Moderate Nrf2 Activation by Genetic Disruption of Keap1 Has Sex-Specific Effects on Bone Mass in Mice

Abstract

Keap1 is a negative controller of the transcription factor Nrf2 for its activity. The Keap1/Nrf2 signaling pathway has been considered as a master regulator of cytoprotective genes, and exists in many cell types including osteoblasts and osteoclasts. Our previous study shows Nrf2 deletion decreases bone formation. Recent studies show hyperactivation of Nrf2 causes osteopenia in Keap1 −/− mice, and Keap1 −/− osteoblasts have significantly less proliferative potential than Keap1 +/− osteoblasts. We aimed to examine if moderate Nrf2 activation by disruption of Keap1 impacts bone metabolism. We examined bone phenotype of Keap1 heterozygotic mice (Ht) in comparison with Keap1 wild type (WT) mice. Deletion or knockdown of Keap1 enhanced the gene expression of Nrf2, ALP and wnt5a in cultured primary osteoblasts compared to WT control. In male mice, compared with their age-matched littermate WT controls, Keap1 Ht mice showed significant increase in bone formation rate (+30.7%, P = 0.0029), but did not change the ultimate force (P < 0.01). The osteoclast cell numbers (−32.45%, p = 0.01) and surface (−32.58%, P = 0.03) were significantly reduced by Keap1 deficiency in male mice. Compared to male WT mice, serum bone resorption marker in male Keap1 Ht mice was significantly decreased. Our data suggest that moderate Nrf2 activation by disruption of Keap1 improved bone mass by regulating bone remodeling in male mice.

NRF2 function in osteocytes is required for bone homeostasis and drives osteocytic gene expression

“…Our results also provide additional information about sexual dimorphism, observed previously after global Nfe2l2 deletion [ [17] , [18] , [19] , [20] , 22 ]. Although in our study deletion was performed only in the osteoblast/osteocyte lineages, dimorphic effects in bone parameters correlate with differential changes in osteoclast numbers.…”

“…To address the unresolved issues of the role of NRF2 in specific bone cells in vivo , we found evidence that NRF2 activity has an indispensable role in the maintenance of bone homeostasis, both in the cortical and trabecular compartments. Previous studies with global deletion of NRF2 have shown sexual dimorphism in the skeletal phenotype [ [17] , [18] , [19] , [20] , 22 ]. Our data confirm this sexual dimorphism, with more profound effects of NRF2 deletion on bone mass in male mice.…”

“…Accumulated nascent NRF2 can then translocate to the nucleus, heterodimerize with sMAF, and activate transcription. The role of NRF2-KEAP1 in bone homeostasis may depend on factors, such as age, sex, or high vs. low activation levels [ 17 , 18 ]. Some studies revealed that deletion of Nfe2l2 in female mice resulted in a deficit in postnatal bone acquisition and increased bone loss [ 17 , 19 ].…”

“…Some studies reported increased bone acquisition and mass [ 17 , 20 ], whereas others showed reduced bone density, load-driven bone formation, and increased radiation-induced bone loss [ 21 , 22 ]. Similarly, moderate NRF2 activation by heterozygous deletion of Keap1 led to sexual dimorphic results with higher bone mass in male but not female mice [ 18 ]. A limitation of these previous studies was that genetic modifications of Nfe2l2 in mice affected all cell types and tissues.…”

NRF2 Is an Upstream Regulator of MYC-Mediated Osteoclastogenesis and Pathological Bone Erosion

“…The inhibitory role of DMF, another NRF2 activator in osteoclastogenesis has been shown in both RANKL- and LPS- induced osteolysis models [ 61 ]. Additionally, various KEAP1 deficiency models in which the levels of NRF2 are elevated report increased osteoclastogenesis both in vitro and in vivo [ 65 , 66 ]. Therefore, identifying the downstream effector of NRF2 is important and activating NRF2 in osteoclast precursor cells could be a therapeutic strategy to modulate pathological bone erosion.…”