Association between histomorphometry and biochemical markers of bone turnover in a longitudinal rat model of parathyroid hormone-related peptide (PTHrP)-mediated tumor osteolysis

Advanced tumor osteopathy is characterized by abnormal bone turnover. Using a rat model of parathyroid hormone-related peptide (PTHrP)-mediated tumor osteolysis, the aim of the present study was to define the sequential changes in, and the association between, biochemical and histomorphometric indices of bone metabolism during the early stages of developing tumor osteopathy. Eight-month-old Wistar rats (n = 48) were subcutaneously inoculated with either 2 × 106 cells of the Walker carcinosarcoma 256, or saline on day 0, and treated with either saline or the bisphosphonate ibandronate until killing on day 8. Serum calcium (sCa), alkaline phosphatase (sTAP), and osteocalcin (sOC) and urinary calcium (uCa), deoxypyridinoline (uDPD), and pyridinoline (uPYD) were measured daily. In a second semilongitudinal experiment (n = 70), the number of osteoclasts and osteoblasts (N.Oc, N.Ob), trabecular bone volume (BV/TV), and osteoid volume (O.Ar) were assessed by histomorphometry. In untreated tumor-bearing animals, osteoclast numbers increased by 74% on day 3 (5.4 ± 2.4 vs. 3.1 ± 1.5/mm2, p< 0.05), and trabecular bone volume fell by 24% on day 4 (12.5 ± 2.0 vs. 15.8 ± 1.2%, p< 0.05). Both time course and magnitude of these changes were closely reflected by an increase in uDPD (0.46 ± 0.14 vs. 0.31 ± 0.15 nmol/12 h, p< 0.05) and uPYD on day 4 (1.44 ± 0.25 vs. 1.03 ± 0.3 nmol/12 h, p< 0.05), sCa (3.8 ± 0.52 vs. 3.0 ± 0.13 mmol/L, p< 0.01), and uCa (0.13 ± 0.08 vs. 0.03 ± 0.01 mmol/12 h, p< 0.001) on day 6, and sTAP (254 ± 127 vs. 120 ± 40 U/L, p< 0.001) on day 7 (mean ± SD), whereas sOC remained unchanged until day 8. When combining the results of the two experiments, a high correlation was found between the number of osteoclasts and the urinary excretion of PYD (r = 0.91) and DPD (r = 0.89). Treatment with ibandronate delayed hypercalcemia, abolished hypercalciuria, and accelerated bone resorption. We conclude that osteoclast activation is an early event in PTHrP-mediated osteolysis, which is closely reflected by the renal excretion of pyridinium cross-links of type I collagen. Therefore, specific biochemical markers of collagen breakdown may be useful as early indicators of developing tumor osteopathy.