Highlight
Stimulating thyrotropin receptor antibodies (TSHR antibodies) upregulate both TSHR and insulin-like growth factor-1 receptor (IGF-1R) expression on fibroblasts, key players in thyroid eye disease (TED). This effect is mediated primarily through receptor recycling rather than increased gene transcription and can be inhibited by blocking endocytosis, unveiling potential therapeutic avenues.
Study Background
Thyroid eye disease (TED) is a complex autoimmune disorder affecting retroorbital tissues, leading to inflammation, fibrosis, and disfiguring proptosis. Central to its pathogenesis are autoantibodies targeting the thyrotropin receptor (TSHR) on orbital fibroblasts. Recent evidence points to a synergistic role of insulin-like growth factor 1 receptor (IGF-1R) signaling alongside TSHR activation, exacerbating tissue remodeling and inflammation in TED. However, the mechanisms by which stimulating TSHR autoantibodies modulate receptor expression, especially in fibroblasts, remain incompletely understood. Elucidating these processes holds clinical significance, as it may inform targeted therapies addressing receptor expression and signaling in TED.
Study Design
This investigation employed in vitro cellular models, including murine 3T3-L1 fibroblasts and human orbital fibroblasts, to assess the impact of TSH and stimulating TSHR antibodies on receptor expression. A novel hinge-region monoclonal antibody (MC-1), which binds TSHR without interfering with hormone or autoantibody interaction, was utilized to quantify receptor expression accurately.
Key methodologies included: dose-dependent treatments with recombinant TSH and stimulating TSHR antibody M22; flow cytometry and real-time quantitative polymerase chain reaction (RT-qPCR) to measure TSHR and IGF-1R expression; differentiation confirmation of 3T3-L1 fibroblasts via Oil Red O staining; and pharmacological inhibition experiments to dissect underlying mechanisms—specifically, blockade of protein synthesis and endocytosis. Molecular dynamics simulations evaluated the accessibility of binding sites on TSHR in the presence of MC-1.
Key Findings
Dose-Dependent Upregulation of TSHR and IGF-1R Expression
Exposure to TSH induced a significant, dose-dependent increase in detectable TSHR expression on fibroblasts as measured by MC-1 binding. Correspondingly, IGF-1R expression was similarly upregulated in a parallel dose-dependent manner. Treatment with the stimulating TSHR antibody M22 replicated these effects, with both TSHR and IGF-1R expression peaking approximately 48 hours post-treatment.
Mechanisms Underlying Receptor Upregulation
Contrary to expectations, RT-qPCR analyses did not reveal significant transcriptional upregulation of TSHR or IGF-1R mRNAs, suggesting that the increased receptor levels arose from post-transcriptional processes. Inhibition of protein synthesis failed to reduce receptor induction, further supporting a non-transcriptional mechanism.
Crucially, pharmacological inhibition of endocytosis markedly attenuated the enhanced expression of both receptors. These findings implicate receptor recycling from intracellular compartments to the cell surface as the principal mechanism driving increased receptor availability, rather than new receptor biosynthesis.
Implications for Receptor Complex Formation
The concurrent upregulation of TSHR and IGF-1R is consistent with their previously reported physical and functional receptor complex formation. This receptor cooperation may underpin the additive retroorbital signaling observed in TED, amplifying pathogenic fibroblast activation.
Expert Commentary
These observations provide important mechanistic insights into autoantibody-mediated receptor modulation in TED. By demonstrating that stimulating TSHR antibodies not only activate receptors but also promote their recycling and surface expression, the study highlights an underappreciated aspect of disease pathogenesis.
Targeting receptor recycling and endocytosis pathways introduces a novel therapeutic strategy to diminish aberrant TSHR and IGF-1R signaling in orbital fibroblasts. This could complement existing approaches like IGF-1R blocking antibodies (e.g., teprotumumab), currently approved for TED, by reducing receptor availability and downstream signaling.
However, translation to clinical application requires further validation of these mechanisms in vivo and assessment of safety profiles for endocytosis inhibitors in relevant tissues. Additionally, the absence of transcriptional changes invites further inquiry into the intracellular trafficking pathways and molecular players involved in receptor recycling.
Conclusion
Stimulating thyroid-stimulating hormone receptor autoantibodies enhance surface expression of both TSHR and IGF-1R on fibroblasts predominantly via receptor recycling mechanisms. This dual receptor upregulation facilitates additive signaling implicated in thyroid eye disease pathogenesis. Inhibition of endocytosis effectively diminishes receptor expression, unveiling promising therapeutic targets. These findings deepen understanding of TED molecular pathology and suggest novel intervention strategies to improve patient outcomes.
Funding and Clinical Trials
The study was published in the American Thyroid Association’s journal; specific funding information and clinical trial registration details were not provided in the source publication.
References
- Xiang P, Mezei M, Latif R, Davies TF. Stimulating Thyrotropin Receptor Antibodies Enhance the Expression of Both Thyrotropin Receptors and Insulin-Like Growth Factor 1 Receptors in Fibroblasts. Thyroid. 2026 Jun 30; PMID: 42374926. Available from: https://pubmed.ncbi.nlm.nih.gov/42374926/
- Douglas RS. Teprotumumab, signaling pathways, and thyroid eye disease: an emerging therapeutic paradigm. J Clin Endocrinol Metab. 2020;105(7):2237-2239.
- Smith TJ. Insulin-like growth factor-I receptor signaling and its therapeutic inhibition in thyroid-associated ophthalmopathy. J Clin Endocrinol Metab. 2015;100(6):2273-2284.