Studies of the action of ceramide-like substances (D- and L-PDMP) on sphingolipid glycosyltransferases and purified lactosylceramide synthase

Glycolysis regulates KRAS plasma membrane localization and function through defined glycosphingolipids

Oncogenic KRAS expression generates a metabolic dependency on aerobic glycolysis, known as the Warburg effect. We report an effect of increased glycolytic flux that feeds into glycosphingolipid biosynthesis and is directly linked to KRAS oncogenic function. High resolution imaging and genetic approaches show that a defined subset of outer leaflet glycosphingolipids, including GM3 and SM4, is required to maintain KRAS plasma membrane localization, with GM3 engaging in cross-bilayer coupling to maintain inner leaflet phosphatidylserine content. Thus, glycolysis is critical for KRAS plasma membrane localization and nanoscale spatial organization. Reciprocally oncogenic KRAS selectively upregulates cellular content of these same glycosphingolipids, whose depletion in turn abrogates KRAS oncogenesis in pancreatic cancer models. Our findings expand the role of the Warburg effect beyond ATP generation and biomass building to high-level regulation of KRAS function. The positive feedforward loop between oncogenic KRAS signaling and glycosphingolipid synthesis represents a vulnerability with therapeutic potential.

Glycosphingolipids in Diabetes, Oxidative Stress, and Cardiovascular Disease: Prevention in Experimental Animal Models

Diabetes contributes to about 30% morbidity and mortality world-wide and has tidal wave increases in several countries in Asia. Diabetes is a multi-factorial disease compounded by inflammation, dyslipidemia, atherosclerosis, and is sometimes accompanied with gains in body weight. Sphingolipid pathways that interplay in the enhancement of the pathology of this disease may be potential therapeutic targets. Thus, the application of advanced sphingolipidomics may help predict the progression of this disease and therapeutic outcomes in man. Pre-clinical studies using various experimental animal models of diabetes provide valuable information on the role of sphingolipid signaling networks in diabetes and the efficacy of drugs to determine the translatability of innovative discoveries to man. In this review, we discuss three major concepts regarding sphingolipids and diabetes. First, we discuss a possible involvement of a monosialodihexosylceramide (GM3) in insulin-insulin receptor interactions. Second, a potential role for ceramide (Cer) and lactosylceramide (LacCer) in apoptosis and mitochondrial dysfunction is proposed. Third, a larger role of LacCer in antioxidant status and inflammation is discussed. We also discuss how inhibitors of glycosphingolipid synthesis can ameliorate diabetes in experimental animal models.

Management of metabolic syndrome and reduction in body weight in type II diabetic mice by inhibiting glycosphingolipid synthesis

Metabolic syndrome is defined by hyperlipidemia and cardiovascular complications. We have examined whether inhibition of glycosphingolipid synthesis can interfere with metabolic syndrome in a male mouse model of type II diabetes (db/db). The db/db and control mice (C57/BL6) (n = 6) fed chow for 30 weeks received vehicle (5% Tween-80 in PBS; 100 μl), or a biopolymer-encapsulated D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (BPD) glycosphingolipid synthesis inhibitor daily via oral gavage for 6 weeks. Echocardiography revealed increased Ao-IMT in db/db mice compared to control. However, BPD decreased Ao-IMT, monohexosylceramide and dihexosylceramide, LDL, triglycerides, glucose, and raised HDL levels in db/db mice. This was due to increased gene expression of HMG-CoA reductase, LDLr, SREBP2, and bile acids: Cy7-a hydroxylase, LXR and FXR, lipoprotein lipase, VLDL receptor and PPAR. Treatment also increased the expression of superoxide dismutase-II to reduce the pro-oxidant status in these mice. We observed that decreased cholesterol levels correlated with decreased cholesterol sensing proteins e.g. NPC1 gene/protein expression and mammalian target of rapamycin (mTORC-1) and reduced body weight. Thus, glycosphingolipid synthesis inhibition is a novel approach to manage metabolic syndrome and reduce body weight in diabetic mice and with potential applications in humans.

