The action of GH/IGF-I/IGFBP in osteoblasts and osteoclasts

Bone remodelling is a cyclical phenomenon consisting of osteoclastic bone resorption followed by osteoblastic bone formation. Although recent evidence suggests that GH participates in bone remodelling, the exact mechanism remains unclear. The present series of in vitro studies aimed to clarify how GH affects bone formation and resorption. GH binding sites were found to be present in osteoblastic MC3T3-E1 cells. Bovine GH (bGH) increased DNA synthesis, stimulated alkaline phosphatase activity and enhanced both type I procollagen mRNA expression and collagen synthesis. GH also increased the expression of both IGF-I and IGF-binding protein-5 mRNA as well as the release of IGF-I from these cells. The addition of IGF-I or recombinant IGFBP-5 alone significantly increased ALP activity and type I procollagen mRNA expression. These findings indicate that GH acts directly on osteoblasts to stimulate bone formation and that IGF-I and IGFBP-5 are involved in GH-stimulated bone formation. GH also stimulated pit formation on dentine slices and osteoclast differentiation in stromal cell-containing mouse bone cell cultures, whereas it did not affect the bone-resorbing activity of isolated rabbit osteoclasts. The addition of IGF-I or rIGFBP-5 alone exhibited similar effects. These stimulatory effects of GH on pit formation and osteoclast differentiation were significantly blocked in the presence of neutralizing anti-IGF-I antibody. PCR products corresponding in size to the mouse GH receptor were detected in osteoclast precursor cells. GH stimulated osteoclast-like cell formation from these cells in the absence of stromal cells, and these osteoclast-like cells formed pits on dentine slices in the presence of MC3T3-G2/PA-6 stromal cells. These findings indicate that GH stimulates osteoclastic bone resorption through both its direct and indirect action on the maturation of osteoclast precursor cells and through its indirect activation of mature osteoclasts, possibly via stromal cells. In conclusion, GH stimulates osteoclastic bone resorption as well as osteoblastic bone formation in vitro, and locally produced IGF-I and/or IGFBP-5 are involved in the stimulation of bone remodelling by GH.