A ceramide analogue inhibits T cell proliferative response through inhibition of glycosphingolipid synthesis and enhancement of N,N-dimethylsphingosine synthesis

The ceramide analogue 1-phenyl-2-(decanoylamino)-3-morpholino-1-propanol (PDMP) (particularly the D-threo isomer, D-PDMP) caused inhibition of cell growth in some types of cells, and this growth-inhibitory effect has been attributed to inhibition of UDP-Glc:Cer beta-Glc transferase, resulting in reduced glycolipid synthesis and increased free ceramide [Inokuch, J., & Radin, N. S. (1987) J. Lipid Res. 28, 565-571; Okada, Y., et al. (1988) FEBS Lett. 235, 25-29]. In view of increasing evidence that the T cell proliferative immune response is modulated by glycosphingolipids (GSLs), the reagent D-PDMP was used to evaluate the role of GSLs in this respect. Con A induced or PHA-induced mitogenesis of C3H/HeJ mouse splenocytes, as well as IL2-dependent CTLL cell growth, were strongly inhibited in a dose-dependent manner when cells were preincubated in the presence of 5-10 microM D-PDMP, but not with its stereoisomer L-PDMP. Closely associated with this growth-inhibitory effect in the presence of D-PDMP, levels of essentially all GSLs, including GM3 and other gangliosides, were greatly reduced, whereas ceramide accumulated. Importantly, metabolically labeled radioactive bands, corresponding to free sphingosine and N-monomethylsphingosine, were found to be present in very small quantities (5-12%) relative to the band corresponding to N,N-dimethylsphingosine (DMS), which showed significant accumulation in D-PDMP-treated lymphocytes. The quantity of IL2 receptors and their affinity to IL2 on T cells did not change, but IL2-dependent tyrosine phosphorylation was greatly stimulated, following D-PDMP treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of steroidogenic response to corticotropin in mouse adrenal tumor cells (Y-1) by the ionophore A23187. Role of protein biosynthesis.

Addition of the ionophore A23187 to Y-1 mouse adrenal tumor cells in monolayer culture inhibits steroidogenesis and the steroidogenic response to corticotropin (50% inhibition at 1 . 10(-7)M). Inhibition is rapid in onset and is not overcome by addition of external Ca2+. The ionophore also inhibits stimulation of steroid synthesis by cyclic AMP. A23187 inhibits incorporation of the amino acid lysine into protein by Y-1 cells and the dose dependence of this inhibition closely resembles that of the inhibition of the steroidogenic response to corticotropin. Addition of A23187 to a subcellular system for protein synthesis prepared from Y-1 cells, inhibits incorporation of the amino acid phenylalanine into protein and this effect is not overcome by high concentrations of Ca2+. The inhibitory effect of A23187 on the response to corticotropin, like that response itself, takes place at some part of steroid synthesis after entry of cholesterol into the cells and before the side-chain cleavage of cholesterol. These studies confirm the importance of protein synthesis in the response to corticotropin and demonstrate that the effect of protein synthesized under the influence of corticotropin is exerted at some point in the events which bring substrate (cholesterol) to the mitochondrial side-chain cleavage enzyme system. It is also shown that A23187 inhibits protein synthesis, and hence the response to corticotropin, by a mechanism which is independent of the concentration of available Ca2+.     

Lactosylceramide is essential for the osteoclastogenesis mediated by macrophage-colony-stimulating factor and receptor activator of nuclear factor-kappa B ligand.

Glycosphingolipids and their metabolites play important roles in a variety of biological processes. Several signal molecules are localized in a glycolipid-enriched microdomain on the cell surface, and their signals are regulated by the glycolipid composition. However, the function of glycolipids in osteoclastogenesis has not been clearly understood. We found that D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), a glucosylceramide synthase inhibitor, completely inhibits the osteoclast formation induced by macrophage-colony-stimulating factor and receptor activator of nuclear factor-kappa B ligand (RANKL) in a dose-dependent manner. Expression of RANK, the receptor of RANKL, induced by macrophage colony-stimulating factor, was reduced markedly in D-PDMP-treated cells. d-PDMP also inhibited the phosphorylation of the inhibitor of nuclear factor-kappa B and extracellular signal-regulated kinase 1/2 induced by RANKL. In several experiments with the addition of glycolipids to D-PDMP-treated purified bone marrow cells, lactosylceramide (LacCer) strongly affected the differentiation into tartrate-resistant acid phosphatase mononucleated cells, but not positive multinucleated cells. GM3 and GM1 also recovered, but less effectively compared with LacCer. Moreover, exogenous LacCer recovered the reduced expression of RANK and the phosphorylation of inhibitor of NF-kappa B and extracellular signal-regulated kinase 1/2 after stimulation by RANKL at the same level of cells without D-PDMP treatment. Our data suggest that glycosphingolipids, especially LacCer, are necessary for the initiation step of RANKL-induced osteoclastogenesis.     

