The inhibition of megalin-mediated transcytosis of thyroglobulin as a mechanism for insulin resistance-induced hypothyroidism in male rats

The aim of this study was to evaluate the impact of insulin resistance on thyroid function and explore megalin-targeting strategies to mitigate hypothyroidism in diabetes. Using a high-fat diet-induced insulin resistance model in male Sprague-Dawley rats, we observed structural thyroid abnormalities and reduced expression of thyroglobulin (Tg), thyroid peroxidase (TPO), and megalin, alongside hypothyroid serum markers. To validate these findings in vitro, hyperinsulinemic FRTL-5 cells exhibited parallel declines in Tg, TPO, and megalin expression. Additionally, the effects of metformin treatment and megalin blockade via siRNA were explored. Mechanistically, insulin resistance inhibited both megalin expression and transcytosis via phosphatidylinositol-3-kinase (PI3K) pathway, thereby reducing triiodothyronine (T3) release. CRISPR/Cas9-mediated megalin overexpression reversed this inhibition, confirming its essential role in T3 regulation. Building on these mechanistic insights, we explored therapeutic interventions. Megalin overexpression rescued T3 release, while metformin upregulated megalin expression in vivo. Crucially, siRNA-mediated megalin blockade abolished metformin's therapeutic effects, demonstrating pathway specificity. Our findings establish megalin inhibition as a critical pathway linking insulin resistance and hypothyroidism in diabetes, with metformin exerting therapeutic effects through megalin upregulation. This highlights megalin as a promising target for managing endocrine comorbidities.