3,3’,5-triiodo-l-thyronine and 3,5-diodo-l-thyronine differentially modulate hepatic mitochondrial quality control in hypothyroid rats

Objective: The maintenance of healthy and functional mitochondrial network via mitochondrial quality control (QC) mechanisms, is critical throughout life to respond to physiological adaptations and stress. Due to their role in energy production, mitochondria are exposed to high amounts of reactive oxygen species making their DNA (mtDNA) particularly vulnerable to oxidative damage. Mitochondrial dysfunction causes altered QC mechanisms (i.e. altered biogenesis, dynamics, autophagy/mitophagy) and mtDNA damage and depletion, and in some cases, mtDNA release. When this occurs, mtDNA released from mitochondria into the extracellular and cytosol environment plays a central role in the damage-associated molecular patterns (DAMPs) through the activation of cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway, acting as an inflammatory trigger. Both 3,5-diiodo-L-thyronine (3,5-T2) and 3,5,3’-triiodo-L-tyronine (T3) have been shown to influence the mitochondrial QC system. However, the underlying mechanisms are poorly understood and likely differentiated when comparing the two iodothyronines. Here, by using a rat model of chemically induced hypothyroidism, we investigated the effect of administration of either 3,5-T2 or T3 on some key factors related to inflammation, mtDNA damage and mitochondrial QC system in the liver. Methods: Hypothyroidism was induced by propylthiouracil and iopanoic acid; 3,5-T2 and T3 were intraperitoneally administered to hypothyroid rats for 1 week at 25 and 15 µg/100 g BW, respectively. Factors linked to hepatic inflammation (i.e. cGAS-STING pathways) were investigated. The status of mtDNA damage/repair and mitochondrial QC mechanisms (biogenesis, dynamics, and mitophagy) were studied. Results: We showed an increase in mtDNA damage in the liver of hypothyroid rats accompanied by a significant reduction of mtDNA copy number, suggesting a reduction in mitochondrial biogenesis. Moreover, in hypothyroid rats, we found increased protein expression of both cGAS and cSTING, indicating activation of DAMPs pathways. The administration of either 3,5-T2 or T3 affected QC mechanisms ameliorating mitochondria fitness. Both iodothyronines enhanced mitochondrial copy number, reduced the mtDNA lesion frequency and oxidative damage, induced mtDNA repair mechanism and mitochondriogenesis, being T3 more effective than 3,5-T2. Also mitochondrial dynamics and autophagy were influenced. Of note, 3,5-T2, but not T3, reverted the activation of inflammatory triggers. Conclusion: The reported data highlight new molecular mechanisms underlying the effect elicited by the administration of naturally occurring iodothyronines to hypothyroid rats on liver pathways related to QC to preserve mitochondrial health.