Effects of diabetes on rodent cardiac thyroid hormone receptor and isomyosin expression

Previous studies show that diabetes induces marked transformations in cardiac myosin heavy chain (MHC) gene expression that are somehow linked to the cellular action of thyroid hormone 3,5,3'-triiodothyronine (T3). In this study, we tested the hypothesis that diabetes induces a reduced expression of thyroid hormone receptors (TRs), which are known to be important transcription factors interacting with thyroid response elements (TREs) in the promoter region of both alpha- and beta-MHC genes. Adult female rats were randomly assigned to either a normal control (NC) or diabetic (D) group. Three and/or six weeks after induction of diabetes via streptozotocin injection, the hearts of the animals were analyzed for MHC and TR mRNA isoforms expression, nuclear T3 binding, and nuclear extract interaction with a palindromic TRE. Results showed that diabetes induced significant alteration in alpha- and beta-MHC expression. Northern blot analyses indicated no diabetes-associated differences in TR isoform mRNA signals. Cardiac nuclear T3 binding studies suggested no differences in either the binding capacity or the equilibrium binding constant among the two groups, indicating no changes in either the number of nuclear TRs or their affinity for T3. Furthermore, gel mobility shift assays detected no difference between NC and D groups for cardiac nuclear extract binding to palindromic TRE. Collectively, these findings suggest that, whereas diabetes exerts a profound effect on cardiac isomyosin gene expression, the underlying mechanism, although dependent on factors linked to T3 function, does not involve alterations in TR expression.