Specific Isoforms of Protein Kinase G Downregulate the Transcription of Cyclin D1 in NIH3T3

To elucidate the role of PKG isoforms in transcriptional control of cyclin D1, we employed a series of expression vectors of PKG la and PKG 1 beta which encode HA-tagged wild type and constitutively active (SD and Delta N) mutants. Our present study demonstrates that both the constitutively active mutants of PKG 1 beta downregulate the transcription of cyclin D1 when transiently transfected in NIH3T3 cells, whereas PKG 1 alpha mutants show weak inhibition. We further studied the transcriptional regulators of cyclin D1, such as, c-fos, NF-kappa B, and CRE by using the luciferase reporter assay. Constitutively active mutants of PKG 1 beta showed marked transcriptional downregulation of c-fos in NIH3T3 cells, whereas PKG 1 alpha downregulated c-fos to a lesser extent. We also found that the constitutively active mutants of PKG negatively regulated the activation of NF-kappa B and CRE, suggesting their involvement in the regulation of cyclin D1.