The role on bone marrow-derived cells in lung repair and regeneration.

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All-trans-retinoic-acid (ATRA) is known to reverse the anatomic and physiologic signs of pulmonary emphysema. However, the origin of the progenitor cells involved in this lung regeneration remains unclear. Recently, it was shown that bone marrow could be the source of progenitor cells for several cell types. Mice with elastase-induced emphysema were treated with ATRA, granulocyte-colony stimulating factor (G-CSF), or a combination of both. ATRA or G-CSF promoted lung regeneration and increased BMC numbers in alveoli. Combined treatment of both had an additive effect, which indicated that BMC mobilization might be important in lung regeneration. In addition, Circulating endothelial progenitor cells (EPCs) play a pivotal role in angiogenesis. Hepatocyte growth factor (HGF) is known to induce proliferation and motility in endothelial cells, and to play a role in mitogenic and morphogenic actions. However, the role of HGF in EPC mobilization has not been dearly described yet. We investigated the effect of HGF on mobilizing EPCs and on angiogenesis in elastase-induced lung injury. HGF significantly increased the triple-positive (Sca-1+, Flk-1+, and c-kit+) fraction in peripheral mononuclear cells in mice. The bone marrow-derived cells were recruited into the injured lungs, where they differentiated to capillary endothelial cells. HGF induced proliferation of both bone marrow-derived and resident endothelial cells in the alveolar wall. In conclusion, the present study suggests that HGF induces EPC mobilization from the bone marrow, and enhances the proliferation of endothelial cells in vivo. These complex effects induced by HGF orchestrate pulmonary regeneration in emphysematous lung parenchyma.