Cation‐selective transporters are critical to the AMPK‐mediated antiproliferative effects of metformin in human breast cancer cells

• Published in: International journal of cancer 2016-05, Vol.138 (9), p.2281-2292
• Main Authors: Cai, Hao, Zhang, Yunhui, Han, Tianxiang (Kevin), Everett, Ruth S., Thakker, Dhiren R.
• Format: Article
• Language: English
• Subjects: AMP-Activated Protein Kinases - metabolism; AMPK pathway; Animals; Antineoplastic Agents - pharmacology; antiproliferative activity; Breast cancer; Breast Neoplasms - pathology; Cancer; Cell Line, Tumor; Cell Proliferation - drug effects; Female; Kinases; Mammary Neoplasms, Experimental - drug therapy; Mammary Neoplasms, Experimental - pathology; Medical research; metformin; Metformin - pharmacology; Mice; Mice, Nude; Organic Cation Transport Proteins; Phosphorylation; Ribosomal Protein S6 Kinases, 70-kDa - metabolism; Signal Transduction - drug effects; TOR Serine-Threonine Kinases - metabolism; transporters; Tumors; Xenograft Model Antitumor Assays
• ISSN: 0020-7136; 1097-0215
• DOI: 10.1002/ijc.29965

The antidiabetic drug metformin exerts antineoplastic effects against breast cancer and other cancers. One mechanism by which metformin is believed to exert its anticancer effect involves activation of its intracellular target, adenosine monophosphate‐activated protein kinase (AMPK), which is also implicated in the antidiabetic effect of metformin. It is proposed that in cancer cells, AMPK activation leads to inhibition of the mammalian target of rapamycin (mTOR) and the downstream pS6K that regulates cell proliferation. Due to its hydrophilic and cationic nature, metformin requires cation‐selective transporters to enter cells and activate AMPK. This study demonstrates that expression levels of cation‐selective transporters correlate with the antiproliferative and antitumor efficacy of metformin in breast cancer. Metformin uptake and antiproliferative activity were compared between a cation‐selective transporter‐deficient human breast cancer cell line, BT‐20, and a BT‐20 cell line that was engineered to overexpress organic cation transporter 3 (OCT3), a representative of cation‐selective transporters and a predominant transporter in human breast tumors. Metformin uptake was minimal in BT‐20 cells, but increased by >13‐fold in OCT3‐BT20 cells, and its antiproliferative potency was >4‐fold in OCT3‐BT20 versus BT‐20 cells. This increase in antiproliferative activity was associated with greater AMPK phosphorylation and decreased pS6K phosphorylation in OCT3‐BT20 cells. In vitro data were corroborated by in vivo observations of significantly greater antitumor efficacy of metformin in xenograft mice bearing OCT3‐overexpressing tumors versus low transporter‐expressing wildtype tumors. Collectively, these findings establish a clear relationship between cation‐selective transporter expression, the AMPK‐mTOR‐pS6K signaling cascade, and the antiproliferative activity of metformin in breast cancer. What's new? Metformin, a diabetes drug, is widely known for its antitumor activities, which are effected through direct and indirect pathways. The direct pathway, which involves intracellular activation of adenosine monophosphate‐activated protein kinase (AMPK), may depend on transporter‐mediated uptake of metformin. Here, cation‐selective transporter expression was found to correlate with transporter‐mediated metformin uptake, metformin‐mediated modulation of the AMPK‐pS6K‐mTOR pathway, and metformin antiproliferative activity in human breast cancer cell lines. Transporter