| AMP-activated protein kinase (AMPK) and the NAD+-dependent histone/protein deacetylase Sirtuin 1 (SIRT1) are metabolic sensors that can increase each other's activity. They are also both targets for the treatment of metabolic diseases based in part on their ability to decrease hepatosteatosis. The abundance of the tumor suppressor protein p53, a target of both AMPK and SIRT1, is increased in the livers of mice with obesity and hepatic steatosis. The anti-diabetic drug metformin activates AMPK and SIRT1 and decreases hepatic lipids, but its effect on p53 has yet to be examined. We found that in a human hepatoblastoma cell line (HepG2 cells), AMPK activation by metformin or incubation in a low glucose medium decreases the abundance of p53. In addition, metformin treatment increased SIRT1 activity, as evidenced by decreased histone H3 acetylation, and decreased p53 acetylation at lysine 382, a SIRT1-targeted site. DCF fluorescence analysis revealed that cytosolic oxidative stress, which can trigger p53 accumulation, was also diminished by metformin, although mitochondrial superoxide content, as assessed by MitoSOX dye, was increased. The effect of metformin on p53 abundance was abolished by inhibition of the ubiquitin ligase murine double minute 2 (MDM2), overexpression of a dominant-negative AMPK, and shRNA-mediated knockdown of SIRT1. These results suggest that metformin increases MDM2-mediated degradation of p53 by activating AMPK, decreasing cytosolic oxidative stress, and increasing SIRT1-mediated deacetylation. In additional studies, the ability of metformin to decrease plasminogen activator inhibitor-1 (PAI-1) secretion and cellular triglyceride content was attenuated in cells overexpressing p53. The latter effect was associated with increased glucose utilization, decreased SIRT1 abundance and AMPK activity, and was reversed by treatment with the SIRT1 activator SRT2183. Overall these findings suggest that metformin-induced decreases in p53 may have beneficial effects on hepatocellular metabolic signaling. Importantly, metformin has been shown to inhibit cellular proliferation, thus any decrease in p53 abundance that occurs with metformin treatment is not likely to increase the risk of tumorigenesis. |
