Targeting PAR1 activation in JAK2V617F-driven philadelphia-negative myeloproliferative neoplasms: Unraveling its role in thrombosis and disease progression

Philadelphia chromosome-negative myeloproliferative neoplasms (Ph^-MPNs) are clonal disorders marked by high morbidity and mortality, driven by uncontrolled myeloid proliferation from hematopoietic stem/progenitor cells (HSCs) and associated with a significant risk of thrombosis. This study explored the relationship between JAK2V617F and protease-activated receptor 1 (PAR1) by examining PAR1 expression and activation across various hematopoietic stem/progenitor cell (HSPC) subgroups, assessing their contribution to the hypercoagulable state in Ph^-MPNs. We investigated the effects of thrombin, a PAR1 antagonist (vorapaxar), and a JAK2 inhibitor (ruxolitinib) on Ph^-MPN cells. Mononuclear cells (MNCs) were isolated from Ph-MPN patients (n = 18), cord blood (CB) samples (n = 5) and healthy volunteers (n = 11). Specific subpopulations were sorted and analyzed for PAR1 expression and JAK2V617F status using qRT-PCR. PAR1 expression changes, along with other PAR pathway-related genes, were assessed post-treatment. Our results revealed that most PAR1⁺ cells (∼95 %) co-expressed CD34⁺, with a smaller JAK2V617F⁺ PAR1⁺ population lacking CD34. PAR1 expression was significantly higher in Ph-MPN MNCs compared to CB (p = 0.0005), particularly in EMP, HSC/EPC, and EPC subsets. Thrombin treatment reduced surface PAR1 expression, while PAR1 antagonist treatment further decrease the expression level. Combined PAR1 antagonist and ruxolitinib treatment significantly downregulated PAR1 expression (p < 0.0001), and several PAR-pathway-related genes were notably downregulated after treatment. This study highlights that elevated PAR1 expression in primitive hematopoietic subpopulations is linked to disease progression and thrombosis in Ph^-MPNs, suggesting PAR1 as a potential therapeutic target. Combining PAR1 antagonists with JAK2 inhibitors shows promise in reducing PAR1 expression and mitigating thrombotic events in Ph^-MPN patients.