Involvement of p21waf1/cip1 expression in the cytotoxicity of the potent histone deacetylase inhibitor spiruchostatin B towards susceptible NALM-6 human B cell leukemia cells

Spiruchostatin B (SP-B) is a potent histone deacetylase (HDAC) inhibitor that has potential for the chemotherapy of leukemia. The aim of this study was to study the susceptibility of human leukemia cell lines to SP-B. We found that NALM-6 human B cell leukemia cells are the most susceptible to SP-B. There was a low correlation between the expression of HDAC1 mRNA and HDI susceptibility of leukemia cells. NALM-6 has higher endogenous p21waf1/cip1 mRNA expression than other leukemia cells. SP-B-induced cytotoxicity was mediated by induction of histone acetylation via inhibition of HDACs, and this effect of SP-B was associated with apoptosis, which was mediated by caspase activation in NALM-6 cells. SP-B time-dependently increased the size of the sub-G1 (apoptotic) peak, and this effect correlated with SP-B induction of cellular apoptotic features such as changes in nuclear morphology. SP-B significantly increased p21waf1/cip1 expression prior to induction of apoptosis. In conclusion, NALM-6 cells, which have a higher expression of p21waf1/cip1 mRNA than other leukemia cell lines, were susceptible to SP-B-induced cytotoxicity that resulted in induction of apoptosis. Our findings may be useful when establishing a therapeutic strategy based on SP-B.