Thyroid Hormone Deiodination in Fish

Abstract

We review the experimental evidence accumulated within the past decade regarding the physiologic, biochemical, and molecular characterization of iodothyronine deiodinases (IDs) in piscine species. Agnathans, chondrichthyes, and teleosts express the three isotypes of IDs: ID1, ID2, and ID3, which are responsible for the peripheral fine-tuning of thyroid hormone (TH) bioactivity. At the molecular and operational level, fish IDs share properties with their corresponding vertebrate counterparts. However, fish IDs also exhibit discrete features that seem to be distinctive for piscine species. Indeed, teleostean ID1 is conspicuously resistant to propylthiouracil (PTU) inhibition, and its response to thyroidal status differs from that exhibited by other ID1s. Moreover, both the high level of ID2 activity and its expression in the liver of teleosts are unique among vertebrates. The physiologic role of iodothyronine deiodination in functions regulated by TH in fish is not entirely clear. Nevertheless, current experimental evidence suggests that IDs may coordinate and facilitate, in a tissue-specific fashion, the action of iodothyronines and other hormones involved in such processes.

Parental Transfer of Polybrominated Diphenyl Ethers (PBDEs) and Thyroid Endocrine Disruption in Zebrafish

“…Our results are thus consistent with a previous report, which showed that hyperthyroidism suppresses Dio2 mRNA in fish. 30 It is also worth noting that different responses of Dio1 and Dio2 were observed in the present study. The upregulation of Dio1 gene expressions could possibly be explained as increased TH degradation and downregulation of Dio2 in response to hyperthyroidism.…”

“…In fish, it has been demonstrated that Dio2 plays a pivotal role in producing active T3, allowing an adequate availability of local and systemic T3. 30 Dio1 is mainly expressed in the kidney and is thought to play a minimal role in plasma TH homeostasis, while it has a considerable influence on iodine recovery and TH degradation. 30,31 In our study, both Dio1 and Dio2 were significantly down-regulated in the adult fish liver.…”

“…30 Dio1 is mainly expressed in the kidney and is thought to play a minimal role in plasma TH homeostasis, while it has a considerable influence on iodine recovery and TH degradation. 30,31 In our study, both Dio1 and Dio2 were significantly down-regulated in the adult fish liver. Our results are consistent with previous reports showing that hyperthyroidism suppresses Dio1 and Dio2 activities and expression of their mRNA, while hypothyroidism increases them in fish.…”

“…Our results are consistent with previous reports showing that hyperthyroidism suppresses Dio1 and Dio2 activities and expression of their mRNA, while hypothyroidism increases them in fish. 30,31 The down-regulation of gene expression of the deiodinases may indicate a regulatory role in response to increased THs, indicating that the observed effects have functional consequences and indicate a true hyperthyroid state.…”

Exposure to DE-71 alters thyroid hormone levels and gene transcription in the hypothalamic–pituitary–thyroid axis of zebrafish larvae

“…In fish, 2 deiodinase enzymes (Deio1 and Deio2) convert T4 to T3 by removing iodine from the outer ring of T4 (Orozco and Valverde, 2005). Deio1 plays a minimal role in plasma TH homeostasis, while it has a considerable influence on iodine recovery and TH degradation (Van der Geyten et al, 2001).…”