Sabrina Christensen and Alexandra Lee, Linh Ho
California Northstate University College of Medicine
California Northstate University College of Pharmacy
Sirtuin 3 (Sirt3) is a mitochondrial deacetylase enzyme in metabolic tissues that is decreased in diabetes. Sirt3 knockout mice exhibit insulin resistance, glucose intolerance, and accelerated obesity. Elucidating Sirt3’s molecular mechanism in regulating insulin sensitivity in adipocytes can contribute to the effort of targeting Sirt3 for treatment of obesity and type 2 diabetes.
3T3-L1 cells were treated with increasing concentrations of Sirt3 inhibitor (3-TYP) or Sirt3 activator (Honokiol). Quantitative PCR investigated changes in adipocyte gene markers including peroxisome proliferator-activated receptor gamma (PPARγ), and in lipolysis gene markers including ATP-citrate lyase (ACL) and lipoprotein lipase (LPL). Western blot identified Sirt3’s role in the insulin signaling pathway. Data analysis used Graphpad Prism Software. Statistical analyses used a two-tailed unpaired followed by post hoc Dunnette’s multiple comparisons.
There was an increase in FoxO1/FoxO3a/FoxO4 phosphorylation, IGF-IRb (type I insulin-like growth factor receptor beta), mTOR (mammalian target of rapamycin), and IRB (insulin receptor beta) in Honokiol-treated cells. There was a significant decrease in LPL and PPARγ.
In contrast, FoxO1/FoxO3a/FoxO4 phosphorylation, mTOR and IRB protein expression were decreased in 3-TYP treated cells. There was a significant increase in LPL and a slight increase in PPARγ.
Sirt3 inhibitor decreased LPL, which is associated with insulin resistance, obesity, dyslipidemias, etc. In sharp contrast, Sirt3 activator increased LPL, indicating that activation improves insulin resistance. Additionally, Sirt3 activation upregulated insulin pathway by activating insulin receptors and the downstream member mTOR. These results indicate that Sirt3 activation can improve insulin resistance whereas inhibition leads to insulin resistance.
Keywords: Sirtuin 3, Insulin-Signaling Pathway