Inhibition of Growth and Modulation of Gene Expression in Human Lung Carcinoma in Athymic Mice by Site-selective 8-Cl-Cyclic Adenosine Monophosphate

Site-selective cyclic AMP (cAMP) analogues inhibit growth and induce changes in morphology in a spectrum of human cancer cell lines (D. Katsaros etal., FEBS Lett., 223:97, 1987). The cellular events underlying such effects of cAMP analogues include differential regulation of type I versus type II cAMP-dependent protein kinase isozymes (S. Ally etal., Proc. Natl. Acad. Sci. USA, 85: 6319, 1988). Infusion (i.p.) of 8-C1-cAMP, the most potent site-selective cAMP analogue, for 7 days produced regression of LX-1 lung carcinoma in athymic mice in a dose-dependent manner. The tumor regression correlated with the changing levels of cAMP receptor proteins, RI and RH6 the regulatory subunits of cAMP-dependent protein kinase type I and type II, respectively. By photoaffinity labeling with 8-N₃-XP]cAMP and immunoblotting with a monospecific anti-RII antibody, RI_α (M_r 49,000) and RII_Β, (M_r 51,000) were identified in the untreated control tumors. 8-Cl-cAMP treatment induced a rapid increase of both R_α, and RII_Β in tumor cytosols and translocation (within 1 h) of only RH_Β, from the cytosol to the nucleus. RII_Β, in both cytosols and nuclei remained elevated during 8-Cl-cAMP treatment, whereas RI_α in the cytosols gradually decreased with time of treatment after its initial transient increase. Northern blot analyses demonstrated that the RII_Β, mRNA level increased within 6 h of 8-C1-cAMP treatment and remained elevated during treatment, whereas the RI_α mRNA level decreased to below that of the untreated control tumor level after its transient increase during 1–6 h of treatment. 8-Cl-cAMP treatment also caused a sharp decrease in both N-ros and c-myc mRNA levels. These results suggest that the fundamental basis for the antineoplastic activity of 8-Cl-cAMP may reside in the restoration of normal gene regulation in neoplasms in which cAMP receptor proteins play a role.