Transcriptional regulation of the human neutral ceramidase gene

Measurement of neutral ceramidase activity in vitro and in vivo

Neutral ceramidase is a hydrolase of ceramide that has been implicated in multiple biologic processes, including inflammation and oncogenesis. Ceramides and other sphingolipids, belong to a family of N-acyl linked lipids that are biologically active in signaling, despite their limited structural functions. Ceramides are generally pro-apoptotic, while sphingosine and sphingosine-1-phosphate (S1P) exert proliferative and pro-oncogenic effects. Ceramidases are important regulators of ceramide levels that hydrolyze ceramide to sphingosine. Thus, ceramidase inhibition significantly increases the quantities of ceramide and its associated signaling. To better understand the function of ceramide, biochemical and cellular assays for enzymatic activity were developed and validated to identify inhibitors of human neutral ceramidase (nCDase). Here we review the measurement of nCDase activity both in vitro and in vivo.

Role of Ceramidases in Sphingolipid Metabolism and Human Diseases

Human pathologies such as Alzheimer’s disease, type 2 diabetes-induced insulin resistance, cancer, and cardiovascular diseases have altered lipid homeostasis. Among these imbalanced lipids, the bioactive sphingolipids ceramide and sphingosine-1 phosphate (S1P) are pivotal in the pathophysiology of these diseases. Several enzymes within the sphingolipid pathway contribute to the homeostasis of ceramide and S1P. Ceramidase is key in the degradation of ceramide into sphingosine and free fatty acids. In humans, five different ceramidases are known—acid ceramidase, neutral ceramidase, and alkaline ceramidase 1, 2, and 3—which are encoded by five different genes (ASAH1, ASAH2, ACER1, ACER2, and ACER3, respectively). Notably, the neutral ceramidase N-acylsphingosine amidohydrolase 2 (ASAH2) shows considerable differences between humans and animals in terms of tissue expression levels. Besides, the subcellular localization of ASAH2 remains controversial. In this review, we sum up the results obtained for identifying gene divergence, structure, subcellular localization, and manipulating factors and address the role of ASAH2 along with other ceramidases in human diseases.

Functions of neutral ceramidase in the Golgi apparatus

Ceramidases hydrolyze ceramides into sphingosine and fatty acids, with sphingosine being further metabolized into sphingosine-1-phosphate (S1P); thus, ceramidases control the levels of these bioactive sphingolipids in cells and tissues. Neutral ceramidase (nCDase) is highly expressed in colorectal tissues, and a recent report showed that nCDase activity is involved in Wnt/β-catenin signaling. In addition, the inhibition of nCDase decreases the development and progression of colorectal tumor growth. Here, to determine the action of nCDase in colorectal cancer cells, we focused on the subcellular localization and metabolic functions of this enzyme in HCT116 cells. nCDase was found to be located in both the plasma membrane and in the Golgi apparatus, but it had minimal effects on basal levels of ceramide, sphingosine, or S1P. Cells overexpressing nCDase were protected from the cell death and Golgi fragmentation induced by C6-ceramide, and they showed reduced levels of C6-ceramide and higher levels of S1P and sphingosine. Furthermore, compartment-specific metabolic functions of the enzyme were probed using C6-ceramide and Golgi-targeted bacterial SMase (bSMase) and bacterial ceramidase (bCDase). The results showed that Golgi-specific bCDase also demonstrated resistance against the cell death stimulated by C6-ceramide, and it catalyzed the metabolism of ceramides and produced sphingosine in the Golgi. Targeting bSMase to the Golgi resulted in increased levels of ceramide that were attenuated by the expression of nCDase, also supporting its ability to metabolize Golgi-generated ceramide. These results are critical in understanding the functions of nCDase actions in colorectal cancer cells as well as the compartmentalized pathways of sphingolipid metabolism.

The Arabidopsis ceramidase AtACER functions in disease resistance and salt tolerance

Ceramidases hydrolyze ceramide into sphingosine and fatty acids. In mammals, ceramidases function as key regulators of sphingolipid homeostasis, but little is known about their roles in plants. Here we characterize the Arabidopsis ceramidase AtACER, a homolog of human alkaline ceramidases. The acer-1 T-DNA insertion mutant has pleiotropic phenotypes, including reduction of leaf size, dwarfing and an irregular wax layer, compared with wild-type plants. Quantitative sphingolipid profiling showed that acer-1 mutants and the artificial microRNA-mediated silenced line amiR-ACER-1 have high ceramide levels and decreased long chain bases. AtACER localizes predominantly to the endoplasmic reticulum, and partially to the Golgi complex. Furthermore, we found that acer-1 mutants and AtACER RNAi lines showed increased sensitivity to salt stress, and lines overexpressing AtACER showed increased tolerance to salt stress. Reduction of AtACER also increased plant susceptibility to Pseudomonas syringae. Our data highlight the key biological functions of ceramidases in biotic and abiotic stresses in plants.

