The interactions between mTOR and NF‐κB: A novel mechanism mediating mechanical stretch‐stimulated osteoblast differentiation

Abstract

Mechanical stretch is known to promote osteoblast differentiation in vitro and accelerate bone regeneration in vivo, whereas the relevant mechanism remains unclear. Recent studies have shown the importance of reciprocal interactions between mammalian target of rapamycin (mTOR) and nuclear factor kappa B (NF-κB; two downstream molecules of Akt) in the regulation of tumor cells. Thus, we hypothesize that mTOR and NF-κB as well as their interconnection play a critical role in mediating stretch-induced osteogenic differentiation in osteoblasts. We herein found that mechanical stretch (10% elongation at six cycles/min) significantly promoted the expression of osteoblast differentiation-related markers (including ALP, BMP2, Col1α, OCN, and Runx2) in osteoblast-like MG-63 cells, accompanied by increased mTOR phosphorylation and NF-κB p65 phosphorylation and nuclear translocation. Blockade of mTOR by antagonist or small interfering RNA suppressed osteogenesis-related gene expression in response to mechanical stretch, whereas inhibition of NF-κB further increased stretch-induced osteoblast differentiation. Moreover, inhibition of mTOR decreased the phosphorylation of NF-κB, and blockade of NF-κB reduced the mTOR activation in MG63 cells under mechanical stretch. Coinhibition of mTOR and NF-κB abolishes the alteration of osteogenic differentiation induced by single mTOR or NF-κB inhibition under mechanical stretch, which is equivalent to the noninhibition level for osteoblasts under mechanical stretch. The expression levels of osteogenic differentiation in osteoblasts after inhibition of Akt were similar to those after co-inhibition of mTOR and NF-κB under mechanical stretch. This study for the first time reveals the reciprocal interconnection between mTOR and NF-κB in osteoblasts under mechanical stretch and indicates that mTOR and NF-κB as well as their interactions play a key role in the regulation of cellular homeostasis of osteoblasts in response to mechanical stretch. These findings are helpful for enriching our basic knowledge of the molecular mechanisms of osteoblast mechanotransduction, and also providing insight

The Akt–mTORC1 pathway mediates Axl receptor tyrosine kinase-induced mesangial cell proliferation

“…The PI3K/Akt signaling pathway can induce cell type‐ and upstream signaling‐dependent activation of both the NF‐κB and mTOC1 pathways. 23 Cross regulation of the mTORC1 and NF‐κB pathways has recently been demonstrated in cancer cells 24,25 . To determine if similar coregulation exists in Axl‐activated mesangial cells, we evaluated the effects of adding small molecule inhibitors for Akt (AT7867, abbreviated AT), NFκ‐B (BMS345541, abbreviated BMS), or mTOR (rapamycin, abbreviated rap) to mesangial cell cultures prior to Axl activation.…”

“…23 Cross regulation of the mTORC1 and NF-κB pathways has recently been demonstrated in cancer cells. 24,25 To determine if similar coregulation exists in Axl-activated mesangial cells, we evaluated the effects of adding small molecule inhibitors for Akt , A1, and A2). This suggests that Akt is upstream of mTOR and NF-κB in the Axl signaling cascade.…”

Endogenous Aβ induces osteoporosis through an mTOR-dependent inhibition of autophagy in bone marrow mesenchymal stem cells (BMSCs)

“…Additionally, rapamycin, an mTOR inhibitor, increased BMD. These findings suggested that mTOR played a negative role in bone grafting mediated by osteoblasts by inhibiting the differentiation or the bone formation at late stages (30)(31)(32).…”

The role of mechanobiology in bone and cartilage model systems in characterizing initiation and progression of osteoarthritis

“…For example, in osteocytes, cell stretching (5% elongation over 1-20 min) has an anti-apoptotic effects through ERK1/2 activation. 198 In osteoblasts, numerous studies applying cyclical stretching have shown that osteoblastic maturation is accelerated, osteogenic gene expression is increased, and ECM deposition enhanced, [199][200][201] while in osteoclasts, bone resorbing activity is increased by cyclic stretching in 2D cultures. 202 These devices have been limited by their relatively low through-put capabilities.…”