Potentiation of sodium butyrate-induced apoptosis by vanadate in human promyelocytic leukemia cell line HL-60

Vanadate (10 μM), a potent inhibitor of tyrosine phosphatase, added simultaneously potentiated the sodium butyrate (BuONa)-induced growth inhibition. Furthermore, at 1 mM BuONa alone, after 96 h of incubation, about 20 ± 5% of cells exhibited the morphological characteristic of apoptosis, as established by nuclear changes (condensed and fragmented nuclei) and decrease in cell size. After treatment of cells with 1 mM BuONa in the presence of 10 μM vanadate, apoptotic cells became more abundant; 90 ± 3% of cells presented morphological characteristics of apoptosis after 96 h of incubation. Flow cytometric measurement of DNA content demonstrated the accumulation of cells in G₁ phase after 72 h of incubation with 1 mM BuONa alone. In the presence of vanadate (10 μM), accumulation of cells in G₁ phase appeared after shorter times of incubation (48 h) with BuONa. A substantial increase in the proportion of cells with degraded DNA characteristic of apoptosis was observed after 48- to 72-h incubation with BuONa in the presence of vanadate. BuONa-induced apoptosis was accompanied by the increase of tyrosine phosphorylation of cellular proteins pp37 and pp97. Our results raised the possibility that regulation of tyrosine phosphorylation of pp37 and pp97 is an important event that heralds the BuONa-induced apoptosis.