A rapid and simple assay method for UDP-glucose:ceramide glucosyltransferase

Selective action of the iminosugar isofagomine, a pharmacological chaperone for mutant forms of acid-beta-glucosidase

Gaucher disease is a lysosomal glycolipid storage disorder characterized by defects in acid-beta-glucosidase (GlcCerase), the enzyme responsible for the catabolism of glucosylceramide. We recently demonstrated that isofagomine (IFG), an iminosugar that binds to the active site of GlcCerase, enhances the folding, transport and activity of the N370S mutant form of GlcCerase. In this study we compared the effects of IFG on a number of other glucosidases and glucosyltransferases. We report that IFG has little or no inhibitory activity towards intestinal disaccharidase enzymes, ER alpha-glucosidase II or glucosylceramide synthase at concentrations previously shown to enhance N370S GlcCerase folding and trafficking in Gaucher fibroblasts. Furthermore, treatment of wild type fibroblasts with high doses of IFG did not alter the processing of newly synthesized N-linked oligosaccharides. These findings support further evaluation of IFG as a potential therapeutic agent in the treatment of some forms of Gaucher disease.

Overcoming resistance to gamma-rays in squamous carcinoma cells by poly-drug elevation of ceramide levels

Recent strategies to sensitize radioresistant tumours are based on combining gamma-irradiation with inducers of apoptosis. We report that the combination of three inhibitors of sphingolipid metabolism, DL-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol.HCl(DL-PDMP)+imipramine +/- D-erythro-2-(N-myristoylamino)-1-phenyl-1-propanol (D-MAPP), with 10-Gy irradiation triggers both mitotic and apoptotic killing in radioresistant SQ20B squamous carcinoma cells. In these cells, apoptosis is defective due to a lack of ceramide generation upstream, which cannot be explained by sphingomyelinase (neutral and acidic) deficiency or rapid derivation to the sphingolipid pathway. We present evidence of a functional transduction death pathway when ceramide generation is restored, which involves the mitochondrial-mediated pathway coupled to alterations in redox status and to executive caspases activation. The poly-drug treatment restored apoptosis to levels similar to those observed in radiosensitive SCC61 squamous carcinoma cells. Simultaneous exposure to gamma-irradiation and poly-drug treatment acted synergistically in SQ20B cells to produce a marked increase in both mitochondrial dysfunction and caspase cleavage, which led to a 7.8-fold increase in apoptosis within 48 h, relative to irradiated cells. Moreover, the results suggest that the ceramide released by irradiation or poly-drug treatment converges upon common cellular targets. Modulation of endogenous ceramide levels by inhibitors of sphingolipid metabolism may represent a new cellular target for the sensitization of radioresistant tumours to gamma-ray therapy.

The mitochondria-associated endoplasmic-reticulum subcompartment (MAM fraction) of rat liver contains highly active sphingolipid-specific glycosyltransferases

Although most glycosphingolipids (GSLs) are thought to be located in the outer leaflet of the plasma membrane, recent evidence indicates that GSLs and their precursor, ceramide, are also associated with intracellular organelles and, particularly, mitochondria. GSL biosynthesis starts with the formation of ceramide in the endoplasmic reticulum (ER), which is transported by controversial mechanisms to the Golgi apparatus, where stepwise addition of monosaccharides on to ceramides takes place. We now report the presence of GSL-biosynthetic enzymes in a subcompartment of the ER previously characterized and termed 'mitochondria-associated membrane' (MAM). MAM is a membrane bridge between the ER and mitochondria that is involved in the biosynthesis and trafficking of phospholipids between the two organelles. Using exogenous acceptors coated on silica gel, we demonstrate the presence of ceramide glucosyltransferase (Cer-Glc-T), glucosylceramide galactosyltransferase and sialyltransferase (SAT) activities in the MAM. Estimation of the marker-enzyme activities showed that glycosyltransferase activities could not be ascribed to cross-contamination of MAM by Golgi membranes. Cer-Glc-T was found to have a marked preference for ceramide bearing phytosphingosine as sphingoid base. SAT activities in MAM led to the synthesis of G(M3) ganglioside and small amounts of G(D3). G(M1) was also synthesized along with G(M3) upon incubation of the fraction with exogenous unlabelled G(M3), underlying the presence of other sphingolipid-specific glycosyltransferases in MAM. On the basis of our results, we propose MAM as a privileged compartment in providing GSLs for mitochondria.

Temporal and quantitative expression of the myelin-associated lipids, ethanolamine plasmalogen, galactocerebroside, and sulfatide, in the differentiating CG-4 glial cell line

We determined the expression of three myelin-typical lipids in the continuous CG-4 glial cell line of oligodendrocyte progenitor cells, as the cells differentiated into oligodendrocytes. On 6 different days during the first 9 days of oligodendrocyte development, cells were labeled for 24 h with [3H]ethanolamine to label ethanolamine plasmalogens or with [3H]galactose to label the galactocerebroside and sulfogalactocerebroside; and the amount of labeled lipid expressed on each day was determined. Each labeled lipid was expressed with its own specific time course and in a defined amount on each day of differentiation. Increased labeling of plasmalogens and sulfogalactocerebroside started at early developmental stages, and increased labeling of galactocerebroside started at later stages. The results indicate that the differentiating CG-4 cell line provides a valuable system to investigate factors affecting the early time course of myelin-lipid expression and the amounts expressed.

Induction of ceramide glucosyltransferase activity in cultured human keratinocytes. Correlation with culture differentiation

Ceramides are the major component of the extracellular lipids that comprise the epidermal permeability barrier. They are derived from glucosylceramides (GlcCer) upon their extrusion from lamellar granules into the extracellular space in the upper layers of the epidermis. To better understand the regulation of the unique pathway for ceramide production in epidermis, we have studied the activity of the enzyme responsible for GlcCer synthesis, ceramide glucosyltransferase (CerGlc transferase), during keratinocyte culture differentiation. Human keratinocyte cultures were expanded in low calcium keratinocyte growth medium (KGM) and then switched to either normal calcium KGM (nKGM) or "complete" Dulbecco's modified Eagle's medium/Ham's F-12 (3:1) supplemented with 10% fetal bovine serum (cDMEM). At 7 and 10 days after the medium switch, electron microscopy revealed that cDMEM cultures were more fully differentiated morphologically and contained numerous lamellar granules. The GlcCer/DNA content of cDMEM cultures increased to 6 times that of day 0 cultures and was nearly 4 times greater than that of nKGM cultures, whereas the total lipid/DNA content of cDMEM cultures increased to only 1.8 times that of day 0 cultures and was approximately 1.2 times that of nKGM cultures. CerGlc transferase activity/DNA increased 6 times in cDMEM cultures but <1.5 times in nKGM cultures. By contrast, beta-glucocerebrosidase activity, which is responsible for the conversion of GlcCer to ceramide, increased to a similar extent in both differentiating culture systems. Treatment of cultures with the reversible CerGlc transferase inhibitor, DL-threo-1-phenyl-2-(palmitoylamino)-3-morpholino-1-propanol, prevented the increase of GlcCer in cDMEM cultures, and blocked conversion of exogenously added ceramide to GlcCer. A low level of CerGlc transferase activity, relative to that in differentiated keratinocytes, was detected in cultures of other human cell types. These results indicate that CerGlc transferase activity is induced during epidermal differentiation and that regulation of this enzyme may be an important determinant of the specialized production and compartmentalization of epidermal sphingolipids.