Targeting HIF-2a in skeletal progenitors and their descendants for the radioprotection of bone
Radiotherapy continues to be a common treatment modality for cancer despite skeletal complications. Currently, there are no effective treatments for radiation-induced bone loss. Radiation induces substantial changes to the bone microenvironment; however, the effects of these changes on skeletal progenitors and their descendants remain elusive. Following radiation, LeprCre skeletal progenitors and their descendants, localize to hypoxic regions and exhibit HIF-2a stabilization, thus implicating a role for HIF-2a in radiation-induced bone loss. To test whether HIF-2a deficiency rescues radiation-induced bone loss, we conditionally ablated HIF-2a in LeprCre-expressing cells and their descendants. Compared to irradiated controls, irradiated LeprCre;Hif-2afl/fl knock out mice demonstrate increased bone mass with values comparable to non-irradiated animals. Mechanistically, HIF-2a negatively regulates osteoblastogenesis, mineralization, and number of skeletal progenitors. To test whether HIF-2a can be targeted therapeutically, we administered PT2399, a potent and selective HIF-2a inhibitor whose structural analog belzutifan, has recently received FDA approval. Administration of PT2399 was sufficient to prevent radiation-induced bone loss in a single-limb irradiation model. As ~90% of patients on HIF-2a inhibitor treatment develop anemia, we constructed a PT2399-loaded bone-targeting nanocarrier (Aln-PT-NC) to increase on-target efficacy and reduce off-target kidney toxicity. Delivery of Aln-PT-NC completely prevented radiation-induced bone loss while reducing drug accumulation in the kidney. In this study, we identify HIF-2a as a negative regulator of bone formation in LeprCre skeletal progenitors and their descendants following radiotherapy and developed a bone-targeting HIF-2a therapeutic. Together, these studies highlight the clinical potential of Aln-PT-NC to be translated into a treatment regimen for bone radioprotection. Overall design: To detemine the impact of loss of HIF-2 in LeprCre expressing cells after radiation exposure. Control mice (LepRCre;tdTomato+) and KO mice (LepRCre;tdTomato;HIF-2fl/fl) mice were exposed to 0 Gy or 4 Gy total body irradiation. 3 or 14 days post IR, long bones were collected and bone marrow was aspirated. Bone marrow cells were then sorted by FACS and we collected TdTomato+ cells for  RNA extraction. We then perfomed gene profiling anaysis on collected samples