Five classes of TH‐binding proteins. The thyroid gland secreted TH (predominantly T4 in mammals) into the blood, where it binds THDPs (1). TH can dissociate from THDPs and enter cells by passive diffusion, or via TH transporter proteins (2). Within the cell, THs can be deiodinated by deiodinases (3) and bind cytosolic TH‐binding proteins (4). Within the nucleus, T3 binds TH receptors (TRs) (5). NB: deiodinases D1, D2 and D3 have different locations with a cell; TRs change their conformation upon binding to DNA. inline image, albumin; inline image, transthyretin (TTR); inline image, TBG; inline image, TH transporter; inline image, deiodinase; inline image, cytosolic TH‐binding protein; inline image, TR; inline image, TR bound to DNA.
Transthyretin / Thyroxine-binding globulin TBG
Plasma Membrane Transporters
BOTH TRANSLATION START SITES IN THE HUMAN MCT8
GENE ARE USED TO PRODUCE TWO MCT8 PROTEIN
ISOFORMS IN VITRO
E. C. FRIESEMA, W. E. VISSER, T. J. VISSER
Internal Medicine, Erasmus MC, Rotterdam, Netherlands
MCT8 is an important thyroid hormone transporter as mutations are
associated with severe psychomotor retardation and elevated serum
T3 levels with low to normal fT4 levels. MCT8 knockout mice show
the same changes in thyroid parameters, but lack the neurological
defects. The human (h) MCT8 gene contains two possible translation
start sites (TLSs). However, mouse MCT8 has only one TLS, which
corresponds to the second TLS in hMCT8. So far, we have only
studied the hMCT8 protein generated from the second TLS. mRNA
expression data from human liver revealed the presence of mRNA
species containing both TLSs. Long hMCT8 cDNA was constructed
by extending the 50-end of our short hMCT8 cDNA with a 312 bp
fragment. Transport studies were performed in transiently transfected
COS1 cells using both hMCT8 constructs. We found that T3
uptake in cells transfected with long hMCT8 was only 12% lower than
in cells with short hMCT8. Western blotting showed that transfection
with long hMCT8 gives rise to two hMCT8 protein bands of 61 and
69 kDa, the latter being more abundant than the former. The 61 kDa
band represents short hMCT8 (539 amino acids) and the 69 kDa band
represents long hMCT8 (613 amino acids). These results suggest that
long hMCT8 is capable of transporting thyroid hormone, but to study
this more specifically, we are currently mutating the second TLS to
prevent the synthesis of short MCT8. We found that both TLSs in the
hMCT8 gene are used to produce two isoforms that may be differentially
expressed in human tissues. Our findings might also explain
the neurological differences between humans and mice with MCT8
mutations as the long hMCT8 that does not exist in mice might play
an important role in human brain development.
Efflux from the cell
Thyroid hormone export from cells: contribution of P-glycoprotein. 2005
J Endocrinol. 2005 Apr;185(1):93-8.
Mitchell AM, Tom M, Mortimer RH.
Verapamil inhibits tri-iodothyronine (T3) efflux from several cell types, suggesting the involvement of multidrug resistance-associated (MDR) proteins in T3 transport. The direct involvement of P-glycoprotein (P-gp) has not, however, been investigated. We compared the transport of 125I-T3 in MDCKII cells that had been transfected with mdr1 cDNA (MDCKII-MDR) versus wild-type MDCKII cells (MDCKII), and examined the effect of conventional (verapamil and nitrendipine) and specific MDR inhibitors (VX 853 and VX 710) on 125I-T3 efflux. We confirmed by Western blotting the enhanced expression of P-gp in MDCKII-MDR cells. The calculated rate of 125I-T3 efflux from MDCKII-MDR cells (around 0.30/min) was increased twofold compared with MDCKII cells (around 0.15/min). Overall, cellular accumulation of 125I-T3 was reduced by 26% in MDCKII-MDR cells compared with MDCKII cells, probably reflecting enhanced export of T3 from MDCKII-MDR cells rather than reduced cellular uptake, as P-gp typically exports substances from cells. Verapamil lowered the rate of 125I-T3 efflux from both MDCKII and MDCKII-MDR cells by 42% and 66% respectively, while nitrendipine reduced 125I-T3 efflux rate by 36% and 48% respectively, suggesting that both substances inhibited other cellular T3 transporters in addition to P-gp. The specific MDR inhibitors VX 853 and VX 710 had no effect of 125I-T3 efflux rate from wild-type MDCKII cells but reduced 125I-T3 export in MDCKII-MDR cells by 50% and 53% respectively. These results have provided the first direct evidence that P-gp exports thyroid hormone from cells.
T4 uptake review 2001
Two stereospecific binding sites for each T4 and T3 have been detected in cell membranes and on intact cells from humans andother species.
The apparent Michaelis-Menten values of the high-affinity, low-capacity binding sites for T4 and T3 are in
the nanomolar range, whereas the apparent MichaelisMentenvalues of the low-affinity, high-capacity binding sites
are usually in the lower micromolar range.
Cellular uptake of T4 and T3 by the high-affinity sites is energy, temperature, and often Na
dependent and represents the translocation of thyroid hormone over the plasma membrane. Uptake by the lowaffinity
sites is not dependent on energy, temperature, andNa
and represents binding of thyroid hormone to proteins
associated with the plasma membrane.
In rat erythrocytes
and hepatocytes, T3 plasma membrane carriers have been tentatively
identified as proteins with apparent molecular
masses of 52 and 55 kDa. In different cells, such as rat erythrocytes,
pituitary cells, astrocytes, and mouse neuroblastoma
cells, uptake of T4 and T3 appears to be mediated largely by
system L or T amino acid transporters.
Efflux of T3 from different
cell types is saturable, but saturable efflux of T4 has not
yet been demonstrated.
Saturable uptake of T4 and T3 in the brain occurs both via the blood-brain barrier and the choroid
plexus-cerebrospinal fluid barrier.
Thyroid hormone uptakein the intact rat and human liver is ATP dependent and rate
limiting for subsequent iodothyronine metabolism.
In starvation and nonthyroidal illness in man, T4 uptake in the liver is
decreased, resulting in lowered plasma T3 production. Inhibition
of liver T4 uptake in these conditions is explained by
liver ATP depletion and increased concentrations of circulating
inhibitors, such as 3-carboxy-4-methyl-5-propyl-2-furanpropanoic
acid, indoxyl sulfate, nonesterified fatty acids, and
bilirubin. Recently, several organic anion transporters and L
type amino acid transporters have been shown to facilitate
plasma membrane transport of thyroid hormone.
Competitive inhibition of thyroid hormone uptake into cultured rat brain astrocytes by bilirubin and bilirubin conjugates. 1993