Crosstalk between tumor cells and endothelial cells via the tetraspanin D6.1A
Zusammenfassung der Projektergebnisse
Rapidly growing tumors essentially depend on angiogenesis. We had noted that a tumor overexpressing the tetraspanin Tspan8 is a powerful angiogenesis inducer in the tumor and tumor-free organs. Aiming to answer the question, how a metastasis-associated molecule could support angiogenesis in tumor-free tissue, we identified tumor exosomes as the relevant entity. Meanwhile, tumor exosomes are considered as most powerful intercellular communicators. Thus, we extended our study to search, whether exosomes may also be engaged in the crosstalk between metastasizing tumor cells and the host. Using a non-metastatic and a metastatic subline of a rat pancreatic adenocarcinoma, we focused on three questions: 1) Why do only Tspan8-expressing exosomes induce angiogenesis? 2) How do exosomes communicate with selected target cells? 3) Do exosomes support the establishment of a premetastatic niche? We demonstrate for the first time that exosomes suffice to induce angiogenesis, where Tspan8 expressing exosomes drive endothelial cell progenitors into differentiation and promote resting endothelial cell activation. The particular contribution of Tspan8 relies on its mode of recruitment into exosomes, which is accompanied by a reorganization of the Tspan8 web, such that Tspan8 becomes associated with the alpha4 integrin chain. It is this complex, which is decisive for the preferential interaction with resting endothelial cells and endothelial cell progenitors. We further exemplify with Tspan8 that tetraspanins, constitutively expressed at a high level in exosomes, play a decisive role in exosome generation. They contribute to the exosome protein profile, which contains the tetraspanin membrane complexes of the donor cell, as well as to the selective recruitment of mRNA and miRNA. Of particular importance is the finding that tetraspanins also are engaged in exosome binding to selective targets. This is due to the exosomal tetraspanin complexes binding to target cell protein complexes, which are also located in internalization prone membrane domains. The requirement for fitting receptor and ligand complexes supports target cell selectivity, provides a signaling hub and facilitates exosome uptake, where mostly exosomal miRNA appears to account for target cell reprogramming. Finally, we were the first to demonstrate that exosomes play an essential role in premetastatic niche formation, which requires in addition a cooperation of exosomes with the tumor and the host matrix. The alliance between exosomes and the extracellular matrix promotes matrix modulation such that it supports tumor and host cell motility and activation. Meanwhile, we confirmed that in premetastatic niche formation, too, the metastasis promoting tetraspanins Tspan8 and CD151 play a central role. Taken together, besides unraveling for the first time the importance of exosomes in angiogenesis and premetastatic niche formation, it is particularly the demonstration that exosomal tetraspanin-complexes account for target cell selection, which opens the path generating tailored exosomes as selective therapeutics.
