β-Hydroxyisovaleryshikonin is a novel and potent inhibitor of protein tyrosine kinases

Abstract

β-Hydroxyisovalerylshikonin (β-HIVS), a compound isolated from Lithospermium radix, most efficiently induced cell-death in two lines of lung cancer cells, namely, NCI-H522 and DMS114, whereas shikonin was effective against a wide variety of tumor cell lines. During our studies of the mechanism of action of β-HIVS on tumor cells, we found that this compound inhibited protein tyrosine kinase (PTK) activity. The tyrosine kinase activities of a receptor for EGF (EGFR) and v-Src were strongly inhibited and that of KDR/Flk-1 was weakly inhibited by Β-HIVS. The inhibition by β-HIVS of the activities of EGFR and v-Src was much stronger than that by shikonin. The IC50 values of β-HIVS for EGFR and v-Src were approximately 0.7 μM and 1 μM, respectively. Moreover, the inhibition of v-Src by β-HIVS was non-competitive with respect to ATP. These results strongly suggest that the action of β-HIVS, as well as that of shikonin, involves the inhibition of PTK, and they also suggest the possibility of producing a novel group of PTK inhibitors based on shikonin as the parent compound.