Megakaryocytic IGF1 coordinates activation and ferroptosis to safeguard hematopoietic stem cell regeneration after radiation injury.

BACKGROUND: Hematopoietic stem cell (HSC) regeneration underlies hematopoietic recovery from myelosuppression, which is a life-threatening side effect of cytotoxicity. HSC niche is profoundly disrupted after myelosuppressive injury, while if and how the niche is reshaped and regulates HSC regeneration are poorly understood. METHODS: A mouse model of radiation injury-induced myelosuppression was built by exposing mice to a sublethal dose of ionizing radiation. The dynamic changes in the number, distribution and functionality of HSCs and megakaryocytes were determined by flow cytometry, immunofluorescence, colony assay and bone marrow transplantation, in combination with transcriptomic analysis. The communication between HSCs and megakaryocytes was determined using a coculture system and adoptive transfer. The signaling mechanism was investigated both in vivo and in vitro, and was consolidated using megakaryocyte-specific knockout mice and transgenic mice. RESULTS: Megakaryocytes become a predominant component of HSC niche and localize closer to HSCs after radiation injury. Meanwhile, transient insulin-like growth factor 1 (IGF1) hypersecretion is predominantly provoked in megakaryocytes after radiation injury, whereas HSCs regenerate paralleling megakaryocytic IGF1 hypersecretion. Mechanistically, HSCs are particularly susceptible to megakaryocytic IGF1 hypersecretion, and mTOR downstream of IGF1 signaling not only promotes activation including proliferation and mitochondrial oxidative metabolism of HSCs, but also inhibits ferritinophagy to restrict HSC ferroptosis. Consequently, the delicate coordination between proliferation, mitochondrial oxidative metabolism and ferroptosis ensures functional HSC expansion after radiation injury. Importantly, punctual IGF1 administration simultaneously promotes HSC regeneration and hematopoietic recovery after radiation injury, representing a superior therapeutic approach for myelosuppression. CONCLUSIONS: Our study identifies megakaryocytes as a last line of defense against myelosuppressive injury and megakaryocytic IGF1 as a novel niche signal safeguarding HSC regeneration.

Long-acting PGE2 and Lisinopril Mitigate H-ARS.

Thrombocytopenia is a major complication in hematopoietic-acute radiation syndrome (H-ARS) that increases the risk of mortality from uncontrolled hemorrhage. There is a great demand for new therapies to improve survival and mitigate bleeding in H-ARS. Thrombopoiesis requires interactions between megakaryocytes (MKs) and endothelial cells. 16, 16-dimethyl prostaglandin E2 (dmPGE2), a longer-acting analogue of PGE2, promotes hematopoietic recovery after total-body irradiation (TBI), and various angiotensin-converting enzyme (ACE) inhibitors mitigate endothelial injury after radiation exposure. Here, we tested a combination therapy of dmPGE2 and lisinopril to mitigate thrombocytopenia in murine models of H-ARS following TBI. After 7.75 Gy TBI, dmPGE2 and lisinopril each increased survival relative to vehicle controls. Importantly, combined dmPGE2 and lisinopril therapy enhanced survival greater than either individual agent. Studies performed after 4 Gy TBI revealed reduced numbers of marrow MKs and circulating platelets. In addition, sublethal TBI induced abnormalities both in MK maturation and in in vitro and in vivo platelet function. dmPGE2, alone and in combination with lisinopril, improved recovery of marrow MKs and peripheral platelets. Finally, sublethal TBI transiently reduced the number of marrow Lin-CD45-CD31+Sca-1- sinusoidal endothelial cells, while combined dmPGE2 and lisinopril treatment, but not single-agent treatment, accelerated their recovery. Taken together, these data support the concept that combined dmPGE2 and lisinopril therapy improves thrombocytopenia and survival by promoting recovery of the MK lineage, as well as the MK niche, in the setting of H-ARS.

Astragalus polysaccharide has a protective effect on hematopoiesis in an irradiated mouse model and decreases apoptosis in megakaryocytes.

