Production and action of transforming growth factor-beta in human osteoblast cultures: dependence on cell differentiation and modulation by calcitriol.

Abstract

Transforming growth factor beta (TGF-beta) plays an important role in skeletal remodelling. However, few studies have examined its effects on cultured human osteoblasts. Our aim is to characterise the biological effects of TGF-beta1 on human osteoblasts and to examine the interaction between TGF-beta1 and calcitriol. In vitro study employing two models of normal human osteoblasts: human bone marrow stromal cells [hMS/(OB)] containing osteoprogenitor cells and trabecular bone osteoblasts (hOB), which are mature osteoblasts. A reverse-transcriptase-polymerase-chain-reaction assay was employed to measure steady state mRNA levels of TGF-beta(s) isoforms and receptors. Effects of short-term treatment of TGF-beta1 on osteoblast proliferation and differentiation markers were assessed. The effect of cotreatment of calcitriol (10-8 M) and TGF-beta1 on osteoblast differentiation was also determined. Both hMS(OB) and hOB cells expressed mRNA transcripts of TGF-beta1, TGF-beta2, TGF-beta 3, TGF-beta type I and type II receptors. TGF-beta 1 stimulated osteoblast proliferation in hMS(OB) and in hOB cultures. In hOB cultures, TGF-beta1 stimulated AP production and cotreatment with calcitriol induced a synergistic increase in AP levels to 250 +/- 61% of calcitriol-treated controls. Effects of TGF-beta1 and calcitriol were less pronounced in hMS(OB) cultures. TGF-beta1 inhibited collagen type I production in hMS(OB) cells and these effects were abolished in presence of calcitriol. In presence of calcitriol, TGF-beta1 increased collagen type I production in hOB cells. In both hOB and hMS(OB) cultures, TGF-beta1 inhibited osteocalcin production. TGF-beta increases osteoblastic cell proliferation irrespective of the differentiation state. In presence of calcitriol, it initiates osteoblast cell differentiation and matrix formation. As TGF-beta inhibits osteocalcin production, other factors are necessary for inducing terminal differentiation of osteoblasts. The observed effects of TGF-beta on human osteoblasts in vitro may represent important regulatory steps in controlling osteoblast cell proliferation and differentiation in vivo.