Thyrocyte cell survival and adaptation to chronic endoplasmic reticulum stress due to misfolded thyroglobulin

The Oxidoreductase Retinol Saturase in Thyroid Gland Is Regulated by Hypothyroidism and Iodide Overload and Its Deletion Impairs Metabolic Homeostasis in Mice

Aims: Thyroid hormones (TH) are major regulators of cell differentiation, growth, and metabolic rate. TH synthesis in the thyroid gland requires high amounts of H2O2 to oxidize iodide for the iodination of thyroglobulin (TG). Retinol Saturase (RetSat) is an oxidoreductase implicated in dihydroretinol formation and cellular sensitivity toward peroxides and ferroptosis. RetSat is highly expressed in metabolically active organs where it regulates lipid metabolism and the production of reactive oxygen species. Due to the high expression of RetSat in the thyroid gland and its role in peroxide sensitivity, we investigated the regulation and function of RetSat in the thyroid gland in appropriate mouse models. Results: RetSat is strongly expressed in thyrocytes, induced by hypothyroidism, and decreased by iodide overload in mice. Thyrocyte-specific deletion of RetSat increased circulating thyroid-stimulating hormone levels, altered thyroid morphology, and disturbed metabolic homeostasis in a diet- and sex-dependent manner without major effects on the concentrations of circulating TH. Moreover, deletion of RetSat increased TG protein levels but lowered TG iodination upon iodide overload. In cultured thyrocytes, acute RetSat depletion altered the expression of genes involved in TH biosynthesis and the response to endoplasmic reticulum stress. Innovation: This is the first report that specifically dissects the regulation and function of the oxidoreductase RetSat in the thyroid gland. Conclusion: Deletion of RetSat in thyrocytes induces compensatory feedback mechanisms to maintain TH homeostasis in mice. We conclude that RetSat in the thyroid gland is required for TH biosynthesis and secretion and metabolic homeostasis in mice. Antioxid. Redox Signal. 00, 000-000.

Defective Thyroglobulin: Cell Biology of Disease

The primary functional units of the thyroid gland are follicles of various sizes comprised of a monolayer of epithelial cells (thyrocytes) surrounding an apical extracellular cavity known as the follicle lumen. In the normal thyroid gland, the follicle lumen is filled with secreted protein (referred to as colloid), comprised nearly exclusively of thyroglobulin with a half-life ranging from days to weeks. At the cellular boundary of the follicle lumen, secreted thyroglobulin becomes iodinated, resulting from the coordinated activities of enzymes localized to the thyrocyte apical plasma membrane. Thyroglobulin appearance in evolution is essentially synchronous with the appearance of the follicular architecture of the vertebrate thyroid gland. Thyroglobulin is the most highly expressed thyroid gene and represents the most abundantly expressed thyroid protein. Wildtype thyroglobulin protein is a large and complex glycoprotein that folds in the endoplasmic reticulum, leading to homodimerization and export via the classical secretory pathway to the follicle lumen. However, of the hundreds of human thyroglobulin genetic variants, most exhibit increased susceptibility to misfolding with defective export from the endoplasmic reticulum, triggering hypothyroidism as well as thyroidal endoplasmic reticulum stress. The human disease of hypothyroidism with defective thyroglobulin (either homozygous, or compound heterozygous) can be experimentally modeled in thyrocyte cell culture, or in whole animals, such as mice that are readily amenable to genetic manipulation. From a combination of approaches, it can be demonstrated that in the setting of thyroglobulin misfolding, thyrocytes under chronic continuous ER stress exhibit increased susceptibility to cell death, with interesting cell biological and pathophysiological consequences.

Haizao Yuhu decoctions including three species of glycyrrhiza protected against propylthiouracil-induced goiter with hypothyroidism in rats via the AMPK/mTOR pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Glycyrrhiza and sargassum are among the 18 incompatible medicaments according to traditional Chinese medicine (TCM) theory. Although it contains glycyrrhiza and sargassum, Haizao Yuhu decoction (HYD) is a classic prescription widely used as TCM to treat goiter. According to the Chinese Pharmacopoeia, glycyrrhiza is divided into three varieties: Glycyrrhiza uralensis Fish., Glycyrrhiza glabra L., and Glycyrrhiza inflata Bat. Whether the three varieties of glycyrrhiza have different efficacy or toxicity when applied in the HYD is unknown.

AIM OF THE STUDY

To explore whether the HYDs comprising three varieties of glycyrrhiza have different efficacy or toxicity when used to treat goiter in rats and the underlying mechanisms of these HYDs.

