BK Induces cPLA2 Expression via an Autocrine Loop Involving COX-2-Derived PGE2 in Rat Brain Astrocytes

Bradykinin (BK) is a proinflammatory mediator and elevated in several brain injury and inflammatory diseases. The deleterious effects of BK on brain astrocytes may aggravate brain inflammation mediated through the upregulation of cytosolic phospholipase A(2) (cPLA(2))/cyclooxygenase-2 (COX-2)-derived prostaglandin E-2 (PGE(2)) production. However, the signaling mechanisms underlying BK-induced cPLA(2) expression in brain astrocytes remain unclear. Herein, we investigated the effects of activation of cPLA(2)/COX-2 system on BK-induced cPLA(2) upregulation in rat brain astrocytes (RBA-1). The data obtained with Western blotting, RT-PCR, and immunofluorescent staining analyses showed that BK-induced de novo cPLA(2) expression was mediated through activation of cPLA(2)/COX-2 system. Upregulation of native cPLA(2)/COX-2 system by BK through activation of PKC delta, c-Src, MAPKs (ERK1/2 and JNK1/2) cascades led to PGE(2) biosynthesis and release. Subsequently, the released PGE(2) induced cPLA(2) expression via the same signaling pathways (PKC delta, c-Src, ERK1/2, and JNK1/2) and then activated the cyclic AMP response element-binding protein (CREB) via B2 BK receptor-mediated cPLA(2)/COX-2 system-derived PGE(2)/EP-dependent manner. Finally, upregulation of cPLA(2) by BK may promote more PGE(2) production. These results demonstrated that in RBA-1, activation of CREB by PGE(2)/EP-mediated PKC delta/c-Src/MAPK cascades is essential for BK-induced de novo cPLA(2) protein. More importantly, upregulation of cPLA(2) by BK through native cPLA(2)/COX-2 system may be a positive feedback mechanism that enhances prolonged brain inflammatory responses. Understanding the mechanisms of cPLA(2)/COX-2 system upregulated by BK on brain astrocytes may provide rational therapeutic interventions for brain injury and inflammatory diseases.