An Intact N Terminus Is Required for the Anabolic Action of Parathyroid Hormone on Adult Female Rats

Abstract

Intermittent administration of parathyroid hormone (PTH) peptides increases bone density in animal and human models of osteoporosis. In vitro studies have demonstrated that PTH analogs lacking the first two amino acids can stimulate cell proliferation in certain cell systems, whereas fragments with an intact N terminus can be antimitogenic. We have tested whether the truncated PTH(3-38) fragment may be a better "anabolic analog" than PTH(1-38) by monitoring bone density and biomechanical properties of the femur in 6-month-old ovariectomized (OVX) rats. Either PTH fragment was administered subcutaneously (8 g/100 g of body weight) 5 days/week, for 4 weeks, starting 1 week after surgery. During the entire study, untreated OVX rats lost 12.1 ؎ 4.4% of their initial bone density. PTH(1-38) reversed the initial bone loss, leading to complete restoration of presurgery values after 4 weeks of treatment. Conversely, administration of PTH(3-38) resulted in 13.2 ؎ 5.8% bone loss, while continuous estrogen infusion (10 g/kg/day) prevented bone loss but did not reverse it. Sham-operated animals also experienced significant bone loss in the vehicle and PTH(3-38)-treated groups (؊4.5 ؎ 6.7%, and ؊7.6 ؎ 2.8%, respectively), whereas a significant gain in bone density (؉4.4 ؎ 5.6%) was observed in the rats treated with PTH(1-38). A bone quality factor (index of strain energy loss) and the impact strength (resistance to fracture) were 25% and 44% lower in femurs explanted from OVX animals treated with either vehicle or PTH(3-38), compared with sham-operated animals. On the contrary, no difference was observed between OVX and control animals after treatment with PTH(1-38), indicating a preservation of the capacity to withstand mechanical stress. Thus, PTH(1-38) counteracts estrogen-dependent loss of mineral density and bone biomechanical properties and increases bone density in estrogen-replete animals. An intact N terminus sequence is necessary for this anabolic action of PTH. (J Bone Miner Res 1997;12:384-392)

Osteoblast expression of an engineered G s -coupled receptor dramatically increases bone mass

“…Our studies demonstrate that osteoblast expression of Rs1, an engineered G s -coupled RASSL with constitutive G s activity, induces a dramatic anabolic bone effect that is significantly different from previous models (10)(11)(12)(13)(14)(15). The ColI(2.3)ϩ/Rs1ϩ mice display several unique and unexpected characteristics, including a crucial temporal requirement for Rs1 expression, an unusually large amount of bone mineral accrual, and effects on bone macroarchitecture.…”

“…Mice expressing a constitutively active PTHR1 in osteoblasts show increased trabecular bone volume and decreased cortical bone thickness at 12 weeks of age, with grossly normal femur shape and size (10,11). In addition, models using PTH peptide fragments that selectively activate PTHR1-linked G s signaling (12)(13)(14)(15) suggest that G s signaling can increase bone formation. Because a mouse model with constitutively active GNAS in osteoblasts has not been developed, the direct role of activated G s signaling in osteoblasts has not been clearly tested.…”

CREM deficiency in mice alters the response of bone to intermittent parathyroid hormone treatment

“…However, despite this knowledge, the function of CREM factors in bone remains largely unknown. It is generally accepted that cAMP-mediated PKA activation through PTH1 receptors in osteoblasts is both necessary and sufficient for the anabolic effect of PTH in vivo (16,17), although the role of PKC activation cannot be excluded (25). Our previous initial pilot experiment did not reveal an obvious bone phenotype of Crem KO mice.…”

“…Cells of the osteoblast lineage contain PTH receptors and serve as the primary target cells of PTH signaling in bone (15). Although PTH activates both the cAMPprotein kinase A (PKA) and PKC pathways in osteoblasts, cAMP-PKA signaling has been shown to be critical for the anabolic effect of PTH on bone mass acquisition (16,17). It is shown that the time course of RANKL and OPG expression is different between acute and sustained PTH treatment in mice (18).…”

Molecular and cellular mechanisms of the anabolic effect of intermittent PTH

“…Daily injections of PTH(1-34) or PTH(1-31) produced an equivalent anabolic effect in rats; however, whereas PTH(1-34) activates both cAMP production and PKC, PTH (1-31) only activates cAMP production [41]. On the other hand, PTH(3-38), which activates PKC but not cAMP production, did not cause an anabolic effect [42]. Therefore, PTHstimulated cAMP production is sufficient for initiation of signaling cascades that increase osteoblast number, but activation of PKC is not.…”