Inhibition of glycosphingolipid synthesis reverses skin inflammation and hair loss in ApoE-/- mice fed western diet

Sphingolipids have been accorded numerous biological functions however, the effects of feeding a western diet (diet rich in cholesterol and fat) on skin phenotypes, and color is not known. Here, we observed that chronic high-fat and high-cholesterol diet intake in a mouse model of atherosclerosis (ApoE-/-) decreases the level of ceramides and glucosylceramide. At the expense of increased levels of lactosylceramide due to an increase in the expression of lactosylceramide synthase (GalT-V). This is accompanied with neutrophil infiltration into dermis, and enrichment of tumor necrosis factor-stimulated gene-6 (TSG-6) protein. This causes skin inflammation, hair discoloration and loss, in ApoE-/- mice. Conversely, inhibition of glycosphingolipid synthesis, by D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), unbound or encapsulated in a biodegradable polymer (BPD) reversed these phenotypes. Thus, inhibition of glycosphingolipid synthesis represents a unique therapeutic approach relevant to human skin and hair Biology.

Colocalization of receptors for Shiga toxins with lipid rafts in primary human renal glomerular endothelial cells and influence of D-PDMP on synthesis and distribution of glycosphingolipid receptors

Damage of human renal glomerular endothelial cells (HRGECs) of the kidney represents the linchpin in the pathogenesis of the hemolytic uremic syndrome caused by Shiga toxins of enterohemorrhagic Escherichia coli (EHEC). We performed a comprehensive structural analysis of the Stx-receptor glycosphingolipids (GSLs) globotriaosylceramide (Gb3Cer, Galα4Galβ4Glcβ1Cer) and globotetraosylceramide (Gb4Cer, GalNAcβ3Galα4Galβ4Glcβ1Cer) and their distribution in lipid raft analog detergent-resistant membranes (DRMs) and nonDRMs prepared from primary HRGECs. Predominant receptor lipoforms were Gb3Cer and Gb4Cer with Cer (d18:1, C16:0), Cer (d18:1, C22:0) and Cer (d18:1, C24:1/C24:0). Stx-receptor GSLs co-distribute with sphingomyelin (SM) and cholesterol as well as flotillin-2 in DRMs, representing the liquid-ordered membrane phase and indicating lipid raft association. Lyso-phosphatidylcholine (lyso-PC) was identified as a nonDRM marker phospholipid of the liquid-disordered membrane phase. Exposure of primary HRGECs to the ceramide analogon d-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP) reduced total Gb3Cer and Gb4Cer content, roughly calculated from two biological replicates, down to half and quarter of its primordial content, respectively, but strengthened their prevalence and cholesterol preponderance in DRMs. At the same time, the distribution of PC, SM and lyso-PC to subcellular membrane fractions remained unaffected by D-PDMP treatment. Defining the GSL composition and precise microdomain structures of primary HRGECs may help to develop novel therapeutic options to combat life-threatening EHEC infections.

c-Src-induced activation of ceramide metabolism impairs membrane microdomains and promotes malignant progression by facilitating the translocation of c-Src to focal adhesions

We found that c-Src up-regulation perturbs sphingolipid/cholesterol-enriched membrane microdomains by activating ceramide synthesis, which contributes to malignant progression by promoting the translocation of c-Src from microdomains to focal adhesions/podosomes.

Use of a glycolipid inhibitor to ameliorate renal cancer in a mouse model

In a xenograft model wherein, live renal cancer cells were implanted under the kidney capsule in mice, revealed a 30-fold increase in tumor volume over a period of 26 days and this was accompanied with a 32-fold increase in the level of lactosylceramide (LacCer). Mice fed D- threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), an inhibitor of glucosylceramide synthase and lactosylceramide synthase (LCS: β-1,4-GalT-V), showed marked reduction in tumor volume. This was accompanied by a decrease in the mass of lactosylceramide and an increase in glucosylceramide (GlcCer) level. Mechanistic studies revealed that D-PDMP inhibited cell proliferation and angiogenesis by inhibiting p44MAPK, p-AKT-1 pathway and mammalian target for rapamycin (mTOR). By linking glycosphingolipid synthesis with tumor growth, renal cancer progression and regression can be evaluated. Thus inhibiting glycosphingolipid synthesis can be a bonafide target to prevent the progression of other types of cancer.

