Effects of dexamethasone and transforming growth factor-β2 on group II phospholipase A2 mRNA and activity levels in interleukin 1β- and forskolin-stimulated mesangial cells

Abstract

The expression of 14 kDa group II phospholipase A2 [also referred to as secretory PLA2 (sPLA2)] is induced in rat glomerular mesangial cells by exposure to inflammatory cytokines and forskolin, a cAMP elevating agent. Previously we have shown that dexamethasone and transforming growth factor-β2 (TGF-β2) suppress sPLA2 protein synthesis and enzyme activity induced by cytokines and forskolin. The regulation of sPLA2 by pro-inflammatory cytokines suggests that the enzyme may play a role in glomerular inflammatory reactions. In order to understand the regulation of sPLA2 in more detail, we investigated whether dexamethasone and TGF-β2 also suppress sPLA2 mRNA after its induction by either interleukin-1β (IL-1β) or forskolin. We found that IL-1β-induced sPLA2 mRNA in rat mesangial cells is not down-regulated by pretreatment of the cells with dexamethasone, even at a concentration of 10 μM, which dramatically decreases sPLA2 protein levels and activity. Metabolic labelling experiments indicated that the decreased sPLA2 levels under these conditions can be explained by inhibition of the rate of sPLA2 synthesis from the elevated mRNA levels. In contrast, the forskolin-induced elevation of sPLA2 mRNA is inhibited by dexamethasone in a concentration-dependent manner. Likewise, TGF-β2 inhibits the elevation of sPLA2 mRNAs induced by either IL-1β or forskolin. The decrease in sPLA2 mRNA caused by TGF-β2 corresponds with the decrease in sPLA2 enzyme levels and activity. These data suggest that cytokine- and forskolin-induced sPLA2 expression is tightly controlled via both transcriptional and post-transcriptional mechanisms. Furthermore, we show that pretreatment of mesangial cells with epidermal growth factor prior to stimulation with IL-1β or forskolin had no suppressing effect on sPLA2 levels or enzyme activity, as has been reported previously for osteoblasts.