Induction of Apoptosis in the Bone Marrow Promotes Regenerative Actions of Parathyroid Hormone (PTH) in Bone.

Abstract

PTH has bone regenerative properties via mechanisms that remain elusive. This study aimed to determine the impact of marrow apoptosis on PTH anabolic actions. Studies were in conformance with FASEB animal use principles. Adult mice were treated with pro‐apoptotic etoposide (ETOP) or vehicle (VEH), daily 5d then 3d/wk for 6wks. PTH or VEH was administered daily for: VEH, PTH, ETOP and PTH+ETOP and skeletal phenotyping, flow cytometry, and serum bone markers analyzed. MicroCT and histomorphometry of tibiae revealed ETOP decreased and PTH increased bone volume and area, the greatest increase with PTH + ETOP. ETOP (5d) increased early marrow apoptotic cells (AnnexinV+PI‐) as well as MER+CD11b+F4/80+ and MER+CD68+F4/80+ myeloid cells. At 6wks, CD68+, Gr1+CD11b+F4/80+, CD45+and Lin−CD29+sca1+ cells were increased in ETOP w/ or w/o PTH. Serum TRAcP5b was increased in ETOP and to a greater extent with PTH. P1NP levels were increased in PTH and PTH+ETOP. PTH+ETOP mice had reduced osteoclasts per tibial bone vs. VEH or ETOP. In another model, luciferase+bone marrow stromal cells were implanted to form ectopic ossicles and bone growth tracked via bioluminescence. ETOP mice had increased luciferase vs. VEH, and ETOP augmented the PTH increase. These data suggest that induction of apoptosis provides a conducive environment for PTH pro‐regenerative actions, effects which may be due to increased macrophage efferocytosis.