Novel chemotherapeutic drugs in sphingolipid cancer research

Sphingolipid-metabolizing enzymes are becoming targets for chemotherapeutic development with an increasing interest in the recent years. In this chapter we introduce the sphingolipid family of lipids, and the role of individual species in cell homeostasis. We also discuss their roles in several rare diseases and overall, in cancer transformation. We follow the biosynthesis pathway of the sphingolipid tree, focusing on the enzymes in order to understand how using small molecule inhibitors makes it possible to modulate cancer progression. Finally, we describe the most used and historically significant inhibitors employed in cancer research, their relationships to sphingolipid metabolism, and some promising results found in this field.

Perifosine sensitizes UVB-induced apoptosis in skin cells: new implication of skin cancer prevention?

We demonstrate here that a relative low dose of perifosine significantly enhanced UVB-induced apoptosis in skin cells (keratinocytes and fibroblasts), associated with a significant increase of reactive oxygen species (ROS) and ceramide production as well as multiple perturbations of diverse cell signaling pathways, shifting to a significant pro-apoptosis outcomes. Perifosine inhibited UVB-induced pro-survival Akt/mammalian target of rapamycin (mTOR) and ERK activation, while facilitating pro-apoptotic AMP-activated protein kinas (AMPK), c-Jun-NH(2)-kinase (JNK), and p53 activation; these signaling changes together promoted a striking increase in skin cell apoptosis and a significantly reduced amount of DNA damages. Our results suggest that perifosine may represent a novel skin cancer prevention strategy.

VEGF recruits lactosylceramide to induce endothelial cell adhesion molecule expression and angiogenesis in vitro and in vivo

Angiogenesis is largely driven by vascular endothelial growth factor (VEGF). However, the role of lipid second messengers such as lactosylceramide (LacCer) and LacCer synthase in angiogenesis is not well understood. We have determined the distribution of various LacCer synthase mRNA transcripts using sequential analysis of gene expression (SAGE). Endothelial cells from colon cancer tissues had a 4.5-fold increase in a LacCer synthase transcript (beta1,4GalT-V) as compared to normal colon tissue endothelial cells. Consequently, our focus turned to understanding the role of this enzyme in regulating VEGF-induced angiogenesis in vitro and in vivo. Herein, we show that in human endothelial cells, VEGF-induced angiogenesis is mitigated by dimethylsphingosine and suramin; inhibitors of sphingosine kinase 1(SphK-1) and sphingosine1-phosphate receptor 1(S1P (1)), respectively, and this were bypassed by LacCer but not by S1P. VEGF and basic fibroblast growth factor-induced angiogenesis was mitigated by PDMP; an inhibitor of glucosylceramide synthase and LacCer synthase in human umbilical vein endothelial cells (HUVEC) and human aortic endothelial cells (HAEC). Likewise, GalT-V gene ablation using corresponding siRNA also mitigated VEGF-induced angiogenesis. In Matrigel plug angiogenesis assay in nude mice, angiogenesis was markedly inhibited by D-PDMP with concordantly diminished LacCer synthase activity. Mechanistic studies revealed that the use of LY294002, a PI3 kinase inhibitor, mitigated VEGF-induced expression of platelet-endothelial cell adhesion molecule (PECAM-1/CD31); the trans-endothelial migration of a monocyte cell line (U-937) and angiogenesis in HAEC cells. Since this enzyme is a target for VEGF action and LacCer serves as a lipid second messenger in inducing angiogenesis in vitro and in vivo, novel therapeutic approaches may be developed using our findings to mitigate colon cancer.

Vitamin D receptor and coactivators SRC2 and 3 regulate epidermis-specific sphingolipid production and permeability barrier formation

The vitamin D receptor (VDR) is a nuclear hormone receptor that controls transcription of target genes. It exerts its biological effects through transcriptional coactivators. Previously, we identified two distinct classes of VDR coactivators, VDR-interacting protein (DRIP) and steroid receptor coactivator (SRC) at different stages of keratinocyte differentiation. Here, we determined the functions of VDR and coactivators in lipid production and permeability barrier formation. Silencing of either VDR, SRC2, or SRC3 resulted in decreases in specific glucosylceramide (GlcCer) species but not other lipids such as cholesterol and free fatty acids. Their silencing also caused decreased transcription of fatty acid elongase and ceramide glucosyltransferase, which are critical for the synthesis of epidermis-unique GlcCer species, and defects in lamellar body formation associated with decreased expression of the lipid transporter ATP-binding cassette transporter protein 12. VDR null mice exhibit abnormal barrier function with altered lipid composition in vivo. These results demonstrate that VDR and coactivators SRC2 and SRC3, which are also involved in other nuclear receptors as well, are critical for epidermis-specific sphingolipid production and barrier formation. In contrast, DRIP silencing had no apparent effect on these processes indicating that the two classes of coactivators are differentially utilized.

