Mitochondrial ferritin is a nuclear encoded iron-storage protein localized in mitochondria. It has anti-oxidant properties related to its ferroxidase activity, and it is able to sequester iron avidly into the organelle. The protein has a tissue-specific pattern of expression and is also highly expressed in sideroblasts of patients affected by hereditary sideroblastic anemia and by refractory anemia with ringed sideroblasts. The present study examined whether mitochondrial ferritin has a role in the pathogenesis of these diseases. Design and MethodsWe analyzed the effect of mitochondrial ferritin over-expression on the JAK2/STAT5 pathway, on iron metabolism and on heme synthesis in erythroleukemic cell lines. Furthermore its effect on apoptosis was evaluated on human erythroid progenitors. Results Data revealed that a high level of mitochondrial ferritin reduced reactive oxygen species andStat5 phosphorylation while promoting mitochondrial iron loading and cytosolic iron starvation. The decline of Stat5 phosphorylation induced a decrease of the level of anti-apoptotic BclxL transcript compared to that in control cells; however, transferrin receptor 1 transcript increased due to the activation of the iron responsive element/iron regulatory protein machinery. Also, high expression of mitochondrial ferritin increased apoptosis, limited heme synthesis and promoted the formation of Perls-positive granules, identified by electron microscopy as iron granules in mitochondria. ConclusionsOur results provide evidence suggesting that Stat5-dependent transcriptional regulation is displaced by strong cytosolic iron starvation status induced by mitochondrial ferritin. The protein interferes with JAK2/STAT5 pathways and with the mechanism of mitochondrial iron accumulation.Key words: mitochondrial ferritin, iron acculumation, hereditary sideroblastic anemia, refractory anemia with ringed sideblasts, ROS, iron starvation, JAK2/STA5. 2011;96(10):1424-1432 doi:10.3324/haematol.2011 This is an open-access paper. Citation: Santambrogio P, Erba BG, Campanella A, Cozzi A, Causarano V, Cremonesi L, Gallì A, Della Porta MG, Invernizzi R, and Levi S. Over-expression of mitochondrial ferritin affects the JAK2/STAT5 pathway in K562 cells and causes mitochondrial iron accumulation. Haematologica Over-expression of mitochondrial ferritin affects the JAK2/STAT5 pathway in K562 cells and causes mitochondrial iron accumulation

Primary myelofibrosis is characterized by the development of fibrosis in the bone marrow that contributes to ineffective hematopoiesis. Bone marrow fibrosis is the result of a complex and not yet fully understood interaction among megakaryocytes, myeloid cells, fibroblasts, and endothelial cells. Here, we report that >30% of the endothelial cells in the small vessels of the bone marrow and spleen of patients with primary myelofibrosis have a mesenchymal phenotype, which is suggestive of the process known as endothelial-to-mesenchymal transition (EndMT). EndMT can be reproduced in vitro by incubation of cultured endothelial progenitor cells or spleen-derived endothelial cells with inflammatory cytokines. Megakaryocytes appear to be implicated in this process, because EndMT mainly occurs in the microvessels close to these cells, and because megakaryocyte-derived supernatant fluid can reproduce the EndMT switch in vitro. Furthermore, EndMT is an early event in a JAK2-V617F knock-in mouse model of primary myelofibrosis. Overall, these data show for the first time that microvascular endothelial cells in the bone marrow and spleen of patients with primary myelofibrosis show functional and morphologic changes that are associated to the mesenchymal phenotype.

Our findings suggest that the FtMt mutation may determine a condition similar to haploinsufficiency resulting in a reduced protection from iron-dependent oxidative stress in mitochondria.

SummaryIn myelodysplastic syndromes with ring sideroblasts (MDS-RS), the iron deposited in the mitochondria of RS is present in the form of mitochondrial ferritin (FTMT), but it is unknown whether FTMT overexpression is the cause or the result of mitochondrial iron deposition. Lentivirus FTMTtransduced CD34+ bone marrow cells from seven healthy donors and CD34 + cells from 24 patients with MDS-RS were cultured according to a procedure that allowed the expansion of high numbers of erythroid progenitors. These cells were used to investigate the possible influence of experimentally-induced FTMT overexpression on normal erythropoiesis and the functional effects of FTMT in sideroblastic erythropoiesis. In MDS-RS progenitors, FTMT overexpression was associated with reduced cytosolic ferritin levels, increased surface transferrin receptor expression and reduced cell proliferation; FTMT effects were independent of SF3B1 mutation status. Similarly, FTMT overexpressing normal erythroid progenitors were characterized by reduced cytosolic ferritin content and increased CD71 expression, and also by higher apoptotic rate in comparison with the FTMT-controls. Significantly lower levels of STAT5 phosphorylation following erythropoietin stimulation were found in both sideroblastic and normal FTMT + erythroid cells compared to the FTMT-cells. In conclusion, experimental overexpression of FTMT may modify mitochondrial iron availability and lead to ineffective erythropoiesis.

Primary myelofibrosis (PMF) is a Philadelphia-negative (Ph−) myeloproliferative disorder, showing abnormal CD34+ progenitor cell trafficking, splenomegaly, marrow fibrosis leading to extensive extramedullary haematopoiesis, and abnormal neoangiogenesis in either the bone marrow or the spleen. Monocytes expressing the angiopoietin-2 receptor (Tie2) have been shown to support abnormal angiogenic processes in solid tumors through a paracrine action that takes place in proximity to the vessels. In this study we investigated the frequency of Tie2 expressing monocytes in the spleen tissue samples of patients with PMF, and healthy subjects (CTRLs), and evaluated their possible role in favouring spleen angiogenesis. We show by confocal microscopy that in the spleen tissue of patients with PMF, but not of CTRLs, the most of the CD14+ cells are Tie2+ and are close to vessels; by flow cytometry, we found that Tie2 expressing monocytes were Tie2+CD14lowCD16brightCDL62−CCR2− (TEMs) and their frequency was higher (p = 0.008) in spleen tissue-derived mononuclear cells (MNCs) of patients with PMF than in spleen tissue-derived MNCs from CTRLs undergoing splenectomy for abdominal trauma. By in vitro angiogenesis assay we evidenced that conditioned medium of immunomagnetically selected spleen tissue derived CD14+ cells of patients with PMF induced a denser tube like net than that of CTRLs; in addition, CD14+Tie2+ cells sorted from spleen tissue derived single cell suspension of patients with PMF show a higher expression of genes involved in angiogenesis than that found in CTRLs. Our results document the enrichment of Tie2+ monocytes expressing angiogenic genes in the spleen of patients with PMF, suggesting a role for these cells in starting/maintaining the pathological angiogenesis in this organ.