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

Abstract

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. Breast

Metformin Accumulation Correlates with Organic Cation Transporter 2 Protein Expression and Predicts Mammary Tumor Regression In Vivo

“…It is possible that alternative transporters may also play a role in tumor cell metformin uptake contributing to direct metformin effects in vivo . A recent study found overexpressing OCT3 in BT-20 breast cancer cells (triple negative) positively correlated with metformin uptake, increasing metformin uptake >13-fold and decreasing proliferation 4-fold, which established a clear relationship between rate of selective metformin uptake and suppression of breast cancer cell proliferation (34). …”

“…In concordance, hormonal regulation of kidney OCT2 expression was demonstrated in rats; testosterone upregulated OCT2 and estradiol moderately down-regulated OCT2 (35). Other studies identified variability in transporter expression profiles among luminal and basal (triple negative) breast cancer cell lines, with some triple negative lines showing higher levels of transporter expression, suggesting that heterogeneity in breast cancer tissue may also play a role in response to metformin (34). Triple negative breast cancer cells are more sensitive to metformin as compared to luminal lines (7).…”

Effect of Liver Disease on Hepatic Transporter Expression and Function

“…32 Classic substrates include tetraethylammonium (TEA) and the parkinsonian neurotoxin 1-methyl-4-phenylpyridinium (MPP + ); clinically relevant drug substrates include famotidine, ranitidine, and metformin. 33–35 …”

Repurposing metformin for the prevention of cancer and cancer recurrence

“…These include the organic cation transporters (OCT) 1–3, plasma monoamine transporter (PMAT) and the multidrug and toxin extrusion protein (MATE) 1 and 2 [1, 9]; OCT1 is critical for uptake into hepatocytes [10]. The expression of metformin transporters is variable across tumour cell lines and has been shown to be downregulated in breast tumour tissues in comparison to adjacent nonmalignant tissues [11]. In human cancer cell lines, which are known to express OCT1, metformin has been shown to decrease cell proliferation through inhibition of Complex I [12].…”