Effects of the glucosphingolipid synthesis inhibitor, PDMP, on lysosomes in cultured cells

The Glucosylceramide Synthase Inhibitor PDMP Causes Lysosomal Lipid Accumulation and mTOR Inactivation

For many years, the biology of glycosphingolipids was elucidated with the help of glucosylceramide synthase (GCS) inhibitors such as 1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP). Additionally, PDMP gained interest because of its chemosensitizing effects. Several studies have successfully combined PDMP and anti-cancer drugs in the context of cancer therapy. However, the mechanism of action of PDMP is not fully understood and seems to go beyond glycolipid inhibition. Here, we used a functionalized sphingosine analogue (pacSph) to investigate the acute effects of PDMP on cellular sphingolipid distribution and found that PDMP, but not other GCS inhibitors, such as ND-DNJ (also called Miglustat), induced sphingolipid accumulation in lysosomes. This effect could be connected to defective export from lysosome, as monitored by the prolonged lysosomal staining of sphingolipids as well as by a delay in the metabolic conversion of the pacSph precursor. Additionally, other lipids such as lysobisphosphatidic acid (LBPA) and cholesterol were enriched in lysosomes upon PDMP treatment in a time-dependent manner. We could further correlate early LBPA enrichment with dissociation of the mechanistic target of rapamycin (mTOR) from lysosomes followed by nuclear translocation of its downtream target, transcription factor EB (TFEB). Altogether, we report here a timeline of lysosomal lipid accumulation events and mTOR inactivation arising from PDMP treatment.

An update on sphingosine-1-phosphate receptor 1 modulators

Sphingolipids represent an essential class of lipids found in all eukaryotes, and strongly influence cellular signal transduction. Autoimmune diseases like asthma and multiple sclerosis (MS) are mediated by the sphingosine-1-phosphate receptor 1 (S1P₁) to express a variety of symptoms and disease patterns. Inspired by its natural substrate, an array of artificial sphingolipid derivatives has been developed to target this specific G protein-coupled receptor (GPCR) in an attempt to suppress autoimmune disorders. FTY720, also known as fingolimod, is the first oral disease-modifying therapy for MS on the market. In pursuit of improved stability, bioavailability, and efficiency, structural analogues of this initial prodrug have emerged over time. This review covers a brief introduction to the sphingolipid metabolism, the mechanism of action on S1P₁, and an updated overview of synthetic sphingosine S1P₁ agonists.

Pharmacological Modulation of Glycosphingolipid Metabolism

The experimental approach to deplete cellular glycosphingolipids (GSLs) with the specific inhibitors of glycosphingolipid biosynthesis has the potential to identify functions of endogenous GSLs. Most GSLs are derived from glucosylceramide (GlcCer). D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP) inhibits GIcCer synthase and has been used extensively to study the biological functions of living cells. D-PDMP inhibits mTORC1 activity, which is independent of its inhibitory activity on GlcCer synthase. We also developed an analog of D-PDMP, D-threo-1-phenyl-2-benzyloxycarbonylamino-3-pyrrolidino-1-propanol (D-PBPP) lacking the effect on mTORC1. Here, we summarize the effects of D-PDMP and D-PBPP on the metabolism of GSLs and cell growth.

PDMP, a ceramide analogue, acts as an inhibitor of mTORC1 by inducing its translocation from lysosome to endoplasmic reticulum

