Susanne Schiffmann

Chain length-specific properties of ceramides.

Ceramides are a class of sphingolipids that are abundant in cell membranes. They are important structural components of the membrane but can also act as second messengers in various signaling pathways. Until recently, ceramides and dihydroceramides were considered as a single functional class of lipids and no distinction was made between molecules with different chain lengths. However, based on the development of high-throughput, structure-specific and quantitative analytical methods to measure ceramides, it has now become clear that in cellular systems the amounts of ceramides differ with respect to their chain length. Further studies have indicated that some functions of ceramides are chain-length dependent. In this review, we discuss the chain length-specific differences of ceramides including their pathological impact on Alzheimers disease, inflammation, autophagy, apoptosis and cancer.

Ceramide synthases and ceramide levels are increased in breast cancer tissue

Several in vitro studies have correlated dysfunction of the sphingolipid-signaling pathway with promotion of tumor cell growth as well as progression and resistance of tumors to chemotherapeutic agents. As ceramides (Cer) constitute the structural backbones of all sphingolipids, we investigated the endogenous ceramide levels in 43 malignant breast tumors and 21 benign breast biopsies and compared them with those of normal tissues using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). The total ceramide levels in malignant tumor tissue samples were statistically significantly elevated when compared with normal tissue samples. Upregulation of the total ceramide level averaged 12-fold and 4-fold higher than normal tissue samples, for malignant tumors and benign tissues, respectively. Specifically, the levels of C(16:0)-Cer, C(24:1)-Cer and C(24:0)-Cer were significantly raised in malignant tumors as compared with benign and normal tissue. The augmentation of the various ceramides could be assigned to an increase of the messenger RNA levels of ceramide synthases (CerS) LASS2 (longevity assurance), LASS4 and LASS6. Notably, elevated levels of C(16:0)-Cer were associated with a positive lymph node status, indicating a metastatic potential for this ceramide. Moreover, the levels of C(18:0)-Cer and C(20:0)-Cer were significantly higher in estrogen receptor (ER) positive tumor tissues as compared with ER negative tumor tissues. In conclusion, progression in breast cancer is associated with increased ceramide levels due to an upregulation of specific LASS genes.

Long chain ceramides and very long chain ceramides have opposite effects on human breast and colon cancer cell growth.

Ceramides are known to be key players in intracellular signaling and are involved in apoptosis, cell senescence, proliferation, cell growth and differentiation. They are synthesized by ceramide synthases (CerS). So far, six different mammalian CerS (CerS1-6) have been described. Recently, we demonstrated that human breast cancer tissue displays increased activity of CerS2, 4, and 6, together with enhanced generation of their products, ceramides C(16:0), C(24:0), and C(24:1). Moreover, these increases were significantly associated with tumor dignity. To clarify the impact of this observation, we manipulated cellular ceramide levels by overexpressing ceramide synthases 2, 4 or 6 in MCF-7 (breast cancer) and HCT-116 (colon cancer) cells, respectively. Overexpression of ceramide synthases 4 and 6 elevated generation of short chain ceramides C(16:0), C(18:0) and C(20:0), while overexpression of ceramide synthase 2 had no effect on ceramide production in vivo, presumably due to limited substrate availability, because external addition of very long chain acyl-CoAs resulted in a significant upregulation of very long chain ceramides. We also demonstrated that upregulation of CerS4 and 6 led to the inhibition of cell proliferation and induction of apoptosis, whereas upregulation of CerS2 increased cell proliferation. On the basis of our data, we propose that a disequilibrium between ceramides of various chain length is crucial for cancer progression, while normal cells require an equilibrium between very long and long chain ceramides for normal physiology.

Sphingosine kinase 2 deficient tumor xenografts show impaired growth and fail to polarize macrophages towards an anti-inflammatory phenotype

A challenging task of the immune system is to fight cancer cells. However, a variety of human cancers educate immune cells to become tumor supportive. This is exemplified for tumor‐associated macrophages (TAMs), which are polarized towards an anti‐inflammatory and cancer promoting phenotype. Mechanistic explanations, how cancer cells influence the macrophage phenotype are urgently needed to address potential anti‐cancer strategies along this line. One potential immune modulating compound, sphingosine‐1‐phosphate (S1P), was recently highlighted in both tumor growth and immune modulation. Using a xenograft model in nude mice, we demonstrate a supportive role of sphingosine kinase 2 (SphK2), one of the S1P‐producing enzymes for tumor progression. The growth of SphK2‐deficient MCF‐7 breast tumor xenografts was markedly delayed when compared with controls. Infiltration of macrophages in SphK2‐deficient and control tumors was comparable. However, TAMs from SphK2‐deficient tumors displayed a pronounced anti‐tumor phenotype, showing an increased expression of pro‐inflammatory markers/mediators such as NO, TNF‐α, IL‐12 and MHCII and a low expression of anti‐inflammatory IL‐10 and CD206. These data suggest a role for S1P, generated by SphK2, in early tumor development by affecting macrophage polarization.

