Thyroid hormones are crucial hormones that primarily regulate the metabolism of entire body cells. In this study, Sprague-Dawley rats were grouped into sham thyroidectomy (Sham Tx), thyroidectomy (Tx), Tx with thyroxine replacement (Tx + T4), and PTU injection (PTU) groups. Metabolic parameters were measured by means of metabolic cages for 14 days. After 14 days, the rats were sacrificed while the levels of plasma or serum TSH and growth-related molecules, such as active and total ghrelin, GH, and IGF-1, were assayed. The results revealed that hypothyroid rats tended to eat less food and experienced substantial body weight gain, whereas the rats with T4 replacement tended to eat more food OPEN ACCESSMolecules 2014, 19 11179 while continuing to lose weight. In hypothyroid rats, the growth-related molecules, such as active ghrelin and total ghrelin secretion, were enhanced, and the ghrelin receptors were also up-regulated. However, circulating GH levels were not elevated and IGF-1 secretion was inhibited in hypothyroid rats. In the Tx + T4 group, the changes of active ghrelin, total ghrelin, GHS-R expression, and IGF-1 were reversed, whereas the GH secretion was higher than that of the Sham Tx group and hypothyroid groups. This study resulted in the novel finding that the ghrelin/GHS-R axis and GH/IGF-1 axis are interrupted in hypothyroid rats.

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Glucosinolates (GLSs) are a group of secondary metabolites that are involved in the defense of herbivores. In Arabidopsis thaliana, Glucosinolate Transporter 1 (AtGTR1) transports GLSs with high affinity via a proton gradient-driven process. In addition to transporting GLSs, AtGTR1 also transports phytohormones, jasmonic acidisoleucine (JA-Ile), and gibberellin (GA). However, little is known about the mechanisms underlying the broad substrate specificity of AtGTR1. Here, we characterized the substrate preference of AtGTR1 by using a yeast uptake assay, and the results revealed that GLS transport rates are negatively correlated with the hydrophobicity of substrates. Interestingly, the AtGTR1 showed a higher substrate affinity for GLSs with higher hydrophobicity, suggesting a hydrophobic substrate binding pocket. In addition, a competition assay revealed that the presence of JA, salicylic acid (SA), and indole-3-acetic acid (IAA) inhibits the transport of GLSs in yeast, suggesting a potential regulatory mechanism of AtGTR1. To further characterize the functional properties of AtGTR1, mutagenesis experiments confirmed that the conserved EXXEK motif and Arg166 are essential for the GLS transport function. In addition, the purified AtGTR1 adopts a homodimeric conformation, which is possibly regulated by phosphorylation on Thr105. The phosphomimetic mutation, T105D, reduced its protein expression and completely abrogated its GLS transport function, indicating the essential role of phosphorylation on AtGTR1. In summary, this study investigated various factors associated with the GLS transport and increased our knowledge on the substrate preferences of AtGTR1. These findings contribute to understanding how the distribution of defense GLSs is regulated in plants and could be used to improve crop quality in agriculture.

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