Thyroid Hormones Transport
Thyroid Hormones

Author: Gianpiero Pescarmona
Date: 26/09/2009

Description

Plasma Transport

transthyretin

Cell 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.

Attachments
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MOT8-MOT10-600.gifgp26/09/2009
MOT8-MOT10.gifgp26/09/2009
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