Mammalian or mechanistic target of rapamycin complex 1 (mTORC1) is a master regulator of cell growth, metabolism, and cell differentiation. Recent studies have revealed that the recruitment of mTORC1 to lysosomes is essential for its activation. The ceramide analogue 1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP), a well known glycosphingolipid synthesis inhibitor, also affects the structures and functions of various organelles, including lysosomes and endoplasmic reticulum (ER). We investigated whether PDMP regulates the mTORC1 activity through its effects on organellar behavior. PDMP induced the translocation of mTORC1 from late endosomes/lysosomes, leading to the dissociation of mTORC1 from its activator Rheb in MC3T3-E1 cells. Surprisingly, we found mTORC1 translocation to the ER upon PDMP treatment. This effect of PDMP was independent of its action as the inhibitor, since two stereoisomers of PDMP, with and without the inhibitor activity, showed essentially the same effect. We confirmed that PDMP inhibits the mTORC1 activity based on the decrease in the phosphorylation of ribosomal S6 kinase, a downstream target of mTORC1, and the increase in LC3 puncta, reflecting autophagosome formation. Furthermore, PDMP inhibited the mTORC1-dependent osteoblastic cell proliferation and differentiation of MC3T3-E1 cells. Accordingly, the present results reveal a novel mechanism of PDMP, which inhibits the mTORC1 activity by inducing the translocation of mTOR from lysosomes to the ER.

Palmitoyl-ceramide accumulation with necrotic cell death in A549 cells, followed by a steep increase in sphinganine content

Ceramides (Cers) have recently been identified as key signaling molecules that mediate biological functions such as cell growth, differentiation, senescence, apoptosis, and autophagy. However, the functions of Cer accumulation in necrotic cell death remain unknown. The aim of this study was to clarify the relationship between Cer accumulation with inhibition of the conversion pathway of Cer and concomitant necrotic cell death. In order to minimize the effect of apoptosis against necrotic cell death, A549 cells having the inhibiting effect of caspase 9 by survivin were used in this study. Consequently, Cer accumulation in A549 cells would likely be associated with a pathway other than the mitochondrial caspase-dependent pathway of apoptosis. Here, we showed that the dual addition of a glucosyl-Cer synthase inhibitor and a ceramidase inhibitor to A549 cell culture induced palmitoyl-Cer accumulation with Cer synthase 5 expression and necrotic cell death with lysosomal rupture together with leakage of cathepsin B/alkalization after 2–3 h, although it is unknown in this study whether the necrotic cell death was caused by the lysosomal rupture. This Cer accumulation was followed by a steep increase in sphinganine base levels via the activation of serine palmitoyltransferase activity brought about by the increase in palmitoyl-coenzyme A concentration as a substrate after 5–6 h. The increase in palmitoyl-coenzyme A concentration was achieved by activation of the fatty acid synthetic pathway from acetyl coenzyme A.

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Palmitoylceramide accumulation with necrosis is studied.

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The accumulation is with ceramide synthase 5 expression.

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The accumulation is followed by high sphinganine levels.

Glycosphingolipids mediate pneumocystis cell wall β-glucan activation of the IL-23/IL-17 axis in human dendritic cells

Pneumocystis species are opportunistic fungal organisms that cause severe pneumonia in immune-compromised hosts, with resultant high morbidity and mortality. Recent work indicates that IL-17 responses are important components of host defense against fungal pathogens. In the present study, we demonstrate that cell-surface β-glucan components of Pneumocystis (PCBG) stimulate human dendritic cells (DCs) to secrete IL-23 and IL-6. These cytokines are well established to stimulate a T helper-17 (Th17) phenotype. Accordingly, we further observe that PCBG-stimulated human DCs interact with lymphocytes to drive the secretion of IL-17 and IL-22, both Th17-produced cytokines. The activation of DCs was shown to involve the dectin-1 receptor with a downstream activation of the Syk kinase and subsequent translocation of both the canonical and noncanonical components of the NF-κB transcription factor family. Finally, we demonstrate that glycosphingolipid-rich microdomains of the plasma membrane participate in the activation of DCs by PCBG through the accumulation of lactosylceramide at the cell surface during stimulation with PCBG. These data strongly support the idea that the β-glucan surface components of Pneumocystis drive the activation of the IL-23/IL-17 axis during this infection, through a glycosphingolipid-initiated mechanism.

