Cell-Specific Transport and Thyroid Hormone Receptor Isoform Selectivity Account for Hepatocyte-Targeted Thyromimetic Action of MGL-3196.

Thyroid hormones (THs) and TH receptor-beta (TRß) reduce hepatic triglycerides, indicating a therapeutic potential for TH analogs in liver steatosis. To avoid adverse extrahepatic, especially TRα-mediated effects such as tachycardia and bone loss, TH analogs with combined TRß and hepatocyte specificity are desired. MGL-3196 is a new TH analog that supposedly meets these criteria. Here, we characterize the thyromimetic potential of MGL-3196 in cell-based assays and address its cellular uptake requirements. We studied the contribution of liver-specific organic anion transporters (OATP)1B1 and 1B3 to MGL-3196 action. The TR isoform-specific efficacy of MGL-3196 compared with 3,5,3'-triiodothyronine (T3) was determined with luciferase assays and gene expression analysis in OATP1B1 and OATP1B3 and TRα- or TRß-expressing cells and in primary murine hepatocytes (PMHs) from wild-type and TRß knockout mice. We measured the oxygen consumption rate to compare the effects of MGL-3196 and T3 on mitochondrial respiration. We identified OATP1B1 as the primary transporter for MGL-3196. MGL-3196 had a high efficacy (90% that of T3) in activating TRß, while the activation of TRα was only 25%. The treatment of PMHs with T3 and MGL-3196 at EC50 resulted in a similar induction of Dio1 and repression of Serpina7. In HEK293 cells stably expressing OATP1B1, MGL-3196 had comparable effects on mitochondrial respiration as T3. These data indicate that MGL-3196's hepatic thyromimetic action, the basis for its therapeutic use, results from a combination of hepatocyte-specific transport by OATP1B1 and the selective activation of TRß over TRα.

Canonical Thyroid Hormone Receptor ß Action Stimulates Hepatocyte Proliferation in Male Mice.

CONTEXT: 3,5,3'-L-triiodothyronine (T3) is a potent inducer of hepatocyte proliferation via the Wnt/ß-catenin signaling pathway. Previous studies suggested the involvement of rapid noncanonical thyroid hormone receptor (TR) ß signaling, directly activating hepatic Wnt/ß-catenin signaling independent from TRß DNA binding. However, the mechanism by which T3 increases Wnt/ß-catenin signaling in hepatocytes has not yet been determined. OBJECTIVE: We aimed to determine whether DNA binding of TRß is required for stimulation of hepatocyte proliferation by T3. METHODS: Wild-type (WT) mice, TRß knockout mice (TRß KO), and TRß mutant mice with either specifically abrogated DNA binding (TRß GS) or abrogated direct phosphatidylinositol 3 kinase activation (TRß 147F) were treated with T3 for 6 hours or 7 days. Hepatocyte proliferation was assessed by Kiel-67 (Ki67) staining and apoptosis by terminal deoxynucleotidyl transferase dUTP nick-end labeling assay. Activation of ß-catenin signaling was measured in primary murine hepatocytes. Gene expression was analyzed by microarray, gene set enrichment analysis (GSEA), and quantitative reverse transcription polymerase chain reaction. RESULTS: T3 induced hepatocyte proliferation with an increased number of Ki67-positive cells in WT and TRß 147F mice (9.2% ±â€…6.5% and 10.1% ±â€…2.9%, respectively) compared to TRß KO and TRß GS mice (1.2% ±â€…1.1% and 1.5% ±â€…0.9%, respectively). Microarray analysis and GSEA showed that genes of the Wnt/ß-catenin pathway-among them, Fzd8 (frizzled receptor 8) and Ctnnb1 (ß-catenin)-were positively enriched only in T3-treated WT and TRß 147F mice while B-cell translocation gene anti-proliferation factor 2 was repressed. Consequently, expression of Ccnd1 (CyclinD1) was induced. CONCLUSIONS: Instead of directly activating Wnt signaling, T3 and TRß induce key genes of the Wnt/ß-catenin pathway, ultimately stimulating hepatocyte proliferation via CyclinD1. Thus, canonical transcriptional TRß action is necessary for T3-mediated stimulation of hepatocyte proliferation.