Huangqi, the dried root of Radix Astragali, is an essential herb in Traditional Chinese Medicine and has been used to promote hematopoiesis for centuries. Astragalus polysaccharide (ASPS), the bioactive compound of Huangqi, serves a crucial role in hematopoiesis. The aim of the present study was to investigate the hematopoietic effects, in particular the thrombopoietic effects, and the molecular mechanisms of ASPS using an irradiation­induced myelosuppressive mouse model. Colony­forming unit assays, flow cytometric analysis of apoptosis, ELISAs, Giemsa staining and western blotting were performed to determine the hematopoietic and anti­apoptotic effects of ASPS. The results demonstrated that ASPS enhanced the recovery of red blood cells at day 21 following treatment, as well as platelets and white blood cells at day 14. In addition, ASPS promoted colony formation in all lineages (megakaryocytes, granulocyte monocytes, erythroid cells and fibroblasts). The morphological study of the bone marrow demonstrated that tri­lineage hematopoiesis was preserved in the ASPS­ and thrombopoietin (TPO)­treated groups compared with the control group. The overall cellularity (mean total cell count/area) of the ASPS­treated group was similar to that of the TPO­treated group. Additionally, in vitro experiments indicated that treatment with 100 µg/ml ASPS exhibited the maximum effect on colony formation. ASPS attenuated cell apoptosis in megakaryocytic cells via inhibiting the mitochondrial caspase­3 signaling pathway. In conclusion, ASPS promoted hematopoiesis in irradiated myelosuppressive mice possibly via enhancing hematopoietic stem/progenitor cell proliferation and inhibiting megakaryocytes apoptosis.

Thrombopoietic stimulating activity of rhTyrRS (Y341A).

Tumor radiotherapy induces hematopoietic organ damage and reduces thrombocyte counts. Thrombocytopenia is a common disease. Some studies have shown that tRNA synthetase plays not only catalytic tRNA aminoacylation roles, but also functions similarly to cytokines. Recombinant human tyrosyl-tRNA synthetase with a mutated Y341A (rhTyrRS (Y341A)) promotes megakaryocyte migrate from bone marrow to peripheral blood. It would promote megakaryocytes in the lungs adhering to vascular endothelial cells and resulting in the platelet production. The purpose of this research was to investigate the efficacy of rhTyrRS (Y341A) as a therapy for thrombocytopenia and to explore its mechanism of action. We found platelet number was effectively increased by rhTyrRS (Y341A) via platelet count and reticulated platelets (RPs) flow cytometry. We also demonstrated radiation-induced thrombocytopenia could be prevented by rhTyrRS (Y341A). The results of immunohistochemistry and H&E staining showed the number of pulmonary mature megakaryocytes was significantly increased in rhTyrRS (Y341A) treated groups. In transgenic zebrafish larvae, confocal microscopy results showed rhTyrRS (Y341A) promoted the migration and adhesion of megakaryocytes. These results suggested that rhTyrRS (Y341A) promote megakaryocytes in bone marrow migrating to lungs through blood circulation. rhTyrRS (Y341A) may be an effective medicine which could be used to treat patients suffering from thrombocytopenia.

Effect of radiation-induced endothelial cell injury on platelet regeneration by megakaryocytes.

Thrombocytopenia is an important cause of hemorrhage and death after radiation injury, but the pathogenesis of radiation-induced thrombocytopenia has not been fully characterized. Here, we investigated the influence of radiation-induced endothelial cell injury on platelet regeneration. We found that human umbilical vein endothelial cells (HUVECs) underwent a high rate of apoptosis, accompanied by a significant reduction in the expression of vascular endothelial growth factor (VEGF) at 96 h after radiation. Subsequent investigations revealed that radiation injury lowered the ability of HUVECs to attract migrating megakaryocytes (MKs). Moreover, the adhesion of MKs to HUVECs was markedly reduced when HUVECs were exposed to radiation, accompanied by a decreased production of platelets by MKs. In vivo study showed that VEGF treatment significantly promoted the migration of MKs into the vascular niche and accelerated platelet recovery in irradiated mice. Our studies demonstrate that endothelial cell injury contributes to the slow recovery of platelets after radiation, which provides a deeper insight into the pathogenesis of thrombocytopenia induced by radiation.

