Background & aims: The transcription factor Hath1 plays a crucial role in the differentiation program of the human gut epithelium. The present study was conducted to investigate the molecular mechanism of Hath1 expression and its close association with beta-catenin/glycogen synthase kinase 3beta (GSK3beta) under the Wnt pathway in human colonocytes.
Methods: Tissue distribution of Hath1 messenger RNA in human tissues was examined by Northern blot. Stability of Hath1 protein was analyzed by expression of FLAG-tagged Hath1 in human cell lines. Targeting of Hath1 protein by GSK3beta was determined by specific inhibition of GSK-3beta function. Expression of Hath1 protein in colorectal cancers was examined by immunohistochemistry.
Results: Hath1 messenger RNA expression was confined to the lower gastrointestinal tract in human adult tissues. In colon cancer cells, although Hath1 messenger RNA was also detected, Hath1 protein was positively degradated by proteasome-mediated proteolysis. Surprisingly, the GSK3beta-dependent protein degradation was switched between Hath1 and beta-catenin by Wnt signaling, leading to the dramatic alteration of cell status between proliferation and differentiation, respectively. Hath1 protein was detected exclusively in normal colon tissues but not in cancer tissues, where nuclear-localized beta-catenin was present.
Conclusions: The present study suggests a novel function of the canonical Wnt signaling in human colon cancer cells, regulating cell proliferation and differentiation by GSK3beta-mediated, reciprocal degradation of beta-catenin or Hath1, respectively, which further emphasizes the importance of aberrant Wnt signaling in colonocyte transformation.