Ceramide generated by sphingomyelin hydrolysis and the salvage pathway is involved in hypoxia/reoxygenation-induced Bax redistribution to mitochondria in NT-2 cells

Mammalian ceramide synthases

In mammals, ceramide, a key intermediate in sphingolipid metabolism and an important signaling molecule, is synthesized by a family of six ceramide synthases (CerS), each of which synthesizes ceramides with distinct acyl chain lengths. There are a number of common biochemical features between the CerS, such as their catalytic mechanism, and their structure and intracellular localization. Different CerS also display remarkable differences in their biological properties, with each of them playing distinct roles in processes as diverse as cancer and tumor suppression, in the response to chemotherapeutic drugs, in apoptosis, and in neurodegenerative diseases.

Characteristics of the rat cardiac sphingolipid pool in two mitochondrial subpopulations

Mitochondrial sphingolipids play a diverse role in normal cardiac function and diseases, yet a precise quantification of cardiac mitochondrial sphingolipids has never been performed. Therefore, rat heart interfibrillary mitochondria (IFM) and subsarcolemmal mitochondria (SSM) were isolated, lipids extracted, and sphingolipids quantified by LC-tandem mass spectrometry. Results showed that sphingomyelin (approximately 10,000 pmol/mg protein) was the predominant sphingolipid regardless of mitochondrial subpopulation, and measurable amounts of ceramide (approximately 70 pmol/mg protein) sphingosine, and sphinganine were also found in IFM and SSM. Both mitochondrial populations contained similar quantities of sphingolipids except for ceramide which was much higher in SSM. Analysis of sphingolipid isoforms revealed ten different sphingomyelins and six ceramides that differed from 16- to 24-carbon units in their acyl side chains. Sub-fractionation experiments further showed that sphingolipids are a constituent part of the inner mitochondrial membrane. Furthermore, inner membrane ceramide levels were 32% lower versus whole mitochondria (45 pmol/mg protein). Three ceramide isotypes (C20-, C22-, and C24-ceramide) accounted for the lower amounts. The concentrations of the ceramides present in the inner membranes of SSM and IFM differed greatly. Overall, mitochondrial sphingolipid content reflected levels seen in cardiac tissue, but the specific ceramide distribution distinguished IFM and SSM from each other.

Neurons and oligodendrocytes recycle sphingosine 1-phosphate to ceramide: significance for apoptosis and multiple sclerosis

Both cultured neonatal rat hippocampal neurons and differentiated oligodendrocytes rapidly metabolized exogenous C(2)- and C(6)-ceramides to sphingosine (Sph) and sphingosine 1-phosphate (S1P) but only minimally to C(16-24)-ceramides. Dihydrosphinolipids were unaffected but were increased by exogenous C(6)-dihydroceramide. Conversely, quantitative liquid chromatography-tandem mass spectrometry technology showed that exogenous S1P (0.25-10 microm) was rapidly metabolized to both Sph (a >200-fold increase) and predominantly C(18)-ceramide (a >2-fold increase). Longer treatments with either C(2)-ceramide (>2.5 microm) or S1P (10 microm) led to apoptotic cell death. Thus, there is an active sphingolipid salvage pathway in both neurons and oligodendrocytes. Staurosporine-induced cell death was shown to be associated with decreased S1P and increased Sph and C(16/18)-ceramide levels. The physiological significance of this observation was confirmed by the analysis of affected white matter and plaques from brains of multiple sclerosis patients in which reduced S1P and increased Sph and C(16/18)-ceramides were observed.

17-allylamino-17-demethoxygeldanamycin and MEK1/2 inhibitors kill GI tumor cells via Ca2+-dependent suppression of GRP78/BiP and induction of ceramide and reactive oxygen species