Low-level light treatment ameliorates immune thrombocytopenia.

Immune thrombocytopenia (ITP) is an immune-mediated acquired bleeding disorder characterized by abnormally low platelet counts. We reported here the ability of low-level light treatment (LLLT) to alleviate ITP in mice. The treatment is based on noninvasive whole body illumination 30 min a day for a few consecutive days by near infrared light (830 nm) transmitted by an array of light-emitting diodes (LEDs). LLLT significantly lifted the nadir of platelet counts and restored tail bleeding time when applied to two passive ITP models induced by anti-CD41 antibody. The anti-platelet antibody hindered megakaryocyte differentiation from the progenitors, impaired proplatelet and platelet formation, and induced apoptosis of platelets. These adverse effects of anti-CD41 antibody were all mitigated by LLLT to varying degrees, owing to its ability to enhance mitochondrial biogenesis and activity in megakaryocytes and preserve mitochondrial functions in platelets in the presence of the antibody. The observations argue not only for contribution of mitochondrial stress to the pathology of ITP, but also clinical potentials of LLLT as a safe, simple, and cost-effective modality of ITP.

Noninvasive low-level laser therapy for thrombocytopenia.

Thrombocytopenia is a common hematologic disorder that is managed primarily by platelet transfusions. We report here that noninvasive whole-body illumination with a special near-infrared laser cures acute thrombocytopenia triggered by γ-irradiation within 2 weeks in mice, as opposed to a 5-week recovery time required in controls. The low-level laser (LLL) also greatly accelerated platelet regeneration in the presence of anti-CD41 antibody that binds and depletes platelets, and prevented a severe drop in platelet count caused by a common chemotherapeutic drug. Mechanistically, LLL stimulated mitochondrial biogenesis specifically in megakaryocytes owing to polyploidy of the cells. LLL also protected megakaryocytes from mitochondrial injury and apoptosis under stress. The multifaceted effects of LLL on mitochondria bolstered megakaryocyte maturation; facilitated elongation, branching, and formation of proplatelets; and doubled the number of platelets generated from individual megakaryocytes in mice. LLL-mediated platelet biogenesis depended on megakaryopoiesis and was inversely correlated with platelet counts, which kept platelet biogenesis in check and effectively averted thrombosis even after repeated uses, in sharp contrast to all current agents that stimulate the differentiation of megakaryocyte progenitors from hematopoietic stem cells independently of platelet counts. This safe, drug-free, donor-independent modality represents a paradigm shift in the prophylaxis and treatment of thrombocytopenia.

H2S protects against fatal myelosuppression by promoting the generation of megakaryocytes/platelets.

BACKGROUND: Our previous pilot studies aimed to examine the role of hydrogen sulfide (H2S) in the generation of endothelial progenitor cells led to an unexpected result, i.e., H2S promoted the differentiation of certain hematopoietic stem/progenitor cells in the bone marrow. This gave rise to an idea that H2S might promote hematopoiesis. METHODS: To test this idea, a mice model of myelosuppression and cultured fetal liver cells were used to examine the role of H2S in hematopoiesis. RESULTS: H2S promoted the generation of megakaryocytes, increased platelet levels, ameliorate entorrhagia, and improved survival. These H2S effects were blocked in both in vivo and in vitro models with thrombopoietin (TPO) receptor knockout mice (c-mpl(-/-) mice). In contrast, H2S promoted megakaryocytes/platelets generation in both in vivo and in vitro models with TPO knockout mice (TPO(-/-) mice). CONCLUSIONS: H2S is a novel promoter for megakaryopoiesis by acting on the TPO receptors but not TPO to generate megakaryocytes/platelets.

Intramedullary megakaryocytes internalize released platelet factor 4 and store it in alpha granules.

