Catabolic Effects of Continuous Human PTH (1–38) in Vivo Is Associated with Sustained Stimulation of RANKL and Inhibition of Osteoprotegerin and Gene-Associated Bone Formation

Abstract

Continuous infusion of PTH in vivo results in active bone resorption. To investigate the molecular basis of the catabolic effect of PTH in vivo, we evaluated the role of OPG and RANKL, which are known to influence osteoclast formation and function. Weanling rats fed a calcium-free diet were parathyroidectomized and infused with PTH via an Alzet pump to examine: 1) the changes of serum-ionized calcium and osteoclast number, 2) the expression of OPG/RANKL mRNA and protein, and 3) the expression of osteoblast phenotype bone formation-associated genes such as osteoblast specific transcription factor, osteocalcin, bone sialoprotein, and type I collagen. PTH (1--38) (0.01--20 microg/100 g) continuous infusion for 1--24 h resulted in a dose-dependent increase in serum-ionized calcium in parathyroidectomized rats and a corresponding dose-dependent increase in osteoclast number, indicating an increased bone resorption. At 20 microg/100 g PTH dose level, serum-ionized calcium was 2.1-fold of the vehicle control and not different from the Sham-parathyroidectomized rats, and osteoclast number was 3-fold of the vehicle control and 1.7-fold of the Sham-parathyroidectomized rats. In the distal femur, RANKL mRNA expression was increased (27-fold) and OPG mRNA expression was decreased (4.6-fold). The changes in RANKL and OPG mRNA levels were rapid (as early as 1 h), dose dependent, and sustained over a 24-h period that was examined. Immunohistochemical evaluation of bone sections confirmed that OPG level was reduced in proximal tibial metaphysis upon PTH infusion. Circulating OPG protein level was also decreased by 32% when compared with the parathyroidectomized control. The expression of genes that mark the osteoblast phenotype was significantly decreased [osteoblast specific transcription factor (2.3-fold), osteocalcin (3-fold), bone sialoprotein (2.8-fold), and type I collagen (5-fold)]. These results suggest that the catabolic effect of PTH infusion in vivo in this well-established resorption model is associated with a reciprocal expression of OPG/RANKL and a co-ordinate decrease in the expression of bone formation-related genes. We propose that the rapid and sustained increase in RANKL and decrease in OPG initiate maintain and favor the cascade of events in the differentiation/recruitment and activation of osteoclasts.

gp130 in late osteoblasts and osteocytes is required for PTH-induced osteoblast differentiation

“…Despite these increases in both genotypes, osteoclast numbers were unchanged, as we have previously reported with this low dose of intermittent PTH treatment (Takyar et al 2013, Tonna et al 2014, likely because the inductions of Tnfsf11 and Il6 are transient (Ma et al 2001, Walker et al 2012). IL-6 and RANKL are expressed by a wide range of cells in the bone, including osteoblast lineage cells as well as osteocytes (Lee & Lorenzo 1999, Dai et al 2006, Nakashima et al 2011, Xiong et al 2011, and cells within the bone marrow, including T-cells (Horwood et al 1999, Hirano et al 1986 and, in the case of IL-6, macrophages (Tosato et al 1988).…”

Mediators of the biphasic responses of bone to intermittent and continuously administered parathyroid hormone

“…Continuous PTH decreases expression of OPG and increases expression of RANKL, [Horwood et al, 1998; Lee and Lorenzo, 1999; Kanzawa et al, 2000; Halladay et al, 2002] the major determinants of osteoclast formation (see reviews: Suda et al, 1999; Hofbauer et al, 2000], in a dose‐dependent fashion in vitro, and increases bone turnover and the RANKL/OPG ratio in parathyroidectomized rats in vivo [Ma et al, 2001]. In our system, continuous treatment stimulated RANKL expression, down‐regulated OPG, and induced osteoclast formation.…”

Cellular and Molecular Mechanisms of Bone Remodeling

“…In addition to recruitment of osteoclast precursors, osteoblast expression of the master osteoclastogenesis cytokines, CSF-1, RANKL, and OPG, is also modulated in response to PTH. OPG expression is reduced, and CSF-1 and RANKL production is increased to promote osteoclast formation and subsequent activity (41). CSF-1 and RANKL work in concert; CSF-1 promotes proliferation and survival of osteoclast precur-FIGURE 1.…”