Daphnetin, one of coumarin derivatives, is a protein kinase inhibitor

Protein kinases play key roles in the control of cell proliferation, differ entiation and metabolism. In this work, we studied the effect of coumarin a nd its derivatives, including daphnetin, esculin, 2-OH-coumarin, 4-OH-couma rin and 7-OH-coumarin, on the activity of protein kinases. It was found tha t, in these compounds, only daphnetin was a protein kinase inhibitor. This compound inhibited tyrosine-specific protein kinase, EGF receptor (IC50 = 7 .67 mu M) and serine/ threonine-specific protein kinases, including cAMP-de pendent protein kinase (PKA) (IC50 = 9.33 mu M) and protein kinase C (PKC) (IC50 = 25.01 mu M) in vitro. The inhibition of EGF receptor tyrosine kinas e by daphnetin was competitive to ATP and non-competitive to the peptide su bstrate. The inhibition of EGF-induced tyrosine phosphorylation of EGF rece ptor by daphnetin was not observed in human hepatocellular carcinoma HepG2 cells. The structural comparison of daphnetin with coumarin and other couma rin derivatives suggests that the hydroxylation at C8 may be required for d aphnetin acting as a protein kinase inhibitor. (C) 1999 Academic Press.