title: Smad4 regulates claudin-1 expression in a transforming growth factor-beta-independent manner in colon cancer cells.
abstract: We have recently reported that the expression of a tight junction protein, claudin-1, is increased during colon carcinogenesis and particularly metastatic colorectal cancer. Manipulation of claudin-1 levels in colon cancer cells showed a positive correlation between claudin-1 expression and tumor growth and metastasis. However, the mechanisms underlying the increased claudin-1 expression in colorectal cancer remains unknown. The tumor suppressor Smad4 is a central intracellular signal transduction component of the transforming growth factor-beta (TGF-beta) family of cytokines. Loss of Smad4 protein expression is correlated with poor prognosis and is frequently observed in invasive and metastatic colorectal carcinoma. In the present study, we report an inverse relationship between Smad4 and claudin-1 expression in human colorectal carcinoma tumor samples and in human colon cancer cell lines. We found that the expression of Smad4 in Smad4-deficient but claudin-1-positive SW480 or HT29 colon cancer cell lines down-regulates claudin-1 expression through transcriptional repression by modulating beta-catenin/T-cell factor/lymphocyte enhancer factor activity. Furthermore, this Smad4-dependent inhibition of claudin-1 expression is independent of TGF-beta signaling because Smad4 expression alone is insufficient to restore TGF-beta signaling in the SW480 cells, and the selective TGF-beta receptor kinase inhibitor LY364947 did not prevent the Smad4 suppression of claudin-1 protein expression in either SW480 or HT29 cells. Taken together, these findings suggest a novel mechanism underlying Smad4 tumor-suppressive function through regulation of a potential metastatic modulator, claudin-1, in a TGF-beta-independent manner.
authors.affiliation: Surgical Oncology Research Laboratories, Department of Surgery, Vanderbilt University Medical Center, Nashville, TN 37232, USA.
authors.forename: Sheng-Ru
authors.initials: SR
authors.lastname: Shiou
authors.forename: Amar B
authors.initials: AB
authors.lastname: Singh
authors.forename: Krishnan
authors.initials: K
authors.lastname: Moorthy
authors.forename: Pran K
authors.initials: PK
authors.lastname: Datta
authors.forename: M Kay
authors.initials: MK
authors.lastname: Washington
authors.forename: R Daniel
authors.initials: RD
authors.lastname: Beauchamp
authors.forename: Punita
authors.initials: P
authors.lastname: Dhawan
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D062445:Claudin-1; D015179:Colorectal Neoplasms / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D045325:HCT116 Cells; D019073:HT29 Cells; D006801:Humans; D008565:Membrane Proteins / Q000037:antagonists & inhibitors / Q000096:biosynthesis* / Q000235:genetics; D009361:Neoplasm Invasiveness; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D051381:Rats; D015398:Signal Transduction; D051901:Smad4 Protein / Q000096:biosynthesis* / Q000172:deficiency; D051820:TCF Transcription Factors / Q000378:metabolism; D014158:Transcription, Genetic; D014162:Transfection; D016212:Transforming Growth Factor beta / Q000378:metabolism; D051176:beta Catenin / Q000378:metabolism
chemical_list: C568108:CLDN1 protein, human; D062445:Claudin-1; C568114:Cldn1 protein, rat; D008565:Membrane Proteins; D012333:RNA, Messenger; D051901:Smad4 Protein; D051820:TCF Transcription Factors; D016212:Transforming Growth Factor beta; D051176:beta Catenin