Abstract

BackgroundMetformin, a first-line drug for type 2 diabetes, could induce apoptosis in cancer cells. However, the concentration of glucose affects the effect of metformin, especially low glucose in the culture medium can enhance the cytotoxicity of metformin on cancer cells. Since mitochondria and endoplasmic reticulum is vital for maintaining cell homeostasis, we speculate that low glucose and metformin-induced cell apoptosis may be associated with mitochondria and endoplasmic reticulum. ASK1, as apoptosis signaling regulating kinase 1, is associated with cell apoptosis and mitochondrial damage. This study was designed to investigate the functional significance of ASK1, mitochondria and endoplasmic reticulum and underlying mechanism in low glucose and metformin-induced cell apoptosis.MethodsAn MTT assay was used to evaluate cell viability in SKOV3, OVCAR3 and HO8910 human ovarian cancer cells. Cell apoptosis was analyzed by flow cytometry. The expression of ASK1 was inhibited using a specific pharmacological inhibitor or ASK1-siRNA. Immunofluorescence was used to detect mitochondrial damage and ER stress. Nude mouse xenograft models were given metformin or/and NQDI-1, and ASK1 expression was detected using immunoblotting. In addition, subcellular fractionation of mitochondria was performed to assay the internal connection between ASK1 and mitochondria.ResultsThe present study found that low glucose in culture medium enhanced the anticancer effect of metformin in human ovarian cancer cells. Utilization of a specific pharmacological inhibitor or ASK1-siRNA identified a potential role for ASK1 as an apoptotic protein in the regulation of low glucose and metformin-induced cell apoptosis via ASK1-mediated mitochondrial damage through the ASK1/Noxa pathway and via ER stress through the ROS/ASK1/JNK pathway. Moreover, ASK1 inhibition weakened the antitumor activity of metformin in vivo. Thus, mitochondrial damage and ER stress play a crucial role in low glucose–enhanced metformin cytotoxicity in human ovarian cancer cells.ConclusionsThese data suggested that low glucose and metformin induce cell apoptosis via ASK1-mediated mitochondrial damage and ER stress. These findings indicated that the effect of metformin in anticancer treatment may be related to cell culture conditions.

Highlights

  • Metformin, a first-line drug for type 2 diabetes, could induce apoptosis in cancer cells

  • apoptosis signaling regulating kinase 1 (ASK1) was involved in the anti-tumor effect of metformin in vivo. These data suggested that low glucose and metformin induce cell apoptosis via ASK1-mediated mitochondrial damage and ER stress

  • These results indicated that the combination of low glucose and metformin triggers cell apoptosis through the mitochondria-associated pathway

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Summary

Introduction

A first-line drug for type 2 diabetes, could induce apoptosis in cancer cells. The concentration of glucose affects the effect of metformin, especially low glucose in the culture medium can enhance the cytotoxicity of metformin on cancer cells. This study was designed to investigate the functional significance of ASK1, mitochondria and endoplasmic reticulum and underlying mechanism in low glucose and metformin-induced cell apoptosis. High glucose promotes the proliferation of cancer cells, whereas reduced glucose enhances the cytotoxicity of therapeutic drugs, such as metformin, in several cancers, including ovarian cancer [9]. Zhuang Y et al found low glucose and metformin treatment in cancer cells leads to cell death by decreasing ATP production and inhibiting survival signaling pathways [9]. In the response to metformin-induced energetic stress, the byproducts of mitochondrial respiration, reactive oxygen species (ROS), damage cellular components, such as mitochondria, leading to cell death in high concentrations [18]. ASK1 activation selectively results in sustained Jun N-terminal kinase (JNK) activation, which is associated with ER stress

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