Peroxisomal Localization of a Truncated HMG-CoA Reductase under Low Cholesterol Conditions

Abstract

3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase, HMGCR) is one of the rate-limiting enzymes in the mevalonate pathway required for cholesterol biosynthesis. It is an integral membrane protein of the endoplasmic reticulum (ER) but has occasionally been described in peroxisomes. By co-immunofluorescence microscopy using different HMGCR antibodies, we present evidence for a dual localization of HMGCR in the ER and peroxisomes in differentiated human monocytic THP-1 cells, primary human monocyte-derived macrophages and human primary skin fibroblasts under conditions of low cholesterol and statin treatment. Using density gradient centrifugation and Western blot analysis, we observed a truncated HMGCR variant of 76 kDa in the peroxisomal fractions, while a full-length HMGCR of 96 kDa was contained in fractions of the ER. In contrast to primary human control fibroblasts, peroxisomal HMGCR was not found in fibroblasts from patients suffering from type-1 rhizomelic chondrodysplasia punctata, who lack functional PEX7 and, thus, cannot import peroxisomal matrix proteins harboring a type-2 peroxisomal targeting signal (PTS2). Moreover, in the N–terminal region of the soluble 76 kDa C-terminal catalytic domain, we identified a PTS2-like motif, which was functional in a reporter context. We propose that under sterol-depleted conditions, part of the soluble HMGCR domain, which is released from the ER by proteolytic processing for further turnover, remains sufficiently long in the cytosol for peroxisomal import via a PTS2/PEX7-dependent mechanism. Altogether, our findings describe a dual localization of HMGCR under combined lipid depletion and statin treatment, adding another puzzle piece to the complex regulation of HMGCR.