Characterization of cells resistant to the potent histone deacetylase inhibitor spiruchostatin B (SP-B) and effect of overexpressed p21(waf1/cip1) on the SP-B resistance or susceptibility of human leukemia cells

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 p21(waf1/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 p21(waf1/cip1) was decreased. The introduction of an exogenous p21(waf1/cip1) expression vector restored SP-B induction of NALM-6/SP-B cell apoptosis. Moreover, overexpressed p21(waf1/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 p21(waf1/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 p21(waf1/cip1). Furthermore, introduction of p21(waf1/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 p21(waf1/cip1) on the resistance or susceptibility of human leukemia cells to SP-B.