High serum adiponectin as a biomarker of renal dysfunction: Results from the KNOW-CKD study

High serum adiponectin is noted in several conditions of chronic kidney disease (CKD) and is a predictor for end stage renal disease. However, the relationship between adiponectin level and renal disease progression is not well established. This study aimed to determine the relationship between serum adiponectin levels and CKD progression. This prospective longitudinal study included 2238 patients from the Korean Cohort Study for Outcomes in Patients with Chronic Kidney Disease. Patients were divided into quartiles according to their serum adiponectin level. Composite renal outcome was defined as one or more of the following: initiation of dialysis or transplantation, a two-fold increase in baseline serum creatinine levels, or a 50% decline in the estimated glomerular filtration rate (eGFR) during the follow-up period. A cox proportional hazard ratio model was applied to analyze the relationship between composite renal outcome and serum adiponectin levels. Serum adiponectin level was inversely associated with eGFR (p < 0.001) and positively correlated with urine albumin-creatinine ratio. The highest quartile of serum adiponectin was associated with an increased risk of adverse renal outomes (HR, 1.39; 95%CI, 1.05-1.84; p=0.021). On time-dependent receiver operating characteristic curve analysis, predictive ability of adiponectin for renal outcomes disappeared after adjusting for eGFR. Therefore, serum adiponectin may be a biomarker of renal dysfunction rather than a true risk factor in CKD progression.

In order to quantify the correlation between adiponectin and covariates, we performed multivariable analysis ( Table 2). In model 1, patients in the highest quartile had a significantly higher risk of renal outcome than those in the lowest quartile after adjustment for age and sex (HR, 3.13; 95% CI, 2.42 to 4.05; p < 0.001). In model 2, we added systolic blood pressure (BP), DM, smoking, BMI, LDL, serum albumin, UACR, and drug history of angiotensin converting enzyme inhibitors (ACEi) or angiotensin receptor blockers (ARB). The HR of the highest quartile was 2.29 (95% CI, 1.73 to 3.04; p < 0.001). In a fully adjusted model that added eGFR (model 3), the highest adiponectin quartile was associated with an increased risk of composite renal outcome (HR, 1.39; 95% CI, 1.05-1.84; p = 0.021). In model 3, the risk of composite renal outcome increased by 1% (HR, 1.01; 95% CI, 1.00-1.02; p = 0.029) for every 1 µg/ml increase in baseline adiponectin levels. We also performed univariable analysis for the covariates (Supplementary Table S1).    www.nature.com/scientificreports www.nature.com/scientificreports/ adiponectin for renal outcome (p = 0.435) if eGFR was included. These findings mean that serum adiponectin has a predictive accuracy for renal outcome considering multiple confounders except eGFR. But if considering eGFR, the power of adiponectin as predictor is lost.

Subgroup analysis.
We further analysed the association between the serum adiponectin level and composite renal outcomes in several subgroups (Fig. 4). There were no significant effect modification by CKD stage, age, or sex subgroups.

