Abstract. Diabetes mellitus (DM) is an important factor that contributes to the development of type I endometrial cancer (EC). Previous studies have demonstrated that metformin decreases mortality and risk of neoplasms in patients with DM. Since estrogen and estrogen receptor (ER) expression has been associated with the development of EC, the present study aimed to investigate the effects of metformin on cell proliferation and ER expression in EC cell lines that are sensitive to estrogen. The viability and proliferation of Ishikawa and HEC-1-A cells were measured following treatment with metformin and/or a 5' AMP-activated protein kinase (AMPK) inhibitor (compound C) with or without treatment with estradiol (E2). In addition, the levels of ERα, ERβ, AMPK, ribosomal protein S6 kinase β-1 (p70S6K), myc proto-oncogene protein (c-myc) and proto-oncogene c-fos (c-fos) were measured following treatment. Metformin significantly decreased E2-stimulated cell proliferation; an effect that was rescued in the presence of compound C. Metformin treatment markedly increased the phosphorylation of AMPK while decreasing p70S6K phosphorylation, indicating that metformin exerts its effects through stimulation of AMPK and subsequent inhibition of the mammalian target of rapamycin (mTOR) signaling pathway. In addition, metformin significantly inhibited ERα expression while increasing ERβ expression, whereas treatment with compound C reversed these effects. Reverse transcription-quantitative polymerase chain reaction analysis demonstrated that c-fos and c-myc expression were attenuated by metformin, an effect that was rescued in the presence of compound C. Therefore, metformin regulates the expression of ERs, and inhibits estrogen-mediated proliferation of human EC cells through the activation of AMPK and subsequent inhibition of the mTOR signaling pathway.
IntroductionEndometrial cancer (EC) is the fourth most common cancer in women worldwide and is the most common type of gynecological cancer (1). Patients with high estrogen levels are at increased risk of developing EC since estrogen exhibits growth-promoting properties in EC cells. Thus, estrogen serves as a tumor initiator, since it directly induces DNA mutations in tumor-suppressor genes and oncogenes (2). Upon binding to its receptor, estrogen triggers the transcription of a number of genes. There are two classes of estrogen receptor (ER), ERα and ERβ, which are encoded by the estrogen receptor 1 (ESR1) and ESR2 genes, bind to the same estrogen response elements (EREs) and regulate similar sets of genes (3). However, during the early stages of EC, the expression of ERα is increased compared with that of ERβ (4,5), which activates ERα upon estradiol (E2) binding. This stimulates the expression of estrogen target genes and leads to enhanced proliferation of the previously transformed cells, while causing additional errors in replication and potentially further DNA mutations (2).Previous studies have demonstrated that EC is associated with a shift in the ratio of the two ER subty...