Analysis of the effects of T3, T4 and tetrac on mesenchymal stem cell biology

Mesenchymal stem cells (MSCs) are actively recruited into growing tumour stroma, where they differentiate to carcinoma-associated fibroblasts (CAFs) and form the tumour's fibrovascular network. Thyroid hormones T3 and T4 play a critical role in tumour stroma formation mediated through non-genomic mechanisms via integrin αvβ3. To improve our understanding of thyroid hormone action in tumour stroma formation, we examined the effects of T3, T4 and the integrin-specific inhibitor tetrac on MSC biology. Primary human bone marrow-derived MSCs were treated with T3 or T4 in the presence of hepatocellular carcinoma (HCC) cell-conditioned medium, resulting in stimulation of expression of genes associated with CAF-like differentiation as determined by qPCR and ELISA. These genes include fibroblast surface markers, indicators of tissue remodelling/invasion and neovascularisation and tumour promoting growth factors. In addition, thyroid hormone treatment markedly increased migration of MSCs towards HCC cell-conditioned medium. Further, in a three-dimensional spheroid invasion assay, thyroid hormone-treated MSCs invaded into the centre of HCC cell spheroids whereas untreated control cells remained at the surface. All of these effects were shown to be tetrac-dependent and therefore integrin αvβ3-mediated. In vivo, in the hyperthyroid state both recruitment of MSCs and invasion into HCC xenograft tumours was significantly enhanced compared to euthyroid and in particular hypothyroid mice, where only low levels of MSC recruitment and almost no invasion were detected. Treatment with tetrac almost completely eliminated MSC recruitment. Our data suggest that thyroid hormones T3 and T4 have a profound effect on the biology of MSCs in the tumour microenvironment. These studies significantly improve our understanding of the critical role of T3 and T4 in the regulation of MSC differentiation and migration in the context of tumour stroma formation, as well as the antitumour activity of tetrac.