USP34 regulates endothelial PAR1 mRNA transcript expression and cellular signaling

Signaling by G protein-coupled receptors (GPCRs) is regulated by temporally distinct processes including receptor desensitization, internalization, and lysosomal sorting, and are tightly controlled by posttranslational modifications. While the role of phosphorylation in regulating GPCR signaling is well studied and established, the mechanisms by which other posttranslational modifications, such as ubiquitination, regulate GPCR signaling are not clearly defined. We hypothesize that GPCR ubiquitination and deubiquitination is critical for proper signaling and cellular responses. In the present study, we show that the deubiquitinase ubiquitin-specific protease-34 (USP34) regulates thrombin-stimulated protease-activated receptor-1 (PAR1)-induced p38 autophosphorylation and activation. The PAR1-stimulated p38 signaling pathway is driven by ubiquitination. Interestingly, small interfering RNA-induced knockdown of USP34 expression markedly increased PAR1 cell surface abundance and protein expression without modulating PAR1 ubiquitination or the ubiquitination status of p38 signaling pathway components. In addition, increased PAR1 expression observed in USP34-depleted cells was not caused by altered PAR1 constitutive internalization, agonist-induced internalization, or receptor degradation. Rather, we report that loss of USP34 expression increased mRNA transcript expression of the PAR1-encoding gene, F2R. This study unexpectedly identified a critical role for USP34 in regulation of F2R mRNA transcript expression, which modulates PAR1 cell surface levels and thrombin-stimulated p38 mitogen-activated protein kinase signaling.