System A neutral amino acid transporter regulation by interleukin-1 beta in human osteoarthritic synovial cells: Evidence for involvement of prostaglandin E-2 as a second messenger

We studied the long-terms effects of interleukin-1 beta (IL-1 beta; 3 to 6 h) on alpha-(methylamino) isobutyric acid (MeAlB), a nonmetabolizable amino acid transported by system A. We found that IL-1 beta induced a large decr ease in MeAlB uptake by human osteoarthritic synovial cells and a concomita nt increase in prostaglandin E-2 (PGE(2)) synthesis. Therefore, we investig ated whether PGE(2) acts as a mediator for the long-term action of IL-1 bet a. We found that exogenous PGE(2) inhibited MeAlB uptake, and that AH6809, a PGE(2) receptor antagonist, inhibited IL-1 beta-mediated MeAlB uptake. To identify the enzymes involved in the IL-1 beta-mediated synthesis of PGE(2 ) that inhibits MeAlB uptake, we studied the expression of secreted (s) and cytosolic (c) phospholipase A(2) (PLA(2)). Because both were expressed, we selected a broad spectrum of inhibitors to determine which of the two PLA( 2)s was involved. We used AACOCF3, a cPLA(2) inhibitor, and dithiothreitol (DTT) and bromophenacyl bromide (BPB), which are sPLA(2) inhibitors. Our re sults suggest that the PLA(2) involved in the IL-1 beta-mediated synthesis of PGE(2) was sPLA(2). We also showed the expression of cyclooxygenase (COX )-2 and its partial involvement using a potent selective COX-2 inhibitor, L -745337. These findings provide insight into the mechanisms underlying the IL-1 beta-mediated regulation of transport system A. The Il-1 beta-induced inhibition of MeAlB uptake in human osteoarthritic synovial cells thus seem s to be essentially mediated by PGE(2) production via the activation of sPL A(2) and the partial activation of COX-2. (C) 2000 Wiley-Liss, Inc.