[Mapping Regulatory Elements within 5' and 3' UTRs of SIGLEC15 with a Use of Reporter System]

Siglec-15 is an immune suppressor with broad upregulation on various cancer types and has emerged as a potential target for cancer immunotherapy. However, it remains unclear how SIGLEC15 expression is controlled in normal or cancer cells. In this work, we utilized reporter assays to evaluate the impact of the 5' UTR and the 3' UTR of SIGLEC15 mRNA on gene expression. We found that the 3' UTR dramatically reduced reporter protein production, whereas the 5' UTR showed modest inhibitory effect. Quantification of steady-state mRNA revealed the good coupling of protein amount and mRNA abundance that was associated with the 3' UTR. In contrast, the 5' UTR had little effect on mRNA abundance compared with the empty control. By measuring mRNA half-life, we showed that the 3' UTR markedly promoted mRNA degradation. Testing shortened 3' UTR fragments demonstrated five out of the six having notable inhibitory effect, with the one spanning 993-1317 had the most robust activity. More interestingly, the 993-1317 region contains a predicted 43-nt stem-loop structure that showed apparent inhibitory activity in four cell lines tested. These results suggested that the 3' UTR inhibited reporter gene expression by accelerating mRNA decay possibly via multiple cis-regulatory elements, but the 5' UTR repressed gene expression by inhibiting translation. Thus, our findings provided a clue to the molecular mechanism underlying the regulation of SIGLEC15 expression.