Metformin targets Axl and Tyro3 receptor tyrosine kinases to inhibit cell proliferation and overcome chemoresistance in ovarian cancer cells

Metformin, the most frequently prescribed anti-diabetic drug, has recently been paid attention as a chemotherapeutic agent. In this study, we demonstrated that metformin decreased the viability of parental as well as cisplatin/taxol-resistant ovarian cancer cells. Its anti-proliferative effect was further demonstrated by dose-dependent reduction of the clonogenic ability of the metformin-treated cells. We next observed the effect of metformin on expression of Axl and Tyro3 receptor tyrosine kinases (RTKs) which belong to the TAM subfamily of RTKs transducing pro-survival and antiapoptotic signals. Metformin treatment of ovarian cancer cells decreased both mRNA and protein levels of Axl and Tyro3 in a dose-dependent manner. Axl promoter activity was also inhibited by metformin, indicating that metformin suppresses Axl and Tyro3 expression at the transcriptional level. Metformin treatment was also found to augment its anti-proliferative effect in SKOV3 and taxol-resistant SKOV3/TR cells transfected with Axl and Tyro3 specific siRNAs, siAxl and siTyro3, respectively, suggesting that metformin might target Axl and Tyro3 RTKs to restrain cell proliferation. In parallel, the level of X-linked inhibitor of apoptosis protein (XIAP), an antiapoptotic molecule, was reduced in the metformin-treated cells. Collectively, our data showed that metformin caused reduction of Axl and Tyro3 RTKs' expression, inactivation of downstream effectors, and decrease of anti-apoptotic protein level, forming a potent therapeutic strategy to facilitate its anticancer activity as well as to overcome chemoresistance in human ovarian cancer cells.