Fig. 1: Diagram of apoptosis regulatory mechanisms.

Activation of the extrinsic apoptosis pathway depends on TNFR1-mediated transmission of stimulatory signals such as TNF. Upon binding of TNF to its receptor, RIPK1, TRADD, cIAP1, cIAP2, TRAF2, and TRAF5 are recruited to TNFR1 to form Complex I. cIAP1 and cIAP2 mediate Lys63-linked ubiquitination of RIPK1, which anchors TAK1 and its partners TAB2 and TAB3. The signal is then relayed to IKK (IκBα kinase), which degrades IκBα, the inhibitor of canonical NF-κB in the cytoplasm. Once released from IκBα-mediated inhibition, NF-κB translocates to the nucleus and drives the transcription of pro-survival genes and feedback antagonists. Among the pro-survival NF-κB target genes, A20 and FLIP promote the assembly of a DISC by facilitating interactions among RIPK1 and cytosolic RIPK3, TRADD, FADD, caspase-8, and FLIP. FLIPL can heterodimerize with caspase-8 and promote the cleavage and degradation of CYLD, RIPK1, and RIPK3. However, the DISC can also promote caspase-8 homodimerization and catalytic activation, thereby activating caspase-3 and caspase-7 and triggering apoptosis. The intrinsic apoptotic pathway is primarily triggered by DNA damage and ROS, which leads to the activation of BAX–BAK and results in mitochondrial MOMP, releasing cytochrome c to form apoptotic bodies and induce apoptosis. On the other hand, cytoplasmic Ca²⁺ overload can transport Ca²⁺ into the mitochondria through the VDAC protein in the outer mitochondrial membrane and the MCU protein in the inner mitochondrial membrane. When the Ca²⁺ concentration inside the mitochondria becomes excessive, it can lead to the opening of the mPTP, causing the release of pro-apoptotic factors such as cytochrome c and AIF, thereby triggering apoptosis. IAPs suppress the occurrence of apoptosis, but their inactivation induced by SMAC, HTRA2, and ARTS initiates apoptosis. TNFR1 tumor pyroptosis factor receptor 1, RIPK1 receptor-interacting protein kinase 1, TRADD NF receptor type 1-associated death domain protein, cIAP1 cellular inhibitor of apoptosis protein 1, cIAP2 cellular inhibitor of apoptosis protein 2, TRAF2 TNF receptor‑associated factor 2, TRAF5 TNF receptor‑associated factor 5, RIPK3 receptor‑interacting serine/threonine‑protein kinase 3, TAK1 transforming growth factor-β-activated kinase 1, TAB2 TAK1-associated binding protein 2, TAB3 TAK1-associated binding protein 3, IκBα inhibitor of κ (kappa) B α (alpha), P50 nuclear factor kappa-light-chain-enhancer of activated B cells 1, P65 RELA proto-oncogene, NF-KB Subunit, A20 TNF alpha induced protein 3, NFκB nuclear factor kappa-light-chain-enhancer of activated B Cells, FADD Fas-associated protein with death domain, FLIP Fas-associated death domain-containing protein-associated inhibitor, DISC death-inducing signaling complex, MOMP mitochondrial outer‑membrane permeabilization, Cytc cytochrome c, VDAC voltage‑dependent anion channel, MCU mitochondrial calcium uniporter, mPTP mitochondrial permeability transition pore, AIF apoptosis‑inducing factor, IAPs inhibitor of apoptosis proteins, SMAC second mitochondria‑derived activator of caspase, HTRA2 high temperature requirement protein A2, ARTS apoptosis‑related protein in TGF‑β signaling pathway.