Inhibition of skin carcinomas but not papillomas by sphingosine, N-methylsphingosine, and N-acetylsphingosine

Sphingosine metabolism as a therapeutic target in cutaneous melanoma

Melanoma is by far the most aggressive type of skin cancer with a poor prognosis in its advanced stages. Understanding the mechanisms involved in melanoma pathogenesis, response, and resistance to treatment has gained a lot of attention worldwide. Recently, the role of sphingolipid metabolism has been studied in cutaneous melanoma. Sphingolipids are bioactive lipid effector molecules involved in the regulation of various cellular signaling pathways such as inflammation, cancer cell proliferation, death, senescence, and metastasis. Recent studies suggest that sphingolipid metabolism impacts melanoma pathogenesis and is a potential therapeutic target. This review focuses on defining the role of sphingolipid metabolism in melanoma carcinogenesis, discussing sphingolipid-based therapeutic approaches, and highlighting the areas that require more extensive research.

NRF2, cancer and calorie restriction

The transcription factor NF-E2-related factor (NRF2) is a key regulator of several enzymatic pathways, including cytoprotective enzymes in highly metabolic organs. In this review, we summarize the ongoing research related to NRF2 activity in cancer development, focusing on in vivo studies using NRF2 knockout (KO) mice, which have helped in defining the crucial role of NRF2 in chemoprevention. The lower cancer protection observed in NRF2 KO mice under calorie restriction (CR) suggests that most of the beneficial effects of CR on the carcinogenesis process are likely mediated by NRF2. We propose that future interventions in cancer treatment would be carried out through the activation of NRF2 in somatic cells, which will lead to a delay or prevention of the onset of some forms of human cancers, and subsequently an extension of health- and lifespan.

Sphingosine-1-phosphate signaling and the skin

Sphingolipids have long been viewed as rather passive structural components of cellular membranes. More recently, it has become evident that metabolism of sphingomyelin yields several lipid mediators that evoke diverse and specific responses in different cell types. One sphingomyelin derivate, sphingosine-1-phosphate (S1P), has attracted particular attention for its effect on epidermal cells, which differs from those on most other cell types. S1P inhibits keratinocyte proliferation and induces keratinocyte differentiation and migration, suggesting a role for S1P in the re-epithelialization of wounds. The migratory response involves the phosphorylation and activation of Smad3. In epithelial tumors, S1P signaling has been linked with potential oncogenic effects, but has also been found to inhibit metastasis in a mouse melanoma model. S1P promotes endothelial cell survival, acts as a chemoattractant for vascular cells, and exerts a protective effect on the endothelial barrier. Conversely, S1P receptor knockout leads to embryonic lethality mainly due to impaired vascular maturation. S1P presumably modulates peripheral T-lymphocyte levels by stimulating their egress from lymphoid organs rather than by promoting T-cell proliferation. The S1P analog FTY720 (fingolimod) acts as a functional antagonist by inhibiting lymphocyte egress, and thus holds great promise as an immunosuppressant drug for the prevention of allograft rejection and treatment of T-lymphocyte-driven inflammatory skin diseases, such as lupus erythematosus, psoriasis, and atopic dermatitis. Topical use of S1P and other sphingosine compounds is also under investigation, particularly for the treatment of acne vulgaris.

Sphingomyelin in the suppression of colon tumors: prevention versus intervention

Intestinal cells are regularly exposed to sphingolipid metabolites, i.e., ceramide and sphingoid bases, after hydrolysis of complex sphingolipids from the diet. These metabolites are known regulators of cell growth, differentiation, and death. Non-pharmacological amounts in the diet have been shown to inhibit early stages of chemically induced colon cancer in mice. To distinguish between chemopreventive and chemotherapeutic effects of sphingomyelin supplements, mice were fed sphingomyelin before and after tumor initiation. Both applications drastically reduced tumor formation, without a significant difference among the groups, indicating that sphingolipids are as effective in the chemoprevention of tumors as in early intervention. The normalization of cell proliferation and rate of apoptosis, but not the induction of differentiation, seem to be key players in the suppression of tumor formation by dietary sphingomyelin. This may have implications for the development of a cancer prevention or treatment strategy with sphingolipids as an alternative to conventional drugs.

