Date Published: February 2, 2017
Publisher: Public Library of Science
Author(s): Gretel Gisela Pellegrini, Meloney Cregor, Kevin McAndrews, Cynthya Carolina Morales, Linda Doyle McCabe, George P. McCabe, Munro Peacock, David Burr, Connie Weaver, Teresita Bellido, Valérie Geoffroy.
Accumulation of reactive oxygen species (ROS) is an important pathogenic mechanism underling the loss of bone mass and strength with aging and other conditions leading to osteoporosis. The transcription factor erythroid 2-related factor2 (Nrf2) plays a central role in activating the cellular response to ROS. Here, we examined the endogenous response of bone regulated by Nrf2, and its relationship with bone mass and architecture in the male and female murine skeleton. Young (3 month-old) and old (15 month-old) Nrf2 knockout (KO) mice of either sex exhibited the expected reduction in Nrf2 mRNA expression compared to wild type (WT) littermates. Nrf2 deletion did not lead to compensatory increase in Nrf1 or Nrf3, other members of this transcription factor family; and instead, Nrf1 expression was lower in KO mice. Compared to the respective WT littermate controls, female KO mice, young and old, exhibited lower expression of both detoxifying and antioxidant enzymes; young male KO mice, displayed lower expression of detoxifying enzymes but not antioxidant enzymes; and old male KO mice showed no differences in either detoxifying or antioxidant enzymes. Moreover, old male WT mice exhibited lower Nrf2 levels, and consequently lower expression of both detoxifying and antioxidant enzymes, compared to old female WT mice. These endogenous antioxidant responses lead to delayed rate of bone acquisition in female KO mice and higher bone acquisition in male KO mice as quantified by DXA and μCT, demonstrating that Nrf2 is required for full bone accrual in the female skeleton but unnecessary and even detrimental in the male skeleton. Therefore, Nrf2 regulates the antioxidant endogenous response and bone accrual differently depending on sex and age. These findings suggest that therapeutic interventions that target Nrf2 could be developed to enhance the endogenous antioxidant response in a sex- and age-selective manner.
Oxidative stress results from the imbalance between free radical generation and the scavenging activity of intracellular antioxidant mechanisms. Accumulation of reactive oxygen species (ROS) causing oxidative damage in different tissues occurs with aging, obesity, menopause, and arthritis, and it is also a crucial pathogenic factor in osteoporosis and metabolic bone diseases [1, 2]. Accumulation of ROS increases osteoclast differentiation directly by activating the transcription factor nuclear factor of activated T–cells, cytoplasmic 1 (NFATc1) in pre-osteoclasts and indirectly by enhancing the expression in cells of the osteoblastic lineage of the receptor activator of nuclear factor kappa-B ligand (RANKL) and tumor necrosis factor α (TNFα), which in turn stimulate osteoclastogenesis [3–5]. Conversely, ROS accumulation decreases the number of osteoblasts by inhibiting their proliferation and differentiation and by inducing premature osteoblast apoptosis [6–9]. In addition, ROS induces apoptosis of osteocytes, the most abundant cells in bone that regulate osteoclast and osteoblast function [10, 11].
In this study, we report the endogenous antioxidant response in bone and its correlation with the skeletal phenotype of mice with global deletion of Nrf2, a transcription factor with purported role in bone mass homeostasis. Our findings demostrate that genes responsible for cellular protection from the damaging effects of ROS are regulated by Nrf2 depending on the sex and age. We found that in female young and old mice, Nrf2-dependent mechanisms are responsible for regulating the expression of detoxifying and antioxidant enzymes; in young male mice, Nrf2-dependent mechanisms regulate the expression of detoxifying enzymes whereas Nrf2-independent mechanisms regulate antioxidant enzymes; and lastly, in old male mice, Nrf2-independent mechanisms are responsible for the full antioxidant endogenous response, regulating both detoxifying and antioxidant enzymes in bone (Fig 9). The dissimilar regulation of cytoprotective genes correlates with different effects of Nrf2 on bone accrual and maintenance depending on the sex. Thus, there is delayed rate of bone acquisition in female but higher bone acquisition in male KO mice demonstrating the requirement of Nrf2 for full bone accrual in the female skeleton but not in the male skeleton. Our findings suggest that therapeutic interventions that target Nrf2 could be developed to facilitate the defense against the damagind effects of ROS in a sex- and age-selective manner.