Differential signal pathway activation and 5-HT function: the role of gut enterochromaffin cells as oxygen sensors

摘要

The chemomechanosensory function of the gut enterochromaffin (EC) cell enables it to respond to dietary agents and mechanical stretch. We hypothesized that the EC cell, which also sensed alterations in luminal or mucosal oxygen level, was physiologically sensitive to fluctuations in O_2. Given that low oxygen levels induce 5-HT production and secretion through a hypoxia inducible factor 1alpha (HIF-1alpha)-dependent pathway, we also hypothesized that increasing O_2 would reduce 5-HT production and secretion. Isolated normal EC cells as well as the well-characterized EC cell model KRJ-I were used to examine HIF signaling (luciferase-assays), hypoxia transcriptional response element (HRE)-mediated transcription (PCR), signaling pathways (Western blot), and 5-HT release (ELISA) during exposure to different oxygen levels. Normal EC cells and KRJ-I cells express HIF-1alpha, and transient transfection with Renilla luciferase under HRE control identified a hypoxia-mediated pathway in these cells. PCR confirmed activation of HIF-downstream targets, GLUT1, IGF2, and VEGF under reduced O_2 levels (0.5%). Reducing O_2 also elevated 5-HT secretion (2-3.2-fold) as well as protein levels of HIF-1alpha(1.7-3-fold). Increasing O_2 to 100% inhibited HRE-mediated signaling, transcription, reduced 5-HT secretion, and significantly lowered HIF-1alpha levels (~75% of control). NF-kB signaling was also elevated during hypoxia (1.2-1.6-fold), but no significant changes were noted in PKA/ cAMP. We concluded that gut EC cells are oxygen responsive, and alterations in O_2 levels differentially activate HIF-1alpha and tryptophan hydroxylase 1, as well as NF-kB signaling. This results in alterations in 5-HT production and secretion and identifies that the chemomecha-nosensory role of EC cells extends to oxygen sensing.