Plasma membrane-mediated non-genomic effects of T4, T3 and thyroid hormone metabolite tetrac on different aspects of mesenchymal stem cell biology and their signaling pathways

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In the recent years we and others have shown that mesenchymal stem cells (MSCs) selectively home to sites of injury and play an important role in tissue repair, wound healing and cancer through differentiation and the release of proangiogenic factors. In the last funding period we demonstrated, for the first time, that T4 and T3 are important stimulators of MSC differentiation in fibroblast-/pericyte-like cells, and have direct stimulatory effects on MSC migration, invasion and recruitment. These effects were shown to be mediated through non-genomic mechanisms via avb3 by their tetrac inhibition. One of the possible molecular mechanisms for this observation is T4/T3-mediated stimulation of the hypoxia response network in MSCs which was also demonstrated in our past funding period. As a logical consequence of our past studies, in the next funding period we will analyze in more detail the mechanistic basis for the non-genomic avb3-mediated effects of T4, T3 and tetrac on the migration of MSCs and MSC-mediated angiogenic effects with a special emphasis on characterization of the signaling pathways involved. For this purpose we will analyze the effects of T4, T3 and tetrac on 1) the expression of MSC cell surface markers, integrins and receptors that are associated with MSC migration, 2) the secretion of angiogenic factors by MSCs and their effect on endothelial cell tube formation. Based on preliminary data by others and our own group, the PI3K/Akt, MAPK and canonical Wnt pathways are logical candidates for further analysis as to their involvement in avb3-mediated non-genomic effects of T4, T3 and tetrac in MSC biology. Therefore, in the current proposal these pathways will be investigated by analysis of downstream targets after treatment of MSCs with T4, T3 in the presence or absence of tetrac, and the effect on MSC migration and invasion after application of specific inhibitors of these pathways. In addition, the activation status of the canonical Wnt pathway as well as two central angiogenesis-related pathways, Tie-2 and VEGF signaling pathways, will be assessed by using pathway-responsive reporter assays in vitro and in vivo using NIS as a reporter gene making use of our longstanding expertise in the application of NIS as theranostic gene. Based on the strong analogies between the process of wound healing and tumor stroma formation, the liver cancer xenograft mouse model established in our previous studies will be used as a robust model of chronic wound/tissue repair. These studies will significantly enhance our understanding of the critical role of thyroid hormones T4 and T3 as well as tetrac in regulation of MSC biology in the context of chronic wound/tissue repair. At the same time, our studies will expand our understanding of the evolving role of integrin avb3 as mediator of thyroid hormone action and its therapeutic implications.