Simian virus 40 small tumor antigen inhibits dephosphorylation of protein kinase A-phosphorylated CREB and regulates CREB transcriptional stimulation.
We report that the small tumor (small-t) antigen of simian virus 40 (SV40) forms complexes with nuclear protein phosphatase 2A (PP2A) and regulates the phosphorylation and transcriptional transactivation function of the cyclic AMP (cAMP)-regulatory element binding protein (CREB). PP2A coimmunoprecipitated with small t from nuclear extracts from HepG2 cells expressing small t or from rat liver nuclear extracts to which recombinant small t was added. Protein phosphatase 1 was not detected in small-t immunoprecipitates. In HepG2 cells expressing small t, dibutyryl-cAMP (Bt2cAMP) stimulated the phosphorylation of CREB 65-fold, whereas CREB phosphorylation was stimulated only 5- to 8-fold by Bt2cAMP in cells not expressing small t. Small t also inhibited the dephosphorylation of cAMP-dependent protein kinase (PKA)-phosphorylated CREB in rat liver nuclear extracts. In cells expressing small t, Bt2cAMP-stimulated transcription from the phosphoenolpyruvate carboxykinase (PEPCK) gene promoter was enhanced over the level of transcription from the PEPCK promoter in cells not expressing small t. Small t also enhanced Bt2cAMP-stimulated transcription from a Gal4-responsive promoter in cells expressing a chimeric protein containing the Gal4 DNA-binding domain linked to the CREB transactivation domain. However, small t did not stimulate transcription either from a 5' deletion mutant of the PEPCK promoter that is not able to bind CREB or from the Gal4-responsive promoter in the absence of the Gal4-CREB protein. These data suggest that small t enhances Bt2cAMP-stimulated gene transcription by inhibiting the dephosphorylation of PKA-phosphorylated CREB by nuclear PP2A. These findings support previous observations that nuclear PP2A is the primary phosphatase that dephosphorylates PKA-phosphorylated CREB.
Wheat
W H
WH
Division of Basic Sciences, National Jewish Center for Immunology and Respiratory Medicine, Denver, Colorado 80206.
Roesler
W J
WJ
Klemm
D J
DJ
D000818:Animals; D000952:Antigens, Polyomavirus Transforming / Q000502:physiology*; D003994:Bucladesine / Q000494:pharmacology; D000242:Cyclic AMP / Q000502:physiology; D017362:Cyclic AMP Response Element-Binding Protein / Q000378:metabolism*; D015967:Gene Expression Regulation, Viral; D006801:Humans; D066298:In Vitro Techniques; D009687:Nuclear Proteins / Q000378:metabolism; D010749:Phosphoprotein Phosphatases / Q000378:metabolism; D010766:Phosphorylation; D054645:Protein Phosphatase 1; D054648:Protein Phosphatase 2; D051381:Rats; D015398:Signal Transduction; D014158:Transcription, Genetic; D014407:Tumor Cells, Cultured
D000952:Antigens, Polyomavirus Transforming; D017362:Cyclic AMP Response Element-Binding Protein; D009687:Nuclear Proteins; D003994:Bucladesine; D000242:Cyclic AMP; D010749:Phosphoprotein Phosphatases; D054645:Protein Phosphatase 1; D054648:Protein Phosphatase 2