Augmentation of angiotensinogen expression in the proximal tubule by intracellular angiotensin II via AT_1a/MAPK/NF-кB signaling pathways

Abstract

Long-term angiotensin II (ANG II) infusion significantly increases ANG II levels in the kidney through two major mechanisms: AT1 receptor-mediated augmentation of angiotensinogen (AGT) expression and uptake of circulating ANG II by the proximal tubules. However, it is not known whether intracellular ANG II stimulates AGT expression in the proximal tubule. In the present study, we overexpressed an intracellular cyan fluorescent ANG II fusion protein (Ad-sglt2-ECFP/ANG II) selectively in the proximal tubule of rats and mice using the sodium and glucose cotransporter 2 (sglt2) promoter. AGT mRNA and protein expression in the renal cortex and 24-h urinary AGT excretion were determined 4 wk following overexpression of ECFP/ANG II in the proximal tubule. Systolic blood pressure was significantly increased with a small antinatriuretic effect in rats and mice with proximal tubule-selective expression of ECFP/ANG II (P Ͻ 0.01). AGT mRNA and protein expression in the cortex were increased by Ͼ1.5-fold and 61 Ϯ 16% (P Ͻ 0.05), whereas urinary AGT excretion was increased from 48.7 Ϯ 5.7 (n ϭ 13) to 102 Ϯ 13.5 (n ϭ 13) ng/24 h (P Ͻ 0.05). However, plasma AGT, renin activity, and ANG II levels remained unaltered by ECFP/ANG II. The increased AGT mRNA and protein expressions in the cortex by ECFP/ANG II were blocked in AT1a-knockout (KO) mice. Studies in cultured mouse proximal tubule cells demonstrated involvement of AT1a receptor/MAP kinases/NF-B signaling pathways. These results indicate that intracellular ANG II stimulates AGT expression in the proximal tubules, leading to increased AGT formation and secretion into the tubular fluid, which contributes to ANG II-dependent hypertension.angiotensin II; angiotensinogen; kidney; NF-B; proximal tubule THE IMPORTANT ROLE OF THE renin-angiotensin system (RAS), especially its major effector peptide, angiotensin II (ANG II), in the development and maintenance of hypertension has been well established in all ANG II-dependent hypertension models, including two-kidney, one-clip (2K1C) (5, 30, 46), Ren-2 gene transgenic (40,42,59), and ANG II-infused animals (7,56,61,63). One important and consistent feature of these ANG II-dependent hypertension models is that ANG II levels in the kidneys are increased to a greater extent than can be explained on the basis of elevated plasma ANG II which suppresses renin expression in the juxtaglomerular apparatus (JGA) of the kidney (12,29,44,61). The mechanisms underlying this phenomenon remain incompletely understood. Currently, there are at least two potential mechanisms contributing to augmentation of ANG II levels in the kidney: AT 1 receptor-mediated angiotensinogen (AGT) expression and uptake of circulating ANG II by the proximal tubules. We and others have shown that circulating ANG II or systemically infused ANG II is taken up and accumulated in the proximal tubule of the kidney, including endosomal, mitochondrial, and nuclear compartments (1,15,29,47,61). The uptake of circulating ANG II by the proximal tubule of the kidney appears to be med...

Klotho Ameliorates Medullary Fibrosis and Pressure Natriuresis in Hypertensive Rat Kidneys

“…In addition to AT1R stimulation, the internalized Ang II induces AGT expression. 19 These in vitro data are consistent with the in vivo results that klotho protein supplementation attenuated renal AGT expression and elevations of blood pressure. The present findings also showed that exogenous klotho supplementation inhibited Akt phosphorylation.…”

“…In contrast, β-glucuronidase did not show substantial binding to AT1R (Figure S1). Multiple-way ANOVA for repeated measurements revealed that Ang II induced dose- and time-dependent elevations of IP3 in HK-2 cells 19 and that klotho protein suppressed Ang II-induced IP3 elevations ( P <0.005 for each). At 5 and 10 minutes, Ang II (1 and 3 nM) elicited smaller increments of IP3 in the presence of klotho protein (Figure 1B).…”

“…18 Activation of the renal RAS during Ang II-infused hypertension involves increased renal AGT (angiotensinogen) expression, which is triggered, at least partly, by AT1R (Ang II type-1 receptor) internalization. 19 In the present study, we examined the potential interaction between klotho and the AT1R and determined whether exogenous supplementation of rh-klotho (recombinant human klotho protein, 516 amino acids) ameliorated renal RAS, medullary fibrosis, and pressure natriuresis in early phase of hypertension. 20 …”

Recent Updates on the Proximal Tubule Renin-Angiotensin System in Angiotensin II-Dependent Hypertension

“…4) [21••, 23••, 159••]. The blood pressure-elevating effect of ECFP/ANG II was blocked by concurrent losartan treatment and in AT 1a receptor-knockout mice, suggesting that most of intracellular ANG II-induced effects are still mediated by AT 1 (AT 1a ) receptors [21••, 23••, 160••]. The signaling proteins activated by intracellular ANG II include MAP kinases, NF-κB, and NHE3 signaling [23••, 159••].…”

“…The signaling proteins activated by intracellular ANG II include MAP kinases, NF-κB, and NHE3 signaling [23••, 159••]. Moreover, overexpression of ECFP/ANG II in cultured mouse proximal tubule cells or selectively in the proximal tubules of C57BL/6J mice also induced AGT mRNA and protein expression, suggesting that internalized and/or intracellular ANG II may play an important role in the feed-forward mechanism of ANG II-dependent hypertension by augmenting AGT expression [160••]. …”