Cross-talk between atrial thrombin receptor PAR4 and hepatocyte growth factor receptor c-met in obesity

Purpose: Visceral obesity is a chronic inflammatory and hypercoagulant state that promotes development of cardiovascular diseases including atrial fibrillation (AF). Thrombin supports coagulation-independent inflammation via cellular protease-activated receptors (PAR). In obese human atrial myocardium, expression of PAR4 is elevated and correlates with an activated NLRP3 inflammasome. PAR4-mediated NLRP3 inflammasome activation in atrial cardiomyocytes is not known, nor have potential signaling partners been identified. Thrombin transactivates the hepatocyte-growth factor receptor c-met in some cancer cells, but this has not been verified in the heart. We here  examined PAR4/c-met cross-talk in atrial cardiomyocytes and the possible functional significance in obesity.

Methods and results: In HL-1 atrial cardiomyocytes, thrombin induced caspase-1 auto-activation and IL-1β maturation; IL-1β secretion was evoked by PAR4-activating peptide (AP) but not PAR1-AP. PAR4-AP additionally increased phosphorylated CaMKII-Thr287, mTOR-Ser2481 and Akt-Ser473 while suppressing AMPK-Thr172 phosphorylation. PAR4AP rapidly increased phosphorylation of c-met in HL-1 cells and over time also its transcriptional upregulation. The c-met inhibitor SGX-523 abrogated the effects of PAR4-AP on CaMKII/AKT/mTOR phosphorylation but did not affect PAR4-stimulated IL-1β production. Cardiomyocytes from right atrial appendages (RAA) of obese patients expressed more PAR1 and PAR4 compared to non-obese. Obese whole tissue RAA biopsies contained more IL-1β, phospho-c-met and phospho-mTOR than non-obese RAA; CamKII phosphorylation was not modified. Atria from high fat diet (HFD) versus chow-fed mice also contained more IL-1β, together with higher total and phosphorylated c-met and mTOR; these increases were blunted in PAR4^-/- HFD-fed mice.

Conclusion: Thrombin cross-activates c-met via PAR4 in atrial cardiomyocytes. Trans-activated c-met contributes partially to PAR4-mediated signaling, but NLRP3 inflammasome activation appears to be largely independent of c-met. Abundance of PAR4 and activated c-met increases with obesity, providing therapeutic targets for management of adiposity-driven AF.