BACKGROUND: Megakaryocytes express and store platelet factor 4 (PF4) in alpha granules. In vivo, PF4 is a clinically relevant, negative regulator of megakaryopoiesis and hematopoietic stem cell replication. These findings would suggest a regulated source of free intramedullary PF4. OBJECTIVES: Define the source of free intramedullary PF4 and its intramedullary life cycle. METHODS: We interrogated both murine and human bone marrow-derived cells during megakaryopoiesis in vitro by using confocal microscopy and enzyme-linked immunosorbent assay. With immunohistochemistry, we examined in vivo free PF4 in murine bone marrow before and after radiation injury and in the setting of megakaryocytopenia and thrombocytopenia. RESULTS: Exogenously added human PF4 is internalized by murine megakaryocytes. Human megakaryocytes similarly take up murine PF4 but not the related chemokine, platelet basic protein. Confocal microscopy shows that internalized PF4 colocalizes with endogenous PF4 in alpha granules and is available for release on thrombin stimulation. Immunohistochemistry shows free PF4 in the marrow, but not another alphagranule protein, von Willebrand factor. Free PF4 increases with radiation injury and decreases with megakaryocytopenia. Consistent with the known role of low-density lipoprotein receptor-related protein 1 in the negative paracrine effect of PF4 on megakaryopoiesis, PF4 internalization is at least partially low-density lipoprotein receptor-related protein 1 dependent. CONCLUSIONS: PF4 has a complex intramedullary life cycle with important implications in megakaryopoiesis and hematopoietic stem cell replication not seen with other tested alpha granule proteins.

Hematopoietic stem and progenitor cells regulate the regeneration of their niche by secreting Angiopoietin-1.

Hematopoietic stem cells (HSCs) are maintained by a perivascular niche in bone marrow but it is unclear whether the niche is reciprocally regulated by HSCs. Here, we systematically assessed the expression and function of Angiopoietin-1 (Angpt1) in bone marrow. Angpt1 was not expressed by osteoblasts. Angpt1 was most highly expressed by HSCs, and at lower levels by c-kit(+) hematopoietic progenitors, megakaryocytes, and Leptin Receptor(+) (LepR(+)) stromal cells. Global conditional deletion of Angpt1, or deletion from osteoblasts, LepR+ cells, Nes-cre-expressing cells, megakaryocytes, endothelial cells or hematopoietic cells in normal mice did not affect hematopoiesis, HSC maintenance, or HSC quiescence. Deletion of Angpt1 from hematopoietic cells and LepR(+) cells had little effect on vasculature or HSC frequency under steady-state conditions but accelerated vascular and hematopoietic recovery after irradiation while increasing vascular leakiness. Hematopoietic stem/progenitor cells and LepR(+) stromal cells regulate niche regeneration by secreting Angpt1, reducing vascular leakiness but slowing niche recovery.

Mitoquinone restores platelet production in irradiation-induced thrombocytopenia.

Myelodysplastic syndromes (MDS) are hallmarked by cytopenia and dysplasia of hematopoietic cells, often accompanied by mitochondrial dysfunction and increases of reactive oxygen species (ROS) within affected cells. However, it is not known whether the increase in ROS production is an instigator or a byproduct of the disease. The present investigation shows that mice lacking immediate early responsive gene X-1 (IEX-1) exhibit lineage specific increases in ROS production and abnormal cytology upon radiation in blood cell types commonly identified in MDS. These affected cell lineages chiefly have the bone marrow as a primary site of differentiation and maturation, while cells with extramedullary differentiation and maturation like B- and T-cells remain unaffected. Increased ROS production is likely to contribute significantly to irradiation-induced thrombocytopenia in the absence of IEX-1 as demonstrated by effective reversal of the disorder after mitoquinone (MitoQ) treatment, a mitochondria-specific antioxidant. MitoQ reduced intracellular ROS production within megakaryocytes and platelets. It also normalized mitochondrial membrane potential and superoxide production in platelets in irradiated, IEX-1 deficient mice. The lineage-specific effects of mitochondrial ROS may help us understand the etiology of thrombocytopenia in association with MDS in a subgroup of the patients.