Activation of ceramide synthase 6 by celecoxib leads to a selective induction of C16:0-ceramide

Ceramides serve as bioactive molecules with important roles in cell proliferation and apoptosis. Ceramides (Cer) with different N-acyl side chains (C(14:0)-Cer-C(26:0)-Cer) possess distinctive roles in cell signaling and are differentially expressed in HCT-116 colon cancer cells. Celecoxib, a selective cyclooxygenase-2 (COX-2) inhibitor, exhibiting antiproliferative effects, activates the sphingolipid pathway. To elucidate the mechanism, HCT-116 cells were treated with 50μM celecoxib leading to a significant increase of C(16:0)-Cer. Interestingly, 50μM celecoxib resulted in a 2.8-fold increase of ceramide synthase (CerS) activity as measured by a cell-based activity assay. siRNA against several CerSs revealed that CerS6 was predominantly responsible for the increase of C(16:0)-Cer in HCT-116 cells. Moreover, the silencing of CerS6 partially protected HCT-116 cells from the toxic effects induced by celecoxib. Treatment of cells with celecoxib and fumonisin B1 (inhibitor of CerSs) or myriocin (inhibitor of l-serine palmitoyl transferase) or desipramine (inhibitor of acid sphingomyelinase and acid ceramidase) revealed that the increase of C(16:0)-Cer results predominantly from activation of the salvage pathway. Using the nude mouse model we demonstrated that celecoxib induces also in vivo a significant increase of C(16:0)-Cer in stomach, small intestine and tumor tissue. In conclusion, celecoxib causes a specific increase of C(16:0)-Cer by activating CerS6 and the salvage pathway, which contribute to the toxic effects of celecoxib.

The anti-proliferative potency of celecoxib is not a class effect of coxibs.

Celecoxib, a COX-2 (cyclooxygenase-2)-selective inhibitor (coxib), is the only NSAID (non-steroidal anti-inflammatory drug) that has been approved for adjuvant treatment of patients with familial adenomatous polyposis. To investigate if the anti-proliferative effect of celecoxib extends to other coxibs, we compared the anti-proliferative potency of all coxibs currently available (celecoxib, rofecoxib, etoricoxib, valdecoxib, lumiracoxib). Additionally, we used methylcelecoxib (DMC), a close structural analogue of celecoxib lacking COX-2-inhibitory activity. Due to the fact that COX-2 inhibition is the main characteristic of these substances (with exception of methylcelecoxib), we conducted all experiments in COX-2-overexpressing (HCA-7) and COX-2-negative (HCT-116) human colon cancer cells, in order to elucidate whether the observed effects after coxib treatment depend on COX-2 inhibition. Cell survival was assessed using the WST proliferation assay. Apoptosis and cell cycle arrest were determined using flow cytometric and Western blot analysis. The in vitro results were confirmed in vivo using the nude mouse model. Among all coxibs tested, only celecoxib and methylcelecoxib decreased cell survival by induction of cell cycle arrest and apoptosis and reduced the growth of tumor xenografts in nude mice. None of the other coxibs (rofecoxib, etoricoxib, valdecoxib, lumiracoxib) produced anti-proliferative effects, indicating the lack of a class effect and of a role for COX-2. Our data emphasize again the outstanding anti-proliferative activity of celecoxib and its close structural analogue methylcelecoxib in colon carcinoma models in vitro and in vivo.

Ceramide Synthase 6 Plays a Critical Role in the Development of Experimental Autoimmune Encephalomyelitis

Ceramides are mediators of apoptosis and inflammatory processes. In an animal model of multiple sclerosis (MS), the experimental autoimmune encephalomyelitis (EAE) model, we observed a significant elevation of C16:0-Cer in the lumbar spinal cord of EAE mice. This was caused by a transiently increased expression of ceramide synthase (CerS) 6 in monocytes/macrophages and astroglia. Notably, this corresponds to the clinical finding that C16:0-Cer levels were increased 1.9-fold in cerebrospinal fluid of MS patients. NO and TNF-α secreted by IFN-γ–activated macrophages play an essential role in the development of MS. In murine peritoneal and mouse-derived RAW 264.7 macrophages, IFN-γ–mediated expression of inducible NO synthase (iNOS)/TNF-α and NO/TNF-α release depends on upregulation of CerS6/C16:0-Cer. Downregulation of CerS6 by RNA interference or endogenous upregulation of C16:0-Cer mediated by palmitic acid in RAW 264.7 macrophages led to a significant reduction or increase in NO/TNF-α release, respectively. EAE/IFN-γ knockout mice showed a significant delay in disease onset accompanied by a significantly less pronounced increase in CerS6/C16:0-Cer, iNOS, and TNF-α compared with EAE/wild-type mice. Treatment of EAE mice with l-cycloserine prevented the increase in C16:0-Cer and iNOS/TNF-α expression and caused a remission of the disease. In conclusion, CerS6 plays a critical role in the onset of MS, most likely by regulating NO and TNF-α synthesis. CerS6 may represent a new target for the inhibition of inflammatory processes promoting MS development.