Inhibition of Glycosphingolipid Biosynthesis Reduces Secretion of the β-Amyloid Precursor Protein and Amyloid β-Peptide*[boxs]

Alzheimer disease is associated with extracellular deposits of amyloid beta-peptides in the brain. Amyloid beta-peptides are generated by proteolytic processing of the beta-amyloid precursor protein by beta- and gamma-secretases. The cleavage by secretases occurs predominantly in post-Golgi secretory and endocytic compartments and is influenced by cholesterol, indicating a role of the membrane lipid composition in proteolytic processing of the beta-amyloid precursor protein. To analyze the role of glycosphingolipids in these processes we inhibited glycosyl ceramide synthase, which catalyzes the first step in glycosphingolipid biosynthesis. The depletion of glycosphingolipids markedly reduced the secretion of endogenous beta-amyloid precursor protein in different cell types, including human neuroblastoma SH-SY5Y cells. Importantly, secretion of amyloid beta-peptides was also strongly decreased by inhibition of glycosphingolipid biosynthesis. Conversely, the addition of exogenous brain gangliosides to cultured cells reversed these effects. Biochemical and cell biological experiments demonstrate that the pharmacological reduction of cellular glycosphingolipid levels inhibited maturation and cell surface transport of the beta-amyloid precursor protein. In the glycosphingolipid-deficient cell line GM95, cellular levels and maturation of beta-amyloid precursor protein were also significantly reduced as compared with normal B16 cells. Together, these data demonstrate that glycosphingolipids are implicated in the regulation of the subcellular transport of the beta-amyloid precursor protein in the secretory pathway and its proteolytic processing. Thus, enzymes involved in glycosphingolipid metabolism might represent targets to inhibit the production of amyloid beta-peptides.

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.

Differential chemosensitizing effect of two glucosylceramide synthase inhibitors in hepatoma cells

It has been proposed that ceramide mediates anthracyclin-induced apoptosis and that drug resistance may arise due to upregulated removal of this active lipid through glucosylation. We report that HepG2 hepatoma cells displayed only a modest apoptotic response to doxorubicin treatment, accompanied by a substantial elevation of ceramide levels only at toxic drug concentrations. D,L-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) and D,L-threo-1-phenyl-2-hexadecanoylamino-3-pyrrolidino-1-propanol (PPPP), used at concentrations causing a 90% inhibition of ceramide glucosylation, enhanced doxorubicin-elicited ceramide elevation, but only PDMP potentiated apoptosis. Exogenously administered ceramide had only a marginal apoptotic effect on HepG2 cells; moreover, even in this case, apoptosis was propagated by PDMP but not by PPPP. PDMP moderately inhibited P-glycoprotein activity only at the highest concentration tested, but its chemosensitizing effect was still outstanding at lower concentrations, at which P-gp inhibition was no longer observed. These results demonstrate that the chemosensitizing effect of PDMP is, at least partly, independent from its activity as a glucosylceramide synthase inhibitor. Moreover, P-glycoprotein inhibition is not central to the phenomenon.

Ceramide glycosylation potentiates cellular multidrug resistance

Ceramide glycosylation, through glucosylceramide synthase (GCS), allows cellular escape from ceramide-induced programmed cell death. This glycosylation event confers cancer cell resistance to cytotoxic anticancer agents [Liu, Y. Y., Han, T. Y., Giuliano, A. E., and M. C. Cabot. (1999) J. Biol. Chem. 274, 1140-1146]. We previously found that glucosylceramide, the glycosylated form of ceramide, accumulates in adriamycin-resistant breast carcinoma cells, in vinblastine-resistant epithelioid carcinoma cells, and in tumor specimens from patients showing poor response to chemotherapy. Here we show that multidrug resistance can be increased over baseline and then totally reversed in human breast cancer cells by GCS gene targeting. In adriamycin-resistant MCF-7-AdrR cells, transfection of GCS upgraded multidrug resistance, whereas transfection of GCS antisense markedly restored cellular sensitivity to anthracyclines, Vinca alkaloids, taxanes, and other anticancer drugs. Sensitivity to the various drugs by GCS antisense transfection increased 7- to 240-fold and was consistent with the resumption of ceramide-caspase-apoptotic signaling. GCS targeting had little influence on cellular sensitivity to either 5-FU or cisplatin, nor did it modify P-glycoprotein expression or rhodamine-123 efflux. GCS antisense transfection did enhance rhodamine-123 uptake compared with parent MCF-7-AdrR cells. This study reveals that GCS is a novel mechanism of multidrug resistance and positions GCS antisense as an innovative force to overcome multidrug resistance in cancer chemotherapy.