A study of the effect of sequential injection of 5-androstenediol on irradiation-induced myelosuppression in mice.

Herein, we aimed at examining the therapeutic effects of 5-androstenediol (5-AED), a natural hormone produced in the adrenal cortex, on radiation-induced myelosuppression in C3H/HeN mice. The mice were subjected to whole-body irradiation with a sublethal dose of 5 Gy gamma-irradiation to induce severe myelosuppression, and 5-AED (50 mg/kg) was administered subcutaneously. 5-AED was administrated 1 day before irradiation (pre-treatment) or twice weekly for 3 weeks starting from 1 h after irradiation (post-treatment). Treatment with 5-AED significantly ameliorated the decrease in the peripheral blood neutrophil and platelet populations in irradiated myelosuppressive mice, but had no effect on the lymphocyte population. It also ameliorated hypocellularity and disruption of bone marrow induced by irradiation and led to rapid recovery of myeloid cells. Further, it attenuated the decrease in spleen weight and megakaryocyte and myeloid cell populations in the spleen and promoted multilineage hematopoietic recovery. We found that a single injection of 5-AED produced only a temporary therapeutic effect, while sequential injection of 5-AED after irradiation had a more pronounced and prolonged therapeutic effect and reduced myelosuppression by irradiation. Thus, sequential injection of 5-AED after irradiation has therapeutic potential for radiation-induced myelosuppression when administered continuously and can be a significant therapeutic candidate for the management of acute radiation syndrome, particularly in a mass casualty scenario where rapid and economic intervention is important.

Promoted megakaryocytic differentiation of K562 cells through oxidative stress caused by near ultraviolet irradiation.

Reactive oxygen species (ROS) have been proven to be important activators for various cellular activities, including cell differentiation. Several reports showed the necessity of ROS during cell differentiation of the megakaryocytic (MK) lineage. In this study, we employed near ultraviolet (near-UV) irradiation to generate endogenous oxidative stress in an MK differentiation process of K562 cells with phorbol 12-myristate 13-acetate (PMA) induction. A significant increase in the intracellular ROS level was detected on day 1 after near-UV irradiation. In the initial stage of differentiation, a shifted fraction of G1 and G2 phase cells was obtained using near-UV irradiation, giving an increased percentage of G2 phase cells (up from 31.1 to 68.7%). The near-UV irradiation-induced upregulation of the p21 gene, which is a cell cycle inhibitor, suggested that the G2 phase cells were prevented from undergoing cell division. It was found that the percentage of high ploidy (8N and 16N) cells was enhanced significantly at the later stage of the K562 cell culture with near-UV irradiation. Moreover, time-lapse analysis showed that near-UV irradiation encouraged the expression of CD41, a specific surface marker of megakaryocytes. This is the first report that the elevated oxidative stress through the near-UV irradiation promoted the MK differentiation of PMA-induced K562 cells.

SDF-1 dynamically mediates megakaryocyte niche occupancy and thrombopoiesis at steady state and following radiation injury.

Megakaryocyte (MK) development in the bone marrow progresses spatially from the endosteal niche, which promotes MK progenitor proliferation, to the sinusoidal vascular niche, the site of terminal maturation and thrombopoiesis. The chemokine stromal cell-derived factor-1 (SDF-1), signaling through CXCR4, is implicated in the maturational chemotaxis of MKs toward sinusoidal vessels. Here, we demonstrate that both IV administration of SDF-1 and stabilization of endogenous SDF-1 acutely increase MK-vasculature association and thrombopoiesis with no change in MK number. In the setting of radiation injury, we find dynamic fluctuations in marrow SDF-1 distribution that spatially and temporally correlate with variations in MK niche occupancy. Stabilization of altered SDF-1 gradients directly affects MK location. Importantly, these SDF-1-mediated changes have functional consequences for platelet production, as the movement of MKs away from the vasculature decreases circulating platelets, while MK association with the vasculature increases circulating platelets. Finally, we demonstrate that manipulation of SDF-1 gradients can improve radiation-induced thrombocytopenia in a manner additive with earlier TPO treatment. Taken together, our data support the concept that SDF-1 regulates the spatial distribution of MKs in the marrow and consequently circulating platelet numbers. This knowledge of the microenvironmental regulation of the MK lineage could lead to improved therapeutic strategies for thrombocytopenia.

