Division of Experimental Hematology and Cancer Biology, Children's Hospital Medical Center, University of Cincinnati, Cincinnati, Ohio 45229, USA.
J Biol Chem. 2011 Jul 8;286(27):24068-78. doi: 10.1074/jbc.M111.226951. Epub 2011 May 13.
Adult hematopoietic progenitor cells (HPCs) are maintained by highly coordinated signals in the bone marrow. The molecular mechanisms linking intracellular signaling network of HPCs with their microenvironment remain poorly defined. The Rho family GTPase Rac1/Rac2 has previously been implicated in cell functions involved in HPC maintenance, including adhesion, migration, homing, and mobilization. In the present studies we have identified R-Ras, a member of the Ras family, as a key signal mediator required for Rac1/Rac2 activation. We found that whereas Rac1 activity is up-regulated upon stem cell factor, integrin, or CXCL12 stimulation, R-Ras activity is inversely up-regulated. Expression of a constitutively active R-Ras mutant resulted in down-regulation of Rac1-activity whereas deletion of R-Ras led to an increase in Rac1/Rac2 activity and signaling. R-Ras(-/-) HPCs displayed a constitutively assembled cortical actin structure and showed increased directional migration. Rac1/Rac2 inhibition reversed the migration phenotype of R-Ras(-/-) HPCs, similar to that by expressing an R-Ras active mutant. Furthermore, R-Ras(-/-) mice showed enhanced responsiveness to G-CSF for HPC mobilization and exhibited decreased bone marrow homing. Transplantation experiments indicate that the R-Ras deficiency-induced HPC mobilization is a HPC intrinsic property. These results indicate that R-Ras is a critical regulator of Rac signaling required for HPC migration, homing, and mobilization.
成体造血祖细胞（HPCs）在骨髓中通过高度协调的信号维持。将 HPC 细胞内信号网络与微环境联系起来的分子机制仍未得到明确界定。Rho 家族 GTPase Rac1/Rac2 先前已被牵连到与 HPC 维持相关的细胞功能中，包括黏附、迁移、归巢和动员。在本研究中，我们鉴定出 Ras 家族的成员 R-Ras 是 Rac1/Rac2 激活所需的关键信号介质。我们发现，虽然干细胞因子、整合素或 CXCL12 刺激可上调 Rac1 活性，但 R-Ras 活性却相反地被上调。组成性激活的 R-Ras 突变体的表达导致 Rac1 活性下调，而 R-Ras 缺失则导致 Rac1/Rac2 活性和信号的增加。R-Ras(-/-) HPCs 显示出组成型组装的皮质肌动蛋白结构，并表现出增加的定向迁移。Rac1/Rac2 抑制逆转了 R-Ras(-/-) HPC 的迁移表型，类似于表达 R-Ras 活性突变体的情况。此外，R-Ras(-/-) 小鼠对 G-CSF 引起的 HPC 动员反应增强，骨髓归巢减少。移植实验表明，R-Ras 缺陷诱导的 HPC 动员是 HPC 内在的特性。这些结果表明，R-Ras 是 Rac 信号传导的关键调节剂，对于 HPC 的迁移、归巢和动员是必需的。
