Mechanotransduction in Living Bone: Effects of the Keap1-Nrf2 Pathway

Abstract

The Keap1-Nrf2 pathway regulates a wide range of cytoprotective genes, and has been found to serve a protective and beneficial role in many body systems. There is limited information available, however, about its role in bone homeostasis. While Nrf2 activation has been suggested as an effective method of increasing bone mass and quality, there have been conflicting reports which associate Keap1 deficiency with detrimental phenotypes. As Keap1 deletion is a common method of Nrf2 activation, further study should address the impacts of various methods of regulating Nrf2 expression. Also, little research has been conducted on the specific pathways by which Nrf2 activation improves bone quality. In this study, the effects of alterations to Nrf2 activation levels were explored in two specific and varied scenarios. In the first experiment, moderate Nrf2 activation was achieved via partial deletion of its sequestering protein, Keap1, in an aging mouse model. The hypothesis tested here is that moderate Nrf2 activation improves bone quality by affecting bone metabolism and response to mechanical loading. The results of this first experiment suggest a subtle, sex-specific effect of moderate Nrf2 activation in aging mice which improves specific indices of bone quality to varying degrees, but does not affect loading-induced bone formation. It is likely that the overwhelming phenotypic impacts associated with aging or the systemic effects of global Keap1 deficiency may increase the difficulty in parsing out significant effects that can be attributed solely to Nrf2 activation. In the second experiment, a cell-specific knockout of Nrf2 in the osteocytes was achieved using a Cre/Lox breeding system. The hypothesis tested here is that osteocyte-specific deletion of Nrf2 impairs bone quality by affecting bone metabolism and response to mechanical loading. The results of this experiment suggest an important role of Nrf2 in osteocyte function which improves certain indices of bone quality, which impacts male and female bones in different 7 ways, but did not significantly impact loading-induced bone formation. Further studies should modify the method of Nrf2 activation in an effort to refine the animal model, allowing the effects of Nrf2 to be isolated from the potential systemic effects of Keap1 deletion. Future studies should also utilize other conditional knockout models to elucidate the effects of Nrf2 in other specific cell types.