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Prostaglandin E2 up‐regulates insulin‐like growth factor binding protein‐3 expression and synthesis in human articular chondrocytes by a c‐AMP‐independent pathway: Role of calcium and protein kinase A and C

Insulin‐like growth factor‐1, IGF‐1, is believed to be an important anabolic modulator of cartilage metabolism and its bioactivity and bioavailability is regulated, in part, by IGF‐1 binding protein 3 (IGFBP‐3). Prostaglandin E2 (PGE2) stimulates IGF‐1 production by articular chondrocytes and we determined whether the eicosanoid could regulate IGFBP‐3 and, as such, act as a modifier of IGF‐1 action at a different level. Using human articular chondrocytes in high density primary culture, Western and Western ligand blotting to measure secreted IGFBP‐3 protein, and Northern analysis to monitor IGFBP‐3 mRNA levels, we demonstrated that PGE2 provoked a 3.9 ± 1.1 (n = 3) fold increase in IGFBP‐3 mRNA and protein. This effect was reversed by the Ca++ channel blockers, verapamil and nifedipine, and the Ca++/calmodulin inhibitor, W‐7. The Ca++ ionophore, ionomycin, mimicked the effects of PGE2 as did the phorbol ester PMA, which activates Ca++‐phospholipid‐dependent protein kinase C (PKC). Cyclic AMP mimetics, such as forskolin, IBMX, Ro‐20‐1724, and Sp‐cAMP, inhibited the expression and synthesis of the binding protein. PGE2 did not increase the levels of cAMP or protein kinase A (PKA) activity in chondrocytes. The PGE2 secretagogue, IL‐1β, down‐regulated control levels of IGFBP‐3 which could be completely abrogated by pre‐incubation with the tyrosine kinase inhibitor, erbstatin, and partially reversed (50 ± 8%) by KT‐5720, a PKA inhibitor. These observations suggested that PGE2 does not mediate the effect of its secretagogue and that IL‐1β signalling in chondrocytes may involve multiple kinases of diverse substrate specificities. Dexamethasone down‐regulated control, constitutive levels of IGFBP‐3 mRNA and protein eliminating the previously demonstrated possibility of cross‐talk between glucocorticoid receptor (GR) and PGE2 receptor signalling pathways. Taken together, our results suggest that PGE2 modulates IGFBP‐3 expression, protein synthesis, and secretion, and that such regulation may modify human chondrocyte responsiveness to IGF‐1 and influence cartilage metabolism. © 1996 Wiley‐Liss, Inc.