Killing cancer cells by poly-drug elevation of ceramide levels: a hypothesis whose time has come?

Many papers have shown that sphingolipids control the balance in cells between growth and proliferation, and cell death by apoptosis. Sphingosine-1-phosphate (Sph1P) and glucosylceramide (GlcCer) induce proliferation processes, and ceramide (Cer), a metabolic intermediate between the two, induces apoptosis. In cancers, the balance seems to have come undone and it should be possible to kill the cells by enhancing the processes that lead to ceramide accumulation. The two control systems are intertwined, modulated by a variety of agents affecting the activities of the enzymes in Cer-GlcCer-Sph1P interdependence. It is proposed that successful cancer chemotherapy requires the use of many agents to elevate ceramide levels adequately. This review updates current knowledge of sphingolipid metabolism and some of the evidence showing that ceramide plays a causal role in apoptosis induction, as well as a chemotherapeutic agent.

Effects of the glucolipid synthase inhibitor, P4, on functional and phenotypic parameters of murine myeloma cells

This study describes the effects of the glucolipid synthase inhibitor P4, (DL-threo-1-phenyl-2-palmitoylamino-3-pyrrolidino-1-propanol ), on various functional and phenotypic parameters of 5T33 murine myeloma cells. Cell recovery was reduced by >85% following incubation of the cells for 3 days in the presence of 4 microM P4 (the IC50 concentration). Both cytostatic and cytotoxic inhibition was observed with tumour cell metabolic activity and clonogenic potential reduced to 42% and 14% of controls, respectively, and viability reduced to 52%. A dose-dependent increase in cells undergoing apoptosis (from 7% to 26%) was also found. P4 induced a decrease in the number of cells expressing H-2 Class I and CD44, and a large increase in cells expressing H-2 Class II and the IgG2b paraprotein. It did not affect surface expression of CD45 or CD54 (ICAM-1). Based on these alterations in tumour cell growth, adhesion molecule expression and potential immunogenicity, it is anticipated that P4 will provide a novel therapeutic approach for the treatment of multiple myeloma. In addition, given that essentially all tumours rely heavily on overexpressed or abnormal glucosphingolipids for growth, development and metastasis, glucolipid synthase inhibitors may prove to be universally effective anti-cancer agents.

Differential effects of glycolipid biosynthesis inhibitors on ceramide-induced cell death in neuroblastoma cells

An in vitro model of Gaucher's disease in murine neuroblastoma x rat glioma NG108-15 cells was used to investigate the physiological effects of two specific inhibitors of glucosylceramide synthase, d,l-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (d,l-PDMP) and N-butyldeoxynojirimycin (NB-DNJ), which have been suggested as agents for treatment of glycolipid storage disorders. Incubation of NG108-15 cells with conduritol-B-epoxide, a covalent inhibitor of glucosylceramidase, raised the intracellular concentration of glucosylceramide (GC) by more than fourfold, indicating a glycolipid composition equivalent to that of Gaucher's cells. The level of GC was decreased, and the cells were depleted of gangliosides by postincubation with d,l-PDMP or NB-DNJ. Treatment with d,l-PDMP, but not with NB-DNJ, resulted in a dose-dependent reduction of the growth rate and eventually caused cell death in NG108-15 cells on reaching confluency. An in situ detection assay using terminal nucleotidyltransferase indicated that cell degeneration was accompanied by apoptosis. Lipid analysis by high-performance TLC revealed that on incubation with d,l-PDMP, but not with NB-DNJ, the concentration of endogenous ceramide was elevated by threefold. Ceramide elevation and apoptosis were also observed when NG108-15 cells were incubated with daunorubicin, which was previously reported to induce programmed cell death by stimulation of ceramide synthesis. Structural characterization by HPLC and subsequent laser desorption mass spectrometry revealed that the endogenous ceramide contained fatty acids with chain lengths ranging from C14:0 to C24:0. The results indicate that elevation of levels of these ceramide species by incubation with d,l-PDMP or daunorubicin induces programmed cell death in NG108-15 cells. Because ceramide accumulation and cell death were not observed on incubation with NB-DNJ, its use is suggested to be less toxic than that of d,l-PDMP for treatment of Gaucher's disease and other sphingolipid storage disorders.