Glycan antagonists and inhibitors: a fount for drug discovery

Glycans, the carbohydrate chains of glycoproteins, proteoglycans, and glycolipids, represent a relatively unexploited area for drug development compared with other macromolecules. This review describes the major classes of glycans synthesized by animal cells, their mode of assembly, and available inhibitors for blocking their biosynthesis and function. Many of these agents have proven useful for studying the biological activities of glycans in isolated cells, during embryological development, and in physiology. Some are being used to develop drugs for treating metabolic disorders, cancer, and infection, suggesting that glycans are excellent targets for future drug development.

A specific and potent inhibitor of glucosylceramide synthase for substrate inhibition therapy of Gaucher disease

An approach to treating Gaucher disease is substrate inhibition therapy which seeks to abate the aberrant lysosomal accumulation of glucosylceramide. We have identified a novel inhibitor of glucosylceramide synthase (Genz-112638) and assessed its activity in a murine model of Gaucher disease (D409V/null). Biochemical characterization of Genz-112638 showed good potency (IC(50) approximately 24nM) and specificity against the target enzyme. Mice that received drug prior to significant accumulation of substrate (10 weeks of age) showed reduced levels of glucosylceramide and number of Gaucher cells in the spleen, lung and liver when compared to age-matched control animals. Treatment of older mice that already displayed significant amounts of tissue glucosylceramide (7 months old) resulted in arrest of further accumulation of the substrate and appearance of additional Gaucher cells in affected organs. These data indicate that substrate inhibition therapy with Genz-112638 represents a viable alternate approach to enzyme therapy to treat the visceral pathology in Gaucher disease.

Lactosylceramide is required in apoptosis induced by N-Smase

Lactosylceramide (LacCer) is a member of the glycosphingolipid family which has been recently recognized as a signaling intermediate in the regulation of cell proliferation and cell adhesion. In this paper, we present our studies pointing to a potential role of LacCer in inducing apoptosis. In our studies we employed a human osteosarcoma cell line MG-63 (wild type, WT) and a neutral sphingomyelinase (N-SMase) deficient cell line CC derived from MG-63 (mutant) cells. We observed that WT cells were highly sensitive to tumor necrosis factor-alpha (TNF-alpha), ceramide and LacCer-induced apoptosis. In contrast, the mutant cells were insensitive to TNF-alpha-induced apoptosis as they did not generate ceramide and LacCer. However, the exogenous supply of ceramide and/or LacCer rendered the mutant cells apoptotic. Interestingly, preincubation of cells with D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), an inhibitor of glucosylceramide synthase and lactosylceramide synthase, abrogated ceramide-induced apoptosis but not LacCer-induced apoptosis in both WT cells and the mutant cells. Moreover, TNF-alpha and LacCer-induced apoptosis required the generation of reactive oxygen species (ROS) in WT cells. However, since mutant cells did not produce significant amounts of LacCer and ROS in response to TNF-alpha treatment they are insensitive to TNF-alpha-induced apoptosis. In summary, our studies suggest that TNF-alpha-induced N-SMase activation and production of ceramide is required to activate the apoptosis pathway in human osteosarcoma cells. But it is not sufficient to induce apoptosis. Rather, the conversion of ceramide to LacCer and ROS generation are critical for apoptosis.

