The Tyrosine Kinase c-Met Is Selectively Expressed on Motile Hematopoietic Progenitors Following G-CSF Administration and Promotes Mobilization by Regulating Stem Cell Niche Components.

Application of stress signals such as chemotherapy or repetitive cytokine stimulations induces proliferation, differentiation and mobilization of stem cells from their bone marrow (BM) niches to the circulation, as part of host defense and repair. G-CSF is currently the preferred mobilizing agent used in the clinical setting. We report here that the tyrosine kinase c-Met, is functionally involved in hematopoietic progenitor mobilization by G-CSF administration. Interestingly, c-Met expression was restricted to motile murine and human hematopoietic progenitor cells. While on mouse BM leukocytes c-Met expression was barely detectable during homeostasis, very high levels were documented following G-CSF. Similarly, in the BM of chimeric NOD/SCID mice previously engrafted with human cells, c-Met expression was almost absent on immature human CD34+ and maturing human CD45+ cells from control untreated mice but significant levels of the receptor were detected following G-CSF delivery on both cell populations. This selective expression was associated with increased transcriptional levels of the main regulator of c-Met transcription, hypoxia inducible factor-1 alpha, (HIF-1alpha). More importantly, blockage of c-Met signaling in Balb/c or C57/Bl mice reduced G-CSF-induced mobilization: co-injection of neutralizing c-Met antibodies decreased progenitor cell proliferation in the BM and the release to the peripheral circulation of maturing leukocytes, immature progenitors and primitive Sca⁺/c-kit⁺/lin⁻ cells. Moreover, c-Met neutralization was also accompanied by reduced secretion of the mobilizing protease MMP-9. Chemotaxis to SDF-1 was also affected by c-Met inhibition. In vivo c-Met blockage decreased migration of murine bone marrow cells to a gradient of SDF-1 in vitro, suggesting a role for c-Met in directional migration. Stem cells anchored to their BM niches are mostly non cycling/non motile, however following G-CSF, the niche undergoes dynamic changes which are essential for stem cell proliferation and egress. Of note, neutralizing c-Met during G-CSF mobilization lead to significant changes of key regulatory components of the stem cell niche. While beta catenin was significantly up-regulated following G-CSF treatment, neutralization of c-Met decreased beta catenin expression on BM hematopoietic cells to levels similar to those observed in control untreated mice. The stem cell anchoring molecule angiopoietin-1 and its receptor Tie-2 were also affected following in vivo c-Met inhibition. BM cells obtain from G-CSF treated mice presented low transcriptional levels of these molecules, whereas c-Met neutralization reduced this inhibitory effect exerted by G-CSF. In conclusion, our data identify c-Met as a new player involved in the regulation of several aspects that characterize G-CSF induced mobilization: proliferation and migration of progenitor cells as well as dynamic changes in the stem cell niche which are required for stress induced proliferation and recruitment of stem cells.