Reduction of glycosphingolipid levels in lipid rafts affects the expression state and function of glycosylphosphatidylinositol-anchored proteins but does not impair signal transduction via the T cell receptor

Lipid rafts are highly enriched in cholesterol and sphingolipids. In contrast to many reports that verify the importance of cholesterol among raft lipid components, studies that address the role of sphingolipids in raft organization and function are scarce. Here, we investigate the role of glycosphingolipids (GSLs) in raft structure and raft-mediated signal transduction in T lymphocytes by the usage of a specific GSL synthesis inhibitor, d-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP). Surface GM1 expression and the expression of GSLs in rafts were profoundly reduced by D-PDMP treatment, whereas the expression of other lipid and protein constituents, such as cholesterol, sphingomyelin, Lck, and linker for activation of T cells, was not affected. T cell receptor-mediated signal transduction induced by antigen stimulation or by antibody cross-linking was normal in D-PDMP-treated T cells. In contrast, the signal through glycosylphosphatidylinositol (GPI)-anchored proteins was clearly augmented by D-PDMP treatment. Moreover, GPI-anchored proteins became more susceptible to phosphatidylinositol-specific phospholipase C cleavage in D-PDMP-treated cells, demonstrating that GSL depletion from rafts primarily influences the expression state and function of GPI-anchored proteins. Finally, by comparing the effect of D-PDMP with that of methyl-beta-cyclodextrin, we identified that compared with cholesterol depletion, GSL depletion has the opposite effect on the phosphatidylinositol-specific phospholipase C sensitivity and signaling ability of GPI-anchored proteins. These results indicate a specific role of GSLs in T cell membrane rafts that is dispensable for T cell receptor signaling but is important for the signal via GPI-anchored proteins.     

Fibroblast growth regulatory factor inhibits DNA synthesis in BALB/c 3T3 cells by arresting in G1

A cell surface macromolecular component from quiescent BALB/c 3T3 mouse cells (designated fibroblast growth regulatory factor, FGRF) inhibits DNA synthesis and cell division in growing 3T3 cells. Addition of FGRF to synchronized populations of growing 3T3 cells in the late G1 or early S phase did not inhibit DNA synthesis in the immediate S phase. However, a significant inhibition was observed in the S phase of the next round of cell cycle. Cells exposed to the regulatory factor in late S/early G2 or early G1 showed reduced DNA synthesis in the upcoming S phase; the late S/early G2 cells were more sensitive to inhibition than the cells in the G1. Further, the regulatory factor delayed the progression of G0/G1-arrested cells into the next S phase. These results suggest that the physiological effect of FGRF is to arrest cells in early G1, thus preventing their entry into a new round of cell cycle. In contrast to untransformed 3T3 cells, mouse cells transformed by SV40 were not subjected to growth-arrest by the regulatory factor, although the transformed cells contain active FGRF that inhibits DNA synthesis in growing 3T3 cells.     

Inhibition of G1 phase cyclin dependent kinases by transforming growth factor β1

Transforming growth factor β1 (TGFβ1) inhibits epithelial cell proliferation late in the G1 phase of the cell cycle. We examined the effect of TGFβ1 on known late G1 cell cycle regulators in an attempt to determine the molecular mechanism of growth inhibition by this physiological inhibitor. The results demonstrate the TGFβ1 inhibits the late G1 and S phase specific histone H1 kinase activity of p33^cdk2. This inhibitiion is not dur to TGFβ1's effect on p33^cdk2 synthesis, but rather due to its negative effect on the late G1 phosphorylation of p33^cdk2. It is also shown that TGFβ1 inhibits both late G1 cyclin A and cyclin E associated histon H1 kinase activities. The inhibitor has no effects on the synthesis of cyclin E but to inhibit the synthesis of cyclin A protein in a cell cycle dependent manner. If TGFβ1 is added to cells which have progressed futher than 8 hours into G1, then it is without inhibitory effect on cyclin A synthesis. These effect on TGFβ1 on late G1 cell cycle regulators correlate well with its inhibitory effects on cellular growth and suggest that these G1 cyclin dependent kinases might serve as targets for TGFβ1-mediated growth arrest.     

