Department für Infektiologie, Medizinische Mikrobiologie und Hygiene, Ruprecht-Karls-Universität, Im Neuenheimer Feld 324, Heidelberg, Germany.
Mol Cancer. 2010 Aug 25;9:225. doi: 10.1186/1476-4598-9-225.
RhoH is a constitutively active member of the family of Rho GTPases. Its expression is restricted to the haematopoietic lineage, where it serves as a positive regulator for T cell selection and mast cell function and as a negative regulator for growth-related functions in other lineages. Here, we examined the activation of signal transducer and activator of transcription (STAT) proteins in response to stimulation with interleukin 3 (IL3).
Using the murine IL3-dependent cell line BaF3 we investigated the influence of RhoH protein expression levels on IL3-mediated cellular responses. RhoH overexpressing cells showed lower sensitivity to IL3 and decreased STAT5 activation. SiRNA-mediated repression of RhoH gene expression led to an increase in proliferation and STAT5 activity which correlated with an increased number of IL3 receptor α chain molecules, also known as CD123, expressed at the cell surface. Interestingly, these findings could be reproduced using human THP-1 cells as a model system for acute myeloid leukaemia, where low RhoH levels are known to be an unfavourable prognostic marker. Overexpression of RhoH on the other hand caused an induction of STAT1 activity and western blot analysis revealed that activated STAT1 is phosphorylated on Tyr701. STAT1 is known to induce apoptosis or cell cycle arrest and we detected an upregulation of cyclin-dependent kinase inhibitors (CDKI) p21Cip1 and p27Kip1 in RhoH overexpressing BaF3 cells.
We propose that RhoH functions as a negative regulator for IL3-induced signals through modulation of the JAK-STAT pathway. High levels of RhoH allow the IL3-dependent activation of STAT1 causing decreased proliferation through upregulation of p21Cip1 and p27Kip1. Low RhoH levels on the other hand led to an upregulation of IL3-dependent cell growth, STAT5 activity and an increase of CD123 surface expression, linking RhoH to a CD123/STAT5 phenotype that has been described in AML patients.
RhoH 是 Rho GTPases 家族中的一种组成型激活成员。其表达局限于造血谱系，在那里它作为 T 细胞选择和肥大细胞功能的正调节剂，并作为其他谱系中与生长相关的功能的负调节剂。在这里，我们研究了信号转导和转录激活因子（STAT）蛋白在响应白细胞介素 3（IL3）刺激时的激活。
使用小鼠 IL3 依赖性细胞系 BaF3，我们研究了 RhoH 蛋白表达水平对 IL3 介导的细胞反应的影响。RhoH 过表达细胞对 IL3 的敏感性较低，STAT5 激活减少。RhoH 基因表达的 siRNA 抑制导致增殖和 STAT5 活性增加，这与细胞表面表达的 IL3 受体 α 链分子（也称为 CD123）数量增加相关。有趣的是，这些发现可以使用人类 THP-1 细胞作为急性髓系白血病的模型系统来复制，其中低 RhoH 水平是不利的预后标志物。另一方面，RhoH 的过表达导致 STAT1 活性的诱导，Western blot 分析显示激活的 STAT1 在 Tyr701 上磷酸化。STAT1 已知诱导细胞凋亡或细胞周期停滞，我们在 RhoH 过表达的 BaF3 细胞中检测到细胞周期蛋白依赖性激酶抑制剂（CDKI）p21Cip1 和 p27Kip1 的上调。
我们提出 RhoH 通过调节 JAK-STAT 途径作为 IL3 诱导信号的负调节剂。高水平的 RhoH 允许 IL3 依赖性激活 STAT1，通过上调 p21Cip1 和 p27Kip1 导致增殖减少。另一方面，低 RhoH 水平导致 IL3 依赖性细胞生长、STAT5 活性和 CD123 表面表达增加，将 RhoH 与在 AML 患者中描述的 RhoH/STAT5 表型联系起来。
