Page 32 - Haematologica May 2020

N. Curto-Garcia et al.
this favors MPN HSC growth over normal HSC expan- sion.46 Mullally et al. have described that JAK2V617F-LT- HSC are capable of initiating and promoting the disease, giving a clonal advantage to dominate the niche against WT cells. In addition, mutated LT-HSC could induce fibrotic changes in the bone marrow niche in WT trans- planted mouse models.47 Furthermore, Lundberg et al. proved elevated JAK2 expression levels impact negatively on the repopulation capacity of LT-HSC and will promote the disease expansion.48 Finally, acquisition of other muta- tions, such as TET2 deletions in JAK2 V617F-LT-HSC, gives a clonal advantage favoring the disease progression.49
Regarding the osteoblast-osteoclast axis, it is clear that aberrant functionality of the endosteal osteoblastic niche plays an important role in MPN maintenance and progres- sion. For example, it has been shown in murine models that osteoblast expansion is functionally altered in MPN and promotes the development of fibrosis.50 Over time, disease-driven remodeling of the endosteal niche occurs, leading to a self-reinforcing ‘leukemia-niche’ with impaired normal hematopoiesis. Several mechanisms, as suggested by the authors, are implicated in dysregulated osteoblastic expansion, such as overstimulation of MSC driving production of functionally impaired osteoblasts, resultant direct ‘cell-cell contact’ with mutated MPN HSC, and up-regulated production of TPO, CCL3, TNF-β and Notch, thus inducing a chronic state of ‘inflammation’.51,52 Expression of CXCL12, essential for controlled HSC mobi- lization, as discussed above, is reduced due to this abnor- mal osteoblast functionality. Moreover, Spanoudakis et al. recently showed that monocytes derived from JAK2 V617F (heterozygote)-MPN cells had enhanced osteoclast- formation ability compared to wild-type monocytes. An enriched osteoclast environment additionally favors MPN-associated mutated cell population proliferation and survival.53 Collectively, these findings highlight the impor- tance of the osteoblast-osteoclast axis and its disruption in MPN and how this may be therapeutically exploited.
Clonal-MPN cells additionally have the capacity to dis- rupt the finely balanced vascular niche. By way of exam- ple, JAK2-mutated endothelial cells promote the abnormal proliferation and survival of mutated-HSC whilst inhibit- ing normal HSC functionality. This occurs secondary to alterations within the CXCL12 and stem cell factor path- ways. Vascular endothelial growth factor (VEGF), pro- duced by both the endothelial cells and the ‘mutated’- HSC, supports neo-angiogenesis and increases both sur- vival and proliferation of these HSC. Therefore, a self- reinforcing vascular niche also develops as a favorable environment for MPN mutated-HSC.54 Hypoxia-induced signaling also appears to influence HSC behavior by encouraging quiescence and influences long-term repopu- lating activity.55 Utilizing transgenic MPN-murine-models, it has been shown that downregulation of HIF-1α induces an enhanced MPN phenotype reflected by increased leukocytosis and significant splenomegaly.56
Importantly, BMSC appear pivotal to the development and maintenance of the MPN phenotype. BMSC promote the abnormal expansion of osteoblasts as inflammatory ‘myelofibrotic’ cells; a conversion mediated by dysregula- tion of inflammatory signaling pathways with excess pro- duction of TGF-β1, Notch, IL-6, IL-1β and TNF-β and sec- ondary to direct contact between the clonal MPN-HSC and BMSC.57 Schneider et al. has demonstrated that Gli1+- BMSC participate in the activation of myelofibroblasts.58
Ultimately, the overproduction of inflammatory ‘myelofi- brotic cells’ contributes to progressive BM fibrosis observed in the advanced stages of these diseases.31 At the same time, excessive osteoblast production perpetuates clonal-MPN cell proliferation.50 Ramos et al. recently demonstrated that BMSC derived from MPN patients (mainly PV and ET) present an altered gene and immunophenotypic expression profile compared to those derived from healthy donors. In PV, BMSC show an over- expression of genes involved in cell differentiation and migration such as MYADM, Angiopoietin-1 expression and decreases in CXCL12; that are associated with ‘cross-talk’ between the mutated-HSC and BMSC.59 Angiopoietin-1 participates in both angiogenesis and the quiescence of the HSC. Osteoblast production of angiopoietin-1 facili- tates interaction with Tie-2, resulting in increased adhe- sion of HSC to osteoblasts within the niche.60
More recently, other studies have explored the neuro- hematopoietic axis, demonstrating that the sympathetic nervous system influences bone marrow niche regulation. Arranz et al. elegantly showed that a local neuropathy occurs in MPN-BM, with a reduction in both Nestin+ BMSC and CXCL12 expression and promotion of JAK2+ HSC expansion. The relationship, if any, between this local neuropathy and the patient’s symptomatology and phenotype is still not clear, although it has been described as a possible therapeutic target, as discussed below.31,61 Lastly, an increased understanding of the role of estrogen signaling is emerging. In normal HSC, it has been shown that estrogen receptor stimulation in vivo led to an increased proliferation of quiescent LT-HSC and tamox- ifen induced apoptosis of short-term HSC and multipotent progenitors. In chronic MPN, JAK2-mutated murine mod- els, tamoxifen led to preferential restoration of apoptosis in mutated-HSC.62
Regarding the ECM, clonal-HSC demonstrate dysregu- lated ‘cross-talk’ with augmented levels of cytokines and growth factors within the ECM, enhancing both disease establishment and progression. In MF, there is an intensi- fied deposition of ECM components. Thus, highly fibro- genic TGFβ1 activates fibrosis deposition by two main routes: (i) skewing BMSC activation towards fibroblastic and osteoblastic genesis; and (ii) an augmented production of collagen. Moreover, TGFβ1 levels are intimately linked to megakaryocytic activity.63 Additional growth factors such as PDGF (platelet derived growth factor) and VEGF play a pivotal role in this unbalanced ECM-MPN marrow niche communication. PDGF promotes fibrogenesis by activating both megakaryocytes and fibroblasts whereas VEGF contributes towards megakaryocytic maturation and migration.
Other relevant modifiers of the MPN-associated ECM are matrix metalloproteinases (MMP) and Lysyl Oxidase (LOX).64 In MPN, Wang et al. demonstrated downregula- tion of MMP, supporting the accumulation of ECM sub- stances. Focusing on MF, this study demonstrated decreased MMP3 levels which inversely correlated with increased fibrosis and enhanced expression of tissue inhibitors of the metalloproteinases.65 Both MMP2 and MMP9 are highly expressed in MPN patients and are reduced after treatment with JAK inhibitors.66 LOX is a potent regulator of fibrogenesis and is involved in collagen cross-linking. Previous studies have demonstrated a link between deregulated megakaryocytic production of PDGF, TGF-β1 and IL-1β and augmented LOX activity,
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