Sirt5 Stat3 Acetylation Regulate Its Functions

STAT3 is latent cytoplasmic transcriptional factor of cytokine and growth factor. When cytokine binding activates receptor-associated Janus kinases (Jaks), which phosphorylate one or more receptor chains to create docking sites for STAT3, then Tyr705 and Ser727 of STAT3 can be phosphorylated, whereupon they dissociate from the receptor, form dimers, then translocate to the nucleus and regulate the gene expression. Recently, the researches indicated that Lys685 of STAT3 can be acetylated, which contributed to the formation of stable dimmer and gene transcription.In order to better understand the relationship between acetylation and phosphorylation modification and the importance for regulation of STAT3 activity, we constructed several mutants in this study. Then we used western blot and luciferase report assay to conclude that Lys383, Lys707, Lys709 can also be acetylated. Lys709 mutation will abolish the phosphorylation of Tyr705. It's indicated that acetylation can regulate phosphrylation in this situation. Besides, CBP enhance the phosphorylation of Ser727 and promote the transcriptional activation of STAT3. On the other hand, the transcriptional activity of STAT3 is depressed after acetylated sites mutated. So we concluded that acetylation contributes to STAT3 transcriptional activity.SIRT5 belongs to Class III HDACs. The research indicated that SIRT5 is located in mitochondria, which can regulate deacetylation of CSP1. But its other biological function and characteristics is unclear. In order to analyze whether SIRTs were involved into deacetylation of STAT3, we used RNA silence and over-expression experiments of SIRTs in 293T cells to screen and confirm that SIRT1,SIRT5,SIRT7 of SIRTs family can deacetylate STAT3. SIRT1,SIRT5 can totally reverse the acetylation of STAT3 when CBP is present, but SIRT7 reverse a part of it. SIRT1, SIRT5, SIRT7 can restrain the transcriptional activity of STAT3 through luciferase activity assay. SIRT7 contribute most to decrease transcriptional activity of STAT3, SIRT5 is least and SIRT1 is between them. Next we found that SIRT5 can be acetylated by CBP/p300 and overexpressed SIRT5 is not translocated into nuclei. The conclusions above provide a new fundamental for learning bionomics of SIRT5, and the regulatory mechanism to STAT3. At the same time, it also provides a new direction for tumor therapy through targeting STAT3.