C-type Natriuretic Peptide Attenuates Renal Osteodystrophy Through Inhibiting FGF-23/MAPK Signaling

Object: Renal osteodystrophy(ROD)occurs as early as chronic kidney disease(CKD)stage 2 and seems ubiquitous in almost all pediatric patients with CKD stage 5.Fibroblast growth factor(FGF)-23,a bone-derived endocrine regulator of phosphate homeostasis,is over-expressed in CKD and disturbs osteoblast differentiation and matrix mineralization.On the contrary,C-type natriuretic peptide(CNP)acts as a potent positive regulator of bone growth.We observe whether CNP could attenuate ROD through inhibiting FGF-23 cascades.Methods: 144 male Sprague-Dawley rats weighting 120 to 150 g were randomly divided into three groups: sham-operated group(n=48),uremic group(n=48)and CNPtreated group(n=48).The fasted animals were operated under intra-peritoneal pentobarbital anesthesia(60 mg/kg body weight)and sterile conditions.Ninty-six rats experienced nephrectomy of left kidney and adriamycin(5 mg/kg)injection,were assigned randomly to the uremic group and the CNP-treated group(n=48).The CNPtreated group was given a continuous infusion of CNP(0.05 ?g/kg/min ×1h)through the caudal vein from the day of the disease induction until the day of sacrifice.The other 48 animals were the sham-operated group underwent a sham laparotomy with ureteric manipulation through a midline incision and given an infusion of 0.9% saline through the caudal vein from the day of operation until the day of sacrifice.Subsequently,animals in each group were separated into eight experimental subgroups(n=6),and sacrificed at 24 hours,72 hours,1 week,2 weeks,3 weeks,1 month,2 months and 3 months after operation,respectively.Urine creatinine concentration(UCr)and urinary protein concentration(UPr)were determined by biuret colorimetric method,serum albumin(Alb),blood urea nitrogen(BUN),creatinine(SCr),uric acid(UA),calcium and phosphate were determined by standard enzymatic method.The levels of 25-hydroxyvitamin D [25-(OH)D],parathormone(PTH),osteoblastic and osteolytic proteins were evaluated by enzyme-linked immunosorbent assay(ELISA)kits.Realtime(RT)-PCR,Immunofluorescent staining and Western blotting analysis were used to determine the expressions of FGF-23,Klotho,fibroblast growth factor receptor-1(FGFR-1),STAT-1,RAF-1,ERK,P38 and Col X in bone.Results: In the uremic rats,CNP administration significantly restored renal dysfunction,calcium phosphate metabolic disorders,hypovitaminosis D,secondary hyperparathyroidism and decreased bone turnover markers,and retarded bone pathological progression.More important,within FGF-23/mitogen-activated protein kinase(MAPK)signaling,the fibroblast growth factor receptor-1(FGFR-1),Klotho and alternative(STAT-1/phospho-STAT-1)elements were up-regulated by CNP,whereas FGF-23,RAF-1/phospho-RAF-1 and downstream(ERK/phospho-ERK and P38/phospho-P38)elements were paradoxically under-expressed in bone tissue.Conclusion: Therefore,CNP exerts a therapeutic effect on ROD through inhibition of FGF-23/MAPK signaling at RAF-1 level.