mTORC1 Inhibits NF-κB/NFATc1 Signaling and Prevents Osteoclast Precursor Differentiation, In Vitro and In Mice

Abstract

The mechanistic target of rapamycin complex 1 (mTORC1) is a critical sensor for bone homeostasis and bone formation; however, the role of mTORC1 in osteoclast development and the underlying mechanisms have not yet been fully established. Here, we found that mTORC1 activity declined during osteoclast precursors differentiation in vitro and in vivo. We further targeted deletion of Raptor (mTORC1 key component) or Tsc1 (mTORC1 negative regulator) to constitutively inhibit or activate mTORC1 in osteoclast precursors (monocytes/macrophages), using LyzM-cre mice. Osteoclastic formation was drastically increased in cultures of Raptor deficient bone marrow monocytes/macrophages (BMMs), and Raptor-deficient mice displayed osteopenia with enhanced osteoclastogenesis. Conversely, BMMs lacking Tsc1 exhibited a severe defect in osteoclast-like differentiation and absorptive function, both of which were restored following rapamycin treatment. Importantly, expression of nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) and nuclear factor of activated T cells, cytoplasmic 1 (NFATc1), transcription factors that are essential for osteoclast differentiation was negatively regulated by mTORC1 in osteoclast lineages. These results provide evidence that mTORC1 plays as a critical role as an osteoclastic differentiation-limiting signal and suggest a potential drawback in treating bone loss-related diseases with mTOR inhibitors clinically. © 2017 American Society for Bone and Mineral Research.

mTORC1 signalling is not essential for the maintenance of muscle mass and function in adult sedentary mice

“…Two well‐described transcription factors involved in the slow fibre switch program are NFATc1 and MEF2 . Levels of MEF2A and MEF2D are both increased in RAmKO muscle, and NFATc1 has been shown to be negatively regulated by mTORC1 . Yet, in both cases, it is unclear whether mTORC1 directly affects these proteins.…”

mTORC1 impedes osteoclast differentiation via calcineurin and NFATc1

“…Despite the biological and clinical importance, there are only a few reports on mTORC1 and osteoclastogenesis, including two recent studies presenting conflicting results 36,37 . However, these studies could not provide a unifying explanation for the contradictory clinical observations.…”

“…In another study, Zhang et al deleted Raptor or Tsc1 with LyzM-Cre to inhibit or activate mTORC1 signaling, respectively 37 . In agreement with this study, we observed similar in vivo bone phenotypes as well as ex vivo osteoclast differentiation using LyzM-Cre.…”

“…Zhang et al concluded that mTORC1 inhibits NF-kB/NFATc1 signaling to prevent osteoclast differentiation, but did not provide a clear mechanism for how mTORC1 inhibits NFATc1 37 . First, it is well known that NFATc1 is activated by calcineurin-mediated dephosphorylation and subsequent nuclear translocation 8,9,13–15 .…”

“…As such, rapamycin has been FDA approved as immunosuppressant and cancer drugs. Despite the biological and clinical importance, there are only a few reports on how mTORC1 impacts osteoclastogenesis, including two recent studies presenting conflicting results without clear molecular mechanisms 36,37 . In the present study, we have investigated the roles of mTORC1 signaling in osteoclastogenesis using comprehensive and integrated strategies combining genetic and pharmacological mouse models, gain- and loss-of-function approaches, in vitro and in vivo.…”