Insulin-like growth factor-1, IGF-1, is believed to be an important anabolic modulator of cartilage metabolism whose action is mediated by high affinity cell surface receptors and bioactivity and bioavailability regulated, in part, by IGF-1 binding proteins (IGFBPs). Prostaglandin E2 (PGE2) stimulates collagen and proteoglycan synthesis in cartilage via an autocrine feedback loop involving IGF-1. We determined whether the eicosanoid could regulate IGFBP-4, a major form expressed by chondrocytes and, as such, act as a modifier of IGF-1 action at another level. Using human articular chondrocytes in high-density primary culture, Western and Western ligand blotting to measure secreted IGFBP-4 protein, and Northern analysis to monitor IGFBP-4 mRNA levels, we demonstrated that PGE2 provoked a 2.7 +/- 0.3- and 3.8 +/- 0.5- (n = 3) fold increase in IGFBP-4 mRNA and protein, respectively. This effect was reversed by the Ca(++) channel blocker, verapamil, and the Ca(++)/calmodulin inhibitor, W-7. The Ca(++)ionophore, ionomycin, mimicked the effects of PGE2. The phorbol ester, PMA, which activated phospholipid-dependent protein kinase C (PKC) in chondrocytes, had no effect on IGFBP-4 production. Cyclic AMP mimetics and PKA activators, IBMX, and Sp-cAMP, inhibited the expression of the binding protein as did the PGE2 secretagogue, interleukin-1 beta (IL-beta). The inhibitory effect of the latter cytokine was mediated by a erbstatin/genistein (tyrosine) sensitive kinase. Dexamethasone, an inhibitor of cyclooxygenase (COX-2) expression and PGE2 synthesis, down-regulated control, constitutive levels of IGFBP-4 mRNA and protein, eliminating the previously demonstrated possibility of cross-talk between glucocorticoid receptor (GR) and PGE2-receptor signalling pathways. The results suggest that extracellular signals control IGFBP-4 production by a number of different transducing networks with changes in Ca(++) and calmodulin activity exerting a strong positive influence, possibly maintaining the constitutivity of IGFBP-4 synthesis under basal conditions. PGE2 activation of the IGF-1/IGFBP axis may play a pivotal role in the metabolism of cartilage and possibly connective tissues in general. Eicosanoid biosynthesis may be a rate-limiting step in cartilage repair processes.
