CXCL8/CXCR2 signaling mediates bone marrow fibrosis and is a therapeutic target in myelofibrosis.

Proinflammatory signaling is a hallmark feature of human cancer, including in myeloproliferative neoplasms (MPNs), most notably myelofibrosis (MF). Dysregulated inflammatory signaling contributes to fibrotic progression in MF; however, the individual cytokine mediators elicited by malignant MPN cells to promote collagen-producing fibrosis and disease evolution are yet to be fully elucidated. Previously, we identified a critical role for combined constitutive JAK/STAT and aberrant NF-κB proinflammatory signaling in MF development. Using single-cell transcriptional and cytokine-secretion studies of primary cells from patients with MF and the human MPLW515L (hMPLW515L) murine model of MF, we extend our previous work and delineate the role of CXCL8/CXCR2 signaling in MF pathogenesis and bone marrow fibrosis progression. Hematopoietic stem/progenitor cells from patients with MF are enriched for a CXCL8/CXCR2 gene signature and display enhanced proliferation and fitness in response to an exogenous CXCL8 ligand in vitro. Genetic deletion of Cxcr2 in the hMPLW515L-adoptive transfer model abrogates fibrosis and extends overall survival, and pharmacologic inhibition of the CXCR1/2 pathway improves hematologic parameters, attenuates bone marrow fibrosis, and synergizes with JAK inhibitor therapy. Our mechanistic insights provide a rationale for therapeutic targeting of the CXCL8/CXCR2 pathway among patients with MF.

PRMT5 Inhibition Modulates E2F1 Methylation and Gene-Regulatory Networks Leading to Therapeutic Efficacy in JAK2V617F-Mutant MPN.

We investigated the role of PRMT5 in myeloproliferative neoplasm (MPN) pathogenesis and aimed to elucidate key PRMT5 targets contributing to MPN maintenance. PRMT5 is overexpressed in primary MPN cells, and PRMT5 inhibition potently reduced MPN cell proliferation ex vivo. PRMT5 inhibition was efficacious at reversing elevated hematocrit, leukocytosis, and splenomegaly in a model of JAK2V617F+ polycythemia vera and leukocyte and platelet counts, hepatosplenomegaly, and fibrosis in the MPLW515L model of myelofibrosis. Dual targeting of JAK and PRMT5 was superior to JAK or PRMT5 inhibitor monotherapy, further decreasing elevated counts and extramedullary hematopoiesis in vivo. PRMT5 inhibition reduced expression of E2F targets and altered the methylation status of E2F1 leading to attenuated DNA damage repair, cell-cycle arrest, and increased apoptosis. Our data link PRMT5 to E2F1 regulatory function and MPN cell survival and provide a strong mechanistic rationale for clinical trials of PRMT5 inhibitors in MPN. SIGNIFICANCE: Expression of PRMT5 and E2F targets is increased in JAK2V617F+ MPN. Pharmacologic inhibition of PRMT5 alters the methylation status of E2F1 and shows efficacy in JAK2V617F/MPLW515L MPN models and primary samples. PRMT5 represents a potential novel therapeutic target for MPN, which is now being clinically evaluated.This article is highlighted in the In This Issue feature, p. 1611.

Activation of JAK/STAT Signaling in Megakaryocytes Sustains Myeloproliferation In Vivo.

PURPOSE: The myeloproliferative neoplasms (MPN), including polycythemia vera, essential thrombocythemia, and primary myelofibrosis, are characterized by the expansion of the erythroid, megakaryocytic, and granulocytic lineages. A common feature of these disorders is the presence of abnormal megakaryocytes, which have been implicated as causative agents in the development of bone marrow fibrosis. However, the specific contributions of megakaryocytes to MPN pathogenesis remain unclear. EXPERIMENTAL DESIGN: We used Pf4-Cre transgenic mice to drive expression of JAK2V617F in megakaryocyte lineage-committed hematopoietic cells. We also assessed the critical role of mutant megakaryocytes in MPN maintenance through cell ablation studies in JAK2V617F and MPLW515L BMT models of MPN. RESULTS: JAK2V617F -mutant presence in megakaryocytes was sufficient to induce enhanced erythropoiesis and promote fibrosis, which leads to a myeloproliferative state with expansion of mutant and nonmutant hematopoietic cells. The increased erythropoiesis was associated with elevated IL6 level, which was also required for aberrant erythropoiesis in vivo. Furthermore, depletion of megakaryocytes in the JAK2V617F and MPLW515L BMT models ameliorated polycythemia and leukocytosis in addition to expected effects on megakaryopoiesis. CONCLUSIONS: Our observations reveal that JAK/STAT pathway activation in megakaryocytes induces myeloproliferation and is necessary for MPN maintenance in vivo. These observations indicate that MPN clone can influence the behavior of the wild-type hematopoietic milieu, at least, in part, via altered production of proinflammatory cytokines and chemokines. Our findings resonate with patients who present with a clinical MPN and a low JAK2V617F allele burden, and support the development of MPN therapies aimed at targeting megakaryocytes.