Influence of mature adipocytes on osteoblast proliferation in human primary cocultures

It has been shown that the bone loss occurring with aging in spongy bone is associated with a reduced osteoblastic bone formation and an increased volume of marrow adipose tissue. This observation suggests a relationship between cells from the osteoblastic and adipogenic lineages. The purpose of the present study was to evaluate the influence of mature adipocytes on osteoblastic proliferation and activity in a model of coculture. Human primary osteoblastic (hBOB) cells were derived from femoral bone explants collected in patients undergoing orthopedic surgery. Human stromal osteoblastic (hMSOB) cells were obtained from bone marrow samples collected by aspiration during orthopedic surgery. Extramedullary and medullary mature adipocytes (hAd) showing similar functions, except for their response to insulin, hAd were isolated from mammary adipose tissue collected in women undergoing tumorectomy. Cells were cocultured, with hAd being separated from osteoblastic cells (hBOB or hMSOB) by a porous membrane (0.4 microm). When hBOB cells were seeded on the upper side of the insert and hAd were floating on the lower side, cell contacts between the two cell types were possible through the pores of the membrane. At the end of the experiment, proliferation of the osteoblastic cells was evaluated by [(3)H]-thymidine incorporation and alkaline phosphatase (AP) activity was measured. After 20 h of coculture, proliferation of the hBOB cells was significantly decreased when compared with control hBOB (-40 +/- 6%, p < 0.05). To establish whether or not the influence of hAd on hBOB proliferation required intercellular communications, hAd and hBOB cells were cocultured far from the porous membrane. Six other independent experiments confirmed an inhibition of hBOB proliferation under both experimental conditions (p < 0.05): -35 +/- 7% with possible intercellular contacts, and -30 +/- 7% without any contact. In contrast, the proliferation of hMSOB cells was not significantly modified after coculture with hAd. In addition, the presence of hAd did not significantly modify the AP activity of hBOB (0.163 +/- 0.143 and 0.181 +/- 0.114 nmol/min per microgram of protein in controls and after coculture, respectively). No reproducible effect of hAd-conditioned medium was noted on hBOB- and hMSOB-cell proliferation or hBOB-cell activity. In conclusion, mature adipocytes induced an inhibition of hBOB-cells proliferation, probably mediated by a factor secreted by hAd. This effect may contribute to the age-related reduction of bone formation and bone loss.