Necroptosis is a regulated, caspase-INDEPENDENT lytic cell death — backup death program when apoptosis is blocked (e.g., by viral caspase-8 inhibitors or pharmacological caspase inhibition). The canonical trigger is TNFR1 + caspase-8 inhibition: TNF-α binds TNFR1 → complex I assembly (TRADD + TRAF2/5 + RIPK1 ubiquitinated → NF-κB survival signal). Deubiquitination of RIPK1 (CYLD, A20 regulated) → complex II (RIPK1 + FADD + caspase-8 → apoptosis under normal conditions). When caspase-8 is inhibited or absent → complex IIb (necrosome): RIPK1 + RIPK3 RHIM-domain interaction → amyloid-like filament → RIPK3 autophosphorylation → recruits + phosphorylates MLKL (mixed-lineage kinase-like) at T357/S358 → MLKL trimerization + translocation to plasma membrane → pore formation → osmotic lysis + DAMP release (HMGB1, IL-33, ATP). Differs from pyroptosis (gasdermin-mediated, inflammasome-dependent) and apoptosis (caspase-3/7, no membrane lysis until late). Disease relevance: viral infection (vaccinia caspase-8 inhibitor → triggers necroptosis as antiviral defense; influenza); ischemia-reperfusion (cardiac, brain, kidney); inflammatory bowel disease (intestinal epithelium); psoriasis; ALS; chronic pancreatitis. Therapeutics: necrostatin-1 (RIPK1 inhibitor — preclinical neuroprotection); GSK-872 (RIPK3); GW806742X (MLKL) — all investigational. Several FDA-approved drugs have off-target necroptotic effects: ponatinib (RIPK1/3 + BCR-ABL — CML), dabrafenib (RIPK3 + BRAF — melanoma), sorafenib (multi-kinase + RIPK). Cross-links: apoptosis bcl2 axis (parallel programmed death), pyroptosis gasdermin (parallel lytic death), nfkb signaling (TNFR1 upstream).
Organ Systems
immune-hematologic
integumentary
digestive
Pathway Steps
TNF-α + TNFR1 → complex I (TRADD/TRAF/RIPK1-Ub) → NF-κB survival — via baseline pro-survival outcome of TNF signaling; cIAP1/2 K63-Ub on RIPK1. TNF binding TNFR1 first assembles membrane “complex I” (TRADD, TRAF2/5, RIPK1, cIAPs), where polyubiquitinated RIPK1 signals to NF-κB for survival and inflammation. The default TNF outcome is thus survival — death pathways open only if this checkpoint is disabled, making complex I a life/death gatekeeper.
CYLD / A20 deubiquitination → complex II (RIPK1 + FADD + caspase-8) — via → apoptosis (normal) when caspase-8 intact. If RIPK1 is deubiquitinated (by CYLD/A20), it dissociates into cytosolic “complex II” with FADD and caspase-8. This is the apoptotic complex — active caspase-8 drives apoptosis and, crucially, also cleaves and inactivates RIPK1/RIPK3 to keep necroptosis suppressed.
caspase-8 inhibition / loss → complex IIb / necrosome (RIPK1 + RIPK3 via RHIM) — via amyloid-like filament; switch from apoptosis to necroptosis. When caspase-8 is inhibited or absent (some viral infections, or pharmacologic caspase blockade), RIPK1 and RIPK3 instead associate through their RHIM domains into the necrosome. So caspase-8 activity is what normally keeps necroptosis off — its loss reroutes death from apoptosis to necroptosis.
RIPK3 active → MLKL phosphorylation at T357/S358 — via kinase activity of RIPK3 — necrostatin-1 blocks RIPK1; GSK-872 blocks RIPK3. Within the necrosome, RIPK3 phosphorylates the pseudokinase MLKL at T357/S358. MLKL has no enzymatic activity of its own — it is purely an executioner substrate, and this phosphorylation is the committed, defining step of necroptosis (and a key biomarker of the pathway).
pMLKL → trimerization + plasma membrane translocation → pore — via lytic death; DAMP release (HMGB1, IL-33, ATP) → inflammation. Phosphorylated MLKL oligomerizes (trimerizes) and translocates to the plasma membrane, inserting to form pores that rupture the cell. Unlike apoptosis, this lytic death spills DAMPs to drive inflammation — why necroptosis matters in ischemic injury and inflammatory disease, and is a target (RIPK1 inhibitors).