PROSTAGLANDIN E-2 STIMULATES INSULIN-LIKE-GROWTH-FACTOR BINDING PROTEIN-4 EXPRESSION AND SYNTHESIS IN CULTURED HUMAN ARTICULAR CHONDROCYTES- POSSIBLE MEDIATION BY CA-CALMODULIN REGULATED PROCESSES(+)

Insulin-like growth factor-1, IGF-1, is believed to be an important an abolic modulator of cartilage metabolism whose action is mediated by h igh affinity cell surface receptors and bioactivity and bioavailabilit y regulated, in part by IGF-1 binding proteins (IGFBPs). Prostaglandin E-2 (PGE(2)) stimulates collagen and proteoglycan synthesis in cartil age via an autocrine feedback loop involving IGF-1. We determined whet her the eicosanoid could regulate IGFBP-4, a major form expressed by c hondrocytes and, as such, act as a modifier of IGF-1 action at another level. Using human articular chondrocytes in high-density primary cul ture, Western and Western ligand blotting to measure secreted IGFBP-4 protein, and Northern analysis to monitor IGFBP-4 mRNA levels, we demo nstrated that PGE(2) provoked a 2.7 +/- 0.3- and 3.8 +/- 0.5- (n = 3) fold increase in IGFBP-4 mRNA and protein, respectively. This effect w as reversed by the Ca++ channel blocker, verapamil, and the Ca++/calmo dulin inhibitor, W-7. The Ca++ ionophore, ionomycin, mimicked the effe cts of PGE(2). The phorbol ester, PMA, which activated phospholipid-de pendent protein kinase C (PKC) in chondrocytes, had no effect on IGFBP -4 production. Cyclic AMP mimetics and PKA activators, IBMX, and Sp-cA MP, inhibited the expression of the binding protein as did the PGE(2) secretagogue, interleukin-1 beta (IL-beta). The inhibitory effect of t he latter cytokine was mediated by a erbstatin/genistein (tyrosine) se nsitive kinase. Dexamethasone, an inhibitor of cyclooxygenase (COX-2) expression and PGE(2) synthesis, down-regulated control, constitutive levels of IGFBP-4 mRNA and protein, eliminating the previously demonst rated possibility of cross-talk between glucocorticoid receptor (GR) a nd PGE(2)-receptor signalling pathways. The results suggest that extra cellular signals control IGFBP-4 production by a number of different t ransducing networks with changes in Ca++ and calmodulin activity exert ing a strong positive influence, possibly maintaining the constitutivi ty of IGFBP-4 synthesis under basal conditions. PGE(2) activation of t he IGF-1/IGFBP axis may play a pivotal role in the metabolism of carti lage and possibly connective tissues in general. Eicosanoid biosynthes is may be a rate-limiting step in cartilage repair processes. (C) 1997 Wiley-Liss, Inc.