normal human colonocytes with lambdaCGN activated the Wnt/beta-Catenin cascade and suppressed the expression and secretion of BMP4, inducing significant changes in cellular pathways that are associated with both sporadic and juvenile polyps. CGN may influence development of intestinal polyps in vivo by these mechanisms.