Transport of thyroxine and 3,3',5-triiodothyronine in human umbilical vein endothelial cells

Dissecting thyroid hormone transport and metabolism in dendritic cells

We reported thyroid hormone (TH) receptor expression in murine dendritic cells (DCs) and 3,5,3'-triiodothyronine (T₃)-dependent stimulation of DC maturation and ability to develop a Th1-type adaptive response. Moreover, an increased DC capacity to promote antigen-specific cytotoxic T-cell activity, exploited in a DC-based antitumor vaccination protocol, was revealed. However, putative effects of the main circulating TH, l-thyroxine (T₄) and the mechanisms of TH transport and metabolism at DC level, crucial events for TH action at target cell level, were not known. Herein, we show that T₄ did not reproduce those registered T₃-dependent effects, finding that may reflect a homoeostatic control to prevent unspecific systemic activation of DCs. Besides, DCs express MCT10 and LAT2 TH transporters, and these cells mainly transport T₃ with a favored involvement of MCT10 as its inhibition almost prevented T₃ saturable uptake mechanism and reduced T₃-induced IL-12 production. In turn, DCs express iodothyronine deiodonases type 2 and 3 (D2, D3) and exhibit both enzymatic activities with a prevalence towards TH inactivation. Moreover, T₃ increased MCT10 and LAT2 expression and T₃ efflux from DCs but not T₃ uptake, whereas it induced a robust induction of D3 with a parallel slight reduction in D2. These findings disclose pivotal events involved in the mechanism of action of THs on DCs, providing valuable tools for manipulating the immunogenic potential of these cells. Furthermore, they broaden the knowledge of the TH mechanism of action at the immune system network.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Triiodothyronine stimulates glucose transport in bone cells

Thyroid hormones increase energy expenditure and bone turnover in vivo. To study whether 3,3',5-triiodo-l-thyronine (T₃) stimulates the uptake of glucose in osteoblastic cells, PyMS (a cell line derived from rat bone) cells were kept in serum-free culture medium and treated with T₃. We measured [1-¹⁴C]-2-deoxy-D: -glucose (2DG) uptake and looked for expression of the high-affinity glucose transporters GLUT1 and GLUT3 by northern and western analysis. T₃ did not influence the cell number but slightly (1.3-fold) increased the protein content of the cell cultures. 2DG uptake was low in serum-deprived cell cultures and was increased by T₃ (up to 2.5-fold at 1 nmol l⁻¹ after 4 days) in a dose- and time-dependent manner. Triiodothyronine at 1 nmol l⁻¹ increased GLUT1 and GLUT3 abundance in membranes. Therefore, increased glucose uptake induced by T₃ in osteoblasts may be mediated by the known high-affinity glucose transporters GLUT1 and GLUT3.

The thyroid hormone transporters MCT8 and MCT10 transport the affinity-label N-bromoacetyl-[(125)I]T3 but are not modified by it

Thyroid hormone (TH) transporter proteins mediate transport of TH across the plasma membrane, thereby facilitating its intracellular bioavailability. As only a few transporters have been identified which are relatively specific for TH, including monocarboxylate transporter (MCT) 8 and MCT10, the need for identification of novel specific TH transporters is obvious. A possible strategy to identify TH transporters is their modification with a ligand-derived affinity-label and subsequent identification by mass spectrometry. Previously, N-bromoacetyl (BrAc)-iodothyronines have been reported as useful affinity-labels for human (h) MCT8. In the present study we reinvestigated possible BrAc[(125)I]T3-labeling of hMCT8 and hMCT10. The present study demonstrates that hMCT8 and hMCT10 both facilitate BrAc[(125)I]T3 transport, but are not labeled by BrAc[(125)I]T3. We provide evidence that human protein disulfide isomerase, which molecular mass is similar to hMCT8, is labeled by BrAc[(125)I]T3. In addition, differential inhibitory effects were observed of iodothyronines derivatives with different side chains on T3 transport by hMCT8 and hMCT10. In conclusion, we demonstrated that not hMCT8 and hMCT10, but human protein disulfide isomerase, is labeled by BrAc[(125)I]T3. The usefulness of BrAc[(125)I]T3 as a tool for the identification of novel TH transporters remains to be explored.

Minireview: thyroid hormone transporters: the knowns and the unknowns

The effects of thyroid hormone (TH) on development and metabolism are exerted at the cellular level. Metabolism and action of TH take place intracellularly, which require transport of the hormone across the plasma membrane. This process is mediated by TH transporter proteins. Many TH transporters have been identified at the molecular level, although a few are classified as specific TH transporters, including monocarboxylate transporter (MCT)8, MCT10, and organic anion-transporting polypeptide 1C1. The importance of TH transporters for physiology has been illustrated dramatically by the causative role of MCT8 mutations in males with psychomotor retardation and abnormal serum TH concentrations. Although Mct8 knockout animals have provided insight in the mechanisms underlying parts of the endocrine phenotype, they lack obvious neurological abnormalities. Thus, the pathogenesis of the neurological abnormalities in males with MCT8 mutations is not fully understood. The prospects of identifying other transporters and transporter-based syndromes promise an exciting future in the TH transporter field.