RIPK1 can function as an inhibitor rather than an initiator of RIPK3-dependent necroptosis

Tumour necrosis factor and lipopolysaccharide can promote a regulated form of necrosis, called necroptosis, upon inhibition of caspase activity in cells expressing receptor-interacting serine/threonine kinase (RIPK) 3. Because inhibitors of RIPK1 kinase activity such as necrostatin-1 block necroptosis in many settings, RIPK1 is thought to be required for activation of RIPK3, leading to necroptosis. However, here we show that, although necrostatin potently inhibited tumour necrosis factor-induced, lipopolysaccharide-induced and polyIC-induced necroptosis, RIPK1 knockdown unexpectedly potentiated this process. In contrast, RIPK3 knockdown potently suppressed necroptosis under the same conditions. Significantly, necrostatin failed to block necroptosis in the absence of RIPK1, indicating that its ability to suppress necroptosis was indeed RIPK1-dependent. These data argue that RIPK1 is dispensable for necroptosis and can act as an inhibitor of this process. Our observations also suggest that necrostatin enhances the inhibitory effects of RIPK1 on necroptosis, as opposed to blocking its participation in this process.