Vitamin D(3) and methylenebisphosphonic acid in the correction of mineral metabolism disorders and bone remodeling associated with glucocorticoid-induced osteoporosis

The study was aimed at evaluating therapeutic efficacy of vitamin D3 (VD3, 1000 IU/kg of b.w., 30 days) and sodium salt of methylenebisphosphonic acid (MBPA, 17 mg/kg of b.w., 30 days) monotherapies as well as their effect in combination in preventing mineral metabolism and bone remodeling disturbances associated with glucocorticoid(GC)-induced osteoporosis. Osteoporosis in rats was induced by long-term (30 days) administration of the synthetic glucocorticoid prednisolone (5 mg/kg of b.w.). Calcium and inorganic phosphate levels, activity of alkaline phosphatase (ALP) in serum, bone tissue and bone marrow were determined spectrophotometrically. The protein levels of VD3 receptor (VDR), receptor activator of nuclear factor kappa-B (RANK), its ligand (RANKL), and osteoprotegerin (OPG) in bone tissue were determined by Western blotting. Serum 25-hydroxyvitamin D3 (25OHD3) content was assayed by ELISA. It was shown that prednisolone caused the development of hypocalcemia and hypophosphatemia, increased the alkaline phosphatase activity in the blood serum, while downregulating its activity in bone tissue and bone marrow. GC-induced osteoporosis was accompanied by a profound deficiency of VD3 and a decrease in the content of VDR. Evaluation of the NF-κB-associated cytokine axis RANK/RANKL/OPG, which regulates the balance of osteoblasts/osteoclasts, showed a simultaneous decrease in the RANK content and OPG/RANKL ratio. Vitamin D3 restored mineral metabolism and 25OHD3 level that led to the normalization of VDR-mediated signaling­ and RANK/RANKL/OPG functions in bone tissue. It has been shown that the administration of MBPA had a corrective effect on the content of mineral components in the blood serum and bone tissue, as well as on the activity­ of alkaline phosphatase only in combination with vitamin D3, indicating a low efficiency of bisphosphonate monotherapy in GC-induced vitamin D3 deficiency and osteoporosis. Keywords: bone remode­ling, glucocorticoid-induced osteoporosis, methylenebisphosphonic acid, RANK/RANKL/OPG axis, vitamin D3