Our laboratory has focused on the mechanisms regulating triiodothyronine (T3) homeostasis for over 25 years. Critical to this process are the actions of the iodothyronine deiodinases which function in concert to regulate thyroxine (T4) activation and the inactivation of T4 and T3. This proposal continues this theme. In the first Specific Aim we will continue our investigations of a cell type specific negative thyroid hormone response element (nTRE) in the promoter of the human Type 1 deiodinase (Dl) gene. This thyroid receptor (TR) binding sequence also binds a novel JEG cell transcription factor (JTF) with high affinity and specificity. JTF increases expression of genes containing this sequence and this effect is enhanced by APO-TRs. T3 eliminates this effect. Using DNA affinity matrix techniques we will isolate and identify this newly discovered protein and determine how TR cooperates with it as an example of a specific mechanism for negative regulation of gene expression by thyroid hormone. Uncontrolled, rapid inactivation of thyroid hormone causes hypothyroidism in a syndrome which we recently identified in infants with large hemangiomas. Infantile hemangiomas express Type 3 iodothyronine deiodinase (D3), the major physiological inactivator of 13 and 14, at levels up to 8-fold that in placenta. Large tumors can deiodinate T4 and T3 more rapidly than the infant's thyroid can secrete them.
Specific Aim 2 will elucidate the mechanism for ectopic expression of D3 in these tumors. We will compare hemangioma-derived and normal human capillary endothelial cells to discover pathways which could activate D3 expression analogous to those in placenta.
Specific Aim III is to define the mechanism for the myocardial response of the euthyroid heart to the thyrotoxic state such as occurs in patients with hyperthyroidism. We have developed a novel method for inducing chronic myocardial thyrotoxicosis in mice by use of a transgene in which Type 2 iodothyronine deiodinase (D2) is driven by the alpha-MHC promoter. We will first define the mechanism for the two-fold increase in the cAMP response to forskolin in myocardial membranes from these transgenic mice. We will also document the differences n gene expression profiles between euthyroid and thyrotoxic myocardium from both young and old mice. Only a few genes have been identified which increase their expression significantly between the euthyroid and hyperthyroid state. Identifying such genes in the myocardium will be especially critical to the understanding of the effects of T3 excess on the heart in human hyperthyroidism. These studies will provide new information relevant to both basic and clinical thyroid physiology.
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