Loss of SEL1L or Hrd1 increases intrinsic LOX levels in HEK293 cells independently of proteasomal degradation.

We previously established HEK293 cells deficient in SEL1L, a key component of endoplasmic reticulum-associated protein degradation (ERAD), and attempted to identify factors affected by ERAD dysfunction through comprehensive MS analysis. Among the factors increased by SEL1L-deficiency, lysyl oxidase (LOX) which plays a role in extracellular matrix cross-linking was involved. To elucidate the regulatory mechanisms of LOX mRNA and protein, we applied several ERAD-deficient cells and drug treatments. In SEL1L-deficient cells, LOX mRNA expression levels were observed to increase approximately twofold, and the increase in high molecular weight preproLOX protein was more remarkable in each SEL1L deletion. SEL1L-deficiency also slightly increased the amount of secreted LOX protein cleaved by proteases. Increased proproLOX was also observed in cells lacking Hrd1 that associates with SEL1L. Interestingly, LOX protein was hardly increased in cells deficient for EDEM2 and TXNDC11, which are involved in mannose trimming of N-glycosylated proteins. Since ERAD failure may induce sustained ER stress, we examined the effects of three ER stress inducers on LOX expression. Changes in LOX mRNA following 6-h treatment with each reagent was negligible compared to the major ER stress-inducible mRNA. In contrast, long-term treatment with thapsigargin and tunicamycin increased intracellular LOX protein, but significantly decreased secreted LOX protein. Finally, we examined the effects of proteasome and lysosome inhibitors on LOX expression and, unexpectedly, each reagent hardly increased LOX mRNA or protein levels. These results suggest that expression of intrinsic LOX protein is regulated by SEL1L and Hrd1 in a ubiquitin-proteasome-independent manner.