The equilibrium between long and very long chain ceramides is important for the fate of the cell and can be influenced by co-expression of CerS.

Ceramides are synthesized by six different ceramide synthases (CerS1-6), which differ in their specificity to produce ceramides of distinct chain length. We investigated the impact of CerS-co-transfection on ceramide production and apoptosis and proliferation in HCT-116 cells. Over-expression of CerS4 and CerS6 enhanced the level of C(16:0)-Cer twofold, that of C(18:0)- and C(20:0)-Cer up to sevenfold, in comparison to vector control transfected cells, whereas over-expression of CerS2 had no effect on the level of very long chain ceramide C(24:0)- and C(24:1)-Cer. Instead over-expression of CerS2 together with CerS4 or CerS6 increased the activity of CerS2 against very-long-chain ceramides about twofold. In contrast, co-expression of CerS4 with CerS6 inhibited slightly the production of C20:0-ceramide in comparison to cells over-expressing CerS4 alone, whereas the activity of CerS6 seemed not to be affected by other CerS. Interestingly, down-regulation of ELOVL1 had a comprehensive effect on the synthesis of very long chain ceramides which possibly point to a requirement for ELOVL1 expression for full CerS2-activity. Co-expression of CerS2 with CerS4/CerS6 reversed the inhibitory effect of long chain ceramides on cell proliferation and the induction of apoptosis. Even though we observed a twofold increase in total ceramide levels after co-expression of CerS2 with CerS4/CerS6, we detected no effect on cell proliferation. These data indicate that an increase in ceramide production per se is not critical for cell survival, but the equilibrium between long and very long chain ceramides and possibly protein/protein interactions determine the fate of the cell.

Lack of ceramide synthase 2 suppresses the development of experimental autoimmune encephalomyelitis by impairing the migratory capacity of neutrophils

Ceramide synthases (CerS) synthesise ceramides of defined acyl chain lengths, which are thought to mediate cellular processes in a chain length-dependent manner. In experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS), we observed a significant elevation of CerS2 and its products, C24-ceramides, in CD11b(+) cells (monocytes and neutrophils) isolated from blood. This result correlates with the clinical finding that CerS2 mRNA expression and C24-ceramide levels were significantly increased by 2.2- and 1.5-fold, respectively, in white blood cells of MS patients. The increased CerS2 mRNA/C24-ceramide expression in neutrophils/monocytes seems to mediate pro-inflammatory effects, since a specific genetic deletion of CerS2 in blood cells or a total genetic deletion of CerS2 significantly delayed the onset of clinical symptoms, due to a reduced infiltration of immune cells, in particular neutrophils, into the central nervous system. CXCR2 chemokine receptors, expressed on neutrophils, promote the migration of neutrophils into the central nervous system, which is a prerequisite for the recruitment of further immune cells and the inflammatory process that leads to the development of MS. Interestingly, neutrophils isolated from CerS2 null EAE mice, as opposed to WT EAE mice, were characterised by significantly lower CXCR2 receptor mRNA expression resulting in their reduced migratory capacity towards CXCL2. Most importantly, G-CSF-induced CXCR2 expression was significantly reduced in CerS2 null neutrophils and their migratory capacity was significantly impaired. In conclusion, our data strongly indicate that G-CSF-induced CXCR2 expression is regulated in a CerS2-dependent manner and that CerS2 thereby promotes the migration of neutrophils, thus, contributing to inflammation and the development of EAE and MS.

Dimethylcelecoxib inhibits prostaglandin E2 production.

Dimethylcelecoxib (DMC), a derivative of celecoxib, has been developed to distinguish between the COX-dependent and COX-independent anti-carcinogenic effects of celecoxib. Although DMC has been shown to have no COX-inhibitory activity, it is important to ensure that DMC has no other influence on prostaglandin production. Interestingly, in this study we show that DMC inhibits PGE(2) production in vitro in the low micromolar range in different cancer cell lines. This effect can be at least partly explained by our findings that DMC inhibits microsomal prostaglandin E synthase-1 (mPGES-1) activity in a cell-free assay. Moreover, it prevents mPGES-1 up-regulation after stimulation of HeLa cells with IL-1beta and TNFalpha. Conversely, DMC has no effect on the expression levels of COX-1, COX-2, cytosolic PGES (cPGES) or mPGES-2 in these cells. However, in the cell-free assay DMC inhibits mPGES-1 to a maximum of 65% only and concentrations needed for inhibition of mPGES-1 activity are about 10-fold higher than needed for inhibition of PGE(2) production in cell culture. This suggests that DMC also has an impact on other proteins involved in PGE(2) production. In cell culture experiments the anti-proliferative effect of DMC, measured by the WST-1 assay, seems not to be dependent on PGE(2) inhibition, as DMC was equally effective in unstimulated HeLa cells as well as in stimulated HeLa cells, and the addition of external PGE(2) did not reverse the anti-proliferative effect of DMC in HCA-7 cells. We conclude that DMC is not a suitable non-prostaglandin-inhibiting control substance for research purposes.