Hepatocyte growth factor is involved in formation of osteoclast‐like cells mediated by clonal stromal cells (MC3T3‐G2/PA6)

Abstract

Osteoclast formation from hemopoietic precursors has been shown to require the support of stromal cells in bone tissue. In this study, we demonstrated that hepatocyte growth factor (HGF) is one of the stromal cell‐derived molecules responsible for osteoclast‐like cell formation. For our experimens, we used a coculture system for osteoclastic cell formation and activation in which hemopoietic blast cells are cocultured with calvaria‐derived stromal MC3T3‐G2/PA6 (PA6) cells on dentine slices in the presence of 1, 25‐dihydroxyvitamin D₃ [1,25(OH)₂D₃]. Addition of anti‐HGF neutralizing IgG to the cocultures inhibited the formation of osteoclastic cells and their dentine‐recorbing activity. We detected a single 6.0‐kb transcript for HGF in PA6 cells, and also recognized immunoreactive Mr. 81,000 and 88,000 forms of HGF in conditioned medium (CM) from PA6 cell cultures, the level of which reached 6 ng/ml. Both the CM and HGF stimulated the proliferation of blast cells synergistically with granulocyte‐macrophage colony‐stimulating factor, resulting in an increased number of osteoclast precursors that respond to 1,25(OH)₂D₃ that are tartrate‐resistant acid phosphatase‐positive multinucleate cells in stromal cell‐free blast cell cultures in plastic wells. The effect of the CM was diminished by the addition of anti‐HGF IgG. However, neither the CM nor HGF stimulated the formation of osteoclastic cells and pits on dentine slices in the absence of PA6 cells. These results suggest that although HGF cannot completely replace stromal cells, it is one of the paracrine mediators produced by stromal cells that act on proliferation of osteoclastic cell precursors.