The Effects of Aging and Sex Steroid Deficiency on the Murine Skeleton Are Independent and Mechanistically Distinct

J Bone Miner Res. 2017 Mar;32(3):560-574. doi: 10.1002/jbmr.3014. Epub 2016 Nov 3.

Abstract

Old age and sex steroid deficiency are the two most critical factors for the development of osteoporosis. It remains unknown, however, whether the molecular culprits of the two conditions are similar or distinct. We show herein that at 19.5 months of age-a time by which the age-dependent decline of cortical and cancellous bone mass and cortical porosity were fully manifested in C57BL/6J mice-these animals remained functionally estrogen sufficient. Transgenic mice with conditional expression of mitochondria-targeted catalase-a potent H2 O2 inactivating enzyme-in cells of the myeloid lineage (mitoCAT;LysM-Cre mice) were protected from the loss of cortical, but not cancellous, bone caused by gonadectomy in either sex. Consistent with these findings, in vitro studies with ERα-deficient Prx1+ cells and gonadectomized young adult mice showed that in both sexes decreased ERα signaling in Prx1+ cells leads to an increase in SDF1, a.k.a. CXCL12, an osteoclastogenic cytokine whose effects were abrogated in macrophages from mitoCAT;LysM-Cre mice. In contrast to sex steroid deficiency, the adverse effects of aging on either cortical or cancellous bone were unaffected in mitoCAT;LysM-Cre mice. On the other hand, attenuation of H2 O2 generation in cells of the mesenchymal lineage targeted by Prx1-Cre partially prevented the loss of cortical bone caused by old age. Our results suggest the effects of sex steroid deficiency and aging on the murine skeleton are independent and result from distinct mechanisms. In the former, the prevailing mechanism of the cortical bone loss in both sexes is increased osteoclastogenesis caused by estrogen deficiency; this is likely driven, at least in part, by mesenchymal/stromal cell-derived SDF1. Decreased osteoblastogenesis, owing in part to increased H2 O2, combined with increased osteoclastogenesis caused by aging mechanisms independent of estrogen deficiency, are the prevailing mechanisms of the loss of cortical bone with old age. © 2016 American Society for Bone and Mineral Research.

Keywords: AGING; MOLECULAR PATHWAYS-REMODELING; OSTEOBLASTS; OSTEOCLASTS; SEX STEROIDS.

MeSH terms

  • Aging / physiology*
  • Animals
  • Biomechanical Phenomena
  • Bone Resorption / pathology
  • Bone Resorption / physiopathology
  • Bone and Bones / physiology*
  • Calcification, Physiologic
  • Cancellous Bone / physiology
  • Cell Count
  • Cell Lineage
  • Chemokine CXCL12 / metabolism
  • Cortical Bone / physiology
  • Estrogen Receptor alpha / metabolism
  • Estrogens / metabolism
  • Female
  • Gonadal Steroid Hormones / deficiency*
  • Gonadal Steroid Hormones / metabolism
  • Hydrogen Peroxide / metabolism
  • Male
  • Mesenchymal Stem Cells / metabolism
  • Mice, Inbred C57BL
  • Myeloid Cells / metabolism
  • Osteoclasts / metabolism
  • Ovariectomy
  • Porosity

Substances

  • Chemokine CXCL12
  • Estrogen Receptor alpha
  • Estrogens
  • Gonadal Steroid Hormones
  • Hydrogen Peroxide