Journal of Hypertension:
The proximal tubular renin–angiotensin system during albuminuria
Nakano, Daisuke; Nishiyama, Akira
Department of Pharmacology, Kagawa University Medical School, Kagawa, Japan
Correspondence to Akira Nishiyama, MD, PhD, Department of Pharmacology, Kagawa University Medical School, 1750–1 Ikenobe, Miki-cho, Kita-gun, Kagawa 761–0793, Japan Tel: +81 87 891 2125; fax: +81 87 891 2126; e-mail: firstname.lastname@example.org
The kidneys possess a well controlled barrier that prevents the leakage of albumin from the plasma into urine. Thus, the first sign of renal injury is the appearance of albuminuria. It has been suggested that final concentration of urinary albumin is the difference between albumin leakage at the glomerular filtration barrier and proximal tubular albumin reabsorption. The uptake of filtered albumin in the proximal tubule is a major physiological role of proximal tubular cells (PTCs). Receptor-mediated endocytotic proteins, such as megalin and cubilin, are reported to be responsible for albumin uptake by the PTCs [1,2]. Albumin binds to both megalin and cubilin located on the apical side of PTC membrane and get internalized into the cytosol. Thereafter, albumin gets stored into lysosomes and degraded into small fragments and/or amino acids . A small portion of albumin may also undergo transcytosis from the apical to basolateral membrane [4–6]. It is still unclear whether an increase in glomerular filtration or a decrease in proximal tubule reuptake of albumin contributes to albuminuria [7,8]. However, it is now clear that glomerular filtration of albumin is significantly increased in pathological conditions and, as a result, PTCs may be exposed to greater concentrations of albumin. Previous studies have shown that an increase in albumin exposure may induce PTC stress, leading to PTC apoptosis [9,10] and/or inflammation [11,12], all of which further exacerbate renal injury.
In the current issue of the Journal of Hypertension, Cao et al.  demonstrated that increased concentrations of albumin on the apical side of PTCs augment renin–angiotensin system (RAS) expression via the megalin/cubilin/protein kinase C/NADPH oxidase-dependent pathway. These findings are in agreement with those of previous reports in which an increase in albuminuria was found to be associated with an augmentation of the intrarenal RAS and tubulointerstitial fibrosis [14–19]. Taken together, these findings may help in our understanding of the potential role of albumin-induced PTC stress in the progression of renal injury. One may be curious as to how much albumin is required to stimulate intraproximal tubular RAS in vivo. Cao et al.  examined the effect of 5 mg/ml of rat serum albumin on the expression of RAS components in PTCs. The concentration used was more than 10% of serum albumin concentration (∼35 mg/ml). This suggests that 10% of the total serum albumin needs to pass the glomerular filtration barrier in order to augment intraproximal tubular RAS. A glomerular sieving coefficient of 0.1 is much greater than that of a normal functioning kidney [20,21]. During an overt albuminuric phase, however, the glomerular sieving coefficient of albumin could be greater than 0.1 (based on our unpublished data analyzed with two-photon laser microscopy). Additionally, as discussed by Cao et al. , in-vitro short-term experiments may need a greater concentration of albumin to induce the phenomenon that occurs under in-vivo conditions. Nevertheless, it is possible that albumin leakage from the glomerular filtration barrier stimulates the intraproximal tubular RAS at an early stage of nephropathy. There is also the possibility that increases in albumin concentrations (0.1–10 mg/ml) on the apical side may decrease the albumin binding site on proximal tubules and could limit the detection of the actual amount of albumin uptake . However, Cao et al.  found that there was an albumin dose-dependent increase in the RAS expression in PTCs. These data suggest that proximal tubules reuptake albumin and upregulate the RAS even under the nephritic level of albumin.
