Quercetin, a widely distributed bioflavonoid, inhibits the growth of t
umor cells. The present study was designed to investigate the possible
involvement of apoptosis and heat shock protein in the antitumor acti
vity of quercetin. Treatment with quercetin of K562, Molt-4, Raji, and
MCAS tumor cell lines resulted in morphological changes, including pr
opidium iodide-stained condensed nuclei (intact or fragmented), conden
sation of nuclear chromatin, and nuclear fragmentation. Agarose gel el
ectrophoresis of quercetin-treated tumor cells demonstrated a typical
ladder-like pattern of DNA fragments. In addition, the hypodiploid DNA
peak of propidium iodide-stained nuclei was revealed by flow cytometr
y. Quercetin induced apoptosis in cells at G(1) and S in a dose- and t
ime-dependent manner. The apoptosis-inducing activity of quercetin was
enhanced by cycloheximide and actinomycin D. A nuclease inhibitor, au
rintricarboxylic acid, inhibited quercetin-induced apoptosis, whereas
deprivation of intracellular calcium by EGTA had no effect, 12-O-Tetra
decanoylphorbol-13-acetate and H-7 did not affect the induction of apo
ptosis by quercetin. The synthesis of HSP70 was inhibited by quercetin
when determined by immunocytochemistry, Western blot analysis, and No
rthern blot analysis. Quercetin-treated tumor cells were not induced t
o show aggregation of HSP70 in the nuclei and nucleolus in response to
heat shock, resulting in apoptosis. By contrast, when tumor cells wer
e first exposed to heat shock, no apoptosis was induced by quercetin.
In addition, pretreatment of tumor cells with HSP70 antisense oligomer
that specifically inhibited the synthesis of HSP70 enhanced the subse
quent induction of apoptosis by quercetin. These results suggest that
quercetin displays antitumor activity by triggering apoptosis and that
HSP70 may affect quercetin-induced apoptosis.