ERK1/2 inactivation and p38 MAPK-dependent caspase activation during guanosine 5′-triphosphate-mediated terminal erythroid differentiation of K562 cells

Since differentiation therapy is one of the promising strategies for treatment of leukemia, universal efforts have been focused on finding new differentiating agents. In that respect, it was recently shown that guanosine 5′-triphosphate (GTP) induced the differentiation of K562 cells, suggesting its possible efficiency in treatment of chronic myelogenous leukemia (CML). However, further investigations are required to verify this possibility. Here, the effects of GTP on activation of mitogen-activated protein kinases (MAPKs) and caspases in K562 cells were examined. Exposure of K562 cells to 100 μM GTP markedly inhibited growth (4-70%) and increased percent glycophorin A positive cells after 1-6 days. GTP-induced terminal erythroid differentiation of K562 cells was accompanied with activation of three key caspases, i.e., caspase-3, -6 and -9. More detailed studies revealed that mitochondrial pathway is activated along with down-regulation of Bcl-xL and releasing of cytochrome c into cytosol. Among MAPKs, ERK1/2and p38 were modulated after GTP treatment. Western blot analyses showed that sustained phosphorylation of p38 MAPK was accompanied by a decrease in ERK1/2 activation. These modulatory effects of GTP were observed at early exposure times before the onset of differentiation (3 h), and followed for 24-96 h. Interestingly, inhibition of p38 MAPK pathway by SB202190 impeded GTP-mediated caspases activation and differentiation in K562 cells, suggesting that p38 MAPK may act upstream of caspases in our system. These results point to a pivotal role for p38 MAPK pathway during GTP-mediated erythroid differentiation of K562 cells and will hopefully have important impact on pharmaceutical evaluation of GTP for CML treatment in differentiation therapy approaches.