Another important question is whether the intraproximal tubular RAS is influenced by albumin under pathological conditions. Albumin can be modified via several mechanisms, such as oxidation , glycation  and binding to advanced glycation end products (AGEs)  and free fatty acids [26,27]. Thomas et al.  demonstrated that an infusion of AGE-modified rat serum albumin upregulates various components of the intrarenal RAS, including angiotensinogen, renin, angiotensin-converting enzyme (ACE) and AT1 receptor. Furthermore, an AGE inhibitor, pyridoxamine, prevents these changes, suggesting that the pathological modification of albumin may further augment the albumin-induced increase in intraproximal tubular RAS activity.
In-vitro studies have demonstrated that AT1 receptor stimulation suppresses the cellular uptake of albumin, and the subsequent degradation process . These data suggest that the augmented proximal tubular RAS during albuminuria worsens albumin uptake in the proximal tubule, leading to a further increase in albuminuria. The potential role of proximal tubular RAS in the control of blood pressure has also been indicated. Kobori et al.  found that proximal tubule-specific overexpression of human angiotensinogen with a systemic overexpression of human renin elevates blood pressure with significant increases in kidney angiotensin II levels in mice. Furthermore, renal tubule-specific overexpression of ACE elevates blood pressure and kidney angiotensin II levels, even in systemic ACE knockout mice . It has also been shown that mice overexpressing proximal tubule-specific angiotensinogen develop hypertension, tubular apoptosis, tubulointerstitial fibrosis and albuminuria, all of which are diminished by catalase overexpression . Cao et al.  also demonstrated that 5.0 g/kg per day of albumin induces intraproximal tubular RAS activation and elevates blood pressure in rats, all of which were suppressed by treatment with the antioxidant apocynin. These data support the hypothesis, which was based on the findings of clinical studies , that augmentation of intraproximal tubular RAS impairs renal function and induces hypertension through a reactive oxygen species (ROS)-dependent pathway. The precise mechanism, however, needs to be clarified via additional studies in proximal tubule-specific RAS knockout animals. However, to date, there is still no in-vivo ‘loss-of-function’ evidence demonstrating an association between augmentation of the intraproximal tubular RAS, albuminuria, ROS, renal injury and blood pressure.
It is now generally accepted that the intrarenal RAS plays an important role in the pathogenesis of renal injury through the activation of AT1 receptors. Therefore, assessment of intrarenal RAS is essential in understanding the mechanisms that mediate the pathophysiology of renal function and injury. In the current issue, Cao et al.  demonstrated that albumin exposure to PTCs could trigger the activation of intraproximal tubular RAS. However, as previously mentioned, significant amounts of leaked albumin from glomeruli could be retrieved via the proximal tubules. Therefore, the urinary excretion rate of albumin may not reflect the actual amount of albumin exposure that PTCs undergo. Conversely, urinary angiotensinogen, which is mainly synthesized via a de-novo intraproximal tubule-dependent RAS pathway , could be a potential biomarker for monitoring intrarenal RAS status [35,36]. Thus, it can be speculated that an increase in urinary angiotensinogen may also reflect proximal tubular albumin exposure and may be a predictive marker for chronic kidney disease and hypertension risk.
1 Leheste JR, Melsen F, Wellner M, Jansen P, Schlichting U, Renner-Muller I, et al
. Hypocalcemia and osteopathy in mice with kidney-specific megalin gene defect. FASEB J 2003; 17:247–249.
2 Amsellem S, Gburek J, Hamard G, Nielsen R, Willnow TE, Devuyst O, et al
. Cubilin is essential for albumin reabsorption in the renal proximal tubule. J Am Soc Nephrol 2010; 21:1859–1867.
3 Birn H, Christensen EI. Renal albumin absorption in physiology and pathology. Kidney Int 2006; 69:440–449.
4 van den Eijnden MM, de Bruin RJ, de Wit E, Sluiter W, Deinum J, Reudelhuber TL, Danser AH. Transendothelial transport of renin-angiotensin system components. J Hypertens 2002; 20:2029–2037.
5 Russo LM, Sandoval RM, McKee M, Osicka TM, Collins AB, Brown D, et al
. The normal kidney filters nephrotic levels of albumin retrieved by proximal tubule cells: retrieval is disrupted in nephrotic states. Kidney Int 2007; 71:504–513.
