Effect of mutations in the β1-thyroid hormone receptor on the inhibition of T3 binding by desethylamiodarone

AbstractDesethylamiodarone (DEA) acts as a competitive inhibitor of triiodothyronine (T3) binding to the α1-thyroid hormone receptor (TRα1) but as a non-competitive inhibitor with respect to TRβ1. To gain insight into the position of the binding site of desethylamiodarone on TRβ1 we investigated the naturally occurring mutants Y321C, R429Q, P453A, P453T and the artificial mutants L421R and E457A in the ligand binding domain of human TRβ1. The IC50 values (in μM) of DEA for P453A (50±11) and P453T (55±16) mutant TRβ1 are not different from that for the wild type TRβ1 (56±15), but the IC50 values of R429Q (32±7; P<0.001) and E457A (17±3; P<0.001) are significantly lower than of the wild type. Scatchard plots and Langmuir analyses indicate a non-competitive nature of the inhibition by DEA of T3 binding to all four mutant TRβ1s tested. Mutants P453A and P453T do not influence overall electrostatic potential, and also do not influence the affinity for DEA compared to wild type. Mutant E457A causes a change from a negatively charged amino acid to a hydrophobic amino acid, enhancing the affinity for DEA. Mutant R429Q, located in helix 11, causes an electrostatic potential change from positive to uncharged, also resulting in greater affinity for DEA. We therefore postulate that amino acids R429 and E457 are at or close to the binding site for DEA, and that DEA does not bind in the T3 binding pocket itself, in line with the non-competitive nature of the inhibition of T3 binding to TRβ1 by DEA

Familial dysalbuminemic hyperthyroxinemia: a persistent diagnostic challenge.

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Human metabolic syndrome resulting from dominant-negative mutations in the nuclear receptor peroxisome proliferator-activated receptor-γ

We previously reported a syndrome of severe hyperinsulinemia and early-onset hypertension in three patients with dominant-negative mutations in the nuclear hormone receptor peroxisome proliferator-activated receptor (PPAR)-γ. We now report the results of further detailed pathophysiological evaluation of these subjects, the identification of affected prepubertal children within one of the original families, and the effects of thiazolidinedione therapy in two subjects. These studies 1) definitively demonstrate the presence of severe peripheral and hepatic insulin resistance in the affected subjects; 2) describe a stereotyped pattern of partial lipodystrophy associated with all the features of the metabolic syndrome and nonalcoholic steatohepatitis; 3) document abnormalities in the in vivo function of remaining adipose tissue, including the inability of subcutaneous abdominal adipose tissue to trap and store free fatty acids postprandially and the presence of very low circulating levels of adiponectin; 4) document the presence of severe hyperinsulinemia in prepubertal carriers of the proline-467-leucine (P467L) PPAR-γ mutation; 5) provide the first direct evidence of cellular resistance to PPAR-γ agonists in mononuclear cells derived from the patients; and 6) report on the metabolic response to thiazolidinedione therapy in two affected subjects. Although the condition is rare, the study of humans with dominant-negative mutations in PPAR-γ can provide important insight into the roles of this nuclear receptor in human metabolism

A role for helix 3 of the TR-beta ligand-binding domain in coactivator recruitment identified by characterization of a third cluster of mutations in resistance to thyroid hormone

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