Anti-apoptotic effects of 3,5,3′-tri-iodothyronine in mouse hepatocytes

The present study demonstrates that 3,5,3'-tri-iodothyronine (T-3) in physiological dose range inhibits tumor necrosis factor alpha(TNF alpha)/Fas-induced apoptosis in mouse hepatocytes. T-3 pretreatment prevented Fas-induced early stage of apoptosis signs assessed by flow cytometry analysis of the annexin V positive cell population. T-3 attenuated TNF alpha/Fas-induced cleavage of caspase-8 and DNA fragmentation. We found that T-3 exerted its anti-apoptotic effects by mobilization of several non-genomic mechanisms independent of transcriptional activity. Inhibition of protein kinase A (PKA), extracellular signal-regulated kinase (ERK), and Na+/H+ exchanger blocked T-3-dependent anti-apoptotic effects indicating an involvement of these intracellular targets into T-3-induced signaling cascade. Furthermore, physiological concentrations of T-3, but not reverse T-3, caused increases in intracellular cAMP content and activated PKA. T-3 markedly induced phosphorylation of ERK. We also detected T-3-dependent intracellular alkalinization that abolished TNF alpha-induced acidification. PKA inhibitor KT-5720 blocked T-3-induced activation of ERK and intracellular alkalinization confirming the upstream position of PKA signaling. We further detected that hepatocytes from hypothyroid mice are more sensitive to TNFa/Fas-induced apoptosis than euthyroid animals in vivo. Together, these findings imply that T-3 triggers PKA- and ERK-regulated intracellular pathways capable of driving and ensuring hepatocytes survival in the presence of death receptor ligand-induced damage under chronic inflammatory conditions.