Endocrine Abstracts

Tumour stroma formation is associated with angiogenesis and requires interactions of various different cell types, including endothelial cells and mesenchymal stem cells (MSCs), which are actively recruited into growing tumour stroma. Thyroid hormones T3 and T4 act as non-classical proangiogenic modulators mediated by non-genomic mechanisms via cell surface receptor integrin αvβ3. The deaminated T4 derivative tetrac is a specific inhibitor of thyroid hormone action at the integrin site. The aim of this study was to evaluate the effects of T3 and T4 versus tetrac on MSCs in the context of angiogenesis. Treatment of primary human MSCs with T3 or T4 in the presence of hepatocellular carcinoma (HCC) cell-conditioned medium resulted in stimulation of expression of genes associated with angiogenesis as determined by qPCR. Additional treatment with tetrac reversed these effects. Primary human umbilical vein endothelial cells (HUVECs) were seeded on Matrigel and tube formation was analysed microscopically. Compared to untreated HUVECs, treatment with thyroid hormones and MSC-conditioned medium stimulated tube formation, while tetrac reduced tube formation. As the vascular endothelial growth factor (VEGF) is a critical angiogenesis mediator, we established a reporter gene system by placing the sodium iodide symporter (NIS) gene under control of the VEGF promoter. MSCs transfected with this construct (VEGF-NIS-MSCs) showed enhanced perchlorate-sensitive NIS-mediated iodide uptake activity in vitro after stimulation with HCC cell-conditioned medium and either T3 or T4 that was blocked by tetrac. T3 effects were additionally blocked both by the PI3K pathway inhibitor LY294002 and the ERK1/2 pathway inhibitor RAF265, while T4 effects were only blocked upon RAF265 treatment, supporting integrin αvβ3-dependency. Effects of thyroid hormone on VEGF-driven NIS expression in MSCs in vivo were evaluated by iodide-124 PET in an orthotopic HCC xenograft mouse model. Tumoural radioiodide uptake demonstrated successful tumoural recruitment of VEGF-NIS-MSCs after systemic application followed by VEGF promoter-driven NIS expression. In hyperthyroid animals, a strongly enhanced radioiodide signal was detected in tumours compared to euthyroid and hypothyroid mice, while treatment with tetrac resulted in a markedly reduced signal. These data confirm the in vitro data suggesting significant thyroid hormone-mediated stimulation of VEGF in HCC tumours that was inhibited by tetrac. Our data suggest that thyroid hormones T3 and T4 influence angiogenic signalling in MSCs in an integrin-dependent fashion, providing further evidence of the critical role of thyroid hormones in the regulation of angiogenesis and the anti-angiogenic activity of tetrac in the context of tumour stroma formation.