Cloning and characterization of a novel human alkaline ceramidase. A mammalian enzyme that hydrolyzes phytoceramide

Ceramidases are enzymes involved in regulating cellular levels of ceramides, sphingoid bases, and their phosphates. Based on sequence homology to the yeast alkaline ceramidases YPC1p (Mao, C., Xu, R., Bielawska, A., and Obeid, L. M. (2000) J. Biol. Chem. 275, 6876--6884) and YDC1p (Mao, C., Xu, R., Bielawska, A., Szulc, Z. M., and Obeid, L. M. (2000) J. Biol Chem. 275, 31369--31378), we report the identification and cloning of a cDNA encoding for a novel human alkaline ceramidase (aPHC) that hydrolyzes phytoceramide selectively. Northern blot analysis showed that aPHC was ubiquitously expressed, with the highest expression in placenta. Green fluorescent protein tagging showed that it was localized in both the Golgi apparatus and endoplasmic reticulum. Overexpression of aPHC in mammalian cells elevated in vitro ceramidase activity toward N-4-nitrobenz-2-oxa-1,3-diazole-C(12)-phytoceramide. Its expression in a yeast mutant strain devoid of any ceramidase activity restored the ceramidase activity and caused an increase in the hydrolysis of phytoceramide in yeast cells, thus leading to the decreased biosynthesis of sphingolipids. These data collectively suggest that, similar to the yeast phytoceramidase YPC1p, aPHC has phytoceramidase activity both in vitro and in cells; hence, it is a functional homolog of the yeast phytoceramidase YPC1p. However, in contrast to YPC1p, aPHC exhibited no reverse activity of ceramidase either in vitro or in cells. Biochemical characterization showed that aPHC had a pH optimum of 9.5, was activated by Ca(2+), but was inhibited by Zn(2+) and sphingosine. Substrate specificity showed that aPHC hydrolyzed phytoceramide preferentially. Together, these data demonstrate that aPHC is a novel human alkaline phytoceramidase, the first mammalian alkaline ceramidase to be identified as being specific for the hydrolysis of phytoceramide.

Sphingolipids in food and the emerging importance of sphingolipids to nutrition

Eukaryotic organisms as well as some prokaryotes and viruses contain sphingolipids, which are defined by a common structural feature, i.e. , a "sphingoid base" backbone such as D-erythro-1,3-dihydroxy, 2-aminooctadec-4-ene (sphingosine). The sphingolipids of mammalian tissues, lipoproteins, and milk include ceramides, sphingomyelins, cerebrosides, gangliosides and sulfatides; plants, fungi and yeast have mainly cerebrosides and phosphoinositides. The total amounts of sphingolipids in food vary considerably, from a few micromoles per kilogram (fruits) to several millimoles per kilogram in rich sources such as dairy products, eggs and soybeans. With the use of the limited data available, per capita sphingolipid consumption in the United States can be estimated to be on the order of 150-180 mmol (approximately 115-140 g) per year, or 0.3-0.4 g/d. There is no known nutritional requirement for sphingolipids; nonetheless, they are hydrolyzed throughout the gastrointestinal tract to the same categories of metabolites (ceramides and sphingoid bases) that are used by cells to regulate growth, differentiation, apoptosis and other cellular functions. Studies with experimental animals have shown that feeding sphingolipids inhibits colon carcinogenesis, reduces serum LDL cholesterol and elevates HDL, suggesting that sphingolipids represent a "functional" constituent of food. Sphingolipid metabolism can also be modified by constituents of the diet, such as cholesterol, fatty acids and mycotoxins (fumonisins), with consequences for cell regulation and disease. Additional associations among diet, sphingolipids and health are certain to emerge as more is learned about these compounds.