RESUMEN
Myelofibrosis with myeloid metaplasia or idiopathic myelofibrosis is a myeloproliferative disease. It is known to be a stem-cell disorder that leads to a secondary and reactive stromal reaction in the bone marrow microenvironment that is responsible for impaired haematopoiesis. Although progress has been made in the elucidation of the pathogenesis of idiopathic myelofibrosis, lack of suitable models has limited our understanding of the pathology. The aim of this chapter is to address recent inferred new insights in mouse models into the pathogenesis of osteomyelofibrosis. These insights outline the role of transforming growth factor-beta1 and osteoprotegerin in the promotion of myelofibrosis and osteosclerosis, respectively, paying special regard to the role of abnormal megakaryocyte proliferation and maturation.
Asunto(s)
Mielofibrosis Primaria/complicaciones , Mielofibrosis Primaria/etiología , Animales , Citocinas/fisiología , Modelos Animales de Enfermedad , Megacariocitos/citología , Ratones , Osteosclerosis/etiologíaRESUMEN
The primary genetic lesion(s), as well as the biological processes responsible for the typical structural changes of the bone marrow microenvironment in idiopathic myelofibrosis, are still poorly understood, although a central role in disease pathogenesis has been attributed to the clonal proliferation and defective maturation of megakaryocytes. Two animal models of the disease have been described, that in the last few years significantly contributed to the elucidation of some of the pathogenetic steps of the human disease; these are represented by mice genetically modified to overexpress thrombopoietin and by knock-down mice with defective GATA-1 expression in megakaryocytes (GATA-1(low) mice). This review will outline these murine models, both characterized by extensive accumulation of megakaryocytes in hematopoietic tissues, and illustrate how they provided insights into the identification of some of the molecules and mechanisms responsible for the development of fibrosis and osteosclerosis that present major similarities with those observed in patients with idiopathic myelofibrosis.
Asunto(s)
Proteínas de Unión al ADN , Modelos Animales de Enfermedad , Ratones Noqueados , Ratones Transgénicos , Mielofibrosis Primaria/fisiopatología , Trombopoyetina , Factores de Transcripción , Animales , Proteínas de Unión al ADN/genética , Proteínas de Unión al ADN/inmunología , Proteínas de Unión al ADN/metabolismo , Factores de Unión al ADN Específico de las Células Eritroides , Factor de Transcripción GATA1 , Técnicas de Transferencia de Gen , Ratones , Mielofibrosis Primaria/inmunología , Proteínas/metabolismo , Trombopoyetina/genética , Trombopoyetina/inmunología , Trombopoyetina/metabolismo , Factores de Transcripción/genética , Factores de Transcripción/inmunología , Factores de Transcripción/metabolismoRESUMEN
OBJECTIVE: Osteoprotegerin (OPG) is a soluble member of the tumor necrosis factor receptor superfamily critically involved in the regulation of bone resorption. Within the bone microenvironment, OPG is abundantly produced by osteoblast/stromal cells, and its expression is regulated by transforming growth factor-beta(1) (TGF-beta(1)). However, OPG expression and regulation in primary hematopoietic cells have not been fully investigated. MATERIALS AND METHODS: Opg mRNA was studied in murine hematopoietic cells by semiquantitative reverse transcriptase-polymerase chain reaction. The OPG protein was identified by immunofluorescence labeling and secretion was assessed by enzyme-linked immunosorbent assay. RESULTS: Opg transcripts were detected in platelets, megakaryocytes (MK), monocytes, and B lymphocytes, but not in erythroblasts, neutrophils, and T lymphocytes. Mature MK and proplatelets exhibited strong immunostaining for OPG outside the storage alpha-granules, and secretion was detected in the conditioned medium. To analyze whether opg transcription in MK was influenced by TGF-beta(1), the opg/GpIIb mRNA ratio was compared in cultured MK derived from TGF-beta(1) null mutants and wild-type littermates without or after the addition of bioactive TGF-beta(1). No difference was seen, indicating that opg expression in MK was not modulated by TGF-beta(1). However, mRNA levels were increased when thrombopoietin was present in the culture medium, suggesting that MK maturation was correlated with enhanced opg expression. CONCLUSIONS: With these results we document for the first time that murine MK and platelets express OPG. This suggests a novel role for MK in bone homeostasis, in addition to its role in vascular homeostasis.