New insight into the structure, reaction mechanism, and biological functions of neutral ceramidase

Ceramidase (CDase) is an enzyme that hydrolyzes the N-acyl linkage between the sphingoid base and fatty acid of ceramide. These enzymes are classified into three distinct groups, acid (Asah1), neutral (Asah2), and alkaline (Asah3) CDases, based on their primary structure and optimum pH. Acid CDase catabolizes ceramide in lysosomes and is found only in vertebrates. In contrast, the distribution of neutral and alkaline CDases is broad, with both being found in species ranging from lower eukaryotes to mammals; however, only neutral CDase is found in prokaryotes, including some pathogenic bacteria. Neutral CDase is thought to have gained a specific domain (mucin box) in the N-terminal region after the vertebrate split, allowing the enzyme to be stably expressed at the plasma membrane as a type II membrane protein. The X-ray crystal structure of neutral CDase was recently solved, uncovering a unique structure and reaction mechanism for the enzyme. Neutral CDase contains a zinc ion in the active site that functions as a catalytic center, and the hydrolysis of the N-acyl linkage in ceramide proceeds through a mechanism that is similar to that described for zinc-dependent carboxypeptidase. This review describes the structure, reaction mechanism, and biological functions of neutral CDase in association with the molecular evolution, topology, and mechanical conformation. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

A novel 5'-uncoding region -1248 A>G variation of mitofusin-2 gene is associated with hypertension in Chinese

PURPOSE

Mitofusin2 gene (Mfn2, also named Hyperplasia suppressive gene, HSG) is very important in the origin and development of hypertension. However, the mechanism of Mfn2/HSG expression regulation was not uncovered. This study was designed to explore the association of a novel 5'-uncoding region (UCR) -1248 A>G variation of HSG/Mfn2 gene and hypertension.

MATERIALS AND METHODS

472 healthy, normotensive subjects [normotension (NT) group], 454 prehypertensive subjects [prehypertension (PH) group] and 978 hypertensive patients [essential hypertension (EH) group] were screened for an association study between 5'-UCR -1248 A>G of Mfn2/HSG and hypertension by polymerase chain reaction and DNA sequencing after venous blood was drawn and DNA was extracted.

RESULTS

When comparing the A and G frequency in EH, PH and NT groups, in total, NT group significantly had higher A frequency than in PH group [odds ratio (OR)=1.605, confidence interval (CI) 95%=1.063-2.242, p=0.025] and EH group (OR=5.395, CI 95%=3.783-7.695, p<0.01). When subgrouped by gender, A frequency in NT group was still significantly higher than in EH group (male: OR= 4.264, CI 95%=2.780-6.543, p<0.01; female: OR=8.897, CI 95%=4.686-16.891, p<0.01), but not from PH group, either in male group or in female group. Ordinal Logistic Regression analysis showed that A>G variation was significantly related with blood pressure level (B=-1.271, Wald=40.914, CI 95%=-1.660 - -0.881, p<0.01).

CONCLUSION

5'-UCR -1248 A>G variation of Mfn2/HSG gene was a novel variation and may be associated with hypertension in Chinese.

AP-1 binding transcriptionally regulates human neutral ceramidase

Many forms of cellular stress cause an elevation of endogenous ceramide levels leading to growth arrest or apoptosis. Ceramidases (CDase) play a critical role in regulating apoptosis by hydrolyzing ceramide into sphingosine, a precursor for promitogenic sphingosine-1-phosphate. Growth factor induction of neutral CDase (nCDase) has been shown to have a cytoprotective effect against cytokine-induced increases in ceramide levels. To further define the physiological regulation of nCDase, we identified a 200 bp promoter region and demonstrated that serum activated this proximal promoter, which correlated with a serum-induced increase in human nCDase mRNA expression. Computational analysis revealed a putative cis-element for AP-1, a transcription factor activated by serum. Electrophoretic mobility shift assays demonstrated that the identified transcriptional response element binds to AP-1 transcription factors. RNA interference-mediated knockdown of the AP-1 subunit, c-Jun, inhibited the activity of the human nCDase proximal promoter, whereas, c-Jun overexpression increased promoter activity, which directly correlated with human nCDase mRNA transcription, decreased ceramide mass, and protection against caspase 3/7-dependent apoptosis. Taken together, our findings suggest that c-Jun/AP-1 signaling may, in part, regulate serum-induced human nCDase gene transcription.