Parathyroid hormone (PTH) stimulates hematopoietic cells through mechanisms of action that remain elusive. Interleukin-6 (IL-6) is upregulated by PTH and stimulates hematopoiesis. The purpose of this investigation was to identify actions of PTH and IL-6 in hematopoietic cell expansion. Bone marrow cultures from C57B6 mice were treated with fms-like tyrosine kinase-3 ligand (Flt-3L), PTH, Flt-3L plus PTH, or vehicle control. Flt-3L alone increased adherent and non-adherent cells. PTH did not directly impact hematopoietic or osteoclastic cells but acted in concert with Flt-3L to further increase cell numbers. Flt-3L alone stimulated proliferation, while PTH combined with Flt-3L decreased apoptosis. Flt-3L increased blasts early in culture, and later increased CD45+ and CD11b+ cells. In parallel experiments, IL-6 acted additively with Flt-3L to increase cell numbers and IL-6-deficient bone marrow cultures (compared to wildtype controls) but failed to amplify in response to Flt-3L and PTH, suggesting that IL-6 mediated the PTH effect. In vivo, PTH increased Lin- Sca-1+c-Kit+ (LSK) hematopoietic progenitor cells after PTH treatment in wildtype mice, but failed to increase LSKs in IL-6-deficient mice. In conclusion, PTH acts with Flt-3L to maintain hematopoietic cells by limiting apoptosis. IL-6 is a critical mediator of bone marrow cell expansion and is responsible for PTH actions in hematopoietic cell expansion.

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.