Asunto(s)
Glicoproteínas/genética , Megacariocitos/metabolismo , ARN Mensajero/análisis , Receptores Citoplasmáticos y Nucleares/genética , Animales , Plaquetas/metabolismo , Células Cultivadas , Glicoproteínas/análisis , Ratones , Ratones Endogámicos C57BL , Microscopía Confocal , Osteoprotegerina , Receptores Citoplasmáticos y Nucleares/análisis , Receptores del Factor de Necrosis Tumoral , Reacción en Cadena de la Polimerasa de Transcriptasa Inversa , Transcripción Genética , Factor de Crecimiento Transformador beta/farmacología , Factor de Crecimiento Transformador beta1RESUMEN
Several studies suggest an implication of transforming growth factor-beta1 (TGF-beta1) in the promotion of myelofibrosis associated with hematopoietic malignancies, but the involvement of this cytokine is not fully investigated. To test directly the impact of TGF-beta1 in the pathogenesis of myelofibrosis, bone marrow stem cells from homozygous TGF-beta1 null (TGF-beta1(-/-)) and wild-type (WT) littermates were infected with a retrovirus encoding the murine thrombopoietin (TPO) protein and engrafted into lethally irradiated wild-type hosts for long-term reconstitution. Over the 4 months of follow-up, TPO levels in plasma were markedly elevated in both groups of mice, and animals typically developed a myeloproliferative syndrome characterized by thrombocytosis, leukocytosis, splenomegaly, increased numbers of progenitors in blood, and extramedullary hematopoiesis. Severe fibrosis was observed in spleen and marrow from all the mice engrafted with WT cells. In contrast, none of the mice repopulated with TGF-beta1(-/-) cells (chimerism > 70%) showed deposition of reticulin fibers at any time during the follow-up. In accordance with the development of fibrosis, latent TGF-beta1 levels in plasma and extracellular fluid of the spleen from mice engrafted with WT cells were increased 6-fold and 4-fold, respectively, over levels found in normal hosts, whereas no increase over baseline levels could be demonstrated in animals undergoing transplantation with TGF-beta1(-/-) cells. These data provide evidence that TGF-beta1 produced by hematopoietic cells is pivotal for the pathogenesis of myelofibrosis that develops in mice with TPO overexpression.
Asunto(s)
Mielofibrosis Primaria/inducido químicamente , Trombopoyetina/farmacología , Factor de Crecimiento Transformador beta/fisiología , Animales , Plaquetas/citología , Plaquetas/efectos de los fármacos , Femenino , Supervivencia de Injerto , Trasplante de Células Madre Hematopoyéticas , Células Madre Hematopoyéticas/metabolismo , Masculino , Ratones , Ratones Noqueados , Trastornos Mieloproliferativos/etiología , Mielofibrosis Primaria/etiología , Mielofibrosis Primaria/patología , Trombopoyetina/administración & dosificación , Trombopoyetina/sangre , Transducción Genética , Factor de Crecimiento Transformador beta/genética , Factor de Crecimiento Transformador beta/metabolismo , Factor de Crecimiento Transformador beta1RESUMEN
Myelofibrosis and osteosclerosis are prominent features arising in mice overexpressing thrombopoietin (TPO). The pivotal role of transforming growth factor beta 1 (TGF-beta 1) in the pathogenesis of myelofibrosis has been documented, but the mechanisms mediating osteosclerosis remain unclear. Here, we used mice deficient in osteoprotegerin (OPG), a secreted inhibitor of bone resorption, to determine whether osteosclerosis occurs through a deregulation of osteoclastogenesis. Marrow cells from opg-deficient mice (opg(-/-)) or wild-type (WT) littermates were infected with a retrovirus encoding TPO and engrafted into an opg(-/-) or WT background for long-term reconstitution. The 4 combinations of graft/host (WT/WT, opg(-/-)/opg(-/-), opg(-/-)/WT, and WT/opg(-/-)) were studied. Elevation of TPO and TGF-beta 1 levels in plasma was similar in the 4 experimental groups and all the mice developed a similar myeloproliferative syndrome associated with severe myelofibrosis. Osteosclerosis developed in WT hosts engrafted with WT or opg(-/-) hematopoietic cells and was associated with increased OPG levels in plasma and decreased osteoclastogenesis. In contrast, opg(-/-) hosts exhibited an osteoporotic phenotype and a growth of bone trabeculae was rarely seen. These findings suggest that osteosclerosis in mice with TPO overexpression occurs predominantly via an up-regulation of OPG in host stromal cells leading to disruption of osteoclastogenesis.