PTHrP expression in chondrocytes, regulation by TGF-beta, and interactionsbetween epiphyseal and growth plate chondrocytes

Although PTHrP has been identified as a key regulator of chondrocyte differ entiation in the growth plate, the factors directly regulating PTHrP expres sion have not been identified. Furthermore, while cells from the epiphysis are considered the physiologic source of PTHrP, the relative expression of PTHrP in epiphyseal and growth plate chondrocytes has not been defined. PTH rP expression was examined in chondrocytes isolated from 3- to 5-week-old c hick long bones. The expression of PTHrP mRNA was 10-fold higher in epiphys eal chondrocytes compared to cells from the growth plate. Growth plate chon drocytes were isolated into populations with distinct maturational characte ristics by countercurrent centrifugal elutriation and analyzed for PTHrP ex pression. The expression was highest in the least mature cells and progress ively declined with the onset of maturation. The regulation of PTHrP expres sion was further examined in epiphyseal chondrocytes, Both TGF-beta 1 and c is-retinoic acid stimulation markedly increased PTHrP mRNA levels, while BM P-8 and PTHrP stimulation decreased the expression of this transcript. The effects of TGF-beta 1 (8.9-fold stimulation) and TGF-beta 3 (9.2-fold) were slightly greater than the effects of TGF-beta 2 (4.9-fold). The effect of TGF-beta was dose-dependent and increases could be detected after 68 h of t reatment. To analyze the paracrine effect of epiphyseal and growth plate ch ondrocytes on each other, these cells were placed in coculture and the mRNA from each of the populations was harvested separately after 24 h. Followin g coculture the PTHrP mRNA levels increased in the epiphyseal cells while t he expression of type X collagen and Indian hedgehog transcripts decreased in growth plate chondrocytes. The results demonstrate potentially important paracrine interactions between these cell populations, possibly mediated b y TGF-beta and PTHrP. (C) 2000 Academic Press.