MECHANISM OF ANTITUMOR ACTION OF PKC ACTIVATOR, GNIDIMACRIN

Daphnane-type diterpene gnidimacrin isolated from the Chinese plant St ellera chomaejosme L. is an antitumor agent that activates protein kin ase C (PKC). The mechanism of antitumor action of gnidimacrin and the possible involvement of PKC were examined using sensitive K562 and ref ractory HLE cells. Gnidimacrin did bind to K562 cells 3 times more tha n to HLE cells. Immunoblot analyses revealed pronounced PKC beta II ex pression in gnidimacrin sensitive cell lines including K562 cells, whi le refractory HLE cells strongly expressed PKC alpha, but not PKC beta II. In a 24-hr exposure of K562 cells to gnidimacrin, GI phase arrest and inhibition of cdk2 kinase activity was found at growth-inhibitory concentration (0.0005 mu g/ml). Complete inhibition of cdk2 activity and maximum GI phase arrest were observed at 0.005 mu g/ml, however, t hese biological effects were reduced at 0.05 mu g/ml (260 times the 50 % inhibitory concentration). Cellular PKC after a 24-hr exposure was e xamined by immunoblot analysis and specific binding of [H-3]phorbol-12 ,13-dibutyrate as a ligand of PKC. Expression and the amount of functi onal PKC of K562 cells were not changed at 0.002 mu g/ml, but down-reg ulated to less than 1/10th of the control at 0.05 mu g/ml. The reducti on of biological effects at 0.05 mu g/ml is most likely due to PKC dow n-regulation. Our results suggest that PKC (particularly beta II) is o ne of the major determinants of the ability of cells to respond to gni dimacrin and that the antitumor action might be associated with cell-c ycle regulation through suppression of cdk2 activity. (C) 1998 Wiley-L iss, Inc.