REGULATION OF ALPHA-2A-ADRENERGIC RECEPTOR EXPRESSION IN THE HUMAN COLON-CARCINOMA CELL-LINE HT29 - SCFA-INDUCED ENTEROCYTIC DIFFERENTIATION RESULTS IN AN INHIBITION OF ALPHA-2C10 GENE-TRANSCRIPTION

Previous studies on the intestinal epithelium from various species hav e shown that the number of alpha 2-adrenergic receptors in immature ce lls from the crypts is several-fold higher than in mature cells from t he villi, thus suggesting an inverse relationship between enterocytic differentiation and the expression of this inhibitory receptor. The re ceptor density along the surface-crypt axis of the human colonic mucos a is correlated with the amount of alpha 2C10 mRNA: however, the mecha nisms underlying this regulation remain unknown. The human colonic ade nocarcinoma cell line HT29, which expresses the alpha 2A-adrenergic re ceptor and is able to undergo enterocytic differentiation, is a suitab le model with which to investigate this question in vitro. In this stu dy, we explored the effects of short chain fatty acids (SCFAs), differ entiating agents normally present in the colon lu men, on alpha 2-adre nergic receptor expression, Exposure of HT29 cells to butyrate and pro pionate, but not acetate, resulted in a large diminution of [H-3]RX821 002 binding sites. The reduction of alpha 2-adrenergic receptor number induced by butyrate or propionate was due to decreased amounts of alp ha 2C10 mRNA and was associated with an increase of alkaline phosphata se activity, which reflected the emergence of a more differentiated ph enotype. The changes in alpha 2C10 mRNA level induced by both SCFAs we re dose-dependent, rapid, and reversible and resulted from a diminutio n in the transcription rate of the alpha 2C10 gene. Finally, these eff ects were mimicked by trichostatin A, indicating that they are trigger ed primarily through inhibition of histone deacetylases, The present f indings demonstrate that decrease of alpha 2-adreneric receptor expres sion is a very early event of the HT29 cell differentiation process. T hey also suggest that SCFAs, which originate from bacterial fermentati on of dietary fibers, may play a role in the regulation of tile alpha 2-adrenergic receptivity of colonic mucosa in vivo.