Metformin Inhibits Mammalian Target of Rapamycin–Dependent Translation Initiation in Breast Cancer Cells

Abstract

Metformin is used for the treatment of type 2 diabetes because of its ability to lower blood glucose. The effects of metformin are explained by the activation of AMP-activated protein kinase (AMPK), which regulates cellular energy metabolism. Recently, we showed that metformin inhibits the growth of breast cancer cells through the activation of AMPK. Here, we show that metformin inhibits translation initiation. In MCF-7 breast cancer cells, metformin treatment led to a 30% decrease in global protein synthesis. Metformin caused a dose-dependent specific decrease in cap-dependent translation, with a maximal inhibition of 40%. Polysome profile analysis showed an inhibition of translation initiation as metformin treatment of MCF-7 cells led to a shift of mRNAs from heavy to light polysomes and a concomitant increase in the amount of 80S ribosomes. The decrease in translation caused by metformin was associated with mammalian target of rapamycin (mTOR) inhibition, and a decrease in the phosphorylation of S6 kinase, ribosomal protein S6, and eIF4E-binding protein 1. The effects of metformin on translation were mediated by AMPK, as treatment of cells with the AMPK inhibitor compound C prevented the inhibition of translation. Furthermore, translation in MDA-MB-231 cells, which lack the AMPK kinase LKB1, and in tuberous sclerosis complex 2 null (TSC2 À/À ) mouse embryonic fibroblasts was unaffected by metformin, indicating that LKB1 and TSC2 are involved in the mechanism of action of metformin. These results show that metformin-mediated AMPK activation leads to inhibition of mTOR and a reduction in translation initiation, thus providing a possible mechanism of action of metformin in the inhibition of cancer cell growth. [Cancer Res 2007;67(22):10804-12]

Gene expression analysis of mTOR pathway: association with human longevity

“…Additionally, intermediate phenotype insulin resistance in human diabetes can be prevented by mTOR inhibitors (Krebs et al, 2007). Metformin, a commonly prescribed anti-diabetic drug which inhibits mTORC1 downstream pathways through activation of AMP kinase, has also been shown to reduce tumour growth (Dowling et al, 2007). Human CR studies used as model for down-regulation of mTOR are much more short term in comparison with animal studies, but nevertheless, CR may reduce the risk of age-related diseases even in non-obese humans (Fontana et al, 2004;Holloszy & Fontana, 2007).…”

Diet and tumor LKB1 expression interact to determine sensitivity to anti-neoplastic effects of metformin in vivo

“…In addition, an AMPK-independent, rag GTPasedependent pathway by which metformin inhibits mammalian target of rapamycin complex 1 has recently been described (Kalender et al, 2010). By decreasing mitochondrial ATP production, metformin can indirectly activate LKB1-AMPK signaling in vitro in transformed cells, with consequences including inhibition of protein synthesis (Dowling et al, 2007), proliferation (Zakikhani et al, 2006 and expression of fatty acid synthase (Algire et al, 2010).…”

Metformin Induces G1 Cell Cycle Arrest and Inhibits Cell Proliferation in Nasopharyngeal Carcinoma Cells

“…Metformin may have direct effects on cancer cells through the activation of AMP-activated protein-kinase (AMPK). Furthermore, it is able to inhibit mammalian target of rapamycin complex 1 (mTORC1), a down-stream effector of AMPK and is expected to decrease oncogenic proliferation in some cancers (Dowling et al, 2007;Isakovic et al, 2007;Jiang et al, 2008;Zakikhani et al, 2006).…”

“…The LPM 1 and LMP 2A activate the phosphatidylinositol 3 0 -OH kinase (PI3-K)/protein kinase B (Akt) pathway, which is deregulated in various human malignancies. Recently, metformin has been shown to inhibit cell growth in breast cancer cells through activation of AMPK and inhibition of translation initiation (Dowling et al, 2007;Zakikhani et al, 2006). It may also prevent cell proliferation in prostate and colon cancer cells (Zakikhani et al, 2006) and reduce the risk of human pancreatic cancer in diabetic patients ).…”