The present studies determine in greater detail the molecular mechanisms upstream of the CD95 death receptor by which geldanamycin heat shock protein 90 inhibitors and mitogen-activated protein kinase/extracellular signal-regulated kinase kinase 1/2 (MEK1/2) inhibitors interact to kill carcinoma cells. MEK1/2 inhibition enhanced 17-allylamino-17-demethoxygeldanamycin (17AAG) toxicity that was suppressed in cells deleted for mutant active RAS that were nontumorigenic but was magnified in isogenic tumorigenic cells expressing Harvey RAS V12 or Kirsten RAS D13. MEK1/2 inhibitor and 17AAG treatment increased intracellular Ca(2+) levels and reduced GRP78/BiP expression in a Ca(2+)-dependent manner. GRP78/BiP overexpression, however, also suppressed drug-induced intracellular Ca(2+) levels. MEK1/2 inhibitor and 17AAG treatment increased reactive oxygen species (ROS) levels that were blocked by quenching Ca(2+) or overexpression of GRP78/BiP. MEK1/2 inhibitor and 17AAG treatment activated CD95 and inhibition of ceramide synthesis; ROS or Ca(2+) quenching blocked CD95 activation. In SW620 cells that are patient matched to SW480 cells, MEK1/2 inhibitor and 17AAG toxicity was significantly reduced, which correlated with a lack of CD95 activation and lower expression of ceramide synthase 6 (LASS6). Overexpression of LASS6 in SW620 cells enhanced drug-induced CD95 activation and enhanced tumor cell killing. Inhibition of ceramide signaling abolished drug-induced ROS generation but not drug-induced cytosolic Ca(2+) levels. Thus, treatment of tumor cells with MEK1/2 inhibitor and 17AAG induces cytosolic Ca(2+) and loss of GRP78/BiP function, leading to de novo ceramide synthesis pathway activation that plays a key role in ROS generation and CD95 activation.

A critical role for ceramide synthase 2 in liver homeostasis: II. insights into molecular changes leading to hepatopathy

We have generated a mouse that cannot synthesize very long acyl chain (C22-C24) ceramides (Pewzner-Jung, Y., Park, H., Laviad, E. L., Silva, L. C., Lahiri, S., Stiban, J., Erez-Roman, R., Brugger, B., Sachsenheimer, T., Wieland, F. T., Prieto, M., Merrill, A. H., and Futerman, A. H. (2010) J. Biol. Chem. 285, 10902-10910) due to ablation of ceramide synthase 2 (CerS2). As a result, significant changes were observed in the sphingolipid profile of livers from these mice, including elevated C16-ceramide and sphinganine levels. We now examine the functional consequences of these changes. CerS2 null mice develop severe nonzonal hepatopathy from about 30 days of age, the age at which CerS2 expression peaks in wild type mice, and display increased rates of hepatocyte apoptosis and proliferation. In older mice there is extensive and pronounced hepatocellular anisocytosis with widespread formation of nodules of regenerative hepatocellular hyperplasia. Progressive hepatomegaly and noninvasive hepatocellular carcinoma are also seen from approximately 10 months of age. Even though CerS2 is found at equally high mRNA levels in kidney and liver, there are no changes in renal function and no pathological changes in the kidney. High throughput analysis of RNA expression in liver revealed up-regulation of genes associated with cell cycle regulation, protein transport, cell-cell interactions and apoptosis, and down-regulation of genes associated with intermediary metabolism, such as lipid and steroid metabolism, adipocyte signaling, and amino acid metabolism. In addition, levels of the cell cycle regulator, the cyclin dependent-kinase inhibitor p21(WAF1/CIP1), were highly elevated, which occurs by at least two mechanisms, one of which may involve p53. We propose a functional rationale for the synthesis of sphingolipids with very long acyl chains in liver homeostasis and in cell physiology.

The BCL-2 protein BAK is required for long-chain ceramide generation during apoptosis

The BCL-2 family members BAK and BAX are required for apoptosis and trigger mitochondrial outer membrane permeabilization (MOMP). Here we identify a MOMP-independent function of BAK as a required factor for long-chain ceramide production in response to pro-apoptotic stress. UV-C irradiation of wild-type (WT) cells increased long-chain ceramides; blocking ceramide generation prevented caspase activation and cell death, demonstrating that long-chain ceramides play a key role in UV-C-induced apoptosis. In contrast, UV-C irradiation did not increase long-chain ceramides in BAK and BAX double knock-out cells. Notably, this was not specific to the cell type (baby mouse kidney cells, hematopoietic) nor the apoptotic stimulus employed (UV-C, cisplatin, and growth factor withdrawal). Importantly, long-chain ceramide generation was dependent on the presence of BAK, but not BAX. However, ceramide generation was independent of the known downstream actions of BAK in apoptosis (MOMP or caspase activation), suggesting a novel role for BAK in apoptosis. Finally, enzymatic assays identified ceramide synthase as the mechanism by which BAK regulates ceramide metabolism. There was no change in CerS expression at the message or protein level, indicating regulation at the post-translational level. Moreover, CerS activity in BAK KO microsomes can be reactivated upon addition of BAK-containing microsomes. The data presented indicate that ceramide-induced apoptosis is dependent upon BAK and identify a novel role for BAK during apoptosis. By establishing a unique role for BAK in long-chain ceramide metabolism, these studies further demonstrate that the seemingly redundant proteins BAK and BAX have distinct mechanisms of action during apoptosis induction.

