Abstract 405: Novel Signaling Mechanisms by Which Intracellular Angiotensin II Induces Na+/HCO3- Cotransporter Expression In The Proximal Tubule of The Kidney

Previous studies have shown that endocrine and/or paracrine angiotensin II (ANG II) plays an important role in the regulation of sodium and bicarbonate reabsorption in the proximal tubule of the kidney. However, it is not known whether intracellular (or intracrine) ANG II also plays a role in these responses in the proximal tubule. The present study tested the hypothesis that overexpression of an intracellular cyan fluorescent fusion protein of ANG II (ECFP/ANG II) in the proximal tubule of the kidney induces the expression of the Na+/HCO3- cotransporter via MAPK- and NF-kB signaling pathways. To test the hypothesis, transport-competent mPCTs from wild-type and type 1a ANG II receptor-deficient mice (AT1a-KO) were transfected with ECFP/ANG II, and treated with the AT1 receptor blocker losartan, the MEK1/MEK2 inhibitor U0126, or the NF-κB inhibitor RO 106-9920. In wild-type mPCT cells, the expression of ECFP/ANG II more than doubled total and/or phosphorylated NHE3 antiporter and Na+/HCO3- cotransporter proteins (p<0.01). These response were accompanied by more than threefold increases in phospho-ERK 1/2, p65 subunit of NF-κB, and phospho-IKKα/β (Ser 176/180) proteins (p<0.01). Pretreatment of mPCT cells with losartan, U0126, or RO 106-9920 significantly blocked the effects of ECFP/ANG II (p<0.01). Furthermore, the effects of ECFP/ANG II were significantly attenuated in mPCT cells of AT1a-KO mice (p<0.01),. In wild-type C57BL/6J mice, adenovirus-mediated overexpression of ECFP/ANG II selectively in the proximal tubule of the kidney, driven by the sodium and glucose cotransporter 2 (sglt2) promoter, significantly increased blood pressure, total and/or phosphorylated NHE3 and Na+/HCO3- proteins, and proximal tubular lithium reabsorption (p<0.01). These responses to ECFP/ANG II as observed in C57BL/6J mice were also attenuated in AT1a-KO mice (p<0.01). Our results strongly suggest that intracellular ANG II may induce NHE3 and Na+/HCO3- expression, and increase proximal tubular sodium and bicarbonate reabsorption via AT1a receptor-mediated activation of MAP kinases ERK 1/2 and NF-κB signaling pathways.