Transforming growth factor-? enhances calcitonin-induced cyclic AMP production and the number of calcitonin receptors in long-term cultures of human umbilical cord blood monocytes in the presence of 1,25-dihydroxycholecalciferol

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Abstract
Transforming growth factor-β (TGF-β) is a multifunctional polypeptide, abundant in bone, that regulates both proliferation and differentiation of a wide variety of cells, but its role in osteoclast differentiation remains controversial. We have recently shown that long-term cultures of human cord blood monocytes, in the presence of 1,25 dihydroxycholecalciferol (1,25-(OH)2D3), give rise to cells that express two markers of the osteoclast phenotype, namely, the vitronectin receptor (VNR) and the calcitonin receptor (CTR). TGF-β enhanced the proportion of cells expressing the VNR. In the present study, we investigated the effect of TGF-β on the expression of CTR in cord blood monocytes cultured during 3 weeks in the presence of 1,25-(OH)2D3. When added within the first 2 weeks of culture, TGF-β (500 pg/ml) significantly decreased the cell protein content. TGF-β alone did not stimulate basal cAMP production. The 10 nM-sCT-stimulated cAMP production was enhanced by increasing TGF-β concentrations from 50 pg/ml to 1,000 pg/ml: for 500 pg/ml TGF-β, it was 294 ± 28% vs. 140 ± 25% for control cultures (p < 0.01). The sCT dose-response curves showed a higher cAMP production from 10−9 M to 10−7 M of sCT in the presence of 500 pg/ml TGF-β than in control cultures. The increase was 325 ± 36% in the presence of TGF-β and 195 ± 13% in the absence of TGF-β, for 10−7 M sCT (p < 0.01). This effect of TGF-β on cAMP production was not observed either when it was added to monocyte cultures the last day or 2 hours before the end of the culture or in MCF7, a human breast cancer cell line that expresses CTR. [125I]-sCT binding studies performed on confluent cells showed similar Kd in control and TGF-β-treated cells. By contrast, the CTR number was significantly increased in the presence of TGF-β: 6.1 ± 2 × 104 receptors per cell in control cultures and 28.8 ± 8.1 × 104 receptors per cell in TGF-β-treated cultures (p < 0,05). It is thus suggested that TGF-β increases the number of CTR of these cells that have other features of preosteoclasts. The role of this cytokine on the process of osteoclast differentiation and in bone resorption is thus emphasized.