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Effect of osteogenic protein‐1 on the development and mineralization of primary cultures of avian growth plate chondrocytes: Modulation by retinoic acid

Osteogenic protein‐1 (OP‐1), a member of the TGF‐β family of proteins, induces endochondral bone formation. Here we studied the effect of OP‐1 on the development of primary cultures of avian growth plate (GP) chondrocytes in either serum‐free or serum‐containing medium, in the absence or presence of retinoic acid (RA). OP‐1 was added on day 7 of culture and continued for 7 days, or until the cultures were harvested, typically on day 21. Alone, OP‐1 caused ∼2‐fold increase in proteoglycan synthesis into both the medium and the cell:matrix layer. Additionally, OP‐1 caused a dosage‐dependent increase in alkaline phosphatase (ALP) activity, and an increase in protein, when given from days 7–14 and examined on day 14. This stimulation was greater in cells grown in serum‐free than in serum‐containing media (3–5‐fold vs. 2–3‐fold increase in ALP; ∼40% vs. ∼20% increase in protein). Such stimulation of ALP activity and proteoglycan (PG) synthesis in cultured GP cells indicates that OP‐1 elicits differentiation of chondrocytes. OP‐1 minimally affected cell division (DNA content); however, a slight increase was seen when examined early in the culture. Alone, OP‐1 increased mineral (Ca and Pi) content of the cultures by ∼2‐fold in both types of media. As early as day 14, clusters of mineral encircled many of the OP‐1 treated cells. Thus, as in vivo, OP‐1 strongly promoted mineral formation by the cultured GP chondrocytes. When present together, OP‐1 and RA generally blocked the action of the other. Separately OP‐1 and RA each stimulated protein synthesis, ALP activity, and Ca2+ deposition; together they were inhibitory to each. Also, RA blocked the stimulation of PG synthesis induced by OP‐1; whereas OP‐1 decreased cell division engendered by RA. Thus, this GP chondrocyte culture system is a good model for studying factors that influence differentiation and mineral deposition during bone growth in vivo. J. Cell. Biochem. 67:498–513, 1997. © 1997 Wiley‐Liss, Inc.