Furin-mediated protein processing in infectious diseases and cancer

Braun, Elisabeth; Sauter, Daniel

(a) In a cytokine (e.g. IL-1b)-induced inflammatory state, furin and protease-activated receptor 1 (PAR1) expression are induced. Furin and PAR1 interact with each other and are trapped as inactive proteins in the trans-Golgi network (TGN). As a consequence, PAR1 cannot traffic to the cell surface, where it is usually cleaved by thrombin to induce inflammatory signalling pathways. Moreover, production of infectious HIV-1 particles is impaired because of reduced furin-mediated cleavage of HIV Env. (b) At the same time, cells of the infected host may induce the expression of interferon-stimulated genes such as guanylate-binding proteins 2 and 5 (GBP2 and GBP5). Both proteins colocalise with furin and inhibit its proteolytic activity. As a result, HIV Env maturation is impaired and newly forming viral particles are poorly infectious since they incorporate immature Env glycoproteins. IFN-I/II, type I and type II interferons; STAT, signal transducers and activators of transcription. furin cleaves the toxin precursor at the cell surface. In contrast, penetration of the inhibitor into the cell is essential to prevent the maturation of HIV-1 Env and other viral glycoproteins that are cleaved intracellularly. While some inhibitors (e.g. HA-derived peptides) efficiently enter cells, others were modified to increase their intracellular availability. For example, addition of a decanoyl moiety to CMK inhibitors increases their ability to penetrate cells. 113 Streptamine derivatives may be particularly promising for targeted therapy as the positioning of the guanidyl substituents determines the localisation of the inhibitor to distinct subcellular compartments such as endosomes or the Golgi complex. 29 While many of the inhibitors described above potently reduce furin activity both in vitro and in vivo, most of them also inhibit other proprotein convertases recognising the same or similar polybasic cleavage sites. This limitation is inherent to competitive inhibitors that aim at mimicking the target sequence of furin and may be overcome by allosteric inhibitors that bind furin-specific motifs outside the active site. One example is the nanobody Nb14, which binds to the C-terminal Pdomain of furin, thereby blocking the access of larger substrates to the active site. Notably, Nb14 specifically binds to the P-domain of furin and does not recognise other PCSKs. 114 Instead of targeting furin at the protein level, therapeutic approaches may also aim at targeting its RNA. For example, the endogenous degradation of furin mRNA by Regnase-1 (ZC3H12A) and/or Roquin (RC3H1) 115 could be modulated to interfere with the expression and thus proteolytically active amount of furin. However, modulation of Regnase-1 and Roquin will most likely have off-target effects as both endoribonucleases also degrade additional mRNAs. A more selective RNA-based approach is the silencing of furin via shRNA. In fact, shRNAmediated suppression of furin expression is currently the clinically most advanced therapy targeting this protease. In a phase III clinical trial, patients suffering from metastatic Ewing's sarcoma family of tumors (ESFT) are treated with an immunotherapy that involves the silencing of furin and simultaneous overexpression of GM-CSF. 116 More specifically, tumor cells are extracorporeally transfected and reintroduced as so-called furin knock-down and GM-CSF augmented (FANG) cancer vaccine, also known as Vigil. While GM-CSF boosts the antitumor response by dendriti