Secretory and membrane proteins are vital for cell activities, including intra- and intercellular communication. Therefore, protein quality control in the endoplasmic reticulum (ER) is an essential and crucial process for eukaryotic cells. Endoplasmic reticulum-associated degradation (ERAD) targets misfolded proteins during the protein maturation process in the ER and leads to their disposal. This process maintains the ER productive function and prevents misfolded protein stress (i.e., ER stress). The ERAD-stimulating factor ER degradation-enhancing α mannosidase-like 1 protein (EDEM1) acts on misfolded proteins to accelerate ERAD, thereby maintaining the productivity of the ER. However, the detail mechanism underlying the function of EDEM1 in ERAD is not completely understood due to lack of established physiological substrate proteins. In this study, we attempted to identify substrate proteins for EDEM1 using siRNA. The matrix component thrombospondin-1 (TSP1) and epidermal growth factor receptor (EGFR) were identified as candidate targets of EDEM1. Their protein maturation status and cellular localization were markedly affected by knockdown of EDEM1. We also showed that EDEM1 physically associates with EGFR and enhances EGFR degradation via ERAD. Our data highlight the physiological role of EDEM1 in maintaining specific target proteins and provide a potential approach to the regulation of expression of clinically important proteins.

1) Background: Keloids occur after the failure during the wound-healing process, persist the inflammation and are refractory to various treatments. The pathogenesis of keloids is still unclear. We previously analyzed the gene expression profiles in keloid tissue using microarray and Northern blot analysis and found that HtrA1 was markedly upregulated in the keloid lesions. HtrA1 is a member of the HtrA family of serine protease, has been suggested to play a role in the pathogenesis of various diseases including age-related macular degeneration and osteoarthritis by modulating proteins in extracellular matrix or cell surface. We focused on HtrA1, analyzed the localization and the role in keloid pathogenesis. 2) Methods: Twenty seven keloid patients and seven unrelated patients were enrolled in this study. We performed in situ hybridization analysis, immunohistochemical analysis, western blot analysis and cell proliferation assay. 3) Results: First, the fibroblast-like cells expressed HtrA1 higher in the active keloid lesions than in the surrounding lesions in situ hybridization. Second, the proportion of HtrA1-positive cells in keloid was higher than that of in normal skin significantly in immunohistochemical analysis. Third, HtrA1 protein was up-regulated, relative to normal skin tissue samples in western blot analysis. Finally, silencing of HtrA1 gene expression suppressed the cell proliferation significantly. 4) Conclusion: HtrA1 was highly expressed in keloid tissues and the suppression of HtrA1 gene inhibited the proliferation of keloid-derived fibroblasts. HtrA1 may promote keloid development through accelerating cell proliferation and remodeling keloid-specific extracellular matrix or cell surface molecules. HtrA1 is suggested to have an important role in keloid pathogenesis.