Discussion
In this study, we demonstrated that an increased serum adiponectin level was significantly associated with old age, female sex, hypertension, higher serum creatinine level, higher UACR, lower BMI, reduced eGFR, and lower serum albumin level. The highest serum adiponectin quartile was also associated with an increased risk of adverse renal outcomes on multivariable analysis in a pre-dialysis CKD cohort.
The mechanisms that underly the relationship between high serum adiponectin and kidney disease progression have not been fully understood. Adiponectin, a vasoactive adipokine, circulates as a trimer, hexamer, or higher-molecular mass form in blood 18,19 . Adiponectin functions by activating the adenosine monophosphate-activated protein kinase (AMPK) pathway 20 via the AdipoR receptors, including AdipoR1 and AdipoR2. The AMPK pathway is necessary for the maintenance of normal renal physiology 21 . AMPK normally inhibits protein synthesis and its suppression appears to facilitate high-glucose-induced protein synthesis. AMPK is also a potent oxidative stress inhibitor, controlling the generation of reactive oxygen species 22 . These data indicate that adiponectin could potentially contribute to the maintenance of normal renal function. Adiponectin has anti-inflammatory and insulin-sensitizing properties, as well as cardio-protective effects on endothelial cells 23 .
However, in this study, high adiponectin level was associated with an increased risk of adverse renal outcomes. In view of the clearance of serum adiponectin via glomerular filtration, higher serum adiponectin may be observed in CKD patients due to impaired urinary excretion 24 . Moreover, adiponectin levels decrease after kidney transplantation, implying that the kidneys play an important role in the biodegradation or elimination of this protein 25 .
In this study, higher adiponectin was associated with an increased risk of adverse renal outcomes in pre-dialysis CKD patients after adjusting for accepted risk factors including old age, diabetes, hypertension, smoking, and a sedentary lifestyle. However, after adjusting for the eGFR, the power of adiponectin as a predictor of renal outcomes, disappeared. We speculate that serum adiponectin is biomarker of renal dysfunction rather than a true risk factor, intimately involved in CKD progression. However, further investigation is needed to identify the critical position of adiponectin in the pathogenesis of CKD.
This study has several limitations. Lo et al. reported that the ratio between high-and low-molecular-weight adiponectin may be an important biomarker of high cardiovascular risk 26 . Since we measured total serum adiponectin, the individual ratios could not be determined. Moreover, we did not measure adiponectin levels on follow-up appointments. However, this prospective study has certain strengths including a large sample size, comprehensive CKD presentation, and long-term follow-up with regard to disease outcomes.
High serum adiponectin levels are associated with composite renal outcomes in CKD patients. These findings suggest that adiponectin is a biomarker for CKD progression. We, therefore, propose that serum adiponectin may be used as a biomarker of renal disease outcomes.

Study design and population. The KoreaN cohort Study for Outcomes in patients With Chronic Kidney
Disease (KNOW-CKD) study, launched in 2011, was a patient-based cohort study that enrolled ethnic Korean adults with CKD. Patients aged between 20 and 75 years with various causes of CKD were initially screened for the study. Data were collected by a well-trained study coordinator using a standardized case report form and protocol. The detailed design and methods of KNOW-CKD have been published previously 27 .
All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and national research committees of the participating institute (IRB approval number NCT01630486) and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. The study protocol was approved in 2011 by the institutional review board of each participating clinical centre including the Seoul National University Hospital, Seoul National University Bundang Hospital, Severance Hospital, Kangbuk Samsung Medical Center, Seoul St. Mary's Hospital, Gil Hospital, Eulji General Hospital, Chonnam National University Hospital, and Pusan Paik Hospital. All participating patients provided written informed consent.
A total of 2341 patients with CKD stages ranging from 1 to 5 (predialysis) between April 2011 and February 2016, who voluntarily provided informed consent, were recruited. Among them, 103, who did not satisfy the inclusion criteria or did not have data of isotope dilution mass spectrometry (IDMS)-calibrated creatinine were excluded, leaving 2238 participants. We additionally excluded subjects with missing data for adiponectin and renal outcome; 2087 patients were finally analysed. The mean length of follow-up is 3.56 years.
Data collection. Baseline socio-demographic data including smoking history, anthropometric measurement, medications, and comorbidities were obtained from the electronic data management system developed by the Seoul National University Medical Research Collaborating Center. Blood samples were obtained in serum separation tubes, centrifuged within 1 h for serum separation, and were sent to the central laboratory of the KNOW-CKD study (Lab Genomics, Seongnam, Republic of Korea). Laboratory tests including complete blood count and blood chemistries were performed at baseline and at 6 months for the first year, and then annually thereafter. Serum creatinine concentrations were measured using an isotope-dilution mass spectrometry-traceable method. Total serum adiponectin concentrations were measured using a commercially . Subgroup analysis for the association between adiponectin level and composite renal outcome according to CKD stage (A), age (B), or sex (C). Central markers mean hazard ratio (HR) and the error whiskers mean 95% confidence interval (CI) of composite renal outcome. Adjusted for systolic blood pressure, diabetes mellitus, serum albumin and urine albumin-creatinine ratio.