The glycosphingolipid sulfatide in the islets of Langerhans in rat pancreas is processed through recycling: possible involvement in insulin trafficking

Abstract

In previous studies we have shown that sulfatide (galactosylceramide-3-O-sulfate), in various species, is present in the insulin-producing cells in pancreatic islets of Langerhans. In this study the synthesis of sulfatide in the islets has been investigated by pulse chase labeling at varying glucose levels and in the presence or absence of the glycosphingolipid synthesis inhibitory agents, Brefeldin A, fumonisin B1 and chloroquine and the distribution of sulfatide by immune-electronmicroscopy. The data showed that (1) sulfatide was produced in islets of Langerhans, (2) the main pathway for synthesis was through recycling involving partial degradation in the lysosome, and that (3) high glucose levels, although not primarily reflected in an increased synthesis of sulfatide, lead to an increased expression of mRNA for the UDP-galactose:ceramide galactosyltransferase, producing the immediate precursor of sulfatide. Furthermore, mass spectrometry analyses revealed a high proportion of short chain fatty acids, C16:0 (50%) and no hydroxylated forms and thus special physicochemical properties, indicating important differences between pancreatic and brain/neural sulfatide. Immune electron microscopy revealed an intracellular expression of sulfatide in the secretory granules, the Golgi network and the lysosomes of the islets. These results indicate that sulfatide follows the same intracellular route as insulin and suggest a functional association between these molecules. We have raised the hypothesis that sulfatide possibly plays a role in the trafficking of insulin in the islets of Langerhans in rat pancreas.

Involvement of sulfatide in beta cells and type 1 and type 2 diabetes

“…Sulfatide is primarily produced by a recycling pathway in beta cells [4]. Desulphation takes place in lysosomes and is catalysed by arylsulphatase [12]; the GalCer formed is transported to the Golgi apparatus, where new sulphation by CST takes place [4].…”

“…Insulin stimulates its own secretion via positive feedback [16], whereas Cpeptide inhibits insulin secretion [17]. Sulfatide inhibits insulin release from glucose-activated islets in vitro in a similar fashion, activating potassium channels which subsequently close calcium channels, thereby mediating beta cell rest [18] [4] that involves several enzymatic steps: CGT adds galactose to the ceramide moiety, giving rise to GalCer, which is then sulphated by CST to form sulfatide. Sulphation takes place in the trans-Golgi apparatus, and sulfatide follows insulin trafficking into the secretory granules, as shown in the electron micrograph of human islets (beta cells) (lower panel, indicated by the black arrow).…”

“…Sulfatide is either secreted together with insulin or transported to lysosomal structures (lower panel, indicated by the white arrow). In the lysosomes, sulfatide is degraded to GalCer by arylsulphatase A (ASA) and recycled to the cisGolgi apparatus to serve as a sulfatide precursor [4]. Sulfatide assists proinsulin folding in the Golgi compartment and preserves insulin crystals in the secretory granules, as shown by the scanning electron micrographs (middle panel, left-hand insert).…”

“…In the latter study the authors speculate as to the nature of the endogenous ligand that was loaded in the CD1d receptor. This might indeed have been sulfatide, which is present in the islet [4], can act as a ligand for CD1d [36], and inhibits (as with α-GalCer) diabetes development in the NOD mouse model [23].…”

“…the saturated isoform with 16 carbon atoms is denoted as C16:0 sulfatide. In the brain, long-chain forms are most frequent, whereas the pancreas contains considerable amounts of the short-chain variety (C16:0); in the pancreas of some species, 50% of sulfatide is found in this form [4]. Variations in fatty acid chain length influence the physical chemical properties of the sulfatide molecule.…”