Combined MEK and JAK inhibition reduces osteopontin plasma level and bone marrow fibrosis in a myelofibrosis mouse model

Myelofibrosis (MF), either primary or secondary, is the most aggressive among Philadelphia-negative myeloproliferative neo- plasms (MPNs), since it is characterized by an increased risk of leukemic progression and an inferior life expectancy [1]. Key features of MF include clonal myeloproliferation, extramedullary hematopoiesis, extensive bone marrow (BM) fibrosis deposition, and chronic inflammation [2]. MF progression is primarily driven by neoplastic clone hyperproliferation and the excessive produc- tion of proinflammatory and profibrotic mediators [3]. While considerable progress has been made in identifying the genomic alterations responsible for clonal myeloproliferation, particularly driver mutations in JAK2, MPL, and CALR that activate the JAK/STAT pathway [4], the molecular mechanisms underlying the abnormal mediator production and the disrupted interaction between malignant hematopoietic cells and the BM microenvironment are still not fully understood. Currently, with the exception of hematopoietic stem cell transplantation, no medical or pharma- cologic treatment cures MF. Targeted JAK1/2 inhibitors, while effective in reducing splenomegaly and inflammation-related symptoms, fail to address BM fibrosis and disease burden in most patients [5, 6]. Therefore, the development of novel therapeutic strategies aimed at restoring BM architecture and targeting both clonal myeloproliferation and fibrosis remains a critical unmet need in MF management. We previously showed that the transcription factor MAF, whose levels have been found to be elevated in MF patients, upregulates the production of proinflammatory and profibrotic mediators from differentiated myeloid cells (mainly monocytes and megakaryo- cytes) derived from malignant CD34+ hematopoietic stem and progenitor cells. SPP1 (encoding osteopontin, OPN), a direct target of MAF, was identified as a key player in this process, with OPN overproduction contributing to fibroblast proliferation and col- lagen deposition, thereby driving myelofibrosis. Moreover, OPN plasma levels were significantly elevated in MF patients compared to healthy donors, with even higher levels observed in overtly fibrotic versus pre-fibrotic patients. These elevated OPN levels also correlated with poorer overall survival, highlighting the role of OPN in MF pathogenesis [5]. Building on these results, we further identified that ERK1/ 2 signaling supports OPN production, and pharmacological inhibition with Ulixertinib reduced BM and spleen fibrosis in a thrombopoietin receptor agonist (TPO-RA- treated mouse model of MF. These effects were primarily due to the reduction of OPN expression and function, as similar effects were observed with an anti-OPN neutralizing antibody [6]. It is well established that constitutive activation of the JAK/STAT pathway via driver mutations leads to downstream activation of the mitogen-activated protein kinase (MAPK) pathway, thereby contributing to MF pathogenesis [7], with OPN representing one of the mediators involved in the process.