Differences between disease‐associated endoplasmic reticulum aminopeptidase 1 ( ERAP 1) isoforms in cellular expression, interactions with tumour necrosis factor receptor 1 ( TNF ‐ R 1) and regulation by cytokines

Summary Endoplasmic reticulum aminopeptidase 1 ( ERAP 1) processes peptides for major histocompatibility complex ( MHC ) class I presentation and promotes cytokine receptor ectodomain shedding. These known functions of ERAP 1 may explain its genetic association with several autoimmune inflammatory diseases. In this study, we identified four novel alternatively spliced variants of ERAP 1 m RNA , designated as Δ E xon‐11, Δ E xon‐13, Δ E xon‐14 and Δ E xon‐15. We also observed a rapid and differential modulation of ERAP 1 m RNA levels and spliced variants in different cell types pretreated with lipopolysaccharide ( LPS ). We have studied three full‐length allelic forms of ERAP 1 ( R 127‐ K 528, P 127‐ K 528, P 127‐ R 528) and one spliced variant (Δ E xon‐11) and assessed their interactions with tumour necrosis factor receptor 1 ( TNF ‐ R 1) in transfected cells. We observed variation in cellular expression of different ERAP 1 isoforms, with R 127‐ K 528 being expressed at a much lower level. Furthermore, the cellular expression of full‐length P 127‐ K 528 and Δ E xon‐11 spliced variant was enhanced significantly when co‐transfected with TNF ‐ R 1. Isoforms P 127‐ K 528, P 127‐ R 528 and Δ E xon‐11 spliced variant associated with TNF ‐ R 1, and this interaction occurred in a region within the first 10 exons of ERAP 1. Supernatant‐derived vesicles from transfected cells contained the full‐length and ectodomain form of soluble TNF ‐ R 1, as well as carrying the full‐length ERAP 1 isoforms. We observed marginal differences between TNF ‐ R 1 ectodomain levels when co‐expressed with individual ERAP 1 isoforms, and treatment of transfected cells with tumour necrosis factor ( TNF ), interleukin ( IL )‐1β and IL ‐10 exerted variable effects on TNF ‐ R 1 ectodomain cleavage. Our data suggest that ERAP 1 isoforms may exhibit differential biological properties and inflammatory mediators could play critical roles in modulating ERAP 1 expression, leading to altered functional activities of this enzyme.