Parathyroid hormone [PTH(1-34)] and parathyroid hormone-related protein [PTHrP(1-34)] promote reversion of hypertrophic chondrocytes to a prehypertrophic proliferating phenotype and prevent terminal differentiation of osteoblast-like cells

The effects of parathyroid hormone/parathyroid hormone-related protein (PTH /PTHrP) on late events in chondrocyte differentiation were investigated by a dual in vitro model where conditions of suspension versus adhesion cultur ing are permissive either for apoptosis or for the further differentiation of hypertrophic chondrocytes to osteoblast-like cells. Chick embryo hypertr ophic chondrocytes maintained in suspension synthesized type II and type X collagen and organized their extracellular matrix, farming a tissue highly reminiscent of true cartilage, which eventually mineralized. The formation of mineralized cartilage was associated with the expression of alkaline pho sphatase (ALP), arrest of cell growth, and apoptosis, as observed in growth plates in vivo. In this system, PTH/PTHrP was found to repress type X coll agen synthesis, ALP expression, and cartilage matrix mineralization. Cell p roliferation was resumed, whereas apoptosis was blocked. Hypertrophic chond rocytes cultured in adherent conditions in the presence of retinoic acid un derwent further differentiation to osteoblast-like cells (i.e., they resume d cell proliferation, switched to type I collagen synthesis, and produced a mineralizing bone-like matrix). In this system, PTH addition to culture co mpletely inhibited the expression of ALP and matrix mineralization, whereas cell proliferation and expression of type I collagen were not affected. Th ese data indicate that PTH/PTHrP inhibit both the mineralization of a carti lage-like matrix and apoptosis (mimicked in the suspension culture) and the production of a mineralizing bone-like matrix, characterizing further diff erentiation of hypertrophic chondrocytes to osteoblasts like cells (mimicke d in adhesion culture). Treatment of chondrocyte cultures with PTH/PTHrP re verts cultured cells in states of differentiation earlier than hypertrophic chondrocytes (suspension), or earlier than mineralizing osteoblast-like ce lls (adhesion). However, withdrawal of hormonal stimulation redirects cells toward their distinct, microenvironment-dependent, terminal differentiatio n and fate.