Novel growth inhibitory effect of 8-Cl-cAMP is dependent on serum factors that modulate protein kinase A expression but not the hydrolysis of 8-Cl-cAMP

8-Chloro-cyclic AMP (8-Cl-cAMP) exhibits growth inhibition in vitro and in vivo in a broad spectrum of cancer cell lines. We examined whether the hydrolyzed metabolite is involved in the effect of 8-Cl-cAMP. 8-Cl-cAMP (5 μM, 3 days) exerted varying degrees (0-51%) of growth inhibition on the same cell line cultured in the medium containing different heat-inactivated serum. HPLC analysis neither detected 8-Cl-adenosine in the medium nor demonstrated any correlation between the decrease in 8-Cl-cAMP concentration in the medium and the degree of growth inhibition. The low K(m) phosphodiesterase activity in the sera did not correlate with the varying degrees of growth inhibition. The cAMP-dependent protein kinase RI(α) subunit expression in the same cell line varied widely with the different serum supplements. 8-Cl-cAMP-, but not 8-Cl-adenosine-induced, growth inhibition correlated with the basal levels of RI(α) and specific downregulation of RI(α). 8-Cl-cAMP, but not 8-Cl-phenyl-thio-cAMP or N⁶-benzyl-cAMP, inhibited cell growth in serum-free medium. These results show that 8-Cl-cAMP induces growth inhibition through down-regulation of protein kinase A type I in the absence of its hydrolysis.