6 Pohl M, Kaminski H, Castrop H, Bader M, Himmerkus N, Bleich M, et al
. Intrarenal renin angiotensin system revisited: role of megalin-dependent endocytosis along the proximal nephron. J Biol Chem 2010; 285:41935–41946.
7 Christensen EI, Birn H, Rippe B, Maunsbach AB. Controversies in nephrology: renal albumin handling, facts, and artifacts! Kidney Int 2007; 72:1192–1194.
8 Peti-Peterdi J. Independent two-photon measurements of albumin GSC give low values. Am J Physiol Renal Physiol 2009; 296:F1255–F1257.
9 Thomas ME, Brunskill NJ, Harris KP, Bailey E, Pringle JH, Furness PN, Walls J. Proteinuria induces tubular cell turnover: a potential mechanism for tubular atrophy. Kidney Int 1999; 55:890–898.
10 Erkan E, De Leon M, Devarajan P. Albumin overload induces apoptosis in LLC-PK(1) cells. Am J Physiol Renal Physiol 2001; 280:F1107–F1114.
11 Tang S, Leung JC, Abe K, Chan KW, Chan LY, Chan TM, Lai KN. Albumin stimulates interleukin-8 expression in proximal tubular epithelial cells in vitro and in vivo. J Clin Invest 2003; 111:515–527.
12 Donadelli R, Zanchi C, Morigi M, Buelli S, Batani C, Tomasoni S, et al
. Protein overload induces fractalkine upregulation in proximal tubular cells through nuclear factor kappaB- and p38 mitogen-activated protein kinase-dependent pathways. J Am Soc Nephrol 2003; 14:2436–2446.
13 Cao W, Zhou QG, Nie J, Wang GB, Liu YH, Zhou ZM, Hou FF. Albumin-overload activates intrarenal renin-angiotensin system through PKC and NADPH oxidase dependent pathway. J Hypertens
14 Takase O, Marumo T, Imai N, Hirahashi J, Takayanagi A, Hishikawa K, et al
. NF-kappaB-dependent increase in intrarenal angiotensin II induced by proteinuria. Kidney Int 2005; 68:464–473.
15 Ohashi N, Katsurada A, Miyata K, Satou R, Saito T, Urushihara M, Kobori H. Activation of reactive oxygen species and the renin-angiotensin system in IgA nephropathy model mice. Clin Exp Pharmacol Physiol 2009; 36:509–515.
16 Ohashi N, Katsurada A, Miyata K, Satou R, Saito T, Urushihara M, Kobori H. Role of activated intrarenal reactive oxygen species and renin-angiotensin system in IgA nephropathy model mice. Clin Exp Pharmacol Physiol 2009; 36:750–755.
17 Fan YY, Kohno M, Nakano D, Ohsaki H, Kobori H, Suwarni D, et al
. Cilnidipine suppresses podocyte injury and proteinuria in metabolic syndrome rats: possible involvement of N-type calcium channel in podocyte. J Hypertens 2010; 28:1034–1043.
18 Gonzalez-Villalobos RA, Satou R, Ohashi N, Semprun-Prieto LC, Katsurada A, Kim C, et al
. Intrarenal mouse renin-angiotensin system during ANG II-induced hypertension and ACE inhibition. Am J Physiol Renal Physiol 2010; 298:F150–F157.
19 Fan YY, Kohno M, Hitomi H, Kitada K, Fujisawa Y, Yatabe J, et al
. Aldosterone/mineralocorticoid receptor stimulation induces cellular senescence in the kidney. Endocrinology 2011; 152:680–688.
20 Tojo A, Endou H. Intrarenal handling of proteins in rats using fractional micropuncture technique. Am J Physiol 1992; 263:F601–F606.
21 Tanner GA. Glomerular sieving coefficient of serum albumin in the rat: a two-photon microscopy study. Am J Physiol Renal Physiol 2009; 296:F1258–F1265.
