In Vitro Screening for ToxCast Chemicals Binding to Thyroxine-Binding Globulin

Area of Science:

• Endocrinology
• Toxicology
• Biochemistry

Background:

• Thyroid hormone (TH) carrier proteins are crucial for TH distribution and homeostasis.
• Disruption of TH carrier proteins is a potential mechanism for thyroid system dysfunction.
• Limited data exists on chemicals that bind to TH carrier proteins and displace TH.

Purpose of the Study:

• To screen a large chemical library for compounds that competitively bind to thyroxine-binding globulin (TBG).
• To identify chemicals that interfere with thyroid hormone transport and homeostasis.
• To develop and validate an in vitro assay for high-throughput screening of endocrine disruptors.

Main Methods:

• A fluorescence-based in vitro screening assay was employed.
• Over 1,400 chemicals from U.S. EPA's ToxCast libraries were screened.
• Chemicals showing competitive binding to TBG were further analyzed in concentration-response studies.

Main Results:

• 714 out of 1,400+ chemicals showed inhibition of bound fluorescent ligand at 100 μM.
• 297 chemicals exhibited ≥50% reduction in fluorescence and underwent further testing.
• Ten chemicals demonstrated EC50 values <1 μM, indicating high potency in disrupting TH binding.

Conclusions:

• The developed in vitro assay effectively identifies chemicals that can disrupt thyroid hormone homeostasis.
• Results support prioritizing chemicals for in vivo testing to predict real-world effects.
• This work expands the toolkit for identifying potential endocrine-disrupting chemicals.