The cellular oxygen tension regulates expression of the endoplasmic oxidoreductase ERO1‐Lα

Abstract

The formation of disulfide bonds in the endoplasmic reticulum requires protein disulfide isomerase (PDI) and endoplasmic reticulum oxidoreductin 1 (ERO1) that reoxidizes PDI. We report here that the expression of the rat, mouse and human homologues of ERO1-Like protein a but not of the isoform ERO1-Lb are stimulated by hypoxia in rats vivo and in rat, mouse and human cell cultures. The temporal pattern of hypoxic ERO1-La induction is very similar to that of genes triggered by the hypoxia inducible transcription factor (HIF-1) and is characteristically mimicked by cobalt and by deferoxamine, but is absent in cells with a defective aryl hydrocarbon receptor translocator (ARNT, HIF-1b). We speculate from these findings that the expression of ERO1-La is probably regulated via the HIF-pathway and thus belongs to the family of classic oxygen regulated genes. Activation of the unfolded protein response (UPR) by tunicamycin, on the other hand, strongly induced ERO1-Lb and more moderately ERO1-La expression. The expression of the two ERO1-L isoforms therefore appears to be differently regulated, in the way that ERO1-La expression is mainly controlled by the cellular oxygen tension, whilst ERO1-Lb is triggered mainly by UPR. The physiological meaning of the oxygen regulation of ERO1-La expression likely is to maintain the transfer rate of oxidizing equivalents to PDI in situations of an altered cellular redox state induced by changes of the cellular oxygen tension.Keywords: hypoxia; HIF; protein folding; UPR; PDI.Formation of disulfide bonds is an essential event for the correct folding of proteins in the endoplasmic reticulum. It is well known that this process is catalyzed by protein disulfide-isomerase (PDI) [1]. Until a few a years ago, however, it remained unclear how PDI is reoxidized in this reaction [2]. It was the discovery of the ERO1 (endoplasmic reticulum oxidoreductin) protein in yeast [3,4] which provided evidence that this protein is essential to transfer oxidizing equivalents to PDI [5]. It turned out that ERO1 is a highly conserved endoplasmic protein and for humans and mouse two ERO1-Like proteins have meanwhile been identified, termed ERO1-La [6] and -1b [7]. The ERO1 proteins are probably flavoproteins [8] that covalently bind to PDI [9], what explains their function to transfer oxidizing equivalents to PDI. ERO1-La and -Lb display different tissue distributions [7], and moreover, appear to be differently regulated in their expression. Thus, mainly ERO1-L b transcripts are induced in the course of unfolded protein response [7]. In this pathway accumulation of misfolded proteins in the endoplasmic reticulum induces the expression of a number of proteins including those involved in the correct folding of proteins such as chaperones [10]. How the expression of the ERO1-La protein is regulated is yet unknown. Analyzing the protein expression pattern of a rat vascular smooth muscle cell line, we now found that a ERO1-Like protein highly homologous to mouse and human ERO1-La is strongly upregulated during ce...

Ero1α requires oxidizing and normoxic conditions to localize to the mitochondria-associated membrane (MAM)

“…Second, hypoxia leads to the induction of expression of Ero1α (Gess et al 2003;May et al 2005), which, according to our results, is subsequently secreted. One possible explanation for these observations is that hypoxic cells need to compensate for constant loss of Ero1α during hypoxia.…”

“…The oxidative power of Ero1 proteins originate from their flavin adenine dinucleotide (FAD) prosthetic group that transfers electrons from cysteines to molecular oxygen (Wang et al 2009). Hypoxia, the absence of oxygen, impacts on the functioning of Ero1 proteins in two ways: First, Ero1α, but not Ero1β is a target of hypoxia-inducible factor (HIF-1α)-mediated transcription (Gess et al 2003;May et al 2005). Second, whereas this induction may result in the increased production of growth factors by tumor cells, in vitro experiments suggest that Ero1 proteins cannot promote PDI re-oxidation in the absence of oxygen (May et al 2005;Tu and Weissman 2002).…”

Regulation of the protein disulfide proteome by mitochondria in mammalian cells

“…Key effectors of disulfide formation include the endoplasmic reticulum (ER) resident thiol oxidase ERo1 (4,5), an essential gene in yeast, and the disulfide regulatory system consisting of DsbB protein and the electron transport chain in Escherichia coli (6). Homologs of Ero1 have been identified in mammalian cells, and their overexpression promotes intracellular disulfide formation (7)(8)(9)(10). However, these proteins are not essential for mammalian cell survival, nor has it been demonstrated that they are the primary determinants of cellular disulfide formation.…”

Identification of molecular apocrine breast tumours by microarray analysis

“…The poor survival of the patients with the molecular apocrine profile in the Sorlie and van't Veer studies (Figure 5b) is consistent with the poor prognosis expected for ER-negative, ERBB2-positive tumours. The gene cluster shared by the basal and molecular apocrine tumours (Figure 1c panel 3) includes several genes induced by hypoxia, including ERO1L, ADM and NDRG1 (Cormier-Regard et al, 1998;Park et al, 2000;Gess et al, 2003), which would again be consistent with a poor prognosis. Taken together, these data suggest that the molecular apocrine profile does not identify only patients with classical apocrine carcinomas.…”