In vivo demonstration that human parathyroid hormone 1-38 inhibits the expression of osteoprotegerin in bone with the kinetics of an immediate early gene

Osteoprotegerin (OPG) is a potent inhibitor of osteoclast formation and fun ction. To elucidate how OPG is regulated in bone, we examined (1) the expre ssion and localization of OPG protein in bone tissue, (2) the effect of hum an parathyroid hormone 1-38 (hPTH 1-38) on OPG messenger RNA (mRNA) levels in rat femur metaphyseal and diaphyseal bone, and (3) the effect of hPTH(1- 38) on expression of OPG mRNA in cultured osteoblast-like cells derived fro m the metaphysis and diaphysis, and in ROS 17/2.8 osteosarcoma cells. Becau se PTH has been shown to stimulate osteoblast activity via the cyclic adeno sine monophosphate (cAMP)/protein kinase A (PKA) signal transduction pathwa y we also investigated whether PTH action on OPG in vivo is dependent on ac tivation of cAMP/PKA pathway. Immunohistochemistry was used to evaluate OPG protein expression and Northern blot hybridization was used to analyze OPG mRNA expression both in vivo and in vitro. Immunohistochemistry of OPG pro tein expression in the rat distal femur metaphysis revealed that it was loc alized predominantly in preosteoblasts, osteoblasts, lining cells, and the osteoid layer, with occasional immunoreactivity in osteocytes and cells of the bone marrow. Subcutaneous (sc) administration of a single injection of hPTH(1-38) at 80 mu g/kg induced a rapid and transient decrease in OPG mRNA expression in both metaphyseal and diaphyseal bone, The decrease in OPG me ssage was evident by 1 h and mRNA levels returned to baseline after 3 h, PT H analog PTH(1-31), which stimulates intracellular cAMP accumulation, inhib ited OPG expression, whereas PTH analogs (3-34 and 7-34) that do not stimul ate cAMP production had no effect on expression. In contrast to PTH, prosta glandin E-2 (PGE(2)) had no effect on OPG mRNA expression in vivo in the me taphyseal bone cells, under conditions in which PGE(2) does promote express ion of the c-fos gene. The in vivo effects of hPTH(1-38) on OPG mRNA were c onfirmed in isolated primary osteoblast cultures derived from either metaph yseal or diaphyseal bone as well as in ROS 17/2.8 osteosarcoma cells. We pr opose that the rapid and transient decrease in OPG expression may initiate a cascade of events resulting in the differentiation of osteoclast progenit or. Such a spatially and temporally programmed effect of PTH might contribu te to bone turnover.