Vitamin K-dependent carboxylation of osteocalcin affects the efficacy of teriparatide (PTH1–34) for skeletal repair

Abstract

Teriparatide (PTH1–34) promotes skeletal repair and increases bone mass. Vitamin K is involved in bone mineralization as a coenzyme of γ-carboxylase for Gla proteins, and therefore vitamin K insufficiency caused by malnutrition or therapeutic intake of the vitamin K antagonist warfarin could affect the efficacy of PTH1–34 therapy for bone repair. In the present study, we investigated whether vitamin K influences the efficacy of PTH1–34 therapy for bone repair in a rat osteotomy model. Female 12-week-old Sprague–Dawley rats were subjected to a closed midshaft osteotomy of the femur and randomized into four groups (n=10 per group): vehicle, PTH1–34 (daily 30μg/kg/day subcutaneous injection)+solvent (orally, three times a week), PTH1–34+warfarin (0.4mg/kg/day orally, three times a week), and PTH1–34+vitamin K2 (menatetrenone, 30mg/kg/day orally, three times a week). Serum γ-carboxylated and uncarboxylated osteocalcin (Gla-OC and Glu-OC) levels and radiographic healing were monitored every 2weeks. Skeletal repair was assessed by micro-computed tomography, mechanical testing, and histology at 8weeks after surgery. PTH1–34 amplified the osteotomy-induced increase in Gla-OC and improved the mechanical properties as well as the volumetric bone mineral tissue density of the fracture callus. Concurrent use of warfarin decreased the response to PTH1–34 therapy in terms of mechanical recovery, probably by impairing mineralization due to the lack of Gla-OC. Although the effects of combination therapy with PTH1–34 and vitamin K2 on bone repair did not significantly exceed those of PTH1–34 monotherapy in rats fed sufficient dietary vitamin K, postoperative Gla-OC levels were correlated with the mechanical properties of the osteotomized femur in PTH1–34-treated rats regardless of the use of warfarin or vitamin K2. These findings suggest the importance of vitamin K dependent γ-carboxylation of OC for realizing the full effects of PTH1–34 on skeletal repair.