22 Gekle M, Mildenberger S, Freudinger R, Silbernagl S. Long-term protein exposure reduces albumin binding and uptake in proximal tubule-derived opossum kidney cells. J Am Soc Nephrol 1998; 9:960–968.
23 Anraku M, Kitamura K, Shinohara A, Adachi M, Suenga A, Maruyama T, et al
. Intravenous iron administration induces oxidation of serum albumin in hemodialysis patients. Kidney Int 2004; 66:841–848.
24 Bendayan M, Londono I. Reabsorption of native and glycated albumin by renal proximal tubular epithelial cells. Am J Physiol 1996; 271:F261–F268.
25 Morcos M, Sayed AA, Bierhaus A, Yard B, Waldherr R, Merz W, et al
. Activation of tubular epithelial cells in diabetic nephropathy. Diabetes 2002; 51:3532–3544.
26 Kamijo A, Kimura K, Sugaya T, Yamanouchi M, Hase H, Kaneko T, et al
. Urinary free fatty acids bound to albumin aggravate tubulointerstitial damage. Kidney Int 2002; 62:1628–1637.
27 Souma T, Abe M, Moriguchi T, Takai J, Yanagisawa-Miyazawa N, Shibata E, et al
. Luminal alkalinization attenuates proteinuria-induced oxidative damage in proximal tubular cells. J Am Soc Nephrol 2011; 22:635–648.
28 Thomas MC, Tikellis C, Burns WM, Bialkowski K, Cao Z, Coughlan MT, et al
. Interactions between renin angiotensin system and advanced glycation in the kidney. J Am Soc Nephrol 2005; 16:2976–2984.
29 Hosojima M, Sato H, Yamamoto K, Kaseda R, Soma T, Kobayashi A, et al
. Regulation of megalin expression in cultured proximal tubule cells by angiotensin II type 1A receptor- and insulin-mediated signaling cross talk. Endocrinology 2009; 150:871–878.
30 Kobori H, Ozawa Y, Satou R, Katsurada A, Miyata K, Ohashi N, et al
. Kidney-specific enhancement of ANG II stimulates endogenous intrarenal angiotensinogen in gene-targeted mice. Am J Physiol Renal Physiol 2007; 293:F938–F945.
31 Gonzalez-Villalobos RA, Billet S, Kim C, Satou R, Fuchs S, Bernstein KE, Navar LG. Intrarenal angiotensin-converting enzyme induces hypertension in response to angiotensin I infusion. J Am Soc Nephrol 2011; 22:449–459.
32 Godin N, Liu F, Lau GJ, Brezniceanu ML, Chenier I, Filep JG, et al
. Catalase overexpression prevents hypertension and tubular apoptosis in angiotensinogen transgenic mice. Kidney Int 2010; 77:1086–1097.
33 Ogawa S, Kobori H, Ohashi N, Urushihara M, Nishiyama A, Mori T, et al
. Angiotensin II type 1 receptor blockers reduce urinary angiotensinogen excretion and the levels of urinary markers of oxidative stress and inflammation in patients with type 2 diabetic nephropathy. Biomark Insights 2009; 4:97–102.
34 Kobori H, Nangaku M, Navar LG, Nishiyama A. The intrarenal renin-angiotensin system: from physiology to the pathobiology of hypertension and kidney disease. Pharmacol Rev 2007; 59:251–287.
35 Kobori H, Urushihara M, Xu JH, Berenson GS, Navar LG. Urinary angiotensinogen is correlated with blood pressure in men (Bogalusa Heart Study). J Hypertens 2010; 28:1422–1428.
36 Nishiyama A, Konishi Y, Ohashi N, Morikawa T, Urushihara M, Maeda I, et al
. Urinary angiotensinogen reflects the activity of intrarenal renin-angiotensin system in patients with IgA nephropathy. Nephrol Dial Transplant
© 2011 Lippincott Williams & Wilkins, Inc.