8-Chloro-cyclic AMP (8-Cl-cAMP), which is known to induce growth inhibition, apoptosis, and differentiation in various cancer cell lines, has been studied as a putative anticancer drug. However, the mechanism of anticancer activities of 8-ClcAMP has not been fully understood. Previously, we reported that the 8-Cl-cAMP-induced growth inhibition is mediated by protein kinase C (PKC) activation. In this study, we found that p38 mitogen-activated protein kinase (MAPK) also plays important roles during the 8-Cl-cAMP-induced growth inhibition and apoptosis. SB203580 (a p38-specific inhibitor) recovered the 8-Cl-cAMP-induced growth inhibition and apoptosis, whereas other MAPK inhibitors, such as PD98059 (an extracellular signal-regulated kinase-specific inhibitor) and SP600125 (a c-Jun NH 2 -terminal kinase-specific inhibitor), had no effect. The phosphorylation (activation) of p38 MAPK was increased in a time-dependent manner after 8-ClcAMP treatment. Furthermore, SB203580 was able to block PKC activation induced by 8-Cl-cAMP. However, PKC inhibitor (GF109203x) could not attenuate p38 activation, indicating that p38 MAPK activation is upstream of PKC activation during the 8-Cl-cAMP-induced growth inhibition. 8-Chloroadenosine, a metabolite of 8-Cl-cAMP, also activated p38 MAPK and this activation was blocked by adenosine kinase inhibitor. These results suggest that 8-Cl-cAMP exerts its anticancer activity through p38 MAPK activation and the metabolite(s) of 8-Cl-cAMP mediates this process. (Cancer Res 2005; 65(11): 4896-901)

8-Chloro-cyclic AMP (8-Cl-cAMP) is known to be most effective in inducing growth inhibition and differentiation of a number of cancer cells. Also, its cellular metabolite, 8-Cl-adenosine was shown to induce growth inhibition in a variety of cell lines. However, the signaling mechanism that governs the effects of 8-Cl-cAMP and/or 8-Cl-adenosine is still uncertain and it is not even sure which of the two is the key molecule that induces growth inhibition. In this study using mouse fibroblast DT cells, it was found that adenosine kinase inhibitor and adenosine deaminase could reverse cellular growth inhibition induced by 8-Cl-cAMP and 8-Cl-adenosine. And 8-Cl-cAMP could not induce growth inhibition in the presence of phosphodiesterase (PDE) inhibitor, but 8-Cl-adenosine could. We also found that protein kinase C (PKC) inhibitor could restore this growth inhibition, and both the 8-Cl-cAMP and 8-Cl-adenosine could activate the enzymatic activity of PKC. Besides, after 8-Cl-cAMP and 8-Cl-adenosine treatment, cyclin B was down-regulated and a CDK inhibitor, p27 was up-regulated in a time-dependent manner. These results suggest that it is not 8-Cl-cAMP but 8-Cl-adenosine which induces growth inhibition, and 8-Cl-cAMP must be metabolized to exert this effect. Furthermore, there might exist signaling cascade such as PKC activation and cyclin B down-regulation after 8-Cl-cAMP and 8-Cl-adenosine treatment.