P0871THE IMPACT OF ENDOGENOUS PTH/PTH1R/ATF4 AXIS ON TRABECULAR AND CORTICAL BONE REMODELING AND BONE GROWTH OF YOUNG RATS WITH EXPERIMENTAL CHRONIC KIDNEY DISEASES

Abstract Background and Aims Chronic kidney disease - mineral bone disorder (CKD–MBD) is one of the major clinical complications in patients with chronic kidney disease (CKD). Bone remodeling has been suggested to play a fundamental role in the maintenance of skeletal integrity via a balance between the bone formation/resorption process. The parathyroid hormone (PTH) is a key hormone controlling bone metabolism. PTH is known to affect bone with a net catabolic and anabolic effect but the mechanisms responsible for these differing effects are poorly understood. Previously, we demonstrated the distinct effect of mild degree of CKD on trabecular and cortical bone strength in rapidly growing rats (Pawlak et al. PlosOne, 2016). The aim of the present study was to evaluate the effect of endogenous PTH, its receptor PTH1R and activating transcription factor 4 (ATF4) – the major regulators of the anabolic PTH response in osteoblasts on bone remodeling and growth of young rats with experimental CKD. Method Four-week old Wistar male rats were divided into 2 groups: with CKD induced by surgical 5/6 subtotal nephrectomy, and sham-operated (CON). After one (CON-1; CKD-1) and three months (CON-3; CKD-3) of the surgery the femurs were collected and their lengths were measured. The activity of alkaline phosphatase (ALP), a bone formation marker, and tartrate-resistant acid phosphatase (TRACP5b) reflecting bone resorption were determined in homogenates from trabecular and cortical left femurs. The expression of PTH1R and ATF4 gene was determined by QRT-PCR in right femurs. Serum PTH was analyzed using ELISA kit. Computations were performed using Statistica ver.10 computer software. Results Serum PTH was increased in CKD-3 compared to CKD-1, moreover the slight increase in PTH levels was noted in CKD-3 compared to appropriate controls (both p<0.05). The activity of ALP and TRACP5b in trabecular bone tissue were significantly lower in CKD-3 compared to CKD-1 rats (p<0.01 and p<0.05; respectively). There was a strong positive correlation between ALP and TRACP5b in this bone region (R =0.624, p =0.002), whereas both ALP and TRACP5b were inversely related to serum PTH (R = -0.534, p =0.012 and R = -0.636, p =0.002; respectively). The activity of TRACP5b in cortical bone tissue was significantly higher in CKD-3 compared to CKD-1 group (p<0.05), and it was positively associated with PTH levels (R =0.597, p =0.004). There was no difference in ALP activity between the studied groups, and ALP was not associated with TRACP5b in this bone region. The expression of ATF4 and PTH1R genes was significantly increased in the CKD-3 group compared with the appropriate control (p<0.01 and p<0.05; respectively) and with the CKD-1 group (p<0.05 and p<0.01; respectively). The expression of ATF4 was inversely correlated with ALP and TRACP5b in trabecular bone (R = -0.534, and R = -0.528, both p<0.05), whereas it was positively related to TRACP5b in cortical bone region (R =0.418, p<0.05). The femoral length was significantly increased during 3-month of CKD development (p<0.000), and it was positively associated with PTH levels (R =0.571, p =0.007), cortical TRACP5b activity (R =0.609, p =0.002) and tended to be related with ATF4 gene expression (R =0.409, p =0.065). In contrast, the femoral length was inversely related to ALP activity in trabecular bone tissue (R= -0.656, p =0.0007). Conclusion The endogenous PTH, through PTH1R/ATF4 axis, inhibited trabecular bone remodeling. In contrary, PTH/PTH1R/ATF4 system intensified bone resorption in cortical bone region. This opposite effect of PTH on bone remodeling was associated with the intensification of growth process in the long bones of young rats with CKD.