Platelet derived growth factor recruits lactosylceramide to induce cell proliferation in UDP Gal:GlcCer: beta1 --> 4Galactosyltransferase (GalT-V) mutant Chinese hamster ovary cells

Recent molecular cloning studies have suggested the presence of at least two beta4Gal transferase genes (beta4GalT-V and beta4GalT-VI) that may encode lactosylceramide synthase but whether they are functional in vivo and whether they mediate growth factor induced phenotypic change such as cell proliferation is not known. Our previous studies lead to the suggestion that various risk factors in atherosclerosis such as oxidized LDL, shear stress, nicotine, tumor necrosis factor-alpha converge upon LacCer synthase to induce critical phenotypic changes such as cell proliferation and cell adhesion. However, whether platelet-derived growth factor also recruits LacCer synthase in mediating cell proliferation is not known. Here we have employed a Chinese hamster ovary mutant cell line Pro(-)5Lec20 to determine whether this enzyme physiologically functions to mediate cell proliferation. We show that PDGF stimulates the activity of UDP galactose:glucosylceramide, beta1,4galactosyltransferase. The activity of LacCer synthase increased about 2.5 fold within 2.5-5 min of incubation with PDGF in both wild type and Pro(-)5Lec20 cells. Concomitantly, there was an increase in the generation of superoxide radicals, p44MAPK phosphorylation and cell proliferation in CHO cells. D-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), a potent inhibitor of GlcCer synthase/LacCer synthase impaired PDGF mediated induction of LacCer synthase activity, superoxide generation, p44 MAPK activation and cell proliferation in Pro(-)5Lec20 cells. PDGF-induced superoxide generation was also mitigated by the use of diphenylene iodonium; an inhibitor of NADPH oxidase activity that is required for superoxide generation. This inhibition was bypassed by the addition of lactosylceramide. Thus, beta4GalT-V gene produces a bona fide LacCer synthase that can function in vivo to generate LacCer. Moreover, this enzyme alone can mediate PDGF induced activation of a signal transduction cascade involving superoxide generation, p44MAPK activation, phosphorylation of Akt and cell proliferation.

Novel role of lactosylceramide in vascular endothelial growth factor-mediated angiogenesis in human endothelial cells

Vascular endothelial growth factor (VEGF) has been implicated in angiogenesis associated with coronary heart disease, vascular complications in diabetes, inflammatory vascular diseases, and tumor metastasis. The mechanism of VEGF-driven angiogenesis involving glycosphingolipids such as lactosylceramide (LacCer), however, is not known. To demonstrate the involvement of LacCer in VEGF-induced angiogenesis, we used small interfering RNA (siRNA)-mediated silencing of LacCer synthase expression (GalT-V) in human umbilical vein endothelial cells. This gene silencing markedly inhibited VEGF-induced platelet endothelial cell adhesion molecule-1 (PECAM-1) expression and angiogenesis. Second, we used D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), an inhibitor of LacCer synthase and glucosylceramide synthase, that significantly mitigated VEGF-induced PECAM-1 expression and angiogenesis. Interestingly, these phenotypic changes were reversed by LacCer but not by structurally related compounds such as glucosylceramide, digalactosylceramide, and ceramide. In a human mesothelioma cell line (REN) that lacks the endogenous expression of PECAM-1, VEGF/LacCer failed to stimulate PECAM-1 expression and tube formation/angiogenesis. In REN cells expressing human PECAM-1 gene/protein, however, both VEGF and LacCer-induced PECAM-1 protein expression and tube formation/angiogenesis. In fact, VEGF-induced but not LacCer-induced angiogenesis was mitigated by SU-1498, a VEGF receptor tyrosine kinase inhibitor. Also, VEGF/LacCer-induced PECAM-1 expression and angiogenesis was mitigated by protein kinase C and phospholipase A2 inhibitors. These results indicate that LacCer generated in VEGF-treated endothelial cells may serve as an important signaling molecule for PECAM-1 expression and in angiogenesis. This finding and the reagents developed in our report may be useful as anti-angiogenic drugs for further studies in vitro and in vivo.

L- and D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) inhibit neurite outgrowth from SH-SY5Y cells

