Identification of Human TRIAC Transmembrane Transporters

Area of Science:

• Endocrinology
• Molecular Biology
• Cell Biology

Background:

• 3,5,3 -Triiodothyroacetic acid (TRIAC) is a T3-receptor agonist used to manage T3 resistance and symptoms associated with MCT8 transporter mutations.
• Monocarboxylate transporter 8 (MCT8) is crucial for thyroid hormone (TH) transport into the brain, and its dysfunction limits TH access and function.
• TRIAC can enter the brain independently of MCT8 and influence TH-dependent gene expression.

Purpose of the Study:

• To identify the specific transporters responsible for cellular uptake of TRIAC.
• To understand the mechanisms by which TRIAC enters cells, particularly in the context of MCT8 deficiency.

Main Methods:

• A whole-genome RNAi screen was conducted in HepG2 cells using a T3-receptor-dependent luciferase reporter.
• Hits were validated through counter screens and comparison with T3 effects to exclude off-target effects.
• TRIAC transport activity was biochemically characterized in MDCK1 cells transfected with identified transporter cDNAs.

Main Results:

• SLC22A9 (OAT7) and SLC29A2 (ENT2) were identified as the primary transporters mediating TRIAC cellular uptake.
• Uptake via SLC22A9 was reduced by its substrate estrone-3-sulfate, and uptake via SLC29A2 was inhibited by nitrobenzyl-6-thioinosine.
• ABCD1 was identified as an exporter of TRIAC in transfected cells.

Conclusions:

• SLC22A9 and SLC29A2 are key facilitators of TRIAC cellular entry.
• Understanding TRIAC transporter expression is vital for interpreting its therapeutic effects and limitations.
• While ABCD1 was identified as an exporter, its clinical relevance for TRIAC treatment is likely minimal.