Stimulation of sulfated‐proteoglycan synthesis by forskolin in monolayer cultures of rabbit articular chondrocytes

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Abstract
Forskolin, a plant cardiotonic diterpene, stimulated proteoglycan biosynthesis by chondrocytes in monolayer culture. The quantitative increase in proteoglycans was dependent on the concentration of forskolin, but was relatively independent of the presence of serum. At forskolin concentrations that stimulated proteoglycan synthesis, a significant stimulation of adenylate cyclase and cAMP was also measured. The quantitative increase in proteoglycans was characterized, qualitatively, by an increased deposition of newly synthesized proteoglycan in the cell-associated fraction. An analysis of the most dense proteoglycans (fraction dA1) in the cell-associated fraction showed that more of the proteoglycans eluted in the void volume of a Sepharose CL-2B column, indicating that an increased amount of proteoglycan aggregate was synthesized in forskolin-treated cultures. The proteoglycan monomer dA1D1 secreted into the culture medium of forskolin-stimulated cultures overlapped in hydrodynamic size with that of control cultures, although cultures stimulated with forskolin and phosphodiesterase inhibitors produced even larger proteoglycans. The hydrodynamic size of 35SO4 and 3H-glucosamine-labelled glycosaminoglycans isolated from the dA1D1 fraction of the culture medium was greater in forskolin-treated chondrocytes, especially from those in which phosphodiesterase inhibitors had been added. These results indicated that forskolin, a direct activator of chondrocyte adenylate cyclase mimicked the effects of cAMP analogues on chondrocyte proteoglycan synthesis previously reported. These results implicate activation of adenylate cyclase as a regulatory event in the biosynthesis of cartilage proteoglycans, and more specifically in the production of hydrodynamically larger glycosaminoglycans.