Alterations in the cAMP signal transduction pathway in mouse lung tumorigenesis

Alterations in the cAMP signal transduction pathway are associated with mouse lung neoplasia. cAMP effects are mediated by activating cAMP-dependent protein kinase isozymes, PKA I and PKA II. E9, a tumorigenic cell line, exhibited decreased PKA I levels compared to C10 cells, a nontumorigenic cell line of similar epithelial origin. Western immunoblots of PKA subunit proteins demonstrated low concentrations of both the catalytic (C) and regulatory (R_I) PKA I subunits. Although R_II (regulatory subunit of PKA II) concentrations were similar in both cell lines, R_II from E9 cells was more highly phosphorylated than in C10 cells. R_II phosphorylation status regulates cAMP activation of PKA II. Northern-blot analysis of mRNA content indicated diminished expression of both C and R_I mRNA in E9 relative to C10 cells. Several endogenous PKA substrate proteins present in C10 cells were minimally phosphorylated by PKA in E9 cells. Forskolin, which raises cellular cAMP content, increased phosphorylation of a protein doublet in intact C10 cells, but not in E9 cells. Decreased PKA I expression and alterations in R_II phosphorylation in lung neoplasia may contribute to anomalous regulation by cAMP_I thereby diminishing cAMP-mediated growth inhibitory effects.