Hepatocyte Nuclear Factor 4alpha regulates expression of the serotonin transporter in intestinal epithelial cells.

The serotonin transporter (SERT) functions to regulate the availability of serotonin (5-HT) in the brain and intestine. An intestine specific mRNA variant arising from a unique transcription start site and alternative promoter in the SERT gene has been identified (iSERT: spanning exon 1C). A decrease in SERT is implicated in several gut disorders including inflammatory bowel diseases (IBD). However, little is known about mechanisms regulating the iSERT variant and a clearer understanding is warranted for targeting SERT for the treatment of gut disorders. Current studies examined the expression of iSERT across different human intestinal regions and investigated its regulation by HNF4alpha (Hepatic Nuclear Factor 4alpha), a transcription factor important for diverse cellular functions. iSERT mRNA abundance was highest in the human ileum and Caco-2 cell line. iSERT mRNA expression was down-regulated by loss of HNF4alpha (but not HNF1alpha, HNF1beta or FOXA1) in Caco-2 cells. Overexpression of HNF4alpha increased iSERT mRNA concomitant with an increase in SERT protein. Progressive promoter deletion and site directed mutagenesis revealed that the HNF4alpha response element spans nucleotides -1163 to -1150 relative to the translation start site. SERT mRNA levels in the intestine were drastically reduced in the intestine-specific HNF4alpha knockout mice relative to HNF4a(F/F) mice. Both HNF4alpha and SERT mRNA levels were also downregulated in mouse model of ileitis (SAMP) compared to AKR control mice. These results establish the transcriptional regulation of iSERT at gut specific internal promoter (hSERTp2) and have identified HNF4alpha as a critical modulator of basal SERT expression in the intestine.