The type 2 iodothyronine deiodinase is essential for adaptive thermogenesis in brown adipose tissue

Abstract

Type 2 iodothyronine deiodinase (D2) is a selenoenzyme, the product of the recently cloned cAMP-dependent Dio2 gene, which increases 10- to 50-fold during cold stress only in brown adipose tissue (BAT). Here we report that despite a normal plasma 3,5,3'-triiodothyronine (T3) concentration, cold-exposed mice with targeted disruption of the Dio2 gene (Dio2(-/-)) become hypothermic due to impaired BAT thermogenesis and survive by compensatory shivering with consequent acute weight loss. This occurs despite normal basal mitochondrial uncoupling protein 1 (UCP1) concentration. In Dio2(-/-) brown adipocytes, the acute norepinephrine-, CL316,243-, or forskolin-induced increases in lipolysis, UCP1 mRNA, and O(2) consumption are all reduced due to impaired cAMP generation. These hypothyroid-like abnormalities are completely reversed by a single injection of T3 14 hours earlier. Recent studies suggest that UCP1 is primarily dependent on thyroid hormone receptor beta (TR beta) while the normal sympathetic response of brown adipocytes requires TR alpha. Intracellularly generated T3 may be required to saturate the TR alpha, which has an approximately fourfold lower T3-binding affinity than does TR beta. Thus, D2 is an essential component in the thyroid-sympathetic synergism required for thermal homeostasis in small mammals.

Figure 1

Response of Dio2^–/– and wild-type mice to cold exposure or NE infusion. All results are the mean ± SD of 4–12 mice per time point. *P < 0.05 versus wild-type time point by ANOVA. (a) Core temperature; (b) serum creatine kinase; (c) relative changes in body weight during cold exposure; (d) mitochondrial UCP1 levels by Western analysis and mitochondrial COX-II gene by Southern analysis; (e) temperature changes in IBAT in response to NE infusion. After 10 minutes all Dio2^–/– points are significantly different versus wild-type animals by ANOVA. WT, wild-type; KO, knockout.

Figure 2

Metabolic responses of isolated brown adipocytes from Dio2^–/– and wild-type mice. *P < 0.05 versus wild-type time point by ANOVA. (a–c) O₂ consumption. Each point is the mean ± SD of four measurements of about 50,000 cells. (d–f) Glycerol released during incubation with NE or forskolin at 37°C for 1 hour. (g) The same conditions as in d except that about 500,000 cells were used. (h) Densitometry of the Northern blots shown in g. (i–k) The same conditions as in d except that cAMP is shown.

Figure 3

Role of the D2-generated intracellular T3 in the cellular thermogenesis in brown adipocytes. A plus sign within a circle indicates enhancement of that process by T3.