RESUMEN
The stress sensors ATF6, IRE1, and PERK monitor deviations from homeostatic conditions in the endoplasmic reticulum (ER), a protein biogenesis compartment of eukaryotic cells. Their activation elicits unfolded protein responses (UPR) to re-establish proteostasis. UPR have been extensively investigated in cells exposed to chemicals that activate ER stress sensors by perturbing calcium, N-glycans, or redox homeostasis. Cell responses to variations in luminal load with unfolded proteins are, in contrast, poorly characterized. Here, we compared gene and protein expression profiles in HEK293 cells challenged with ER stress-inducing drugs or expressing model polypeptides. Drug titration to limit up-regulation of the endogenous ER stress reporters heat shock protein family A (Hsp70) member 5 (BiP/HSPA5) and homocysteine-inducible ER protein with ubiquitin-like domain 1 (HERP/HERPUD1) to levels comparable with luminal accumulation of unfolded proteins substantially reduced the amplitude of both transcriptional and translational responses. However, these drug-induced changes remained pleiotropic and failed to recapitulate responses to ER load with unfolded proteins. These required unfolded protein association with BiP and induced a much smaller subset of genes participating in a chaperone complex that binds unfolded peptide chains. In conclusion, UPR resulting from ER load with unfolded proteins proceed via a well-defined and fine-tuned pathway, whereas even mild chemical stresses caused by compounds often used to stimulate UPR induce cellular responses largely unrelated to the UPR or ER-mediated protein secretion.
Asunto(s)
Estrés del Retículo Endoplásmico , Regulación de la Expresión Génica , Respuesta de Proteína Desplegada , Factor de Transcripción Activador 6/genética , Factor de Transcripción Activador 6/metabolismo , Chaperón BiP del Retículo Endoplásmico , Endorribonucleasas/genética , Endorribonucleasas/metabolismo , Células HEK293 , Proteínas de Choque Térmico/genética , Proteínas de Choque Térmico/metabolismo , Humanos , Proteínas Serina-Treonina Quinasas/genética , Proteínas Serina-Treonina Quinasas/metabolismo , eIF-2 Quinasa/genética , eIF-2 Quinasa/metabolismoRESUMEN
Amplification of the candidate oncogene TLOC1/SEC62 in tumors correlates with reduced patient survival. The recently reported role of SEC62 as an autophagy receptor that controls endoplasmic reticulum (ER) size and function might open new scenarios for understanding the phenotypes and treat SEC62high tumors, which are characterized by high ER stress tolerance.
RESUMEN
The endoplasmic reticulum (ER) is a site of protein biogenesis in eukaryotic cells. Perturbing ER homeostasis activates stress programs collectively called the unfolded protein response (UPR). The UPR enhances production of ER-resident chaperones and enzymes to reduce the burden of misfolded proteins. On resolution of ER stress, ill-defined, selective autophagic programs remove excess ER components. Here we identify Sec62, a constituent of the translocon complex regulating protein import in the mammalian ER, as an ER-resident autophagy receptor. Sec62 intervenes during recovery from ER stress to selectively deliver ER components to the autolysosomal system for clearance in a series of events that we name recovER-phagy. Sec62 contains a conserved LC3-interacting region in the C-terminal cytosolic domain that is required for its function in recovER-phagy, but is dispensable for its function in the protein translocation machinery. Our results identify Sec62 as a critical molecular component in maintenance and recovery of ER homeostasis.
