An Irradiation-Altered Bone Marrow Microenvironment Impacts Anabolic Actions of PTH

PTH stimulates bone formation and increases hematopoietic stem cells through mechanisms as yet uncertain. The purpose of this study was to identify mechanisms by which PTH links actions on cells of hematopoietic origin with osteoblast-mediated bone formation. C57B6 mice (10 d) were non-lethally irradiated and then administered PTH for 5-20 d. Irradiation reduced bone marrow cellularity with retention of cells lining trabeculae. PTH anabolic activity was greater in irradiated vs. nonirradiated mice, which could not be accounted for by altered osteoblasts directly or osteoclasts but instead via an altered bone marrow microenvironment. Irradiation increased fibroblast growth factor 2, TGF beta, and IL-6 mRNA levels in the bone marrow in vivo. Irradiation decreased B220 cell numbers, whereas the percent of Lin(-)Sca-1(+)c-kit(+) (LSK), CD11b(+), CD68(+), CD41(+), Lin(-)CD29(+)Sca-1(+) cells, and proliferating CD45(-)Nestin(+) cells was increased. Megakaryocyte numbers were reduced with irradiation and located more closely to trabecular surfaces with irradiation and PTH. Bone marrow TGF beta was increased in irradiated PTH-treated mice, and inhibition of TGF beta blocked the PTH augmentation of bone in irradiated mice. In conclusion, irradiation created a permissive environment for anabolic actions of PTH that was TGF beta dependent but osteoclast independent and suggests that a nonosteoclast source of TGF beta drives mesenchymal stem cell recruitment to support PTH anabolic actions. (Endocrinology 152: 4525-4536, 2011)