Vanadate (10 microM), a potent inhibitor of tyrosine phosphatase, added simultaneously potentiated BuONa-induced (1 mM) apoptosis. The steady-state level of nucleophosmin/B23 mRNA and the total cellular nucleophosmin/B23 protein decreased during the BuONa/vanadate-induced apoptosis. Stabilization and promotor transcriptional activity assays indicate that the decrease in nucleophosmin/B23 mRNA in BuONa/vanadate-treated HL-60 cells was transcriptionally regulated. A decline in telomerase activity was observed in HL-60 cells treated with BuONa/vanadate for 24-96 h. There was virtually no decline of nucleophosmin/B23 mRNA nor the telomerase activities during the growth arrest by serum-starvation. The decrease in nucleophosmin/B23 mRNA expression and telomerase activity in HL-60 cells subsequent to BuONa/vanadate treatment can thus be attributed to cellular apoptosis rather than the growth arrest induced by BuONa/vanadate. Nucleophosmin/B23 antisense oligomer treatment significantly potentiated BuONa-induced apoptosis and inhibition of telomerase activity. Results of this study suggest that nucleophosmin/B23 is one of the key elements in the down-regulation of nucleolar function for cellular apoptosis and mortalization.