Aberrant transforming growth factor-beta signaling in azoxymethane-inducedmouse colon tumors

Alterations in the transforming growth factor-beta (TGF-beta) pathway are i mplicated in the pathogenesis of colorectal cancer. We hypothesize that alt erations in the TGF-beta pathway contribute to differential sensitivity of mice to the colon carcinogen azoxymethane (AOM). A/J (sensitive) and AKR/J (resistant) mice were injected intraperitoneally with AOM (10 mg/kg of body weight once a week for 6 wk). Twenty-four weeks after AOM exposure, mutati onal analysis of TGF-beta type II receptor (T betaR-II) from normal colons and from tumors showed no AOM-induced alterations. A significant decrease ( 1.5-fold, P<0.05) in T<beta>R-II mRNA levels, however, was found in A/J tum ors with the RNase protection assay. Immunofluorescence of T betaR-II showe d marked loss of staining in A/J tumors. The RNase protection assay and seq uence analysis of the downstream signaling molecule Smad3 revealed no carci nogen-induced alterations in either strain. To gain further insight into th e functionality of the pathway, expression of TGF-beta, TGF-beta type I rec eptor (T betaR-1), and several downstream targets of TGF-beta signaling, in cluding Smad7, c-myc, and p15, was examined. Although no alterations in TGF -beta, T betaR-1, or Smad7 were found in tumors, a significant increase in c-myc expression (2.5-fold, P<0.05) and a significant decrease in p15 expre ssion (4.5-fold, P<0.05) were noted. Concomitant repression of T betaR-II a nd overexpression of c-myc may render epithelial cells insensitive to TGF-b eta -mediated growth arrest, a possibility that also is suggested by this m odel. The significant decrease in p15 expression in tumors provides additio nal evidence that TGF-beta signaling may be markedly attenuated during colo n tumorigenesis. (C) 2001 Wiley-Liss, Inc.