Syntenin, a novel target to counteract cancer progression ?

Multiple studies indicate that the gain-of-function of syntenin-1 (hereupon referred to as 'syntenin') acts as a positive regulator of invasion and metastasis, in various tumors. Consistently, syntenin affects several signaling pathways that are of known relevance for the cancer process. For example, syntenin enhances the fibronectin-induced assembly of integrin-β1 signaling complexes with FAK and c-Src, resulting in the activation of FAK and downstream signaling pathways, important for regulating the migration machinery. Syntenin was originally identified in the host laboratory, as a PDZ-scaffold protein that is interacting with the syndecan heparan sulfate proteoglycans, co-receptors for a plethora of adhesion and growth factor receptor systems. Further functional studies established that syntenin controls (at least) two different aspects of vesicular trafficking that are regulated by the small GTPase ADP ribosylation factor 6 (ARF6). Syntenin promotes syndecan recycling from endosomes to the plasma membrane, and also promotes exosome secretion. Syntenin thus allows syndecan cargo, i.e. various signaling receptors to escape degradation and thereby potentially functions as a booster of cellular signaling, both in cis (recycling) and in trans (exosomes). Consistently, syntenin networking with syndecans and ARF6 controls cell migration during vertebrate embryogenesis. Of note, the function of syntenin strictly depends on its PDZ domains that operate as protein-protein and protein-phospholipid (PtdInsPs) interaction modules. Thus, currently available data not solely support a possible role for syntenin in cancer cell growth, invasion and metastasis, but also highlight a most probable molecular basis for this function. The main goal of this work was to clarify whether or not syntenin can be envisioned as a novel therapeutic and pharmacological target in cancer therapy. To test for the potential benefit of syntenin inhibition, I evaluated the impact of syntenin loss-of-function in different cancer cell lines. To test whether syntenin inhibition is safe, I characterized syntenin knock-out mice. To test whether syntenin is druggable, I screened compound libraries for chemicals inhibiting syntenin-syndecan interaction and tested for the selectivity and biological specificity of their effects. Using shRNA, siRNA and CRISPR/Cas9 silencing approaches, I showed that syntenin controls the migration, growth, proliferation and cell cycle progression of different human and mouse cancer cell models. I documented that syntenin knock-out mice are viable and fertile although embryonic fibroblasts isolated from these mice are impaired in their migration and proliferation in vitro. In depth analysis of mouse embryonic fibroblasts revealed that the exosomal function of syntenin is important for the effects on cell migration and proliferation. Surprisingly, it further revealed that syntenin also plays a role in exosomal uptake, by controlling the levels of the syndecan heparan sulfate proteoglycans at the cell surface that function as exosome-receptors. This might in part be explained by the fact that, besides its exosomal function, syntenin also supports the recycling of intact syndecans to the cell surface. Yet, syntenin directly or indirectly also controls syndecan mRNA levels, indicative of feedback loops operating at different levels. Finally, I set up a Homogeneous Time Resolved Fluorescence (HTRF) assay to study syntenin PDZ interactions and screened various protein-protein interaction inhibition (2P2I) oriented libraries of chemicals. I identified several inhibitors and proceeded with one of them, to show it selectively blocks syntenin PDZ domains in vitro and displays syntenin-specific biological effects. In conclusion, I showed (i) that syntenin inhibition could be of potential benefit in various cancers, (ii) that an anti-syntenin therapy holds promises to be relatively safe and might broadly act in exosomal cancer communication (in the emission, but also reception of exosomal signals) and (iii) that PDZ domains might represent a new class of promising therapeutic targets, as specific inhibition of syntenin function is amenable pharmacologically.status: publishe