Impaired hematopoiesis and delayed thrombopoietic recovery following sublethal irradiation in SRC­3 knockout mice.

The objective of the present study was to investigate the role of the steroid receptor coactivator-3 (SRC-3) in hematopoiesis of mouse bone marrow (BM) following total body irradiation (TBI). SRC-3-/­ mice and wild-type (WT) mice were exposed to 4.5 Gy γ rays. Immunoblotting analysis revealed that the SRC-3 protein (p160) levels in normal BM-nucleated cells in WT were higher than in SRC-3-/­ mice. Furthermore, peripheral blood cell counts, BM cellularity and colony-forming unit (CFU) assays were performed following irradiation. The results showed that peripheral blood cells were significantly lower in number and recovered less rapidly in irradiated SRC-3-/­ mice as compared with control animals. BM-nucleated cell and CFU counts were significantly decreased in SRC-3-/­ mice on the 7th and 14th day. Of note, the recovery of platelet (PLT) and megakaryocytic lineage were more depressed than the granulocytic and erythroid lineage in SRC-3-/­ mice. In conclusion, the present study demonstrated that the hematopoietic ability in SRC-3 knockout mice is severely impaired following a sublethal dose of irradiation.

NRF2-mediated Notch pathway activation enhances hematopoietic reconstitution following myelosuppressive radiation.

A nuclear disaster may result in exposure to potentially lethal doses of ionizing radiation (IR). Hematopoietic acute radiation syndrome (H-ARS) is characterized by severe myelosuppression, which increases the risk of infection, bleeding, and mortality. Here, we determined that activation of nuclear factor erythroid-2-related factor 2 (NRF2) signaling enhances hematopoietic stem progenitor cell (HSPC) function and mitigates IR-induced myelosuppression and mortality. Augmenting NRF2 signaling in mice, either by genetic deletion of the NRF2 inhibitor Keap1 or by pharmacological NRF2 activation with 2-trifluoromethyl-2'-methoxychalone (TMC), enhanced hematopoietic reconstitution following bone marrow transplantation (BMT). Strikingly, even 24 hours after lethal IR exposure, oral administration of TMC mitigated myelosuppression and mortality in mice. Furthermore, TMC administration to irradiated transgenic Notch reporter mice revealed activation of Notch signaling in HSPCs and enhanced HSPC expansion by increasing Jagged1 expression in BM stromal cells. Administration of a Notch inhibitor ablated the effects of TMC on hematopoietic reconstitution. Taken together, we identified a mechanism by which NRF2-mediated Notch signaling improves HSPC function and myelosuppression following IR exposure. Our data indicate that targeting this pathway may provide a countermeasure against the damaging effects of IR exposure.

[A comparative study of the myeloprotective effect of dicarbamine and leucostim in experimental post-radiation bone marrow syndrome].

We studied the effect of dicarbamine and leucostim on myelopoiesis in experimental post-radiation bone marrow syndrome. Dicarbamine in different modes of administration and doses provided a high level of protection of proliferating hematopoietic precursors in the early period after radiation, which was reflected in a statistically significant decrease in the depth and duration of post-radiation deficit of cells, such as of granulocytes, lymphocytes, megakaryocytes and erythroid cells. The greatest effect of the drug appeared at a dose of 4 mg/kg (prophylactic administration) and a dose of 15 mg/kg (curative double dose). In the bone marrow of experimental animals leucostim prevented development of post-radiation deficit of granulocytes and lymphocytes to a lesser extent, than dicarbamine, and it was effective for erythroid cells and megakaryocytes.

Megakaryocytes promote murine osteoblastic HSC niche expansion and stem cell engraftment after radioablative conditioning.

