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, we used
guanosine 5'-triphosphate (
GTP) to study its effects on K562 cell line.
GTP, at concentrations between 25-200 microM, inhibited proliferation (3-90%) and induced 5-78% increase in
benzidine-positive cells after 6-days of treatments of K562 cells. Flow cytometric analyses of glycophorine A (GPA) showed that
GTP can induce expression of this marker in more mature erythroid cells in a time- and dose-dependent manner. These effects of
GTP were also accompanied with inhibition of
DNA synthesis (measured by [3H]-
thymidine incorporation) and early S-phase cell cycle arrest by 96 h of exposure. In contrast, no detectable effects were observed when
GTP administered to unstimulated human peripheral blood lymphocytes (PBL). However,
GTP induced an increase in proliferation,
DNA synthesis and viability of
mitogen-stimulated PBL cells. In addition, growth inhibition and differentiating effects of
GTP were also induced by its corresponding
nucleotides GDP, GMP and
guanosine (Guo). In heat-inactivated medium, where rapid degradation of
GTP via extracellular
nucleotidases is slow, the anti-proliferative and differentiating effects of all type of
guanine nucleotides (except Guo) were significantly decreased. Moreover,
adenosine, as an inhibitor of Guo transporter system, markedly reduced the
GTP effects in K562 cells, suggesting that the extracellular degradation of
GTP or its final conversion to Guo may account for the mechanism of
GTP effects. This view is further supported by the fact that
GTP and Guo are both capable of impeding the effects of
mycophenolic acid. In conclusion, our data will hopefully have important impact on
pharmaceutical evaluation of
guanine nucleotides for
leukemia treatments.