Characterization of cells resistant to the potent histone deacetylase inhibitor spiruchostatin B (SP-B) and effect of overexpressed p21waf1/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 highestsusceptibility among a number of leukemia cell lines to spiruchostatin B (SP-B),a potent histone deacetylase (HDAC) inhibitor. We also showed that SP-B-inducedcytotoxicity depended on induction of apoptosis that was mediated by p21waf1/cip1expression. In the present study, we generated and characterized a stable, SP-B-resistantNALM-6 cell line (NALM-6/SP-B) by continuous exposure to SP-B, starting with alow SP-B concentration. NALM-6/SP-B cells were also more resistant to FK228, whichhas a similar chemical structure to SP-B, and were slightly more resistant tothe P-gp substrates doxorubicin and vincristine than parental cells, but displayedsimilar susceptibility to other HDAC inhibitors and to paclitaxel as the parentalcells. 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 anexogenous p21waf1/cip1 expression vector restored SP-B induction of NALM-6/SP-Bcell apoptosis. Moreover, overexpressed p21waf1/cip1 enhanced SP-B induction ofthe apoptosis of the human erythroleukemia leukemia cell line K562 which is lesssusceptible to SP-B than NALM-6 cells. These results suggest that downregulationof p21waf1/cip1, which is a characteristic feature of NALM-6/SP-B cells, was importantfor their resistance to SP-B, and that this SP-B resistance could be overcomeby the introduction of exogenous p21waf1/cip1. Furthermore, introduction of p21waf1/cip1to 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 ofthe effect of overexpressed p21waf1/cip1 on the resistance or susceptibility ofhuman leukemia cells to SP-B.