Concentrations of Adrenocorticotropic Hormone (ACTH

After finding that minimal amounts of adrenocorticotropic hormone (ACTH1-24) prevented osteonecrosis in rabbits, we studied bone formation at nanomolar ACTH1-24, in vivo in rabbits, and in vitro in human osteoblasts. ACTH1-24 in rabbits at 0.6 μg/kg/day had no measurable effect on cortisol. Groups of five rabbits given 0.6 μg/kg/day of ACTH1-24 enhanced trabecular bone in the rabbit femoral head relative to saline-treated controls (controls), increased bone volume/total volume (BV/TV) by micro-computed tomography, p<0.03. Xylenol orange and calcein labeling in vivo showed increased trabecular bone formation with 0.6 μg/kg/day of ACTH1-24, p = 0.0089 versus controls. In contrast, the cortex of the femoral shaft was unaffected, BV/TV p>0.95 ACTH1-24 versus controls. Bone marrow mRNA by PCR showed no change in osteoclast markers, and confirmed increased osteoblast markers, p<0.05. In vitro, ACTH1-24 elevated expression of Collagen 1, alkaline phosphatase (ALP), osteocalcin (BGLAP), and RunX2 in human osteoblasts differentiated on polyethylene terephthalate (PET) membranes. Optimal response was at 10-9 to 10-12 M. VEGF, VEGF receptors FLT-1 and FLK-1, and ACTH1-24 receptors MC2R were upregulated at 10-12 M ACTH1-24. Pathway analysis included increased BMP2, Smad1, Wnt-1, β-Catenin and TGF-β pathways. Because bone-forming osteoblasts are metabolically highly active, we studied mRNA expression of mitochondrial complex 1 (NDUFA5, NDUFS2, NDUFB1, NDUFB6) members with key roles in energy production. This increased at 10-12 M ACTH1-24. An ELISA for mitochondrial complex 1 activity showed maximum activity at 10-9 M and high activity at 10-12 M ACTH1-24. Thus, long-term very low dose ACTH1-24 increases bone formation in vivo and in vitro.