Topic: Thyroid hormone metabolism and action
Title: Gene expression from the imprinted Dio3 locus is associated with cell proliferation of cultured brown adipocytes.
Authors: Hernandez A, Garcia B, & Obregon MJ.
Reference: Endocrinology, May 17, 2007 [Epub ahead of print]
Active thyroid hormones are critical for the differentiation and function of brown adipose tissue. However, we have observed high basal and induced levels of type 3 deiodinase (D3), an enzyme that inactivates thyroid hormones and is coded by the imprinted gene Dio3, in differentiating brown preadipocytes in primary culture.
D3 activity & mRNA expression strongly correlated with the rate of proliferation of undifferentiated precursor cells under various conditions. Furthermore, differentiation of precursor cells to adipocytes was associated with decreased levels of D3 expression, and only very low levels of D3 mRNA were found in mature adipocytes. Dlk1, an inhibitor of adipocyte differentiation and a paternally expressed gene located in the same imprinted domain as Dio3, displayed changes in expression that paralleled those of Dio3. In contrast, a 4 kb transcript for Dio3os, an antisense gene also located in the same imprinted domain, was markedly upregulated in differentiated adipocytes.
D3 expression in differentiating preadipocytes is primarily linked to proliferating cells whereas Dio3os expression is associated with mature adipocytes. Present results suggest that genomic imprinting and gene expression at the Dlk1/Dio3 imprinted domain may play a role in the regulation of adipocyte proliferation and differentiation.
The iodothyronine deiodinase D3 has been much neglected in the past probably because of the believe that the levels of active hormones are regulated through their synthesis rather than their degradation. In contrast to D1 & D2, which are capable of converting the pro-hormone T4 to the active hormone T3, D3 catalyzes the inactivation of T4 and T3 to rT3 and 3,3--T2, respectively, insofar as these metabolites may be regarded as biologically inactive. The type 3 deiodinases are homologous integral membrane proteins containing a catalytic selenocysteine residue in their active centers. Studies over the last decade have increased the appeal of D3 as an important site for the regulation of thyroid hormone bioactivity.
Particular intriguing are the following findings:
- D3 is strongly expressed in different fetal tissues and is down-regulated acutely in the embryonic chicken liver by growth hormone and glucocortico-steroids. Serum T3 levels in the chicken embryo and human fetus are strongly and negatively correlated with hepatic D3 expression.
- Some tumors, including hemangiomas, express very high D3 levels. Patients with hemangiomas may demonstrate severe -consumptive hypothyroidism- due to extensive degradation of thyroid hormone by the tumor.
- D3 shows region-specific and time-dependent expression profiles in the fetal and neonatal human brain. Tissue levels of T3 appear to be depend importantly on local D3 expression.
- In addition to a decreased conversion of T4 to T3, the low T3 syndrome of severe nonthyroidal illness may result from an increased expression of D3 in liver, skeletal muscle, and immune cells.
- The failing heart shows a gene expression profile resembling that seen in hypothyroidism, which may be due at least in part by local degradation of T3 because of a strongly induced D3 expression.
- Compared with D1 and D2 knockout mice, D3 null mice show a much more severe phenotype associated with significant mortality and morbidity, including growth retardation and central hypothyroidism.
- D3 expression is subject to imprinting with predominant expression from the paternal allele. This is similar to regulation of other genes important for fetal development such as IGF-2.
The present study adds further weight to the importance of D3 for thyroid hormone regulation. They show high D3 expression in proliferating pre-adipocytes from brown adipose tissue, and a marked decrease in D3 expression when the cells differentiate into mature adipocytes. The findings confirm the concept emerging from previous studies that in general T3 stimulates the differentiation of cells and decreases their proliferation. Cell proliferation thus requires low intracellular T3 concentrations that - in the presence of ambient T3 levels - can be brought about by high D3 expression. Conversely, differentiation of cells is stimulated by increasing T3 levels resulting from a down-regulation of D3.
Regulation of D3 expression is a complex process. Its gene ( Dio3 ) is located in an imprinted region on mouse chomosome 12 and human chromosome 14. There is at least one other coding gene in this region, Dlk1 , which is also predominantly expressed from the paternal allele. Dlk1 is also termed Pref1 (preadipocyte factor 1), and has been known for some time because of its role in preadipocyte differentiation, showing high expression in preadipocytes and low expression in mature adipocytes. Down-regulation of Dlk1 in preadipocytes also stimulates their differentiation. Present study also demonstrates a strong correlation between D3 and Dlk1 expression in differentiating brown adipocytes. Expression of the paternally imprinted genes in the Dlk1-Dio3 locus is coordinated to expression of non-coding maternally imprinted genes on the opposite DNA strand. One of these is the so called Dio3 opposite strand ( Dio3os ) gene that partially overlaps with the Dio3 gene. Hernandez et al. show up-regulation of Dio3os in differentiating brown adipocytes reciprocal to the down-regulation of D3.
In conclusion, D3 plays a fundamental role in the regulation of tissue growth and differentiation during fetal development, as exemplified by the study of Hernandez et al. It would be very interesting to investigate to what extent their findings also apply to the growth and differentiation of white adipocytes. Their study also underscores the little we still know about the regulation of the expression of the imprinted Dio3 gene.
( Summary and commentary prepared by Theo Visser )
Present summary & commentary are related to Chapter N- 3 (Sections 3c and 3d) of the TDM