Figure 1

Actions of Nec-1 and its derivatives in a mouse system. (a) Chemical structures of Nec-1 derivatives targeting RIPK1: active compound Nec-1, inactive variant Nec-1i and stable variant Nec-1s/7-Cl-O-Nec-1. (b) Effect of Nec-1 derivatives on two pathways involving cell death and inflammation. The TNFR1 signaling pathway mediated by RIPK1 versus the IDO–kynurenine pathway are shown in parallel. TNFR1 signaling can lead to cell survival, apoptosis or necroptosis mediated by formation of three distinct signaling complexes: the membrane-bound Complex I (CI), the cytosolic Complex II or the necrosome, respectively. RIPK1 ubiquitination by cIAPs or LUBAC within the CI functions as a platform to initiate activation of NF-kB and MAPK, resulting in cell survival and inflammation through gene induction. Deubiquitination of RIPK1 by CYLD leads to formation of either CIIa or CIIb (the latter in cIAP-depleted conditions, also called ripoptosome), both leading to apoptosis mediated by caspase-8. Only apoptosis mediated by CIIb is dependent on RIPK1 activity and is thus inhibited by Nec-1. RIPK1 and RIPK3 pro-necrotic activity is restrained by caspase-8 activity in the pro-apoptotic CIIa and CIIb, but inhibition or loss of caspase-8 results in necrosome formation and subsequent necroptosis. This process is dependent on RIPK1 kinase activity and is inhibited by Nec-1. Excess apoptosis or necroptosis may induce inflammation, though in general apoptosis is anti-inflammatory (represented by arrows of different size) through release of specific DAMPs. Thus, all three TNF-induced pathways may result in pathologies associated with cell death and inflammation (X representing harmful effects). Both Nec-1 and Nec-1i also inhibit IDO. The IDO–kynurenine pathway is activated under inflammatory conditions and exerts immunomodulatory functions. Consequently, inhibition of this pathway may also result in pathologies associated with cell death and inflammation. At higher concentrations both Nec-1 and Nec-1i inhibit RIPK1 with equal potency but at low concentration both are toxic (this concentration effect is represented by gradient in a triangle and X represents harmful effects). Nec-1s, which inhibits RIPK1 and subsequent necroptosis, lacks the IDO inhibitory activity and concentration-dependent toxicity in vivo. DAMPs, damage-associated molecular patterns, a, apoptotic, n, necrotic