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

Continuous infusion of PTH in vivo results in active bone resorption. To in vestigate the molecular basis of the catabolic effect of PTH in vivo, we ev aluated the role of OPG and RANKL, which are known to influence osteoclast formation and function. Weanling rats fed a calcium-free diet were parathyr oidectomized and infused with PTH via an Alzet pump to examine: 1) the chan ges of serum-ionized calcium and osteoclast number, 2) the expression of OP G/RANKL mRNA and protein, and 3) the expression of osteoblast phenotype bon e formation-associated genes such as osteoblast specific transcription fact or, osteocalcin, bone sialoprotein, and type I collagen. PTH (1-38) (0.01-2 0 mug/100 g) continuous infusion for 1-24 h resulted in a dose-dependent in crease in serum-ionized calcium in parathyroidectomized rats and a correspo nding dose-dependent increase in osteoclast number, indicating an increased bone resorption. At 20 mug/100 g PTH dose level, serum-ionized calcium was 2.1-fold of the vehicle control and not different from the Sham-parathyroi dectomized rats, and osteoclast number was 3-fold of the vehicle control an d 1.7-fold of the Sham-parathyroidectomized rats. In the distal femur, RANK L mRNA expression was increased (27-fold) and OPG mRNA expression was decre ased (4.6-fold). The changes in RANKL, and OPG mRNA levels were rapid (as e arly as 1 h), dose dependent, and sustained over a 24-h period that was exa mined. Immunohistochemical evaluation of bone sections confirmed that OPG l evel was reduced in proximal tibial metaphysis upon PTH infusion. Circulati ng OPG protein level was also decreased by 32% when compared with the parat hyroidectomized control. The expression of genes that mark the osteoblast p henotype was significantly decreased (osteoblast specific transcription fac tor (2.3-fold), osteocalcin (3-fold), bone sialoprotein (2.8-fold), and typ e I collagen (5-fold)]. These results suggest that the catabolic effect of PTH infusion in vivo in this well-established resorption model is associate d with a reciprocal expression of OPG/RANKL and a co-ordinate decrease in t he expression of bone formation-related genes. We propose that the rapid an d sustained increase in RANKL and decrease in OPG initiate maintain and fav or the cascade of events in the differentiation/recruitment and activation of osteoclasts.