Abstract
Background and Aims
Chronic kidney disease (CKD) is a major public health problem worldwide and refers to a wide range of disorders in bone and mineral metabolism, abnormalities of biochemical parameters and pathological calcification of the blood vessels. Vascular calcification (VC) is a common complication in CKD patients, contributes to cardiovascular disease (CVD), and associates with increased mortality and morbidity. The precise mechanism of VC in CKD is not yet fully understood. Recently discovered molecules such as osteoprotegerin (OPG), its ligand receptor activator of nuclear factor NF-κB ligand (RANKL) and RANK are not only well-known to play a crucial role in bone homeostasis, but they has also been implicated in the process of development of vascular complications However the exact role of OPG/RANKL/RANK axis in the process of VC has not been yet fully assessed. Thus, the aim of this work is to evaluate the role of OPG/RANKL/RANK axis in the process of calcification in CKD.
Method
Seventy two male Wistar rats weighing 260-290 g (8-weeks old) were initially divided into 6 groups containing 12 animals in each group. Rats were divided into six groups: control rats (K4, K6, K8) and CKD rats (B4, B6, B8). Control group rats received standard diet, whereas CKD rats were fed a low adenine – diet containing 0.3 % adenine, 1.0 % Ca, 1.2 % Pi through 4 (K4, B4), 6 (K6, B6) and 8 (K8, B8) weeks. Subsequently, CKD and control rats were sacrificed at weeks 4 (n=24), 6 (n=24) and 8 (n=24). One day before being killed, the rats were placed in metabolic cages for 24-hour urine collection. Thereafter, the rats were anesthetized and samples of blood, as well as aortas were collected. Next, the OPG, RANKL, parathyroid hormone (PTH), 25-hydroxyvitamin D (25(OH)D) and 1,25-dihydroxy vitamin D3 1,25(OH)2D3 concentrations were determined using appropriate ELISA kits. Then the sRANKL/OPG ratio was calculated. The OPG, RANK and RANKL gene expression was assessed using real-time PCR (RT-PCR). The VC was quantified by measurement of the arterial calcium (Ca) and phosphate (Pi) content using flame atomic absorption. Serum levels of urea nitrogen, creatinine, uric acid, Ca, Pi and urinary levels of creatinine, Ca and Pi were measured.
Results
There was a progressive increase in serum urea nitrogen, creatinine, uric acid and PTH of CKD rats in comparison to control values. We also observed significantly decreased levels of 25(OH)D, 1,25(OH)2D and serum Ca. Total Ca content in the aorta was significantly increased in CKD rats in comparison with control group, whereas total Pi content in the aorta was significantly increased only in B8 group in comparison to appropriate controls. There were no differences in serum OPG and sRANKL levels between CKD and control rats. In contrast, we observed decreased OPG, RANKL and RANK gene expression in a B4 group in comparison to appropriate controls, whereas in a B6 group we noticed increased OPG, RANKL and decreased RANK gene expression. B8 group revealed increased RANKL and RANK gene expression, but there were no differences in OPG gene expression between CKD rats and control group. Furthermore, we observed positive correlations between serum sRANKL and OPG and RANK gene expression. Ca and P content in the aorta inversely corelated with RANKL gene expression, whereas positively with OPG gene expression. Serum 25(OH)D concentrations correlated inversely with Ca in aorta. PTH was positively correlated with serum RANKL and OPG and gene expression these cytokines.
Conclusion
Our results suggest that OPG/RANK/RANKL axis may be involved in the process of vascular calcification in chronic kidney disease. However, its role and evaluation of precise mechanism in this field requires further evaluation.
High expression of SLC26A6 in the kidney may contribute to renal calcification via an SLC26A6-dependent mechanism
