Off-target inhibition of NGLY1 by the polycaspase inhibitor Z-VAD-fmk induces cellular autophagy

The polycaspase inhibitor Z-VAD-fmk acts as an inhibitor of peptide: N-glycanase (NGLY1), an endoglycosidase which cleaves N-linked glycans from glycoproteins exported from the endoplasmic reticulum (ER) during ER-associated degradation (ERAD). Both pharmacological N-glycanase inhibition by Z-VAD-fmk and siRNA-mediated knockdown (KD) of NGLY1 induce GFP-LC3-positive puncta in HEK 293 cells. The activation of ER stress markers or induction of reactive oxygen species (ROS) is not observed under either condition. Moreover, Ca2+ handling is unaffected when observing release from intracellular stores. Under conditions of pharmacological NGLY1 inhibition or NGLY1 KD, upregulation of autophagosome formation without impairment of autophagic flux is observed. Enrichment of autophagosomes by immunoprecipitation (IP) and mass spectrometry-based proteomic analysis reveals comparable autophagosomal protein content. Gene ontology analysis of proteins enriched in autophagosome IPs shows overrepresentation of factors involved in protein translation, localization and targeting, RNA degradation and protein complex disassembly. Upregulation of autophagy represents a cellular adaptation to NGLY1 inhibition or KD, and ATG13-deficient mouse embryonic fibroblasts (MEFs) show reduced viability under these conditions. In contrast, treatment with pan-caspase inhibitor, Q-VD-OPh, does not induce cellular autophagy. Therefore, experiments with Z-VAD-fmk are complicated by the effects of NGLY1 inhibition, including induction of autophagy, and Q-VD-OPh represents an alternative caspase inhibitor free from this limitation. Enzymes Peptide:N-glycanase1, Peptide-N(4)-(N-acetyl-beta-glucosaminyl)asparagine amidase [EC:3.5.1.52].

Automated summary

The polycaspase inhibitor Z-VAD-fmk serves as an inhibitor of peptide: N-glycanase (NGLY1), and its inhibition or knockdown induces upregulation of autophagy without affecting autophagic flux, suggesting a cellular adaptation to NGLY1 inhibition. In contrast, treatment with the pan-caspase inhibitor Q-VD-OPh does not induce cellular autophagy.