Osteoprotegerin (OPG) and receptor activator of NF-kB ligand (RANK-L) serum levels in patients on chronic hemodialysis

The mechanisms underlying the skeletal resistance to PTH in patients on chronic hemodialysis (CHD) are not yet fully clarified. Osteoprotegerin (OPG) and receptor activator of NF-kB ligand (RANK-L) modulate the genesis and activity of osteoclasts, however their role in renal osteodystrophy pathogenesis has not been clarified so far. The present study aimed to evaluate OPG and RANK-L serum levels in hemodialysis patients and whether OPG/RANK-L system could have a role in the skeletal resistance to PTH. In fasting blood samples obtained from 60 patients (36 males and 24 females) on CHD for at least 2 yr and from 40 healthy subjects of similar age and gender distribution as controls (CTRs), we measured serum OPG, RANK-L, bone alkaline phosphatase (B-ALP), N-terminal telopeptide of type I collagen (NTx), PTH(1-84), calcium and phosphate. In 30 of 60 hemodialysis patients, a blood sample was also drawn soon after the dialytic session. Serum levels of RANK-L, but not OPG, showed a slight but significant (p<0.05) decrease after the dialytic session. OPG resulted being about six times higher in CHD patients than in CTRs (38.7 +/- 16.2 vs 6.3 +/- 0.17 pg/ml), whereas RAN K-L serum levels were only slightly increased with respect to controls (0.88 +/- 0.47 vs 0.64 +/- 0.38 pmol/l). CHD patients showed serum PTH(1-84) and bone turnover higher than in CTRs. No correlation was found between OPG/RANK-L system and PTH or bone turnover markers. Instead, in the patients with high osteoclast activity (no.=21) OPG/RANK-L ratio was correlated (r=-0.41, p<0.01) with NTx serum levels, whereas in patients with decreased osteoclast activity (no.=39) no relationship was found. In conclusion, our findings showed that, although both OPG and RANK-L are accumulated in hemodialysis patients, only RANK-L and the balance between OPG and RANK-L seem to be related to osteoclast activity