Mechanical Stimulation and IGF‐1 Enhance mRNA Translation Rate in Osteoblasts Via Activation of the AKT‐mTOR Pathway

Insulin‐like growth factor‐1 (IGF‐1) is anabolic for muscle by enhancing the rate of mRNA translation via activation of AKT and subsequent activation of the mammalian target of rapamycin complex 1 (mTOR), thereby increasing cellular protein production. IGF‐1 is also anabolic for bone, but whether the mTOR pathway plays a role in the rate of bone matrix protein production by osteoblasts is unknown. We hypothesized that anabolic stimuli such as mechanical loading and IGF‐1 stimulate protein synthesis in osteoblasts via activation of the AKT‐mTOR pathway. MC3T3‐E1 osteoblasts were either or not subjected for 1 h to mechanical loading by pulsating fluid flow (PFF) or treated with or without human recombinant IGF‐1 (1–100 ng/ml) for 0.5–6 h, to determine phosphorylation of AKT and p70S6K (downstream of mTOR) by Western blot. After 4 days of culture with or without the mTOR inhibitor rapamycin, total protein, DNA, and gene expression were quantified. IGF‐1 (100 ng/ml) reduced IGF‐1 gene expression, although PFF enhanced IGF‐1 expression. IGF‐1 did not affect collagen‐I gene expression. IGF‐1 dose‐dependently enhanced AKT and p70S6K phosphorylation at 2 and 6 h. PFF enhanced phosphorylation of AKT and p70S6K already within 1 h. Both IGF‐1 and PFF enhanced total protein per cell by ∼30%, but not in the presence of rapamycin. Our results show that IGF‐1 and PFF activate mTOR, thereby stimulating the rate of mRNA translation in osteoblasts. The known anabolic effect of mechanical loading and IGF‐1 on bone may thus be partly explained by mTOR‐mediated enhanced protein synthesis in osteoblasts. J. Cell. Physiol. 231: 1283–1290, 2016. © 2015 Wiley Periodicals, Inc.