Inhibition of Protein Degradation Induces Apoptosis through a Microtubule-Associated Protein 1 Light Chain 3-Mediated Activation of Caspase-8 at Intracellular Membranes

Abstract

The accumulation of damaged or misfolded proteins, if unresolved, can lead to a detrimental consequence within cells termed proteotoxicity. Since cancerous cells often display elevated protein synthesis and byproduct disposal, inhibition of the protein degradation pathways is an emerging approach for cancer therapy. However, the molecular mechanism underlying proteotoxicity remains largely unclear. We show here that inhibition of proteasomal degradation results in an increased oligomerization and activation of caspase-8 on the cytosolic side of intracellular membranes. This enhanced caspase-8 oligomerization and activation are promoted through its interaction with the ubiquitin-binding protein SQSTM1/p62 and the microtubuleassociated protein light chain 3 (LC3), which are enriched at intracellular membranes in response to proteotoxic stress. Silencing LC3 by shRNA, or the LC3 mutants defective in membrane localization or p62 interaction fail to induce caspase-8 activation and apoptosis. Our results unveiled a previously unknown mechanism through which disruption of protein homeostasis induces caspase-8 oligomerization, activation, and apoptosis.

Rint1 inactivation triggers genomic instability, ER stress and autophagy inhibition in the brain

“…60 It is often considered that autophagy is associated with survival but in certain situation when it is inhibited at late stages, it could lead to neurodegeneration. 3,63,64 This is consistent with the phenotype in Rint1 Nes-Cre and Rint1 Emx1-Cre cortices, where we observed accumulation of p62, LAMP2 and p-RPS6 as an evidence for autophagy inhibition in late stage. These cells were also positive for Caspase 8 that is known to bind p62 on autophagosomes after inhibition of proteasome.…”

The Quinone Methide Aurin Is a Heat Shock Response Inducer That Causes Proteotoxic Stress and Noxa-dependent Apoptosis in Malignant Melanoma Cells

“…Pharmacological induction of proteotoxic stress is rapidly emerging as a promising strategy for cancer cell-directed chemotherapeutic intervention (3,(7)(8)(9)13). Cumulative experimental evidence suggests that therapeutic impairment of proteostasis may be harnessed for the apoptotic elimination of cancer cells that display constitutively elevated levels of proteotoxic stress originating from endogenous factors such as aneuploidy, mutation-driven expression of misfolded proteins, and adverse conditions associated with the tumor microenvironment (3)(4)(5)(6)(7)(8)(9)(10)(11).…”

“…siRNA Transfection Targeting PMAIP1 Expression-A375 cells were transiently transfected with a 100-nmol pool of four small interfering RNA (siRNA) oligonucleotides (oligos) targeting PMAIP1 or a 100-nmol pool of four non-targeting siRNA oligos using the DharmaFECT 1 transfection reagent (Dharmacon RNA Technologies, Lafayette, CO) following a standard procedure as published recently (13,44). The sequences of siGENOME PMAIP1 SMARTpool (PMAIP1 siRNA; GenBank TM NM021127) were AAACUGAACUUCCGGCAGA, AUUCUGUAUCCAAACUCU, CUGGAAGUCGAGUGUG-CUA, and GCAAGAACGCUCAACCGAG.…”

“…In contrast, it is thought that normal cells display lower constitutive levels of endogenous proteotoxic stress together with a diminished dependence on proteostasis-ensuring mechanisms including proteasomal and heat shock protein chaperone functions. Therefore, constitutively elevated levels of proteotoxic stress may represent a specific molecular vulnerability of malignant cells amenable to chemotherapeutic intervention (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13).…”

Synergistic Myeloma Cell Death via Novel Intracellular Activation of Caspase-10–Dependent Apoptosis by Carfilzomib and Selinexor

“…During proteasome inhibition, and by extension autophagy, ubiquitinated caspase-8 forms an apoptosis-inducing complex on intracellular membranes that depends on LC3 and p62 (25,26). p62 has a newly-appreciated role in modulating cell fate by interacting with key signaling proteins marked by ubiquitination and can promote apoptosis or survival depending on the stimulus (27).…”