In this study, we have demonstrated that mice lacking the thyrotrope-restricted RXR-γ isotype have a phenotype consistent with thyroid hormone resistance and vitamin A deficiency (3
). Vitamin A, retinoids, and thyroid hormone can suppress serum TSH levels and decrease TSH-β subunit gene promoter activity (2
). Retinoids appear to suppress TSH-β promoter activity through the –200 to –149 region of the mouse and rat promoters (5
), which is different than the thyroid hormone response element located near the transcription start site (15
). These data suggest that the retinoid effect is unlikely to occur through an RXR-TR heterodimer as is seen with positively regulated genes. Inspection of the –200 to –149 region of the TSH-β promoter reveals no consensus retinoid elements (DR1 or DR5), but alternate retinoid elements (DR0 and DR7) are present (19
). The precise receptor or receptors mediating this response are currently unknown.
Recent reports have provided further insight into retinoid and potential receptor-mediated suppression of serum TSH and TSH-β promoter activity. In collaboration with Sherman and colleagues (4
), we have shown that the RXR-selective retinoids LG1069 and LG346 (Ligand Pharmaceuticals Inc., San Diego, California, USA) suppress serum TSH levels in humans and decrease TSH-β promoter activity in vitro, suggesting a mechanism using a ligand-bound RXR. Because the RXR-γ isotype has limited tissue distribution including the anterior pituitary and hypothalamus (6
), as well as thyrotropes within the anterior pituitary (5
), we hypothesized that the RXR-γ isotype played a role in the regulation of the pituitary-thyroid axis.
The RXR isotypes may be redundant under certain circumstances, but gene deletion studies have revealed distinct roles for the three major isotypes. RXR-α–deficient mice have severe cardiac and eye developmental defects and do not survive (20
β–deficient mice survive, but males appear to have abnormal spermatogenesis (23
). In this report, we have shown that RXR-
γ–deficient mice appear to survive and develop normally. Pituitary and thyroid histology are normal in these mice, and they reproduce normally. Specific analysis of the pituitary-thyroid axis reveals mild but significant elevations of serum TSH and T4 that do not suppress normally with the administration of T3. These data are consistent with a thyroid hormone resistance phenotype, but this is a more mild phenotype than that seen with lack of the TR-β isoform (24
). Using a different TSH assay, Barros and colleagues did not detect a significant elevation of TSH in these RXR-γ–deficient mice, although TSH levels were higher (24
). Furthermore, they did not observe an additive effect on TSH and T4 elevation when the RXR-
γ–deficient mice were crossed with TR-β–deficient mice, suggesting a dominant effect of TR-β on regulation of the TSH-T4 axis. These data are consistent with the effects of thyroid hormone and vitamin A deficiency seen in other animal studies (25
). In this study, mice were given increasing amounts of T3, which predictably decreased serum TSH and T4 levels in the WT animals. TSH and T4 levels also decreased in RXR-γ–/–
animals in response to T3 administration, but higher doses of T3 were required, suggesting a pattern of central thyroid hormone resistance. Levels of T4 decreased more than did TSH in the RXR-γ–/–
mice, suggesting a reduced effect of TSH on T4 production at the level of the thyroid. RXR-
γ is also expressed in the mouse (7
) and human (B.R. Haugen, unpublished data) thyroid. RXR-
γ may play a separate role in the T4 response to TSH stimulation in the thyroid.
Because mice lacking RXR-γ have mild elevations of TSH and T4, we examined the effects of these hormone changes on peripheral metabolism. Interestingly, mice lacking RXR-γ
had higher metabolic rates than their WT littermates did, suggesting that this gene plays a role in metabolism. One possibility is that the higher T4 levels affect peripheral organs (liver, heart, skeletal muscle) and increase metabolism, as is seen in patients with pituitary resistance to thyroid hormone (13
). Another possibility is that these mice were simply eating more than their WT littermates were eating, which would cause increased metabolic rates. The weights of these animals, however, were not significantly different. Food intake was not directly measured in these experiments. RXR-γ is highly expressed in skeletal muscle (6
), and this receptor isotype may play a direct role in skeletal muscle metabolism such as glucose or fat metabolism, which could account for the changes we have observed. A recent immunohistochemical and in situ hybridization analysis of all six RAR and RXR isotypes in the mouse central nervous system showed that RXR-γ mRNA and protein have high levels of expression in the hypothalamus as well as in the anterior pituitary (14
). Direct hypothalamic effects of RXR-
γ on food intake, metabolism, and TSH regulation by thyrotropin-releasing hormone must therefore also be considered. Future studies of body composition, food intake, heart rate (the RXR-γ2 isoform is expressed in cardiac muscle) and manipulation of the pituitary-thyroid axis will be necessary to determine the role of RXR-
γ on metabolic rate.
In summary, we have demonstrated that mice lacking the RXR-γ isotype have a phenotype consistent with thyroid hormone resistance and vitamin A deficiency. Furthermore, these mice appear to have an increased metabolic rate, which could be caused by the increased T4 levels or direct effects of RXR-γ on organs such as skeletal or cardiac muscle. Because retinoids are being increasingly used as chemotherapeutic and chemopreventive agents, studies defining the precise role of these retinoids and receptors will provide important insight into the effects on the pituitary-thyroid axis and metabolism.