The Effect of PACAP38 on MyD88-Mediated Signal Transduction in Ischemia-/Hypoxia-Induced Acute Kidney Injury

Background/Aims: Toll-like receptor 4 (TLR4) and its adaptor protein MyD88 play an important role in ischemia/reperfusion (I/R) injury in the kidney, and pituitary adenylate cyclase-activating polypeptide (PACAP) could ameliorate renal I/R injury. Methods: Primary cultures of proximal tubule epithelial cells (PTEC) were prepared from wild-type and MyD88 –/– mice, and subjected to hypoxia in vitro. Acute kidney injury (AKI) was induced by I/R in vivo in wild-type mice only. Results: Hypoxia resulted in significant increases in cytokine production and apoptosis/necrosis in wild-type PTEC, but these responses were markedly blunted in MyD88 –/– PTEC. Treatment with PACAP38 before or after hypoxia further suppressed the hypoxia-induced cytokine responses and apoptosis in both MyD88 +/+ and MyD88 –/– PTEC cultures. PACAP38 significantly inhibited TLR4/MyD88/TRAF6 as well as TRIF and IRF3 expression in mouse kidney and PTEC, and inhibited the secretion and mRNA expression of cytokines in kidneys from mice after I/R, paralleling the cytokine responses in vitro. Moreover, treatment with PACAP38 protected mice from renal failure, histological damage, neutrophil influx and tubule cell apoptosis after I/R. Conclusion: Our data reveal that the TLR4-mediated cytokine responses to hypoxia are primarily dependent on MyD88 signaling and highlight the pivotal role of MyD88-dependent mechanisms in the coordination of the innate immune responses to ischemic/hypoxic acute renal tubular injury. The renoprotective effect of PACAP in AKI involves both MyD88-dependent and -independent pathways.