Molecular cloning by hybrid arrest of translation in Xenopus laevis oocytes. Identification of a cDNA encoding the type I iodothyronine 5'-deiodinase from rat liver

Xenopus oocytes as an expression system for plant transporters

The Xenopus oocyte provides a powerful system for the expression and characterisation of plant membrane proteins. Many different types of plant membrane proteins have been expressed and characterised using this system. As there are already several general reviews on the methodology for oocyte expression of channel proteins, we have summarised the particular advantages and disadvantages of using the system for the characterisation of plant cotransporter proteins. As an example of how the system can be used to identify transporters, we describe evidence for a low affinity nitrate transporter in oocytes injected with poly(A) RNA extracted from nitrate-induced barley roots. Furthermore, we describe evidence that the expression of some transporters in oocytes can modify the properties of endogenous membrane proteins. We conclude that although care must be taken in the interpretation of results and in choosing appropriate controls for experiments, oocyte expression is an excellent tool which will have an important role in characterising plant membrane proteins.

Transcriptional activation of type III inner ring deiodinase by growth factors in cultured rat brown adipocytes

The activity of the type III inner ring deiodinase (DIII), which converts T4 and T3 to inactive metabolites, is induced by serum and growth factors in primary cultures of rat brown adipocytes. The contribution of pretranslational mechanisms to this increase in DIII activity was examined in the present studies. DIII mRNA is undetectable in differentiated brown adipocytes when cultured in serum-free medium. However, exposure to epidermal growth factor (EGF), acidic or basic fibroblast growth factors (aFGF or bFGF) increase DIII transcript levels. Lesser inductions are found with platelet-derived growth factor, and insulin-like growth factor I has no effect. Maximal induction of DIII mRNA is obtained after 9 h of exposure to EGF, bFGF, or aFGF at a concentration of 10 ng/ml. The increase in DIII mRNA in response to aFGF, bFGF, and EGF requires gene transcription and protein synthesis, as the inductive effect on mRNA is completely blocked by actinomycin D or cycloheximide. The DIII mRNA half-life is 4 h when stimulated with bFGF and increases to 12 h when 10% serum, EGF, or aFGF is present. In conclusion, EGF, aFGF, and bFGF increase DIII mRNA expression in differentiated brown adipocytes. This effect appears to be exerted at the level of both enhanced transcription and mRNA stabilization.

Thyroid hormones inhibit type 2 iodothyronine deiodinase in the rat cerebral cortex by both pre- and posttranslational mechanisms

The type 2 5'-deiodinase (D2) appears to play an important role in maintaining the intracerebral T3 content relatively constant during changes in thyroidal state. Previous studies have demonstrated that the regulation of this enzyme by thyroid hormone and its analogs occurs at a posttranslational level. The availability of the rat D2 complementary DNA now allows an assessment of whether pretranslational regulation of this enzyme also occurs in the cerebral cortex. In rats rendered hypothyroid by the addition of methimazole to the drinking water, D2 messenger RNA (mRNA) is increased 70% (P = 0.03). Treatment with L-T3 (50 microg/100 g BW) for 4 days results in an 80% decrease in D2 mRNA compared with that in euthyroid controls (P < 0.001). Administration of lower doses of L-T3 (0.25-3 microg/100 g BW x day) is associated with a dose-dependent decrease in cortical D2 mRNA, but little or no change in D2 activity. The decrease in D2 mRNA in response to T3 treatment can be demonstrated within 4 h. Treatment of hypothyroid rats for 2 weeks with graded doses of L-T4 (0.1-1.5 microg/100 g BW x day) results in a significant decrease in cortical D2 activity, but not mRNA. The association between D2 activity and D2 mRNA in euthyroid, hypothyroid, and hormone-treated rats across a full range of thyroidal states suggests that L-T4 treatment is associated with greater changes in cortical D2 activity (via posttranslational effects) than mRNA, whereas L-T3 treatment has a greater effect on decreasing D2 mRNA (i.e. pretranslational effects). In conclusion, these studies demonstrate both pre- and posttranslational regulation of cortical D2 expression. The relative contribution of each mechanism depends on the ambient thyroid hormone concentration.

The deiodinase family of selenoproteins

The realization some forty years ago that several iodothyronine compounds are present in the circulation suggested that deiodination occurs in various tissues. Subsequently, deiodination was indeed documented in in vivo studies. Later, using in vitro assay techniques, three deiodinase processes, termed types 1, 2 and 3, were defined that differed in terms of tissue distribution, reaction kinetics, efficiency of substrate utilization and sensitivity to inhibitors. Although purification of the deiodinase enzymes has continued to be problematic, recent molecular cloning studies have identified cDNAs for these three deiodinase isoforms from multiple species. These cDNAs have provided important insights into the structural characteristics of this family of enzymes. Foremost among the structural features has been the demonstration that all three deiodinase isoforms contain at their active site the uncommon amino acid selenocysteine which is of critical importance to their catalytic activity. The availability of cDNAs for these enzymes provides important reagents for pursuing additional studies aimed at defining their biochemical features and roles in thyroid hormone economy.

Expression of the type II iodothyronine deiodinase in cultured rat astrocytes is selenium-dependent

The iodothyronine deiodinases are a family of selenoproteins that metabolize thyroxine and other thyroid hormones to active and inactive metabolites in a number of tissues including brain. Using primary cultures of rat astroglial cells as a model system, we demonstrate that the mRNA for the type II iodothyronine deiodinase (DII) selenoenzyme is rapidly and markedly induced by forskolin and 8-bromo-cAMP. The induction of DII activity, however, was significantly impaired by culturing cells in selenium-deficient medium for 7 days. Under such conditions, the addition of selenium resulted in a rapid increase in cAMP-induced DII activity that was dose-dependent, with maximal effects noted within 2 h. Cycloheximide blocked this effect of selenium on restoring cAMP-induced DII activity, whereas actinomycin D did not. These data demonstrate that the DII selenoenzyme is expressed in cultured astrocytes and that the induction of DII activity by cAMP analogues appears to be mediated, at least in part, by pretranslational mechanisms. Furthermore, selenium deprivation impairs the expression of DII activity at the level of translation.

Iodothyronine deiodinase

The iodothyronine deiodinases constitute a family of enzymes that catalyze the removal of iodine atoms from various thyroid hormones (THs) in the thyroid gland and extrathyroidal tissues. As such, they are responsible for both the activation and inactivation of these compounds, and are thus important regulators of TH action. Recently, new insights have been gained into the biochemical characteristics of these proteins and their physiologic roles in TH metabolism. In particular, the availability of affinity-labeling techniques, molecular probes, and specific antisera for these enzymes, and the recent identification of the type I deiodinase as a selenoprotein, have ushered in a new era in the study of thyroid hormone deiodination.