Functional interaction of HINT1 with MRTF-A: complex formation, protein stabilisation and transcriptional repression

Histidine triad nucleotide-binding protein 1 (HINT1) is a small and evolutionarily conserved HIT family protein with putative tumor suppressor function. It has been implicated in the negative regulation of several transcription factors, but a role for HINT1 in Myocardin-related transcription factor A (MRTF-A)/serum response factor (SRF) signaling has not been described. Here, we examined whether HINT1 functionally interacts with MRTF-A in fibroblasts. HINT1 reduced MRTF-A-dependent reporter activity in a dose-dependent manner, whereas a histidine triad mutant did not. Repression by HINT1 was retained upon deletion of the N-terminal RPEL motifs of MRTF-A, indicating that the effect was not confined to the inhibition of Rho-actin signaling. In parallel, HINT1-WT increased the MRTF-A protein abundance, which required the C-terminal part of MRTF-A. Co-immunoprecipitation experiments revealed the physical association of HINT1 with MRTF-A, whereas the histidine triad mutant or a C-terminally truncated MRTF-A did not show complex formation. Thus, our work suggests that HINT1 acts as a negative regulator of MRTF-A-dependent transcription by binding to the MRTF-A C-terminal part, which appears to require the histidine triad region and is accompanied by a separable increase in MRTF-A abundance.