Antitumor effects of metformin via indirect inhibition of protein phosphatase 2A in patients with endometrial cancer

OBJECTIVE: Metformin, an antidiabetic drug, inhibits the endometrial cancer cell growth in vivo by improving the insulin resistance; however, its mechanism of action is not completely understood. Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase associated with insulin resistance and type 2 diabetes, and its inhibition restores the insulin resistance. This study investigated the antitumor effect of metformin on endometrial cancer with a focus on PP2A. METHODS: Metformin (1,500–2,250 mg/day) was preoperatively administered to patients with endometrial cancer for 4 to 6 weeks. Expression of the PP2A regulatory subunits, 4 (PPP2R4) and B (PP2A-B), was evaluated using real-time polymerase chain reaction (RT–PCR) and immunohistochemistry (IHC) using paired specimens obtained before and after metformin treatment. The effect of PPP2R4 inhibition with small interfering RNA was evaluated in the endometrial cancer cell lines HEC265 and HEC1B. P values of < .05 were considered statistically significant. RESULTS: Preoperative metformin treatment significantly reduced the expression of PP2A-B, as determined using IHC, and the mRNA expression of PPP2R4, as determined using RT–PCR, in the patients with endometrial cancer. However, metformin could not directly alter the PPP2R4 mRNA levels in the endometrial cancer cell lines in vitro. PPP2R4 knockdown reduced the proliferation and induced the apoptosis by activating caspases 3/7 in HEC265 and HEC1B cells. CONCLUSIONS: Downregulation of the PP2A-B subunit, including PPP2R4, is an important indirect target of metformin. Inhibition of PP2A may be an option for the treatment of endometrial cancer patients with insulin resistance. TRIAL REGISTRATION: This trial is registered with UMIN-CTR (number UMIN000004852).