High glucose down-regulates angiotensin II binding via the PKC-MAPK-cPLA₂ signal cascade in renal proximal tubule cells

Background. It has been reported that renal renin-angiotensin system contributes to the development of diabetic nephropathy. However, the mechanism of angiotensin II receptor regulation in diabetic condition has not been elucidated. Methods. The effects of high glucose on [³H]-arachidonic acid (AA) release and angiotensin II (Ang II) binding and its related signal pathway were examined in primary cultured rabbit renal proximal tubule cells (PTCs). Results. High glucose down-regulated ¹²⁵I-Ang II binding from 12 hours and this response was sustained over 48 hours. Thus, the treatment of 25 mmol/L glucose for 48 hours was used for this study. High glucose-induced down-regulation of ¹²⁵I-Ang II binding was reversed by the removal of extracellular glucose, suggesting a role for glucose specificity. The high glucose-induced down-regulation of ¹²⁵I-Ang II binding was blocked by mepacrine, AACOCF₃, phospholipase A₂ inhibitors, indomethacin, ibuprofen, and cyclooxygenase inhibitors. Indeed, high glucose significantly increased prostaglandin E₂ synthesis. In addition, the high glucose-induced AA release was blocked by PD 98059, a p44/42 mitogen-activated protein kinase (MAPK) inhibitor. PD 98059 also prevented the down-regulation of ¹²⁵I-Ang II binding by high glucose, suggesting a role for p44/42 MAPK. Indeed, high glucose significantly increased p44/42 MAPK activity after the 15-minute time point. Protein kinase C (PKC) inhibitor blocked high glucose-induced activation of p44/42 MAPK, increase of the [³H]-AA release, and down-regulation of ¹²⁵I-Ang II binding. W-7 and KN-62 also blocked the high glucose-induced increase of [³H]-AA release and down-regulation of ¹²⁵I-Ang II binding. However, phospholipase A₂ inhibitor did not block high glucose-induced activation of p44/42 MAPK.