We previously showed that the
B cell leukemia cell line NALM-6 had the highest susceptibility among a number of
leukemia cell lines to
spiruchostatin B (SP-B), a potent
histone deacetylase (
HDAC) inhibitor. We also showed that SP-B-induced cytotoxicity depended on induction of apoptosis that was mediated by p21waf1/cip1 expression. In the present study, we generated and characterized a stable, SP-B-resistant NALM-6 cell line (NALM-6/SP-B) by continuous exposure to SP-B, starting with a low SP-B concentration. NALM-6/SP-B cells were also more resistant to
FK228, which has a similar chemical structure to SP-B, and were slightly more resistant to the P-gp substrates
doxorubicin and
vincristine than parental cells, but displayed similar susceptibility to other
HDAC inhibitors and to
paclitaxel as the parental cells. There was little change in the basal
mRNA expression of HDAC1, p53, Bax, Bcl-2, Fas,
caspase-3, c-Myc and MDR1 in NALM-6/SP-B compared to parental cells, but the
mRNA expression of p21waf1/cip1 was decreased. The introduction of an exogenous p21waf1/cip1 expression vector restored SP-B induction of NALM-6/SP-B cell apoptosis. Moreover, overexpressed p21waf1/cip1 enhanced SP-B induction of the apoptosis of the human
erythroleukemia leukemia cell line K562 which is less susceptible to SP-B than NALM-6 cells. These results suggest that downregulation of p21waf1/cip1, which is a characteristic feature of NALM-6/SP-B cells, was important for their resistance to SP-B, and that this SP-B resistance could be overcome by the introduction of exogenous p21waf1/cip1. Furthermore, introduction of p21waf1/cip1 to other
leukemia cells such as K562 may enhance their susceptibility to SP-B. This is the first report of the characterization of SP-B-resistant cells and of the effect of overexpressed p21waf1/cip1 on the resistance or susceptibility of human
leukemia cells to SP-B.