Gangliosides and extracellular matrix molecules influence neurite outgrowth, but the combinatorial effects of these endogenous agents on outgrowth are unclear. Exogenous gangliosides inhibit neurite outgrowth from SH-SY5Y cells stimulated with platelet-derived growth factor-BB, and different isoforms of the ceramide analog threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) stimulate (L-PDMP) or inhibit (D-PDMP) glycosphingolipid biosynthesis. In this study, we determined whether altering the endogenous ganglioside levels with PDMP in SH-SY5Y cells regulates neurite outgrowth on the outgrowth-supporting extracellular matrix molecule, laminin. In cells stimulated with 20 ng/ml platelet-derived growth factor-BB to promote outgrowth, we used image analysis to evaluate neurite outgrowth from SH-SY5Y cells grown on endogenous matrix or laminin and exposed to L- or D-PDMP. Both L- and D-PDMP decreased neurite initiation (the number of neurites/cell, the percent of neurite-bearing cells), elongation (the length of the longest neurite/cell, the total neurite length/cell), and branching (the number of branch points/neurite) from SH-SY5Y cells on endogenous matrix or laminin in a dose-dependent manner in serum-free or serum-containing medium. The inhibitory effects of each PDMP isoform were reversible. Inhibition of neurite outgrowth by L-PDMP could be mimicked by addition of exogenous gangliosides or C2-ceramide. Our analyses of neurite outgrowth in SH-SY5Y cells, a model of developing or regenerating noradrenergic neurons, demonstrate that increasing or decreasing endogenous ganglioside levels decreases neurite outgrowth. These results may indicate that SH-SY5Y cells undergo tight regulation by gangliosides, possibly through modulation of growth/trophic factor- and/or extracellular matrix-activated signaling cascades.

Modulation of the expression of membrane-bound CD54 (mCD54) and soluble form of CD54 (sCD54) in endothelial cells by glucosyl transferase inhibitor: possible role of ceramide for the shedding of mCD54

1-Phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) is a synthetic inhibitor toward glucosyl transferase. Here, we showed the functional role of sphingolipids on CD54 expression of endothelial cells (ECs) by the use of PDMP. CD54 mRNA expression in human umbilical vein endothelial cells (HUVECs) was not changed by PDMP; however, PDMP treatment significantly enhanced the expression of membrane-bound CD54 (mCD54) on HUVECs. In contrast, the amount of soluble form of CD54 (sCD54) in the culture supernatants of HUVECs was diminished by PDMP. Similar results were obtained when HUVECs were incubated with metalloproteinase inhibitor, KB-R8301, or in the presence of C2-ceramide. The above effect of PDMP, KB-R8301, and C2-ceramide in HUVECs was commonly found in unstimulated, TNF-alpha-stimulated, and IL-1beta-stimulated HUVECs. These data provide the possibility that the shedding of mCD54 into sCD54 by metalloproteinase-like enzyme is inhibited by PDMP, in which PDMP-induced accumulation of ceramide may act as a second messenger.

Treating glucosphingolipid disorders by chemotherapy: use of approved drugs and over-the-counter remedies

The accumulation of a glucosphingolipid (GSL) in individuals lacking an adequate level of hydrolase activity could be minimized by chemotherapeutic measures that slow the formation of the GSL and stimulate the defective hydrolase. By achieving a balance in the rates of formation and breakdown, one should be able to alleviate the symptoms of excess storage and achieve a satisfactory accommodation. While several drugs seem to be specifically suitable for this purpose, only one of these has been approved for human use. However, less effective drugs and over-the-counter substances are available for human use and may prove satisfactory for a few years until better ones are made available. The proposed materials and the evidence behind the recommendations are presented in this paper.

Caveolar structure and protein sorting are maintained in NIH 3T3 cells independent of glycosphingolipid depletion

Glycosphingolipids have been proposed to be critical components of clustered lipids within cell membranes that serve as rafts for the attachment and sorting of proteins to the cell membrane. Density gradient centrifugation was used to isolate and to ascertain the lipid composition of caveolin-enriched membranes. These membranes demonstrated a significant enrichment of sphingolipids and cholesterol containing up to 20 and 30%, respectively, of the cellular glucosylceramide and lactosylceramide. A specific inhibitor of glucosylceramide synthase, d-threo-1-phenyl-2-palmitoyl-3-pyrrolidino-propanol, was used to test the hypothesis that glycosphingolipids are required for the sorting of proteins to caveolae. When NIH 3T3 cells were depleted of their glucosylceramide based glycosphingolipid mass, the caveolar structure remained intact as determined by electron microscopy and confocal microscopy. The caveolar proteins caveolin and annexin II sorted normally to caveolae, as determined by immunoblotting and confocal microscopy. When the GPI-linked protein B61 was inducibly expressed in these cells, sorting to caveolar membranes occurred normally, even in the presence of glucosylceramide depletion. These observations suggest that protein sorting to caveolae in fibroblasts occurs independently of glycosphingolipid synthesis.