Distinct associations between plasma osteoprotegerin, homoarginine and asymmetric dimethylarginine in chronic kidney disease male patients with coronary artery disease

High plasma osteoprotegerin (OPG) and asymmetric dimethylarginine (ADMA) and low homoarginine (hArg) predict adverse renal and cardiovascular (CV) outcomes. In patients with chronic kidney disease and stable coronary artery disease, plasma OPG correlated with hArg (r = − 0.37, P = 0.03) and the hArg/ADMA molar ratio (r = − 0.46, P = 0.009), which was maintained upon adjustment for renal function. Elevated OPG levels and decreased hArg/ADMA ratios independently predicted 4-year composite CV and renal endpoints (CV death or progression to dialysis). Thus, high OPG and low hArg/ADMA ratio, albeit interrelated, appear to independently contribute to adverse clinical outcome.

Associations of CV mortality risk with high OPG (Lewis et al. 2015) or low hArg ) concentrations are considerably stronger in subjects with an estimated GFR (eGFR) below 60 mL/min/1.73 m 2 . We, therefore, hypothesized that the kidneys may play a major role both in the homeostasis and modulation of biological effects of hArg and OPG in CKD patients with CAD. The primary aim of the present study was to test for mutual associations between OPG, hArg and ADMA in plasma of patients with CKD and CAD. We also tested a potential prognostic value of OPG with regard to 1-year renal function decline and 4-year clinical outcome in patients with both CAD and predialysis CKD.

Patients
Forty men with CKD were recruited from non-smoking patients admitted to the Second Department of Cardiology of Jagiellonian University Medical College for elective coronary angiography for stable CAD (Table 1). All CKD subjects were free of heart failure, left ventricular systolic dysfunction (ejection fraction ≥ 50% by echocardiography), clinical instability or coexistent diseases except for wellcontrolled type 2 diabetes mellitus or hypertension. Patients with relevant abnormalities in routine blood assays or with prehospital evidence of unstable creatinine levels were excluded. All patients were on a standard medical therapy recommended by practice guidelines, including low-dose aspirin, statins and angiotensin-converting enzyme inhibitors for at least 3 months prior to the index hospitalization. CKD diagnosis was based on an eGFR value between 15 and 59 mL/min/1.73 m 2 by the CKD-EPI formula, corresponding to eGFR stages G3-G4 according to 2011 KDIGO classification of CKD (Levey et al. 2011). Follow-up data were collected during routine control visits in our outpatient clinic, including control creatinine assay performed 12 ± 1 months after discharge. Patients or their relatives were contacted by telephone for the occurrence of a 4-year composite adverse clinical outcome, i.e., combined progression to dialysis or death from a CV cause, which was then confirmed by the review of medical records.
The study was approved by the Bioethics Committee of the Jagiellonian University (ethical approval No. KBET/364/B/2012) in adherence to the Declaration of Helsinki. All participants provided written informed consent.

Statistical analysis
Data are presented as mean ± SD, median [interquartile range: 25th-75th percentile] or numbers and percentages. Normality was evaluated by Shapiro-Wilk's test. Patients' characteristics were compared according to the 1-year eGFR change (∆eGFR), dichotomized with the reference to the median value of − 0.9 mL/min/1.73 m 2 , or to a 4-year progression to dialysis or CV death. Intergroup differences in continuous data were estimated by two-tailed Student's t test (or Welch's t test in case of inhomogeneous variances assessed by Levene's test) with prior decadic logarithmic (log 10 ) transformation; proportions were compared by Chisquared test. Bivariate Pearson's correlation coefficients (r P ) were calculated. Multiple logistic regression was used to estimate mutual independence of prognosticators with regard to the prediction of the progression to dialysis or the occurrence of CV death during a 4-year follow-up. Two-tailed P values < 0.05 were considered statistically significant.

Results
The concentrations of the biomarkers measured in the plasma samples of the study's patients are summarized in Table 1. The relationships between OPG, hArg and ADMA are illustrated in Fig. 1.

Discussion
To the best of our knowledge, a relationship between hArg and OPG has not been reported so far. Thus, our small study supplements previous observations of a positive correlation of OPG and ADMA in CAD (Morisawa et al. 2015) and essential hypertension (Tsioufis et al. 2011).
The patients' plasma OPG concentrations were within reported ranges (Bjerre et al. 2014;Lewis et al. 2014), hArg levels were lower compared to healthy men or women (Kayacelebi et al. 2014a, b;Atzler et al. 2016), while ADMA was higher than in CAD patients (Thum et al. 2005). The hArg/ADMA molar ratio was lower in our study group versus healthy subjects (Tsikas and Kayacelebi 2014). Plasma nitric oxide (NO) metabolites nitrite and nitrate and the lipid peroxidation biomarker MDA were within references ranges reported previously Tsikas 2017). As ADMA and hArg are formed from Arg in distinctly different pathways involving PRMT1 and AGAT, respectively, our observations suggest diminished AGAT activity, elevated PRMT1 activity, unaltered NOS activity and lipid peroxidation in our patients.
Given that elevated OPG, higher ADMA and lower hArg are established risk factors for renal and CV morbidity and mortality, it may suggest their synergistic contribution to the risk in patients with CKD and coexistent CAD via different detrimental pathways in the kidney and the vasculature. That high OPG and low hArg/ADMA molar ratio independently predicted the composite adverse clinical outcome and their mutual relationship was maintained upon adjustment for eGFR suggests that these prognostic effects were not entirely due to the association of abnormal levels of the biomarkers with impaired renal function (Fliser et al. 2005;Ravani et al. 2005Ravani et al. , 2013März et al. 2010;Røysland et al. 2012;Drechsler et al. 2013;Lewis et al. 2014).
Thus, high OPG and low hArg/ADMA ratio, albeit interrelated, appear to independently contribute to adverse clinical outcome. Admittedly, the underlying mechanisms connecting OPG with hArg and ADMA are still unresolved owing to their multiple, as yet not fully elucidated biological actions. The intriguing findings of our small preliminary study warrant further validation in large cohorts. Fig. 1 Relationships between the plasma concentrations of OPG, hArg and ADMA in the study's patients at baseline. Pearson's correlation coefficients (r P ) between OPG and hArg (a), OPG and ADMA (b), and OPG and the hArg/ADMA molar ratio (c)