Hypoxia-inducible factor-2 alpha mediates senescence-associated intrinsic mechanisms of age-related bone loss

Abstract

Aging is associated with cellular senescence followed by bone loss leading to bone fragility in humans. However, the regulators associated with cellular senescence in aged bones need to be identified. Hypoxia-inducible factor (HIF)-2 alpha regulates bone remodeling via the differentiation of osteoblasts and osteoclasts. Here, we report that HIF-2 alpha expression was highly upregulated in aged bones. HIF-2 alpha depletion in male mice reversed age-induced bone loss, as evidenced by an increase in the number of osteoblasts and a decrease in the number of osteoclasts. In an in vitro model of doxorubicin-mediated senescence, the expression of Hif-2 alpha and p21, a senescence marker gene, was enhanced, and osteoblastic differentiation of primary mouse calvarial preosteoblast cells was inhibited. Inhibition of senescence-induced upregulation of HIF-2 alpha expression during matrix maturation, but not during the proliferation stage of osteoblast differentiation, reversed the age-related decrease in Runx2 and Ocn expression. However, HIF-2 alpha knockdown did not affect p21 expression or senescence progression, indicating that HIF-2 alpha expression upregulation in senescent osteoblasts may be a result of aging rather than a cause of cellular senescence. Osteoclasts are known to induce a senescent phenotype during in vitro osteoclastogenesis. Consistent with increased HIF-2 alpha expression, the expression of p16 and p21 was upregulated during osteoclastogenesis of bone marrow macrophages. ChIP following overexpression or knockdown of HIF-2 alpha using adenovirus revealed that p16 and p21 are direct targets of HIF-2 alpha in osteoclasts. Osteoblast-specific (Hif-2 alpha(fl/fl);Col1a1-Cre) or osteoclast-specific (Hif-2 alpha(fl/fl);Ctsk-Cre) conditional knockout of HIF-2 alpha in male mice reversed age-related bone loss. Collectively, our results suggest that HIF-2 alpha acts as a senescence-related intrinsic factor in age-related dysfunction of bone homeostasis. Bone remodeling: Protein linked to age-related bone loss Drugs directed at a protein called HIF-2 alpha may help combat cellular aging processes in bone. Je-Hwang Ryu from Chonnam National University in Gwangju, South Korea, and colleagues have linked the expression of HIF-2 alpha, a known regulator of bone remodeling, to aging and senescence, a state of irreversible growth arrest in which cells remain metabolically active yet cease dividing. Experimentally targeting HIF-2 alpha in mice, either in bone-forming osteoblast cells or in bone-resorbing osteoclast cells, reversed the gradual and progressive decline of bone tissue associated with aging, showing that senescence-induced HIF-2 alpha activity contributes to imbalances in bone cell turnover. HIF-2 alpha blockers are already in clinical testing for the treatment of cancer; these findings suggest that this class of drugs should now be considered for the treatment of osteoporosis and other bone diseases as well.