Diadenosine tetraphosphate hydrolase is part of the transcriptional regulation network in immunologically activated mast cells

We previously discovered that microphthalmia transcription factor (MITF) and upstream stimulatory factor 2 (USF2) each forms a complex with its inhibitor histidine triad nucleotide-binding 1 (Hint-1) and with lysyl-tRNA synthetase (LysRS). Moreover, we showed that the dinucleotide diadenosine tetraphosphate (Ap₄A), previously shown to be synthesized by LysRS, binds to Hint-1, and as a result the transcription factors are released from their suppression. Thus, transcriptional activity is regulated by Ap₄A, suggesting that Ap₄A is a second messenger in this context. For Ap₄A to be unambiguously established as a second messenger, several criteria have to be fulfilled, including the presence of a metabolizing enzyme. Since several enzymes are able to hydrolize Ap₄A, we provided here evidence that the "Nudix" type 2 gene product, Ap₄A hydrolase, is responsible for Ap₄A degradation following the immunological activation of mast cells. The knockdown of Ap₄A hydrolase modulated Ap₄A accumulation, resulting in changes in the expression of MITF and USF2 target genes. Moreover, our observations demonstrated that the involvement of Ap₄A hydrolase in gene regulation is not a phenomenon exclusive to mast cells but can also be found in cardiac cells activated with the β-agonist isoproterenol. Thus, we have provided concrete evidence establishing Ap₄A as a second messenger in the regulation of gene expression.