ADP-ribosylated proteins as old and new drug targets for anticancer therapy: the example of ARF6.

Post-translational modifications of cellular proteins by mono- or poly-ADP-ribosylation are associated with numerous cellular processes. ADP-ribosylation reactions are important in the nucleus, and in mitochondrial activity, stress response signaling, intracellular trafficking, and cell senescence and apoptosis decisions. These reversible reactions add ADP-ribose to target proteins via specific enzymes to form the ADP-ribosylated protein; the cleaveage of this covalent bond is performed via hydrolases. Deficiencies in these enzymatic activities lead to cell death or tumor formation, thus defining their functional roles and impact on human disease. Unlike mono- ADP-ribosyltransferases, poly-ADP-ribose polymerases (PARPs) have been at the frontline of drug discovery since the 1980s. PARP1 is a valuable therapeutic target, with a central role in responses to DNA damage. With mono-ADP-ribosylation now linked to human diseases, such as inflammation, diabetes, neurodegeneration and cancer metastasis, novel and equally important functions of mono-ADPribosylation in cell signaling pathways can now be defined. Recently, we reported mono-ADP-ribosylation of ADP-ribosylation factor 6 (ARF6), a small G-protein of the Ras superfamily. In addition to its involvement in actin remodeling, plasma membrane reorganization and vesicular transport, ARF6 contributes to cancer progression through activation of cell motility and invasion. Consequently, targeting this modification will counteract the pro-invasive effects of ARF6, providing innovative anti-tumor therapy. This review summarizes our present knowledge of the enzymes and targets involved in ADP-ribosylation reactions, and describes in silico approaches to visualize their site of interaction and to identify the precise site for ADP-ribosylation. This should ultimately improve pharmacological strategies to enhance both anti-tumor efficacy and treatment of a number of inflammatory and neurodegenerative disorders.