Effects of luteolin and quercetin, inhibitors of tyrosine kinase, on cell growth and metastasis-associated properties in A431 cells overexpressing epidermal growth factor receptor

1 Flavonoids display a wide range of pharmacological properties including a nti-inflammatory. Anti-mutagenic, anti-carcinogenic and anti-cancer effects . Here, we evaluated the effects of eight flavonoids on the tumour cell pro liferation, cellular protein phosphorylation, and matrix metalloproteinase (MMPs) secretion. 2 Of the flavonoids examined, luteolin (Lu) and quercetin (Qu) were the two most potent agents, and significantly inhibited A431 cell proliferation wi th IC50 values of 19 and 21 mu M, respectively. 3 The epidermal growth factor (EGF) (10 nM) promoted growth of A431 cells ( + 25 +/- 4.6%) and mediated epidermal growth factor receptor (EGFR) tyrosin e kinase activity and autophosphorylation of EGFR were inhibited by Lu and Qu. At concentration of 20 mu M, both Lu and Qu markedly decreased the leve ls of phosphorylation of A431 cellular proteins, including EGFR. 4 A431 cells treated with Lu or Qu exhibited protuberant cytoplasmic blebs and progressive shrinkage morphology. Lu and Qu also time-dependently induc ed the appearance of a ladder pattern of DNA fragmentation, and this effect was abolished by EGF treatment. 5 The addition of EGF only marginally diminished the inhibitory effect of l uteolin and quercetin on the growth rate of A431 cells, treatment of cellul ar proteins with EGF and luteolin or quercetin greatly reduced protein phos phorylation, indicating Lu and Qu may act effectively to inhibit a wide ran ge of protein kinases, including EGFR tyrosine kinase. 6 EGF increased the levels of matrix metalloproteinase-2 (MMP-2) and matrix metalloproteinase-9 (MMP-9), while Lu and Qu appeared to suppress the secr etion of these two MMPs in A431 cells. 7 Examination of the relationship between the chemical structure and inhibi tory effects of eight flavonoids reveal that the double bond between C2 and C3 in ring C and the OH groups on C3' and C4' in ring B are critical for t he biological activities. 8 This study demonstrates that the inhibitory effects of Lu and Qu, and the stimulatory effects of EGF, on tumour cell proliferation, cellular protein phosphorylation, and MMP secretion may be mediated at least partly through EGFR. This study supports the idea that Lu and Qu may have potential as an ti-cancer and anti-metastasis agents.