Regulation of ceramide channels by Bcl-2 family proteins

Mitochondrial outer membrane permeabilization to proteins, an irreversible step in apoptosis by which critical proteins are released, is tightly regulated by Bcl-2 family proteins. The exact nature of the release pathway is still undefined. Ceramide is an important sphingolipid, involved in various cellular processes including apoptosis. Here we describe the structural properties of ceramide channels and their regulation by the anti-apoptotic and pro-apoptotic proteins of the Bcl-2 family. The evolutionarily conserved regulation of ceramide channels by Bcl-2 family proteins, consistent with their role in apoptosis, lends credibility to the notion that ceramide channels constitute the protein release pathway.

Ceramide synthases: roles in cell physiology and signaling

Ceramide synthases (CerS) are integral membrane proteins of the endoplasmic reticulum. Six mammalian CerS have been described, with each utilizing fatty acyl CoAs of relatively defined chain lengths for N-acylation of the sphingoid long chain base. In this chapter, we review the main functional features of the CerS proteins, discuss their fatty acid specificity, kinetics, tissue distribution and mode of inhibition, as well as possible posttranslational modifications. We then address the reason that mammals contain six distinct CerS, whereas most other enzymes in the sphingolipid biosynthetic pathway only occur in one or two isoforms. Finally, we discuss the putative roles of CerS and the ceramide derived from the CerS, in signaling pathways and in development of disease.

Pharmacologic inhibition of fatty acid oxidation sensitizes human leukemia cells to apoptosis induction

The traditional view is that cancer cells predominately produce ATP by glycolysis, rather than by oxidation of energy-providing substrates. Mitochondrial uncoupling--the continuing reduction of oxygen without ATP synthesis--has recently been shown in leukemia cells to circumvent the ability of oxygen to inhibit glycolysis, and may promote the metabolic preference for glycolysis by shifting from pyruvate oxidation to fatty acid oxidation (FAO). Here we have demonstrated that pharmacologic inhibition of FAO with etomoxir or ranolazine inhibited proliferation and sensitized human leukemia cells--cultured alone or on bone marrow stromal cells--to apoptosis induction by ABT-737, a molecule that releases proapoptotic Bcl-2 proteins such as Bak from antiapoptotic family members. Likewise, treatment with the fatty acid synthase/lipolysis inhibitor orlistat also sensitized leukemia cells to ABT-737, which supports the notion that fatty acids promote cell survival. Mechanistically, we generated evidence suggesting that FAO regulates the activity of Bak-dependent mitochondrial permeability transition. Importantly, etomoxir decreased the number of quiescent leukemia progenitor cells in approximately 50% of primary human acute myeloid leukemia samples and, when combined with either ABT-737 or cytosine arabinoside, provided substantial therapeutic benefit in a murine model of leukemia. The results support the concept of FAO inhibitors as a therapeutic strategy in hematological malignancies.

Bax distribution into mitochondrial detergent-resistant microdomains is related to ceramide and cholesterol content in postischemic hearts

