Effect Of Redox Status On Deiodinase Function And Its Potential Mechanism In HepG2 Cells

Objectives: Thyroid hormone（TH） is important for its role in the regulation of energy metabolism and cellular redox status. The deiodinases are responsible for the metabolism of TH in nearly all peripheral tissues. Emerging studies have reported that the decline in T3 production is probably involved in the occurrence of the nonthyroid illness syndrome（NTIS）, which refers to the characteristic decreases in serum 3,5,3’-triiodothyronine（T3） levels, but normal 3,5,3’,5’-tetraiodothyronine（T4） and thyroid-stimulating hormone（TSH） levels. Thus, the aim of the present study was to clarify the potential relationship between the redox status and the conversion of T4 to T3 by the catalysis of type 1 deiodinase（D1）, as well as its effect on cellular glucose metabolism in HepG2 cells.Methods: HepG2 cells were exposed to 100 μM, 200 μM, 300 μM H₂O₂ for 2 h to establish oxidative injury model, and then cells were divided into five groups: H₂O₂ group, H₂O₂+T3 group, H₂O₂+T4 group, LA group and H₂O₂+T4+LA group. ROS level was detected by DCFHDA method, cell viability was determined by MTT assay, redox indacators, glucose uptake and T3 content were measured by appropriate kits, genes mRNA expression were analyzed by qRTPCR, D1 activity and rT3 content were examined by radioimmunoassay.Results: We observed that T3 but not T4 significantly alleviated H₂O₂ induced oxidative stress and improved cell viability in HepG2 cells by decreasing cellular ROS and MDA levels, increasing T-AOC, GSH-Px activity and GSH/GSSG ratio, upregulating mRNA expression of Nrf2, NQO1, HO-1, Bcl-2 and downregulating Bax, Caspase3 mRNA level. As expected, H₂O₂（200 μM, 300 μM） notably decreased D1 mRNA expression and enzyme activity, and inhibited the induction of D1 by T3 and T4, which is consistent with a significant fall of T3 but not rT3 content. What’s more, LA intervention dramaticlly restored D1 mRNA level and activity, T3 and rT3 content, as well as transcriptional abnormalities of inflammation-associated genes（NF-κB, AP-1, p38 MAPK）, thyroid hormone receptors or relevant genes（TRβ1, TRα1, SRC-1）. Moreover, compared with individual processing of T4, co-treatment of LA and T4 markedly recovered cellular glucose uptake and insulin sensitivity disturbed by H₂O₂, which is in conformity to the transcriptional expression of the key factors in the insulin signaling pathway（INSR, PI3 K, GLUT4） and glucose metabolism-related genes（HK, PK, GS, GLUT2）.Conclusions: T3 exerts notable recovery effect on oxidative stress-induced HepG2 cells much more efficient than T4. H₂O₂ induced oxidative stress significantly inhibited D1 function and the conversion of T4 to T3, which may be partly through an indirect way like activating cellular inflammatory responses and downregulating the transcriptional expression of TRβ1, TRα1, SRC-1. What’s more, LA restores D1 activity and T3 production, and thus promotes cellular glycometabolism.