Altered regulation of mRNA levels encoding the type i isozyme of cAMP-dependent protein kinase in neoplastic mouse lung epithelial cells

Neoplastic mouse lung epithelial cells express greatly diminished activity, protein, and mRNA for the type I isozyme of cAMP-dependent protein kinase (PKA I). To address the mechanism of this decrease, the turnover rate of PKA subunit mRNA was examined. Northern blot analysis of PKA mRNAs from transcriptionally inhibited cells indicated that these messages exhibit different stabilities and that they are more stable in neoplastic E9 cells than in normal C10 cells. This suggests that the mechanism of decreased PKA I mRNA in E9 cells resides at the level of transcription. To examine whether this was due to an altered responsiveness to agents which regulate PKA transcription, PKA levels were experimentally manipulated in C10 and E9 cells by long-term treatment with forskolin or 8-chloro-cAMP. PKA activity and the concentration of RI (regulatory subunit of PKA I) and C (catalytic subunit) are coordinately regulated in both cell lines, but this does not reflect the changes in PKA I subunit mRNAs. RIα mRNA is specifically induced by forskolin in normal C10 cells, but not in E9 cells. Cα mRNA is forskolin-inducible in E9 cells, but this enhanced level of expression does not approach that found constitutively in C10 cells. Thus, while C10 and E9 undergo similar changes in PKA I protein subunits following these treatments, a differential modulation of their PKA I mRNA occurs. These cell-specific mRNA responses to cAMP-mediated induction suggest that the mechanism of the decreased constitutive concentration of PKA I in E9 cells involves altered regulation of PKA I mRNAs.