3-Iodothyronamine and 3,5,3'-triiodo-L-thyronine reduce SIRT1 protein expression in the HepG2 cell line

Background 3-Iodothyronamine (T1AM) is an endogenous messenger chemically related to thyroid hormone. Recent results indicate significant transcriptional effects of chronic T1AM administration involving the protein family of sirtuins, which regulate important metabolic pathways and tumor progression. Therefore, the aim of this work was to compare the effect of exogenous T1AM and 3,5,3'-triiodo-L-thyronine (T3) chronic treatment on mammalian sirtuin expression in hepatocellular carcinoma cells (HepG2) and in primary rat hepatocytes at micromolar concentrations. Materials and methods Sirtuin (SIRT) activity and expression were determined using a colorimetric assay and Western blot analysis, respectively, in cells treated for 24 h with 1-20 μM T1AM or T3. In addition, cell viability was evaluated by the MTTtest upon 24 h of treatment with 0.1-20 μM T1AM or T3. Results In HepG2, T1AM significantly reduced SIRT 1 (20 μM) and SIRT4 (10-20 μM) protein expression, while T3 strongly decreased the expression of SIRT1 (20 μM) and SIRT2 (any tested concentration). In primary rat hepatocytes, T3 decreased SIRT2 expression and cellular nicotinamide adenine dinucleotide (NAD) concentration, while on sirtuin activity it showed opposite effects, depending on the evaluated cell fraction. The extent of MTT staining was moderately but significantly reduced by T1AM, particularly in HepG2 cells, whereas T3 reduced cell viability only in the tumor cell line. Conclusions T1AM and T3 downregulated the expression of sirtuins, mainly SIRT1, in hepatocytes, albeit in different ways. Differences in mechanisms are only observational, and further investigations are required to highlight the potential role of T1AM and T3 in modulating sirtuin expression and, therefore, in regulating cell cycle or tumorigenesis.