Urinary angiotensinogen as a potential biomarker of severity of chronic kidney diseases

Abstract

We previously reported that urinary excretion rates of angiotensinogen (AGT) provide a specific index of the activity of the intrarenal renin-angiotensin system in angiotensin II-dependent hypertensive rats. Meanwhile, we have recently developed direct enzyme-linked immunosorbent assays (ELISAs) to measure plasma and urinary AGT in humans. This study was performed to test a hypothesis that urinary AGT levels are enhanced in chronic kidney disease (CKD) patients and correlated with some clinical parameters. Eighty patients with CKD (37 women and 43 men, from 18 to 94 years old) and seven healthy volunteers (two women and five men, from 27 to 43 years old) were included. Plasma AGT levels showed a normal distribution; however, urinary AGT-creatinine ratios (UAGT/UCre) deviated from the normal distribution. When a logarithmic transformation was executed, Log(UAGT/UCre) levels showed a normal distribution. Therefore, Log(UAGT/UCre) levels were used for further analyses. Log(UAGT/UCre) levels were not correlated with age, gender, height, body weight, body mass index, systolic blood pressure, diastolic blood pressure, serum sodium levels, serum potassium levels, urinary sodium-creatinine ratios, plasma renin activity, or plasma AGT levels. However, Log(UAGT/UCre) levels were significantly correlated positively with urinary albumin-creatinine ratios, fractional excretion of sodium, urinary protein-creatinine ratios, and serum creatinine, and correlated negatively with estimated glomerular filtration rate. Log(UAGT/UCre) levels were significantly increased in CKD patients compared with control subjects (1.8801 ± 0.0885 vs. 0.9417 ± 0.1048; P = .0024). These data confirmed our earlier report and showed that a new ELISA assay is a valid approach for measuring urinary AGT.

Introduction

The renin-angiotensin system (RAS) is well known to play an important role in blood pressure (BP) regulation and fluid and electrolyte homeostasis.¹ In recent years, the focus of interest on the RAS has shifted to a main emphasis on the role of the local/tissue RAS in specific tissues.² Emerging evidence has demonstrated the importance of the tissue RAS in the brain,³ heart,⁴ adrenal glands,⁵ vasculature,6, 7 as well as the kidneys.¹ There is substantial evidence that the major fraction of angiotensin (Ang II) present in renal tissues is generated locally from angiotensinogen (AGT) delivered to the kidney as well as from AGT locally produced by proximal tubule cells.⁸ Renin secreted by the juxtaglomerular apparatus cells into the renal interstitium and vascular compartment also provides a pathway for the local generation of Ang I.⁹ Angiotensin-converting enzyme (ACE) is abundant in the kidney and is present in proximal tubules, distal tubules, and the collecting ducts.¹⁰ Ang I delivered to the kidney can also be converted to Ang II.¹¹ Therefore, all of the components necessary to generate intrarenal Ang II are present along the nephron.¹

Chronic kidney disease (CKD) is widely recognized as a major health problem all over the world. The renoprotective effects of ACE inhibitors and Ang II type 1 receptor blockers on CKD patients are established in diabetic nephropathy12, 13, 14, 15 as well as in nondiabetic nephropathy.16, 17 In order to account for these renoprotective effects, the activated intrarenal RAS was recently proposed to be involved in the progression of renal injury in CKD.¹⁸ The differential regulation of Ang II levels in plasma and kidney is now acknowledged.1, 18 Recently we reported that urinary excretion rates of AGT provide a specific index of intrarenal RAS status in Ang II-dependent hypertensive rats.19, 20, 21, 22, 23 We also recently reported that intrarenal AGT immunoreactivity is enhanced in IgA nephropathy patients.²⁴ Moreover, intrarenal AGT immunoreactivity is significantly correlated positively with urinary occult blood, urinary protein-creatinine ratio (UPro/UCre), and serum creatinine, and correlated negatively with creatinine clearance.²⁴ Recently, we provided evidence demonstrating that urinary AGT levels reflect intrarenal Ang II activity associated with increased risk for deterioration of renal function in CKD patients.²⁵ However, in that study, AGT levels were measured by conversion assay.²⁵ This conversion assay requires three steps. First, samples are incubated with and without exogenous renin. Then, Ang I concentrations in paired test tubes are measured by radioimmunoassay. Finally, converted Ang I is calculated as the difference between paired Ang I concentrations. Therefore, this conversion assay needs time-consuming procedures (∼2 days), indirect measurements, radioisotopes, and purified renin that is expensive and unobtainable. Meanwhile, we recently developed a direct quantitative method to measure plasma and urinary AGT using human AGT enzyme-linked immunosorbent assays (ELISAs).²⁶ In contrast to the aforementioned conversion assay, the newly developed ELISA does not need time-consuming procedures (∼3 hours), indirect measurements, radioisotopes, or purified renin. These data prompted us to measure urinary AGT in CKD patients and investigate correlations with clinical parameters. Therefore, this study was performed to test a hypothesis that urinary AGT levels are enhanced in CKD patients and correlated with some clinical parameters.