Abstract
8‐Cl‐cAMP, which is known to induce differentiation, growth inhibition, and apoptosis in various cancer cells, has been investigated as a putative anti‐cancer drug. Previously, we reported that 8‐Cl‐cAMP and its metabolite 8‐Cl‐adenosine induce growth inhibition and apoptosis through p38 mitogen‐activated protein kinase (MAPK) activation. To further investigate the signal mechanisms that regulate the cellular effects of 8‐Cl‐cAMP, we focused on a small GTPase Rap1 that is known to be involved in growth inhibition and reverse‐transformation. 8‐Cl‐cAMP and 8‐Cl‐adenosine could increase Rap1 activity, which was blocked by ABT702—an adenosine kinase inhibitor. This suggests that 8‐Cl‐cAMP‐induced Rap1 activation is also dependent on the metabolic degradation of 8‐Cl‐cAMP. Overexpression of a constitutively active mutant form of Rap1 (Rap1V12) attenuated cellular growth and soft‐agar colony formation, which was basically the same effect as that observed with the 8‐Cl‐cAMP treatment. Furthermore, the Rap1V12 transfectant showed a high level of p38 MAPK activation. However, 8‐Cl‐cAMP‐induced Rap1 activation was not diminished by SB203580, a p38 MAPK inhibitor, suggesting that Rap1 activation might act upstream of p38 MAPK activation during 8‐Cl‐cAMP‐induced growth inhibition. J. Cell. Physiol. 209: 1039–1045, 2006. © 2006 Wiley‐Liss, Inc.