5′-Untranslated region of the tryptophan hydroxylase-2 gene harbors an asymmetric bidirectional promoter but not internal ribosome entry site in vitro

Tryptophan hydroxylase-2 (TPH2) catalyzes the synthesis of neuronal serotonin, a major neurotransmitter involved in many brain functions and psychiatric disorders. We have previously revealed a critical role of the human TPH2 (hTPH2) 5′-UTR in gene expression regulation. This study aimed to further characterize mechanism(s) by which the hTPH2 5′-UTR regulates gene expression. An internal ribosome entry site (IRES) activity in hTPH2 5′-UTR was suggested by the conventional bicistronic reporter assay; however, further stringent experiments, including in vitro translation, quantitative real-time PCR, Northern blot, ribonuclease protection assay, and monocistronic reporter assay, demonstrated that the hTPH2 5′-UTR harbors a bidirectional promoter, but not IRES, within its downstream segment (61-141). The antisense promoter is much stronger than the sense promoter, but the strength of both promoters are cell-line dependent, with the highest and lowest activities being observed in HEK-293T and SK-N-MC cells, respectively. In accordance with our previous findings, the upstream segment (1-60) of hTPH2 5′-UTR suppresses the neighboring promoter of both direction, independent of the cell line and its location in the 5′- or 3′-flanking regions of the gene. In summary, this study demonstrates that no IRES but an asymmetric bidirectional promoter is present in the downstream segment of hTPH2 5′-UTR, and this promoter is susceptible to a gene silencing effect caused by the upstream segment (1-60) of hTPH2 5′-UTR. Our findings point to the potential involvement of antisense transcription and non-coding RNA in the regulation of TPH2 gene expression.