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

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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 mu M, 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(alpha) 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(alpha) and specific downregulation of RI(alpha). 8-Cl-cAMP, but not 8-Cl-phenyl-thio-cAMP or N-6-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.

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