Apoptosis of Human Myeloid Leukemia Cells Induced by an Inhibitor of Protein Phosphatases (okadaic Acid) is Prevented by Bcl-2 and Bcl-X(L)

Okadaic acid, an inhibitor of serine/threonine protein phosphatases 1 and 2A has been shown to cause mitotic arrest and cell death of HL-60 and K562 cells. HL-60 cells express Bcl-2 and little or no Bcl-X(L), while K562 expresses Bcl-X(L) but not Bcl-2. Since phosphorylation/dephosphorylation reactions have been suggested to be involved in the regulation of Bcl-2, we planned to investigate whether the expression of Bcl-2, Bcl-X(L) and Bax, a protein that antagonizes the antiapoptotic function of Bcl-2, are regulated in myeloid leukemia cell lines (K562, KU812 and HL-60) treated with okadaic acid. Our results indicate that exposure of all three leukemic cell lines to nanomolar concentrations of okadaic acid causes a loss of viability by activation of an apoptotic process accompanied by a marked decrease in the expression of Bcl-2, Bcl-X(L) and Bax at both mRNA and protein level, but not of c-fos, vimentin and epsilon-globin, ruling out a non-specific effect of okadaic acid. Furthermore, constitutive expression of either Bcl-X(L) or Bcl-2 by gene transfer inhibited apoptosis triggered by okadaic acid in K562 cells. Thus, we suggest that protein phosphatases may be involved in maintaining the expression of bcl-2 family genes as part of the survival machinery of the cell.