ROLES OF STAT3 IN REGULATION OF GRANULOPOIESIS AND PATHOLOGY OF NEUTROPHILIA

Abstract

Signal transducer and activator of transcription 3 (STAT3) is activated in response to a variety of cytokines and growth factors, leading to rapid alterations in gene expression and far-reaching consequences for development, cell survival, homeostasis and tumorigenesis. Mice with conditional knockout of STAT3 in different tissues revealed distinct roles of STAT3 depending on the cell type and stimulus. Our lab previously reported that mice with targeted disruption of STAT3 in the bone marrow (BM) developed severe neutrophilia, suggesting that STAT3 might be a negative regulator for granulopoiesis. In this study, the underlying mechanism for the regulatory role of STAT3 in granulopoiesis and neutrophil homeostasis was investigated using the mouse model of BM-specific STAT3 knockout (STAT3-CFF). STAT3-CFF mice exhibited progressive neutrophilia, and BM transplantation experiments revealed that the aberrant neutrophil expansion was originated from the cell autonomous defects in STAT3-deficient hematopoietic stem and progenitor cells (HSPCs). The role of STAT3 in granulopoiesis was then inspected at two different stages, namely early hematopoiesis and terminal granulopoiesis. During early hematopoiesis, a prominent expansion of granulocyte/macrophage progenitors (GMPs) was observed in STAT3-/- HSPCs, at the expense of common lymphoid progenitors (CLPs) and megakaryocyte/erythrocyte progenitors (MEPs), indicating a preferred commitment to the granulocyte/macrophage lineage in the absence of STAT3. Furthermore, the expression of major lineage determinate genes for CLP and MEP was significantly impaired in STAT3-/- HSPCs while key driving factors for myeloid lineage remained unchanged. These data suggests that STAT3 plays an important role in promoting the lineage determinate genes that drives the specification to megakaryocyte/erythrocyte and lymphoid fates during early hematopoiesis. In addition, the progressive expansion of BM granulocytes in STAT3-CFF mice was found accompanied by a downregulation of IRF8, a myeloid suppressor, in STAT3-/- BM immature granulocytes. Moreover, chromatin immunoprecipitation assay and luciferase reporter assay revealed that IRF8 was a potential STAT3 target gene. These observations imply that a STAT3-IRF8 axis may be critical for maintaining the BM granulocyte homeostasis during terminal granulopoiesis. Meanwhile, expression profile of a group of epigenetic factors was screened in STAT3-/- immature granulocytes, and SIRT6 was found to be a potential STAT3 target in terminally differentiated myeloid cells. In conclusion, STAT3 regulates granulopoiesis and neutrophil homeostasis in vivo at multiple stages of hematopoiesis through different effectors.