In this cross-sectional study, we examined serum adiponectin, an anti-inflammatory cytokine involved in the regulation of lipid and glucose metabolism in human diseases. To our knowledge, this is the first study to use the recently developed high-resolution gel filtration assay [19
] to analyze the adiponectin distribution across all three complexes, including a trimer, in a cohort of children with CKD. This method avoids the limitations of other assays where interactions between HMW and albumin-bound trimer can cause cross-reactivity when measuring the HMW complex [25
]. In our subcohort, HMW was the most common complex accounting for approximately one half of the total adiponectin. These results are similar to the only published adiponectin subfraction study of healthy Japanese children aged 9–10 years [26
]. The results also confirmed adult studies [27
] and our previous findings [28
] that the serum level of total adiponectin was increased in children with mild to moderate CKD compared with previously published normal values in children [29
], and it was inversely correlated with kidney function. Adiponectin can also be found in the urine at levels inversely related to GFR [30
]. The fact that adiponectin levels decrease significantly after renal transplantation [31
] suggests that decreased clearance leads to higher levels with progressive CKD. This increase was accompanied by a relative increase in HMW and decrease in LMW complexes in the circulation; the trimeric fraction remained unchanged. The mechanism of this shift in the adiponectin fraction balance might reflect a relatively low clearance of larger HMW oligomers compared with LMW clearance. A recent animal study using fluorescent-labeled recombinant adiponectin indicated that adiponectin is cleared primarily by the liver [33
], with the kidney being the second most common site of clearance. What role the liver versus the kidney plays in the clearance and imbalance of adiponectin complexes in patients with decreased kidney function is unknown. Another possibility arises from the theory of a competitive nature between HMW and LMW fractions. Bouskila et al. [34
] speculated that “the bivalent LMW form of adiponectin could act as an antagonist to adiponectin activity, preventing HMW-mediated receptor clustering on cell membrane.” Thus, with relatively high clearance of LMW complexes, it can be argued that in CKD, the lower concentration of LMW is not able to fully suppress HMW adiponectin. Shen et al. [35
] confirmed that the proportion of HMW was increased in the dialysis group; this was accompanied by up-regulation of the adiponectin/receptor system, possibly as a counterregulatory response to uremic inflammation.
In healthy children, the onset of puberty is associated with the development of sex differences in total adiponectin. Specifically, total adiponectin levels significantly decline in healthy boys in parallel with pubertal development, subsequently leading to reduced adiponectin levels in adolescent boys compared with adolescent girls [36
]. Bush et al. [37
] also noted that adiponectin levels are lower among African American versus Caucasian children. Similar findings were reported by Lee et al. [38
]. Conversely, in this study of children with CKD, no sex- or race-related differences in total, HMW, LMW, or trimer adiponectin levels were observed. As in the general pediatric population, prepubertal and lean children had higher total adiponectin mainly due to a larger percent of HMW complexes than in pubertal and obese children.
How the imbalance between HMWand LMW complexes is associated with CV risk factors in CKD patients remains unclear. Published adult studies evaluating the relationship between total adiponectin, insulin resistance, and inflammation in CKD showed inconsistent and sometimes contradictory results. Guebre-Egziabher et al. [14
] found no relationship between adiponectin and insulin or C-reactive protein (CRP) in CKD stages 2–5. Similarly, Becker et al. [27
] showed no significant relationship between adiponectin and CRP in nondiabetic adults with predialysis CKD. However, in contrast to a previous study, significant inverse associations of adiponectin with insulin were shown. Zoccali et al. [13
] found no association between serum adiponectin and CRP, whereas Stenvinkel et al. [39
] demonstrated significant negative associations between these two biomarkers in dialysis patients. Both of these studies showed significant positive associations between total adiponectin and HDL cholesterol and negative associations between adiponectin and triglycerides. A more recent study of predialysis CKD confirmed a positive correlation with HDL cholesterol and a negative correlation with triglycerides [40
]. In our study, only HMW was significantly associated (in a univariate analysis) with HOMA-IR and HDL cholesterol, confirming previous studies of the importance of the HMW isomer in insulin resistance and dyslipidemia. No association of any of the adiponectin complexes with markers of inflammation was evident in our patients, most likely due to a low degree of inflammation found in the CKiD cohort (data not shown). Interestingly, HMW was independently associated with protein/creatinine ratio in our multivariable analysis, similar to diabetes type 1 patients [41
]. This brings up a question of the role of HMW as a marker of kidney dysfunction.
Unexpectedly, HMW adiponectin and HMW/LMW ratio were positively correlated with systolic BP, whereas LMW adiponectin was inversely correlated with systolic BP and LVM index. These associations were evident even after adjusting for age, sex, and height—factors known to affect the adiponectin level, BP, and LVM index. This contradiction adds to a number of previous publications showing conflicting results when analyzing adiponectin concentrations in relation to CV outcomes in CKD patients. A recent review from Sweden [42
] summarized studies evaluating the role of adiponectin in CV outcomes in CKD and other patient groups. Among ten studies, five (three predialysis CKD and two hemodialysis) showed that low adiponectin levels predict worse clinical outcomes. Five more recent and better-powered studies (three coronary artery disease and congestive heart failure, and two CKD 3–4 and hemodialysis) showed that high levels were associated with worse overall and CV mortality. The authors speculated that a higher adiponectin level may induce protein energy wasting (PEW), a condition associated with malnutrition and inflammation. In this case, the association of higher adiponectin levels with poor outcomes fits well with the theory of reverse epidemiology in advanced CKD. Unfortunately, we could not test this hypothesis, as there were only three participants who met the criteria for PEW syndrome in our subcohort. A more plausible explanation of the positive relationship of HMW adiponectin with higher BP or LVM in CKD children is up-regulation of adiponectin receptors as a compensatory attempt to attenuate endothelial and vascular damage or cardiac hypertrophy. Finally, our findings can now be explained by the results of a recent study [43
] in which the authors, using adiponectin transgenic mice, demonstrated that adiponectin potentially enhances catabolism of the sphingolipid ceramide and increases formation of its metabolite, sphingosine-1-phosphate (S1P). The latter is a growth factor known to be associated with cardiac and vascular hypertrophy.
Our study has important strengths, including the precise measurement of GFR by iohexol clearance; standardized demographic, clinical, laboratory, and echocardiographic measures through the CKiD framework; and the use of a unique assay to measure all three adiponectin complexes. This study is restricted only to children with CKD, a population with much less cumulative CV risk and morbidity compared with adults with CKD. Therefore, this evaluation of the associations of adiponectin complexes avoided multiple confounding variables present in adults. On the other hand, the results cannot be generalized to adults. A control group was also unavailable due to the nature of the parent CKiD study; however, a wide range of GFRs were represented by patients in our study, including those with minimal kidney dysfunction for comparison. This report is also limited by its cross-sectional nature, thus preventing the evaluation of causation between adiponectin fractions and CV risk. This shortcoming will be overcome as the participants in the CKiD study continue longitudinal follow-up with repeated measurements of adiponectin complexes. Finally, the results of this study need to be confirmed in a larger study, as our subcohort comprised only part of the CKiD cohort.
In conclusion, this preliminary cross-sectional analysis demonstrated that the increase in total adiponectin levels with progressive CKD in children is associated with increasing HMW and decreasing LMW complexes. This imbalance may be an important biomarker for increased CV risk despite higher levels of total adiponectin in children with CKD.