Inbition of steroidogenic response to corticotropin in mouse adrenal tumor cells (Y-1 by the ionophore A23187 Role of protein biosynthesis

Addition of the ionophore A123187 to Y-1 mouse adrenal tumor cells in monolayer culture inhibits steroidogenesis and the steroidogenic response to corticotropin (50% inhibition at 1 · 10^?7 M). inhibition is rapid in onset and is not overcome by addition of external Ca²⁺. The ionophore also inhibits stimulation of steroid synthesis by cyclic AMP. A23187 inhibits incorporation of the amino acid lysine into protein by Y-1 cells and the dose dependence of this inhibition closely resembles that of the inhibition of the steroidogenic response to corticotropin. Addition of A23187 to a subcellular system for protein synthesis prepared from Y-1 cells, inhibits incorporation of the amino acid phenylalanine into protein and this effects and this effect is not overcome by high concentrations of Ca²⁺. The inhibitory effect of A23187 on the response to corticotropin, like that response itself, takes place at some part of steriod synthesis after entry of cholesteriol into the cells and before the side-chain cleavage of cholesterol. These studies confirm the importance of protein synthesis in the response to corticotropin and demonstrate that the effect of protein synthesized under the influence of corticotropin is exerted at some point in the events which bring substrate (cholesterol) to the mitochondrial side-chain cleavage enzyme system. It is also shown that A23187 inhibits protein synthesis, and hence the response to corticotropin, by a mechanism which is independent of the concentration of available Ca²⁺.     

Mig-6 plays a critical role in the regulation of cholesterol homeostasis and bile Acid synthesis

The disruption of cholesterol homeostasis leads to an increase in cholesterol levels which results in the development of cardiovascular disease. Mitogen Inducible Gene 6 (Mig-6) is an immediate early response gene that can be induced by various mitogens, stresses, and hormones. To identify the metabolic role of Mig-6 in the liver, we conditionally ablated Mig-6 in the liver using the Albumin-Cre mouse model (Alb(cre/+)Mig-6(f/f); Mig-6(d/d)). Mig-6(d/d) mice exhibit hepatomegaly and fatty liver. Serum levels of total, LDL, and HDL cholesterol and hepatic lipid were significantly increased in the Mig-6(d/d) mice. The daily excretion of fecal bile acids was significantly decreased in the Mig-6(d/d) mice. DNA microarray analysis of mRNA isolated from the livers of these mice showed alterations in genes that regulate lipid metabolism, bile acid, and cholesterol synthesis, while the expression of genes that regulate biliary excretion of bile acid and triglyceride synthesis showed no difference in the Mig-6(d/d) mice compared to Mig-6(f/f) controls. These results indicate that Mig-6 plays an important role in cholesterol homeostasis and bile acid synthesis. Mice with liver specific conditional ablation of Mig-6 develop hepatomegaly and increased intrahepatic lipid and provide a novel model system to investigate the genetic and molecular events involved in the regulation of cholesterol homeostasis and bile acid synthesis. Defining the molecular mechanisms by which Mig-6 regulates cholesterol homeostasis will provide new insights into the development of more effective ways for the treatment and prevention of cardiovascular disease.     

Regulation of bile acid synthesis. V. Inhibition of conversion of 7-dehydrocholesterol to cholesterol is associated with down-regulation of cholesterol 7 alpha-hydroxylase activity and inhibition of bile acid synthesis.