*Diverse changes have been described in mitochondria of apoptotic cells: the phospholipid content is modified, ceramide and GD3 concentrations increase, the cristae structure is modified, and nonresident proteins are recruited into the mitochondrial membranes. In particular, Bax, a Bcl-2 family member protein, moves from the cytosol to the mitochondria, inducing cytochrome c release. Modifications of the content and distribution of specific lipids in the mitochondrial membranes, along with the well-known participation of the mitochondrial permeability transition pore in triggering apoptosis, led us to propose that lipid microdomains in mitochondria could coexist as structural elements with some of the mitochondrial permeability transition pore-forming proteins and with members of the Bcl-2 family. In this work, we demonstrated that Bax was associated preferentially with mitochondrial detergent-resistant membranes (mDRMs) in reperfused rat hearts, a well-known apoptotic model. Bax insertion into mDRMs correlated with cytochrome c release from such mitochondria. Bax location in mDRMs was associated with both the voltage-dependent anion channel and the adenine nucleotide translocator, two mitochondrial permeability transition pore-forming proteins. Interestingly, the voltage-dependent anion channel was more abundant in the mDRM fraction than in the Triton X-100-soluble fraction. Ceramide and cholesterol contents were higher in mDRMs from reperfused hearts. Our results suggest that membrane microenvironments enriched in cholesterol and ceramide in mitochondria favor Bax translocation to this organelle, fostering propagation of the apoptotic cascade.

Ceramide and mitochondria in ischemia/reperfusion

A hallmark of tissue injury in various models of ischemia/reperfusion (IR) is mitochondrial dysfunction and the release of mitochondrial proapoptotic proteins leading to cell death. Although IR-induced mitochondrial injury has been extensively studied and key mitochondrial functions affected by IR are chiefly characterized, the nature of the molecule that causes loss of mitochondrial integrity and function remains obscure. It has become increasingly clear that ceramide, a membrane sphingolipid and a key mediator of cell stress responses, could play a critical role in IR-induced mitochondrial damage. Emerging data point to excessive ceramide accumulation in tissue and, specifically, in mitochondria after IR. Exogenously added to isolated mitochondria, ceramide could mimic some of the mitochondrial dysfunctions occurring in IR. The recent identification and characterization of major enzymes in ceramide synthesis is expected to contribute to the understanding of molecular mechanisms of ceramide involvement in mitochondrial damage in IR. This review will examine the experimental evidence supporting the important role of ceramide in mitochondrial dysfunction in IR to highlight potential targets for pharmacological manipulation of ceramide levels.

AMPK inhibitor Compound C stimulates ceramide production and promotes Bax redistribution and apoptosis in MCF7 breast carcinoma cells

Compound C is commonly used as an inhibitor of AMP-activated protein kinase (AMPK), which serves as a key energy sensor in cells. In this study, we found that Compound C treatment of MCF7 cells led to Bax redistribution from the cytoplasm to mitochondria and cell death. However, this effect does not involve AMPK. In addition, we found that treatment with this compound leads to an enhanced ceramide production. Analyses by quantitative PCR and ceramide synthase activity assay suggest that ceramide synthase 5 (LASS/CerS 5) is involved in Compound C-induced ceramide upregulation. Downregulation of LASS/CerS 5 was found to attenuate Compound C-mediated ceramide production, Bax redistribution, and cell death.

Roles and regulation of secretory and lysosomal acid sphingomyelinase

Acid sphingomyelinase occupies a prominent position in sphingolipid catabolism, catalyzing the hydrolysis of sphingomyelin to ceramide and phosphorylcholine. Enzymatic dysfunction of acid sphingomyelinase results in Niemann-Pick disease, a lysosomal storage disorder characterized at the cellular level by accumulation of sphingomyelin within the endo-lysosomal compartment. Over the past decade interest in the role of acid sphingomyelinase has moved beyond its "housekeeping" function in constitutive turnover of sphingomyelin in the lysosome to include study of regulated ceramide generation. Ceramide functions as a bioactive sphingolipid with pleiotropic signaling properties, and has been implicated in diverse cellular processes of physiologic and pathophysiologic importance. Though many cellular enzymes have the capacity to generate ceramide,there is growing appreciation that "all ceramides are not created equal." Ceramides likely exert distinct effects in different cellular/subcellular compartments by virtue of access to other sphingolipid enzymes (e.g.ceramidases), effector molecules (e.g. ceramide-activated protein phosphatases), and neighboring lipids and proteins (e.g. cholesterol, ion channels). One of the unique features of acid sphingomyelinase is that it has been implicated in the hydrolysis of sphingomyelin in three different settings--the endo-lysosomal compartment,the outer leaflet of the plasma membrane, and lipoproteins. How a single gene product has the capacity to function in these diverse settings, and the subsequent impact on downstream ceramide-mediated biology is the subject of this review.