Metformin triggers apoptosis via endoplasmic reticulum stress in HER2‑positive breast cancer cell lines

The antidiabetic drug metformin has potential as an anticancer agent, particularly due to its observed efficacy in breast cancer. Metformin exerts its cytotoxic effects in the induction of endoplasmic reticulum (ER) stress, which can trigger apoptotic cell death pathways. Therefore, the present study aimed to investigate the dose‑dependent effects of metformin on ER stress and apoptosis in HER2‑positive breast cancer SKBR3 cells. For this purpose, SKBR3 cells were treated with 5, 10 and 20 mM metformin. Cell proliferation was assessed using real‑time cell analysis, while expression levels of ER stress‑associated genes [glucose‑regulated protein 78 kDa (GRP78), PRKR‑like ER kinase (PERK), inositol‑requiring enzyme 1 (IRE1), activating transcription factor 6 (ATF6) and CHOP)] were measured by revese transcription‑quantitative PCR. Apoptosis was analyzed by Annexin V‑FITC/PI flow cytometry in cells treated with 10 and 20 mM metformin. Findings revealed that metformin (5, 10 and 20 mM) dose‑dependently inhibited cell proliferation and activated ER stress pathways. Significant increases were observed in gene expression following treatment with 5, 10 and 20 mM metformin, respectively, including  GRP78 (3.70‑, 5.06‑ and 7.33‑fold; all P<0.0001) PERK (2.48‑, 4.36‑ and 9.11‑fold; all P<0.0001), IRE1 (2.15‑fold, P=0.001; 2.90‑fold, P<0.001; 5.55‑fold, P<0.0001), ATF6 (2.43‑2.44‑ and 3.63‑fold; all P<0.0001) and particularly in pro‑apoptotic CHOP (3.31‑, 27.47‑ and 49.85‑fold; all P<0.0001). Flow cytometry revealed that 10 and 20 mM metformin significantly increased early apoptosis to 6.05% (P<0.001) and 7.28% (P<0.001) and late apoptosis to 13.24% (P<0.001) and 20.59% (P<0.001), respectively, compared with controls (early apoptosis, 0.02%; late apoptosis, 0.05%). The present findings demonstrated that metformin activates ER stress response and induces apoptosis in HER2‑positive breast cancer cells in a dose‑dependent manner. This supports the potential of metformin as an adjuvant therapy, though further in vivo studies are needed to evaluate its clinical applicability.