In the chronic bile fistula rat, the administration of a bolus dose of mevinolinic acid, an inhibitor of HMG-CoA reductase, was followed by rapid down-regulation of cholesterol 7 alpha-hydroxylase activity and a decrease in bile acid synthesis. These observations suggested that either newly synthesized cholesterol or some other metabolite of mevalonate may be involved in the regulation of bile acid synthesis. In order to distinguish between these two alternatives, we carried out experiments in which cholesterol synthesis was blocked by AY9944, a compound that inhibits the conversion of 7-dehydrocholesterol to cholesterol, a last step in the cholesterol biosynthesis pathway. Rats underwent biliary diversion for 72 h at which time they were given intravenously either a bolus dose of AY9944 (1 mg/kg) or control vehicle. At 0 (pre-treatment control), 0.5, 1.5, and 3 h post bolus, livers were harvested and specific activities of cholesterol 7 alpha-hydroxylase were determined. At 1.5, 3, and 6 h post bolus, AY9944 inhibited bile acid synthesis by 19 +/- 6%, 40 +/- 4%, and 41 +/- 6%, respectively, as compared to pretreatment baseline. Cholesterol 7 alpha-hydroxylase activity determined at 0.5, 1.5, and 3 h was decreased by 44 +/- 6%, 44 +/- 2%, and 36 +/- 2%, respectively, as compared to the control value. In in vitro experiments using microsomes from livers of control bile fistula rats, the addition of AY9944 (up to 100 microM) failed to inhibit cholesterol 7 alpha-hydroxylase activity. The results of this study demonstrate that, in the chronic bile fistula rat, acute inhibition of cholesterol synthesis at either early or late steps leads to a rapid down-regulation of cholesterol 7 alpha-hydroxylase activity and decrease in bile acid synthesis.     

Evidence for the inhibition of platelet-derived growth factor induced rat smooth muscle cells DNA synthesis by fenofibric acid at the Go/G1 cell cycle level

It has been proposed that a platelet-derived growth factor (PDGF) may play an important role in the genesis of atherosclerosis by promoting proliferation of smooth muscle cells. The present study shows evidence that fenofibric acid inhibits the growth promoting activity of PDGF on cultured smooth muscle cells.When smooth muscle cells are in a quiescent state by feeding them a PDGF -and lipoproteins- deficient medium, addition of a platelet extract induces DNA synthesis in a synchronous fashion. A 6 hours' exposure of cells to this extract is sufficient for this effect. Fenofibric acid could inhibit this synthesis when the compound was present in the culture medium concomitantly with platelet extract. This result suggests that fenofibric acid target is a cellular event attendant on PDGF-induced growth promotion. Fenofibric acid is a hypolipidemic drug which inhibits 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMGCoA reductase) activity, the limiting step of endogenous cholesterol synthesis. If any endogenous cholesterol is available, smooth muscle cell cholesterol needs for proliferation can be supplied by LDL-cholesterol. As the addition of LDL to the culture medium did not overcome fenofibric acid inhibition, this demonstrates a cholesterol independent mechanism for the anti-PDGF growth promoting activity.The fact that fenofibric acid, a hypolipidemic drug, can also inhibit growth promoting activity of PDGF suggests that this drug can be effective on the prevention of atherosclerosis and cardiovascular diseases by at least two independent mechanisms.     

Niemann-pick type C1 (NPC1) overexpression alters cellular cholesterol homeostasis

The Niemann-Pick type C1 (NPC1) protein is a key participant in intracellular trafficking of low density lipoprotein cholesterol, but its role in regulation of sterol homeostasis is not well understood. To characterize further the function of NPC1, we generated stable Chinese hamster ovary (CHO) cell lines overexpressing the human NPC1 protein (CHO/NPC1). NPC1 overexpression increases the rate of trafficking of low density lipoprotein cholesterol to the endoplasmic reticulum and the rate of delivery of endosomal cholesterol to the plasma membrane (PM). CHO/NPC1 cells exhibit a 1.5-fold increase in total cellular cholesterol and up to a 2.9-fold increase in PM cholesterol. This increase in PM cholesterol is closely paralleled by a 3-fold increase in de novo cholesterol synthesis. Inhibition of cholesterol synthesis results in marked redistribution of PM cholesterol to intracellular sites, suggesting an unsuspected role for NPC1 in internalization of PM cholesterol. Despite elevated total cellular cholesterol, CHO/NPC1 cells exhibit increased cholesterol synthesis, which may be attributable to both resistance to oxysterol suppression of sterol-regulated gene expression and to reduced endoplasmic reticulum cholesterol levels under basal conditions. Taken together, these studies provide important new insights into the role of NPC1 in the determination of the levels and distribution of cellular cholesterol.     