PDMP blocks brefeldin A-induced retrograde membrane transport from golgi to ER: evidence for involvement of calcium homeostasis and dissociation from sphingolipid metabolism

In this study, we show that an inhibitor of sphingolipid biosynthesis, D,L-threo-1-phenyl-2- decanoylamino-3-morpholino-1-propanol (PDMP), inhibits brefeldin A (BFA)-induced retrograde membrane transport from Golgi to endoplasmic reticulum (ER). If BFA treatment was combined with or preceded by PDMP administration to cells, disappearance of discrete Golgi structures did not occur. However, when BFA was allowed to exert its effect before PDMP addition, PDMP could not "rescue" the Golgi compartment. Evidence is presented showing that this action of PDMP is indirect, which means that the direct target is not sphingolipid metabolism at the Golgi apparatus. A fluorescent analogue of PDMP, 6-(N-[7-nitro-2,1, 3-benzoxadiazol-4-yl]amino)hexanoyl-PDMP (C6-NBD-PDMP), did not localize in the Golgi apparatus. Moreover, the effect of PDMP on membrane flow did not correlate with impaired C6-NBD-sphingomyelin biosynthesis and was not mimicked by exogenous C6-ceramide addition or counteracted by exogenous C6-glucosylceramide addition. On the other hand, the PDMP effect was mimicked by the multidrug resistance protein inhibitor MK571. The effect of PDMP on membrane transport correlated with modulation of calcium homeostasis, which occurred in a similar concentration range. PDMP released calcium from at least two independent calcium stores and blocked calcium influx induced by either extracellular ATP or thapsigargin. Thus, the biological effects of PDMP revealed a relation between three important physiological processes of multidrug resistance, calcium homeostasis, and membrane flow in the ER/ Golgi system.

Altered ceramide and sphingosine expression during the induction phase of ischemic acute renal failure

Recent evidence indicates that a "sphingomyelin signaling pathway" exists: in response to heterogeneous influences, sphingomyelin is hydrolyzed, liberating ceramide, and subsequently its sphingoid base, sphingosine. Ceramide and sphingosine can influence diverse cellular processes, including cell differentiation, proliferation, protein trafficking, and apoptosis. Each of these processes have important implications for post-ischemic acute renal failure (ARF). However, sphingosine and ceramide expression during the induction of ischemic/reperfusion injury have not been previously assessed. To this end, CD-1 mice were subjected to 45 minutes of unilateral renal ischemia +/- reperfusion, followed by cortical sphingosine, ceramide, and sphingomyelin assessments. Contralateral kidneys served as controls. Ischemia caused approximately 50% sphingosine and ceramide decrements. During reperfusion, sphingosine rebounded to normal values. Conversely, ceramide rose to, and was maintained at, supranormal levels (approximately 175% of controls). Subsequent studies performed with hypoxic or oxygenated isolated proximal tubules suggested that these changes: (1) had a multifactorial basis; (2) were partially simulated by enhanced PLA2 activity; (3) and were dissociated from alterations in net sphingomyelin content. To assess the potential pathogenic relevance of the documented ceramide increments, cultured human proximal tubule (HK-2) cells were subjected to ATP depletion/Ca2+ ionophore- or PLA2-induced attack with or without exogenous C2 ceramide loading. Ceramide worsened both forms of injury without exerting an independent lethal effect. Conversely, ceramide markedly attenuated arachidonic acid cytotoxicity. This occurred without any decrease in arachidonate uptake, suggesting a direct cytoprotective effect.

