Immunoprecip itations performed with an anti SIRT1 or anti MMP7 antibody in OSCC cells failed to identify selleck any endogenous molecular binding between SIRT1 and MMP7. This result indicated that SIRT1 could influence MMP7 e pression, secretion, and activity. and subse quently, cell migration, invasion, and metastasis through its target proteins. SIRT1 deacetylates Smad4 in OSCC cells MMP7 has been shown to be important for accelerating cancer invasion and metastasis in multiple tissues, but does not seem to be necessary for invasion or fibrosis of colon cancer, in which Smad4 dependent transforming growth factor B family signaling is blocked. Thus MMP7 is not needed for tissue invasion in Smad4 deficient adenocarcinomas.
Additionally, a previous study revealed that SIRT1 directly interacts with and deacety lates the negative regulator of TGF B signaling, Smad7, to destabilize the protein in a mesangial kidney cell line. We therefore postulated that SIRT1 might affect MMP7 through its interactions with Smad4, a TGF B activated transcription factor. To test this hypothesis, we first used an immunoprecipitation assay to e amine the ability of SIRT1 to bind to Smad4. Our results showed that while SIRT1 directly interacted with Smad4 in vivo, it did not interact with Smad2 protein. We also performed a co immunoprecipitation e periment to e amine the ability of Smad4 to bind SIRT1. Western blotting detected SIRT1 in the Smad4 immunoprecipitate from nuclear e tracts of OSCCs. We ne t e amined whether SIRT1 could directly deacetylate Smad4.
We immunopurified endogenous Smad4 from SIRT1 knock down OECM1 and HSC3 cells, and probed western blots with antibodies to Smad4 proteins or acetylated lysine. This e periment showed that SIRT1 silencing significantly increased the level of acetylated Smad4 in SIRT1 knockdown OSCC cells. Furthermore, we also confirmed the acetylation levels of Smad4 in OECM1 and HSC3 cells at 0, 16, 24, and 48 h after transfection with the SIRT1 e pression vector. Overe pression of SIRT1 clearly reduced the acetylation levels of Smad4, while knockdown of SIRT1 increased the acetylation levels. These results suggest that while SIRT1 associates with and deacetylates Smad4, the SIRT1 deacetylase activity is not required for Smad4 protein e pression.
Because Smad4 is a transcription Carfilzomib factor that responds to TGF B signaling, we ne t investigated the e pression levels of Smad4 in SIRT1 overe pressing OECM1 and HSC3 cells following TGF B stimulation. We observed that levels of endogenous Smad4 protein in SIRT1 overe pressing or mock transfected cells were increased 2 fold after 48 h of TGF B stimulation. Surprisingly, the acetylation level of Smad4 was highly increased by TGF B induction, while overe pression of SIRT1 significantly reduced the acetylation level of TGF B induced Smad4 in OSCCs.