Abstract A31 The COX-2 inhibitor, celecoxib, inhibits prostaglandin E2 (PGE2) production and intestinal adenoma formation in mice and in humans. Previously, we showed that treatment of Min/+ mice with dietary celecoxib for 3 weeks reduced PGE2 and induced tumor regression, whereas treatment for 5 months resulted in increased PGE2 and tumor growth (Carothers et al. Cancer Res. 2006; 66: 6432-8). In the small intestine, cross-talk between epithelial and stromal cells occurs across the extracellular matrix (ECM). Heparan sulfate proteoglycans (HSPGs) within the ECM sequester and modulate the activities of growth factors and latent TGFβ ligands. TGFβ stimulates differentiation of fibroblasts into PGE2-producing myofibroblasts. These stromal cells play important roles in intestinal tumorigenesis and in the chemopreventive effect of nonsteroidal anti-inflammatory drugs. We tested the hypothesis that celecoxib resistance in the intestine of Min/+ mice involves ECM regulation of HSPGs and TGFβ signaling in epithelial and stromal cells. Using immunohistochemistry (IHC), we compared the non-tumor mucosa and adenomas of untreated Min/+ controls with Min/+ mice treated with celecoxib for 3 weeks and for 5 months. We performed IHC for the HSPG membrane protein syndecan-1, cytokines TGFβ1, 2, 3 (TGFβ), and the nuclear downstream TGFβ effector, Smad4. IHC was also performed to characterize subepithelial stromal cells using F4/80, a macrophage-specific marker, as well as α-smooth muscle actin (αSMA) and vimentin, myofibroblast-specific markers. In comparison to untreated Min/+ small intestine, celecoxib treatment for 3 weeks increased membrane-localized syndecan-1, TGFβ, and nuclear Smad4 in crypt and villus enterocytes. Under these conditions, few αSMA+ and vimentin+ myofibroblasts or F4/80+ macrophages were present in the pericryptal submucosa and lamina propria. Thus, brief treatment may sequester latent TGFβ and growth factors in the ECM, blocking their downstream signaling. A reciprocal effect was produced by 5 month treatment, in which there was markedly reduced expression of membrane-localized syndecan-1, TGFβ, and nuclear Smad4 in the crypt and villus enterocytes. There was also a dramatic increase in the number of myofibroblasts in the lamina propria and pericryptal submucosa (P<.01). Thus, celecoxib resistance was associated with TGFβ-induced differentiation of fibroblasts into PGE2-producing myofibroblasts. These results suggest that the duration of celecoxib treatment affects the cross-talk between epithelial and stromal cells through the ECM, providing a novel mechanism for celecoxib resistance. Moreover, elevated numbers of myofibroblasts in the human colon may be predictive of chemoprevention failure. Citation Information: Cancer Prev Res 2008;1(7 Suppl):A31.