IN CONCLUSION

(1) sphingosine and ceramide fluxes are hallmarks of early ischemic/reperfusion injury; (2) these changes occur via divergent metabolic pathways; and (3) that ceramide increments can affect divergent injury pathways, and that sphingosine and ceramide have potent cell signaling effects, suggest that the currently documented sphingosine/ ceramide fluxes could have important implications for the induction phase and evolution of post-ischemic ARF.

Agents that reverse multidrug resistance, tamoxifen, verapamil, and cyclosporin A, block glycosphingolipid metabolism by inhibiting ceramide glycosylation in human cancer cells

We have previously shown that multidrug-resistant cancer cells display elevated levels of glucosylceramide (Lavie, Y., Cao, H., Bursten, S. L., Giuliano, A. E., and Cabot, M. C. (1996) J. Biol. Chem. 271, 19530-19536). In this study we used the multidrug-resistant human breast cancer cell line MCF-7-Adriamycin-resistant (AdrR), which exhibits marked accumulation of glucosylceramide compared with the parental MCF-7 wild type (drug-sensitive) cell line, to define the relationship between glycolipids and multidrug resistance (MDR). Herein it is shown that clinically relevant concentrations of tamoxifen, verapamil, and cyclosporin A, all circumventors of MDR, markedly decrease glucosylceramide levels in MCF-7-AdrR cells (IC50 values, 1. 0, 0.8, and 2.3 microM, respectively). In intact cells, tamoxifen inhibited glycosphingolipid synthesis at the step of ceramide glycosylation. In cell-free assays for glucosylceramide synthase, tamoxifen (1:10 molar ratio with ceramide) inhibited glucosylceramide formation by nearly 50%. In cell cultures, inhibition of glucosylceramide synthesis by tamoxifen is correlated with its ability to sensitize MCF-7-AdrR cells to Adriamycin toxicity. Moreover, treatment of cells with 1-phenyl-2-palmitoylamino-3-morpholino-1-propanol, an inhibitor of glucosylceramide synthesis, likewise sensitized MCF-7-AdrR cells to Adriamycin. It is concluded that high cellular levels of glucosylceramide are correlated with MDR, and that glycolipids are a target for the action of MDR-reversing agents such as tamoxifen. The data entertain the notion that drug resistance phenomena are aligned with cell capacity to metabolize ceramide.

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.

Treatment of Gaucher disease with an enzyme inhibitor

The hypothesis is offered predicting that Gaucher patients could be treated with a drug that slows the synthesis of glucosylceramide, the lipid that accumulates in this disorder. The present therapeutic approach involves augmenting the defective enzyme, glucosylceramide beta-glucosidase, with exogenous beta-glucosidase isolated from human tissue. This spectacularly expensive mode of treatment should be replaceable with a suitable enzyme inhibitor that simply slows formation of the lipid and matches the rate of synthesis with the rate of the defective, slowly working beta-glucosidase. Several drugs that possess this ability are available, the best known of which is 1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP), a designer inhibitor that resembles the synthase's substrate and product. PDMP has been found to be effective in mice, rats, fish, and a wide variety of cultured cells. Its use, at suitable dosages, seems to be harmless, although long-term tests have not been made. The lack of suitable animal models of Gaucher disease has made it difficult to test the hypothesis adequately, but PDMP does rapidly lower the levels of glucosylceramide in normal animal tissues and the animals evidently do well with the lowered levels of glucosylceramide and its more complex glycolipid metabolites.