Addition of excess thyroid hormone induces detrimental changes in human ex vivo full thickness osteochondral explants

Purpose: By applying 3D in vitro chondrogenesis in a model using human bone marrow mesenchymal stem cells (hBMSC), it was demonstrated that excess thyroid hormone (T3) and the upregulation of deiodinase iodothyronine type-2 (D2) gene (DIO2), had similar detrimental effects on cartilage matrix deposition. Moreover, an in vivo rat model further indicated that this unfavourable effect of DIO2 upregulation on articular cartilage is likely modulated by mechanical loading. In the current study our aim was to investigate the effect of excess T3 in a human ex vivo explant model in interaction with mechanical loading. Methods: Full thickness osteochondral explants were isolated from the macroscopically preserved condyles of three human osteoarthritic (OA) donors included in the RAAK study. From day 0 to 5 the explants were kept under standardized conditions in chondrogenic medium to normalize expression levels. All explants were loaded from day 6 to 9, with a regime of 1 hz (Mach-1 mechanical testing system, Biomomentum) at 30% strain, during 10 min. To determine the effect of thyroid hormone, 10 nM T3 was added to the culture medium. As primary output, RT-PCR was performed to measure changes in COL2A1 (anabolism), MMP13 (catabolism) and EPAS1 (hypertrophy) gene expression. To determine structural damage, paraffin sections of the cartilage were stained with hematoxylin and eosin (HE), Safranin O and Alcian blue staining. Results: In our human explants, excess T3 resulted in a significant upregulation of EPAS1 (P ¼ 0.037) whereas for MMP13 and COL2A1 suggestive evidence for upregulation was found (P ¼ 0.065 and P ¼ 0.065, respectively). Additionally, staining with Safranin O, confirmed that excess T3 was detrimental to cartilage homeostasis as reflected by a loss of proteoglycans. Conclusions: Similar to our 3D in vitro chondrogenesis model with hBMSC, we here show that excessive T3 while applying 30% strain is detrimental to cartilage matrix homeostasis in a human explant culture model. The DIO2 gene, resulting in excessive T3 levels, has previously been identified as a susceptibility gene for OA and our results here further recognize the thyroid homeostasis as a potential therapeutic target of OA.