Anticancer effect of luteolin is mediated by downregulation of TAM receptor tyrosine kinases, but not interleukin-8, in non-small cell lung cancer cells.

TAM receptor tyrosine kinases (RTKs), Tyro3, Axl and MerTK, transduce diverse signals responsible for cell survival, growth, proliferation and anti-apoptosis. In the present study, we demonstrated the effect of luteolin, a flavonoid with antioxidant, anti-inflammatory and anticancer activities, on the expression and activation of TAM RTKs and the association with its cytotoxicity in non-small cell lung cancer (NSCLC) cells. We observed the cytotoxic effect of luteolin in parental A549 and H460 cells as well as in cisplatin-resistant A549/CisR and H460/CisR cells. Exposure of these cells to luteolin also resulted in a dose‑dependent decrease in clonogenic ability. Next, luteolin was found to decrease the protein levels of all three TAM RTKs in the A549 and A549/CisR cells in a dose‑dependent manner. In a similar manner, in H460 and H460/CisR cells, the protein levels of Axl and Tyro3 were decreased following luteolin treatment. In addition, Axl promoter activity was decreased by luteolin, indicating that luteolin suppresses Axl expression at the transcriptional level. We next found that luteolin abrogated Axl phosphorylation in response to growth arrest-specific 6 (Gas6), its ligand, implying the inhibitory effect of luteolin on Gas6-induced Axl activation. Ectopic expression of Axl was observed to attenuate the antiproliferative effect of luteolin, while knockdown of the Axl protein level using a gold nanoparticle-assisted gene delivery system increased its cytotoxicity. In contrast to the inhibitory effect of luteolin on the expression of TAM RTKs, interleukin-8 (IL-8) production was not decreased by luteolin in H460 and H460/CisR cells, while IL-8 production/cell was increased. Collectively, our data suggest that TAM RTKs, but not IL-8, are promising therapeutic targets of luteolin to abrogate cell proliferation and to overcome chemoresistance in NSCLC cells.