Aberrant transforming growth factor-beta signaling in azoxymethane-induced mouse colon tumors.
Alterations in the transforming growth factor-beta (TGF-beta) pathway are implicated in the pathogenesis of colorectal cancer. We hypothesize that alterations 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, mutational analysis of TGF-beta type II receptor (TbetaR-II) from normal colons and from tumors showed no AOM-induced alterations. A significant decrease (1.5-fold, P < 0.05) in TbetaR-II mRNA levels, however, was found in A/J tumors with the RNase protection assay. Immunofluorescence of TbetaR-II showed marked loss of staining in A/J tumors. The RNase protection assay and sequence analysis of the downstream signaling molecule Smad3 revealed no carcinogen-induced alterations in either strain. To gain further insight into the functionality of the pathway, expression of TGF-beta, TGF-beta type I receptor (TbetaR-I), and several downstream targets of TGF-beta signaling, including Smad7, c-myc, and p15, was examined. Although no alterations in TGF-beta, TbetaR-I, 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 expression (4.5-fold, P < 0.05 ) were noted. Concomitant repression of TbetaR-II and overexpression of c-myc may render epithelial cells insensitive to TGF-beta-mediated growth arrest, a possibility that also is suggested by this model. The significant decrease in p15 expression in tumors provides additional evidence that TGF-beta signaling may be markedly attenuated during colon tumorigenesis.
Guda
K
K
Center for Molecular Medicine, University of Connecticut Health Center, Farmington, CT 06030-3101, USA.
Giardina
C
C
Nambiar
P
P
Cui
H
H
Rosenberg
D W
DW
D000236:Adenoma / Q000139:chemically induced / Q000235:genetics; D000818:Animals; D001397:Azoxymethane; D002273:Carcinogens; D018797:Cell Cycle Proteins; D003110:Colonic Neoplasms / Q000139:chemically induced* / Q000235:genetics / Q000473:pathology; D050762:Cyclin-Dependent Kinase Inhibitor p15; D004268:DNA-Binding Proteins / Q000096:biosynthesis / Q000235:genetics; D015536:Down-Regulation / Q000187:drug effects; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D008297:Male; D051379:Mice; D008805:Mice, Inbred A; D008806:Mice, Inbred AKR; D017346:Protein Serine-Threonine Kinases; D016271:Proto-Oncogene Proteins c-myc / Q000096:biosynthesis / Q000235:genetics; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D000077294:Receptor, Transforming Growth Factor-beta Type II; D018125:Receptors, Transforming Growth Factor beta / Q000096:biosynthesis / Q000235:genetics; D015398:Signal Transduction / Q000187:drug effects*; D051900:Smad3 Protein; D051906:Smad7 Protein; D015534:Trans-Activators / Q000096:biosynthesis / Q000235:genetics; D014157:Transcription Factors / Q000096:biosynthesis / Q000235:genetics; D016212:Transforming Growth Factor beta / Q000502:physiology*; D025521:Tumor Suppressor Proteins
D002273:Carcinogens; C493968:Cdkn2b protein, mouse; D018797:Cell Cycle Proteins; D050762:Cyclin-Dependent Kinase Inhibitor p15; D004268:DNA-Binding Proteins; D016271:Proto-Oncogene Proteins c-myc; D012333:RNA, Messenger; D018125:Receptors, Transforming Growth Factor beta; D051900:Smad3 Protein; C497791:Smad3 protein, mouse; D051906:Smad7 Protein; C497807:Smad7 protein, mouse; D015534:Trans-Activators; D014157:Transcription Factors; D016212:Transforming Growth Factor beta; D025521:Tumor Suppressor Proteins; D017346:Protein Serine-Threonine Kinases; D000077294:Receptor, Transforming Growth Factor-beta Type II; D001397:Azoxymethane