MATERIALS AND METHODS

For two weeks, the goiter model was replicated by intragastric propylthiouracil (PTU) administration. Samples were divided into the control group, model group, euthyrox group, HYD with glycyrrhiza uralensis (HYD-U) group, HYD with glycyrrhiza glabra (HYD-G) group, and HYD with glycyrrhiza inflata (HYD-I) group. After four weeks of treatment, body weight, rectal temperature, thyroid/liver/kidney coefficient, thyroid/liver/kidney function, thyroid/liver/kidney histomorphology, and thyroid ultrastructure were evaluated. Then, real-time quantitative reverse-transcription polymerase chain reaction (RTqPCR), Western blot, and immunofluorescence analyses were performed to detect genes and proteins affecting autophagy and apoptosis in thyroid cells in the AMP-activated Protein Kinases (AMPK)/Mammalian target of rapamycin (mTOR) pathway.

RESULTS

All three HYDs increased thyroid hormones (THs) levels, relieved thyroid pathological tissue and ultrastructure, and activated vital proteins and genes in the AMPK/mTOR pathway. Comparisons among the efficacy of the three HYDs indicated that HYD-U restored the THs most effectively; however, no difference in the anti-goiter effect was observed. Moreover, the three HYDs resulted in no toxicity and promoted the recovery of impaired liver and kidney function caused by PTU. Comparisons among the recovery effects of the three HYDs on the liver and kidney were the same.

CONCLUSION

Our experiments demonstrated that the three HYDs had outstanding anti-goiter effects and protected liver and kidney function. Their anti-goiter effects were attributed to AMPK/mTOR pathway-induced autophagy and apoptosis. HYD-U resulted in the best THs recovery. It was further indicated that in our present study, glycyrrhiza and sargassum were compatible in the three HYDs, thereby suggesting their safety of compounding in HYD and providing a basis for the research of the 18 incompatible medicaments.

Maintaining the thyroid gland in mutant thyroglobulin-induced hypothyroidism requires thyroid cell proliferation that must continue in adulthood

Congenital hypothyroidism with biallelic thyroglobulin (Tg protein, encoded by the TG gene) mutation is an endoplasmic reticulum (ER) storage disease. Many patients (and animal models) grow an enlarged thyroid (goiter), yet some do not. In adulthood, hypothyroid TG^cog/cog mice (bearing a Tg-L2263P mutation) exhibit a large goiter, whereas adult WIC rats bearing the TG^rdw/rdw mutation (Tg-G2298R) exhibit a hypoplastic thyroid. Homozygous TG mutation has been linked to thyroid cell death, and cytotoxicity of the Tg-G2298R protein was previously thought to explain the lack of goiter in WIC-TG^rdw/rdw rats. However, recent studies revealed that TG^cog/cog mice also exhibit widespread ER stress–mediated thyrocyte death, yet under continuous feedback stimulation, thyroid cells proliferate in excess of their demise. Here, to examine the relative proteotoxicity of the Tg-G2298R protein, we have used CRISPR–CRISPR-associated protein 9 technology to generate homozygous TG^rdw/rdw knock-in mice in a strain background identical to that of TG^cog/cog mice. TG^rdw/rdw mice exhibit similar phenotypes of defective Tg protein folding, thyroid histological abnormalities, hypothyroidism, and growth retardation. TG^rdw/rdw mice do not show evidence of greater ER stress response or stress-mediated cell death than TG^cog/cog mice, and both mouse models exhibit sustained thyrocyte proliferation, with comparable goiter growth. In contrast, in WIC-TG^rdw/rdw rats, as a function of aging, the thyrocyte proliferation rate declines precipitously. We conclude that the mutant Tg-G2298R protein is not intrinsically more proteotoxic than Tg-L2263P; rather, aging-dependent difference in maintenance of cell proliferation is the limiting factor, which accounts for the absence of goiter in adult WIC-TG^rdw/rdw rats.

Differential gene expression profile of multinodular goiter

Introduction

The goiter, a neglected heterogeneous molecular disease, remains a major indication for thyroidectomies in its endemic regions.

Objectives

This study analyzed differential gene expression in surgical specimens diagnosed with multi nodular and compared the data to that of thyroid tissue without multinodular goiter from patients undergoing thyroidectomy in Manaus-AM, Brazil using RNA-seq technology.

Methodology

The transcriptome information of the surgical specimen fragments with and without multinodular goiter was accessed by Illumina HiSeq 2000 New Generation Sequencing (NGS) using the RNA-seq NEBNext^® Ultra^™ RNA Library Prep Kit for Illumina^®—#E7530L protocol and differential gene expression analysis.

Results

Differences were found between the gene expression profiles of the diseased tissues and those of the healthy control tissues; at least 70 genes were differentially expressed. The HOTS gene was expressed only in multinodular goiter tissues (p < 0.05).

Conclusion

These results demonstrate that the gene expression profile of multinodular goiter is pro-tumoral and that HOTS can play a central role in multinodular goiter development.

