Thyroid Deficiency Before Birth Alters the Adipose Transcriptome to Promote Overgrowth of White Adipose Tissue and Impair Thermogenic Capacity

Abstract

Background: Development of adipose tissue before birth is essential for energy storage and thermoregulation in the neonate and for cardiometabolic health in later life. Thyroid hormones are important regulators of growth and maturation in fetal tissues. Offspring hypothyroid in utero are poorly adapted to regulate body temperature at birth and are at risk of becoming obese and insulin resistant in childhood. The mechanisms by which thyroid hormones regulate the growth and development of adipose tissue in the fetus, however, are unclear. Methods: This study examined the structure, transcriptome, and protein expression of perirenal adipose tissue (PAT) in a fetal sheep model of thyroid hormone deficiency during late gestation. Proportions of unilocular (UL) (white) and multilocular (ML) (brown) adipocytes, and UL adipocyte size, were assessed by histological and stereological techniques. Changes to the adipose transcriptome were investigated by RNA sequencing and bioinformatic analysis, and proteins of interest were quantified by Western blotting. Results: Hypothyroidism in utero resulted in elevated plasma insulin and leptin concentrations and overgrowth of PAT in the fetus, specifically due to hyperplasia and hypertrophy of UL adipocytes with no change in ML adipocyte mass. RNA sequencing and genomic analyses showed that thyroid deficiency affected 34% of the genes identified in fetal adipose tissue. Enriched Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) pathways were associated with adipogenic, metabolic, and thermoregulatory processes, insulin resistance, and a range of endocrine and adipocytokine signaling pathways. Adipose protein levels of signaling molecules, including phosphorylated S6-kinase (pS6K), glucose transporter isoform 4 (GLUT4), and peroxisome proliferator-activated receptor γ (PPARγ), were increased by fetal hypothyroidism. Fetal thyroid deficiency decreased uncoupling protein 1 (UCP1) protein and mRNA content, and UCP1 thermogenic capacity without any change in ML adipocyte mass. Conclusions: Growth and development of adipose tissue before birth is sensitive to thyroid hormone status in utero. Changes to the adipose transcriptome and phenotype observed in the hypothyroid fetus may have consequences for neonatal survival and the risk of obesity and metabolic dysfunction in later life.

Keywords: adipose; fetus; insulin; insulin-like growth factor; leptin; thyroid hormone; uncoupling protein.

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Mean (± SEM) measurements of (A) perirenal adipose tissue (PAT) composition, (B) relative PAT mass and (C) unilocular (UL) adipocyte perimeter in sham and thyroidectomised (TX) fetuses at 129 and 143 days of gestation (dGA). * Significantly different from sham fetuses at same gestational age; + significantly different from fetuses at 129 dGA in the same treatment group, P<0.05. Representative histological images of perirenal adipose tissue taken from (D) sham and (E) TX sheep fetuses at 143 dGA. Haematoxylin and eosin stain.

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KEGG pathway and biological process (BP) bar plots using RNA-sequencing data from perirenal adipose tissue taken from sham and thyroidectomised (TX) fetuses at 129 and 143 days of gestation (dGA). Selected relevant KEGG (A) and BP ontology (B) pathway bar plots indicating the number of up and down-regulated genes when the data were compared by treatment (TX and sham); the red and blue bars represent up and down-regulated genes, respectively.

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Mean (± SEM) abundance of (A) uncoupling protein-1 (UCP1) mRNA, (B) UCP1 mRNA relative to the percentage volume of multilocular (ML) adipose tissue, and (C) UCP1 protein relative to the percentage volume of ML adipose tissue and (D) citrate synthase (CS) activity, in perirenal adipose tissue taken from sham and thyroidectomised (TX) fetuses at 129 and 143 days of gestation (dGA). * Significantly different from sham fetuses at same gestational age; + significantly different from fetuses at 129 dGA in the same treatment group, P<0.05. AU, arbitrary units.

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Mean (± SEM) mRNA abundance of (A) iodothyronine deiodinase-1 (DIO1), (B) DIO2, (C) thyroid hormone receptor α1 (TRα1), (D) TRα2 and (E) TRβ in perirenal adipose tissue taken from sham and thyroidectomised (TX) fetuses at 129 and 143 days of gestation (dGA). * Significantly different from sham fetuses at same gestational age; + significantly different from fetuses at 129 dGA in the same treatment group, P<0.05. AU, arbitrary units.