The mechanism of granulopoiesis and leukemogenesis by the activation of Stat3

Jak-Stat通路在细胞因子信号传导中起重要作用。G-CSF促进颗粒生成，Stat3是G-CSF激活的主要Stat蛋白。经G-CSF处理后，IL-3依赖性细胞系32D克隆3（32Dcl3）分化为中性粒细胞，表达显性阴性Stat3 （32Dcl3/DNStat3）的32Dcl3细胞在G-CSF中增殖而不分化。g - csf刺激细胞系的基因表达谱显示，Stat3的激活上调了C/EBP_α的表达。激活Stat3结合C/EBP_α，导致C/EBP_α转录活性增强。G-CSF刺激后32Dcl3/DNStat3细胞条件表达C/EBP_α可消除G-CSF依赖性细胞增殖并诱导粒细胞分化。这些结果表明Stat3在G-CSF信号通路中的主要作用之一是增强C/EBP_α的功能。用Stat3条件缺陷小鼠检测Stat3在体内的功能。造血细胞缺乏Stat3的小鼠表现为中性粒细胞减少，并且在缺乏Stat3的小鼠中观察到g - csf诱导的祖细胞增殖。在stat3缺失小鼠的造血细胞中，观察到微量的SOCS3（一种颗粒生成的负调节因子），并且G-CSF刺激不诱导SOCS3的表达。Stat3缺失骨髓细胞在基础条件下表现出明显的ERK1/2激活，G-CSF刺激可增强并维持ERK1/2。此外，加入MEK1抑制剂可显著降低stat3缺陷骨髓细胞对G-CSF的增殖增强反应。Stat3在体内通过诱导SOCS3表达作为G-CSF信号的负调节因子，ERK激活是诱导造血细胞响应G-CSF增殖的主要因素。

The Jak-Stat pathway plays an essential role in cytokine signaling. G-CSF promotes granulopoiesis, and Stat3 is the principle Stat protein activated by G-CSF. Upon treatment with G-CSF, the IL-3 dependent cell line 32D clone 3(32Dcl3) differentiates into neutrophils, and 32Dcl3 cells expressing dominant-negative Stat3 (32Dcl3/DNStat3) proliferate in G-CSF without differentiation. Gene expression profile of G-CSF-stimulated cell lines revealed that the expression of C/EBP_α was up-regulated by the activation of Stat3. And activated Stat3 bound to C/EBP_α, leading to the enhancement of the transcription activity of C/EBP_α. Conditional expression of C/EBP_α in 32Dcl3/DNStat3 cells after G-CSF stimulation abolishes the G-CSF dependent cell proliferation and induces granulocytic differentiation. These results show that one of the major roles of Stat3 in G-CSF signaling pathway is to augment the function of C/EBP_α.The function of Stat3 in vivo was examined using the Stat3 conditional deficient mice. Mice deficient for Stat3 in hematopoietic cells show neutrocytosis, and G-CSF-induced proliferation of progenitor cells was observed in Stat3-deficient mice. In hematopoietic cells from Stat3-deficient mice, trace levels of SOCS3, a negative regulator of granulopoiesis, were observed, and SOCS3 expression was not induced by G-CSF stimulation. Stat3 null bone marrow cells displayed a significant activation of ERK1/2 under basal conditions, and it was enhanced and sustained by G-CSF stimulation. Furthermore, the augmented proliferation of Stat3-deficient bone marrow cells in response to G-CSF was dramatically decreased by addition of a MEK1 inhibitor. Stat3 functions in vivo as a negative regulator of G-CSF signaling by inducing SOCS3 expression, and ERK activation is the major factor responsible for inducing the proliferation of hematopoietic cells in response to G-CSF.