Thyroid hormone synthesis continues despite biallelic thyroglobulin mutation with cell death

Complete absence of thyroid hormone is incompatible with life in vertebrates. Thyroxine is synthesized within thyroid follicles upon iodination of thyroglobulin conveyed from the endoplasmic reticulum (ER), via the Golgi complex, to the extracellular follicular lumen. In congenital hypothyroidism from biallelic thyroglobulin mutation, thyroglobulin is misfolded and cannot advance from the ER, eliminating its secretion and triggering ER stress. Nevertheless, untreated patients somehow continue to synthesize sufficient thyroxine to yield measurable serum levels that sustain life. Here, we demonstrate that TG^{W2346R/W2346R} humans, TG^cog/cog mice, and TG^rdw/rdw rats exhibited no detectable ER export of thyroglobulin, accompanied by severe thyroidal ER stress and thyroid cell death. Nevertheless, thyroxine was synthesized, and brief treatment of TG^rdw/rdw rats with antithyroid drug was lethal to the animals. When untreated, remarkably, thyroxine was synthesized on the mutant thyroglobulin protein, delivered via dead thyrocytes that decompose within the follicle lumen, where they were iodinated and cannibalized by surrounding live thyrocytes. As the animals continued to grow goiters, circulating thyroxine increased. However, when TG^rdw/rdw rats age, they cannot sustain goiter growth that provided the dying cells needed for ongoing thyroxine synthesis, resulting in profound hypothyroidism. These results establish a disease mechanism wherein dead thyrocytes support organismal survival.

Structure and genetic variants of thyroglobulin: Pathophysiological implications

Thyroglobulin (TG) plays a main role in the biosynthesis of thyroid hormones (TH), and, thus, it is involved in a wide range of vital functions throughout the life cycle of all vertebrates. Deficiency of TH production due to TG genetic variants causes congenital hypothyroidism (CH), with devastating consequences such as intellectual disability and impaired growth if untreated. To this day, 229 variations in the human TG gene have been identified while the 3D structure of TG has recently appeared. Although TG deficiency is thought to be of autosomal recessive inheritance, the introduction of massive sequencing platforms led to the identification of a variety of monoallelic TG variants (combined with mutations in other thyroid gene products) opening new questions regarding the possibility of oligogenic inheritance of the disease. In this review we discuss remarkable advances in the understanding of the TG architecture and the pathophysiology of CH associated with TG defects, providing new insights for the management of congenital disorders as well as counseling benefits for families with a history of TG abnormalities. Moreover, we summarize relevant aspects of TH synthesis within TG and offer an updated analysis of animal and cellular models of TG deficiency for pathophysiological studies of thyroid dyshormonogenesis while highlighting perspectives for new investigations. All in all, even though there has been sustained progress in understanding the role of TG in thyroid pathophysiology during the past 50 years, functional characterization of TG variants remains an important area of study for future advancement in the field.

Cell death-associated lipid droplet protein CIDE-A is a noncanonical marker of endoplasmic reticulum stress

Secretory protein misfolding has been linked to ER stress and cell death. We expressed a TGrdw transgene encoding TG-G(2298)R, a misfolded mutant thyroglobulin reported to be linked to thyroid cell death. When the TGrdw transgene was expressed at low level in thyrocytes of TGcog/cog mice that experienced severe ER stress, we observed increased thyrocyte cell death and increased expression of CIDE-A (cell death-inducing DFFA-like effector-A, a protein of lipid droplets) in whole thyroid gland. Here we demonstrate that acute ER stress in cultured PCCL3 thyrocytes increases Cidea mRNA levels, maintained at least in part by increased mRNA stability, while being negatively regulated by activating transcription factor 6 - with similar observations that ER stress increases Cidea mRNA levels in other cell types. CIDE-A protein is sensitive to proteasomal degradation yet is stabilized by ER stress, and elevated expression levels accompany increased cell death. Unlike acute ER stress, PCCL3 cells adapted and surviving chronic ER stress maintained a disproportionately lower relative mRNA level of Cidea compared with that of other, classical ER stress markers, as well as a blunted Cidea mRNA response to a new, unrelated acute ER stress challenge. We suggest that CIDE-A is a novel marker linked to a noncanonical ER stress response program, with implications for cell death and survival.

The Probable, Possible, and Novel Functions of ERp29

The luminal endoplasmic reticulum (ER) protein of 29 kDa (ERp29) is a ubiquitously expressed cellular agent with multiple critical roles. ERp29 regulates the biosynthesis and trafficking of several transmembrane and secretory proteins, including the cystic fibrosis transmembrane conductance regulator (CFTR), the epithelial sodium channel (ENaC), thyroglobulin, connexin 43 hemichannels, and proinsulin. ERp29 is hypothesized to promote ER to cis-Golgi cargo protein transport via COP II machinery through its interactions with the KDEL receptor; this interaction may facilitate the loading of ERp29 clients into COP II vesicles. ERp29 also plays a role in ER stress (ERS) and the unfolded protein response (UPR) and is implicated in oncogenesis. Here, we review the vast array of ERp29’s clients, its role as an ER to Golgi escort protein, and further suggest ERp29 as a potential target for therapies related to diseases of protein misfolding and mistrafficking.