Deficiency of interleukin-1 receptor antagonist deteriorates fatty liver and cholesterol metabolism in hypercholesterolemic mice

Although the anti-inflammatory effect of interleukin-1 (IL-1) receptor antagonist (IL-1Ra) has been described, the contribution of this cytokine to cholesterol metabolism remains unclear. Our aim was to ascertain whether deficiency of IL-1Ra deteriorates cholesterol metabolism upon consumption of an atherogenic diet. IL-1Ra-deficient mice (IL-1Ra(-/-)) showed severe fatty liver and portal fibrosis containing many inflammatory cells following 20 weeks of an atherogenic diet when compared with wild type (WT) mice. Expectedly, the levels of total cholesterol in IL-1Ra(-/-) mice were significantly increased, and the start of lipid accumulation in liver was observed earlier when compared with WT mice. Real-time PCR analysis revealed that IL-1Ra(-/-) mice failed to induce mRNA expression of cholesterol 7alpha-hydroxylase, which is the rate-limiting enzyme in bile acid synthesis, with concurrent up-regulation of small heterodimer partner 1 mRNA expression. Indeed, IL-1Ra(-/-) mice showed markedly decreased bile acid excretion, which is elevated in WT mice to maintain cholesterol level under atherogenic diet feeding. Therefore, we conclude that the lack of IL-1Ra deteriorates cholesterol homeostasis under atherogenic diet-induced inflammation.     

The Niemann-Pick C1 gene is downregulated by feedback inhibition of the SREBP pathway in human fibroblasts

The Niemann-Pick C1 (NPC1) protein regulates the transport of cholesterol from late endosomes/lysosomes to other compartments responsible for maintaining intracellular cholesterol homeostasis. The present study examined the expression of the NPC1 gene and the distribution of the NPC1 protein that resulted from the transport of LDL-derived cholesterol through normal human fibroblasts. A key finding was that the transport of cholesterol from late endosomes/lysosomes to the sterol-regulatory pool at the endoplasmic reticulum, as determined by feedback inhibition of the sterol-regulatory element binding protein (SREBP) pathway, was associated with the downregulation of the NPC1 gene. Consistent with these results, fibroblasts incubated with LDL had decreased amounts of SREBP protein that interacted with sterol-regulatory element (SRE) sequences positioned within the NPC1 gene promoter region. Finally, partial colocalization of the NPC1 protein with late endosomes/lysosomes and distinct regions of the endoplasmic reticulum suggested that the NPC1 protein may facilitate the transport of cholesterol directly between these two compartments. Together, these results indicate that the transport of LDL-derived cholesterol from late endosomes/lysosomes to the sterol-regulatory pool, known to be regulated by the NPC1 protein, is responsible for promoting feedback inhibition of the SREBP pathway and downregulation of the NPC1 gene.     

Use of a novel anti-proliferative compound coated on a biopolymer to mitigate platelet-derived growth factor-induced proliferation in human aortic smooth muscle cells: comparison with sirolimus

Drug eluting stents (DES) have become a common mode of treatment for stenosis in coronary arteries. However, currently, the use of sirolimus/paclitaxel-coated DES has come under scrutiny, because of their pro-thrombotic effects leading to potential adverse outcomes in the long run. We have previously documented that d-threo-1-phenyl-2-decanoylamino-3-morholino propanol (D-PDMP); an inhibitor of glucosylceramide synthase and lactosylceramide (LacCer) synthase markedly inhibited platelet-derived growth factor (PDGF)-induced cell proliferation. We have fabricated DES wherein, D-PDMP or sirolimus was coated on to a double layer of poly (lactic-co-glycolic acid) on a bare metal stent. The in vitro release of D-PDMP from biopolymer and its consequent effect on PDGF induced proliferation and apoptosis was assessed in human aortic smooth muscle cells (ASMC). D-PDMP was released from biopolymers in a dose-dependent fashion and was accompanied with a decrease in PDGF-induced cell proliferation, but not apoptosis. In contrast, sirolimus markedly increased apoptosis in these cells in addition to inhibiting proliferation. Our mechanistic studies revealed that D-PDMP, but not sirolimus decreased the cellular level of glucosyl and lactosylceramide that accompanied inhibition of PDGF-induced cell proliferation. Our short-term (14 days) in vivo studies in rabbits also attested to the safety and biocompatibility of the D-PDMP coated stents. Our data reveal the superiority of D-PDMP coated biopolymers over sirolimus coated biopolymers in mitigating ASMC proliferation. Such D-PDMP coated stents may be useful for localized delivery of drug to mitigate neo-vascular hyperplasia and other proliferative disorders. Yong-Dan Tang, Ambarish Pandey, Subbu S. Venkatraman, and Subroto Chatterjee contributed equally to this work.