Successful hematopoietic stem cell (HSC) transplantation requires donor HSC engraftment within specialized bone marrow microenvironments known as HSC niches. We have previously reported a profound remodeling of the endosteal osteoblastic HSC niche after total body irradiation (TBI), defined as relocalization of surviving megakaryocytes to the niche site and marked expansion of endosteal osteoblasts. We now demonstrate that host megakaryocytes function critically in expansion of the endosteal niche after preparative radioablation and in the engraftment of donor HSC. We show that TBI-induced migration of megakaryocytes to the endosteal niche depends on thrombopoietin signaling through the c-MPL receptor on megakaryocytes, as well as CD41 integrin-mediated adhesion. Moreover, niche osteoblast proliferation post-TBI required megakaryocyte-secreted platelet-derived growth factor-BB. Furthermore, blockade of c-MPL-dependent megakaryocyte migration and function after TBI resulted in a significant decrease in donor HSC engraftment in primary and competitive secondary transplantation assays. Finally, we administered thrombopoietin to mice beginning 5 days before marrow radioablation and ending 24 hours before transplant to enhance megakaryocyte function post-TBI, and found that this strategy significantly enhanced donor HSC engraftment, providing a rationale for improving hematopoietic recovery and perhaps overall outcome after clinical HSC transplantation.

Effects of radiation on the maturation of megakaryocytes.

Megakaryocytes are generated by the differentiation of megakaryocytic progenitors; however, little information has been reported regarding how ionizing radiation affects the differentiation pathway and cellular responses. Human leukemia K562 cells have been used as a model to study megakaryocytic differentiation. In the present study, to investigate the effects of radiation on phorbol 12-myristate 13-acetate (PMA)-induced megakaryocytic differentiation of K562 cells, the cellular processes responsible for the expression of CD41 antigen (GPIIb/IIIa), which is reported to be expressed early in megakaryocyte maturation, were analyzed. The expression of CD41 antigens was significantly increased 72 h after treatment with both 4 Gy X-irradiation and PMA. In this fraction, two populations, CD41(low) and CD41(high) cells, were detected by flow cytometry. The CD41(high) cells sustained intracellular ROS at the initial level for up to 72 h, but CD41(low) cells had reduced ROS by 48 h. The maximum suppressive effect on CD41 expression was observed when N-acetyl cysteine, which is known to act as a ROS scavenger, was administered 48 h after PMA stimulation. When K562 cells were pretreated with mitogen-activated protein kinase (MAPK) pathway inhibitors, an ERK1/2 inhibitor and a p38 MAPK inhibitor, followed by X-irradiation and PMA stimulation, the reactivity profiles of both inhibitors showed the involvement of MAPK pathway. There is a possibility that the K562 cell population contains at least two types of radiosensitive megakaryocytic progenitors with respect to ROS production mechanisms, and intracellular ROS levels determine the extent of CD41 expression.

Megakaryocytic differentiation in human chronic myelogenous leukemia K562 cells induced by ionizing radiation in combination with phorbol 12-myristate 13-acetate.

Differentiation-induction therapy is an attractive approach in leukemia treatment. It has been suggested that the accumulation of intracellular reactive oxygen species (ROS) is involved in megakaryocytic differentiation induced by phorbol 12-myristate 13-acetate (PMA) in the K562 leukemia cell line. Therefore, a ROS-inducible technique could be a powerful method of differentiation induction. Accordingly, we hypothesized that ionizing radiation contributes to the acceleration of megakaryocytic differentiation through the accumulation of intracellular ROS in leukemia cells. In the present study, ionizing radiation was shown to promote PMA-induced megakaryocytic differentiation. Cells with high CD41 expression sustained intracellular ROS levels effectively. The enhancement of differentiation by ionizing radiation was found to be regulated through the mitogen-activated protein kinase (MAPK) pathway, involving both extracellular signal-regulated protein kinase 1/2 (ERK1/2) and p38 MAPK. Ionizing radiation also controlled mRNA expression of the oxidative stress response gene heme oxygenase-1 (HO1). Consequently, we concluded that intracellular ROS, increased by ionizing radiation, modulate megakaryocytic differentiation downstream of the MAPK pathway.