Downregulation of cell cycle proteins by protein kinase G

초록

Protein kinase G (PKG) has been shown to be involved in induction of apoptosis and growth inhibition. To understand the roles of specific PKG isoforms in the transcriptional control of cell cycle genes, we constructed a series of expression vectors of PKG1-a and PKG1-b which encode HA-tagged wild type, constitutively active and dominant negative mutants and examined the effects of these constructs on the transcription of cyclin D1 and other cell cycle genes. Our present study demonstrates that the constitutively active mutants of PKG1-b downregulate the transcription of cyclin D1 when transfected in NIH3T3 cells. We also examined the effects of PKG constructs on several enhancer elements found in cyclin D1 promoter, such as, c-fos, c-jun, SRE and TRE using the luciferase reporter assay. Constitutively active mutants of PKG1-b showed marked transcriptional downregulation of c-fos in NIH3T3 cells, whereas PKG1-a showed lesser extent. We found that the constitutively active mutants of PKG1-b inhibit the activation of SRE and TRE, suggesting their involvement in the regulation of cyclin D1. These results show that the activation of PKG1-b isoform may be responsible for the downregulation of cyclin D1 and provide a novel anti-proliferative signaling pathway in mammalian cells.