The sphingolipid anteome: implications for evolution of the sphingolipid metabolic pathway

Sphingolipid metabolism and regulated cell death in malignant melanoma

Malignant melanoma (MM) is a highly invasive and therapeutically resistant skin malignancy, posing a significant clinical challenge in its treatment. Programmed cell death plays a crucial role in the occurrence and progression of MM. Sphingolipids (SP), as a class of bioactive lipids, may be associated with many kinds of diseases. SPs regulate various forms of programmed cell death in tumors, including apoptosis, necroptosis, ferroptosis, and more. This review will delve into the mechanisms by which different types of SPs modulate various forms of programmed cell death in MM, such as their regulation of cell membrane permeability and signaling pathways, and how they influence the survival and death fate of MM cells. An in-depth exploration of the role of SPs in programmed cell death in MM aids in unraveling the molecular mechanisms of melanoma development and holds significant importance in developing novel therapeutic strategies.

How has the evolution of our understanding of the compartmentalization of sphingolipid biosynthesis over the past 30 years altered our view of the evolution of the pathway?

Sphingolipids are unique among cellular lipids inasmuch as their biosynthesis is compartmentalized between the endoplasmic reticulum (ER) and the Golgi apparatus. This compartmentalization was first recognized about thirty years ago, and the current review not only updates studies on the compartmentalization of sphingolipid biosynthesis, but also discusses the ramifications of this feature for our understanding of how the pathway could have evolved. Thus, we augment some of our recent studies by inclusion of two further molecular pathways that need to be considered when analyzing the evolutionary requirements for generation of sphingolipids, namely contact sites between the ER and the Golgi apparatus, and the mechanism(s) of vesicular transport between these two organelles. Along with evolution of the individual enzymes of the pathway, their subcellular localization, and the supply of essential metabolites via the anteome, it becomes apparent that current models to describe evolution of the sphingolipid biosynthetic pathway may need substantial refinement.

Mechanism of Fumonisin Self-Resistance: Fusarium verticillioides Contains Four Fumonisin B1-Insensitive-Ceramide Synthases

Fusarium verticillioides produces fumonisins, which are mycotoxins inhibiting sphingolipid biosynthesis in humans, animals, and other eukaryotes. Fumonisins are presumed virulence factors of plant pathogens, but may also play a role in interactions between competing fungi. We observed higher resistance to added fumonisin B1 (FB1) in fumonisin-producing Fusarium verticillioides than in nonproducing F. graminearum, and likewise between isolates of Aspergillus and Alternaria differing in production of sphinganine-analog toxins. It has been reported that in F. verticillioides, ceramide synthase encoded in the fumonisin biosynthetic gene cluster is responsible for self-resistance. We reinvestigated the role of FUM17 and FUM18 by generating a double mutant strain in a fum1 background. Nearly unchanged resistance to added FB1 was observed compared to the parental fum1 strain. A recently developed fumonisin-sensitive baker's yeast strain allowed for the testing of candidate ceramide synthases by heterologous expression. The overexpression of the yeast LAC1 gene, but not LAG1, increased fumonisin resistance. High-level resistance was conferred by FUM18, but not by FUM17. Likewise, strong resistance to FB1 was caused by overexpression of the presumed F. verticillioides "housekeeping" ceramide synthases CER1, CER2, and CER3, located outside the fumonisin cluster, indicating that F. verticillioides possesses a redundant set of insensitive targets as a self-resistance mechanism.

The Sphinx and the egg: Evolutionary enigmas of the (glyco)sphingolipid biosynthetic pathway

In eukaryotes, the de novo synthesis of sphingolipids (SLs) consists of multiple sequential steps which are compartmentalized between the endoplasmic reticulum and the Golgi apparatus. Studies over many decades have identified the enzymes in the pathway, their localization, topology and an array of regulatory mechanisms. However, little is known about the evolutionary forces that underly the generation of this complex pathway or of its anteome, i.e., the metabolic pathways that converge on the SL biosynthetic pathway and are essential for its activity. After briefly describing the pathway, we discuss the mechanisms by which the enzymes of the SL biosynthetic pathway are targeted to their different subcellular locations, how the pathway per se may have evolved, including its compartmentalization, and the relationship of the pathway to eukaryogenesis. We discuss the circular interdependence of the evolution of the SL pathway, and comment on whether current Darwinian evolutionary models are able to provide genuine mechanistic insight into how the pathway came into being.

Emerging roles and therapeutic potentials of sphingolipids in pathophysiology: emphasis on fatty acyl heterogeneity

Sphingolipids not only exert structural roles in cellular membranes, but also act as signaling molecules in various physiological and pathological processes. A myriad of studies have shown that abnormal levels of sphingolipids and their metabolic enzymes are associated with a variety of human diseases. Moreover, blood sphingolipids can also be used as biomarkers for disease diagnosis. This review summarizes the biosynthesis, metabolism, and pathological roles of sphingolipids, with emphasis on the biosynthesis of ceramide, the precursor for the biosynthesis of complex sphingolipids with different fatty acyl chains. The possibility of using sphingolipids for disease prediction, diagnosis, and treatment is also discussed. Targeting endogenous ceramides and complex sphingolipids along with their specific fatty acyl chain to promote future drug development will also be discussed.

Fanning the Flames of Endoplasmic Reticulum (ER) Stress: Can Sphingolipid Metabolism Be Targeted to Enhance ER Stress-Associated Immunogenic Cell Death in Cancer?

The three arms of the unfolded protein response (UPR) surveil the luminal environment of the endoplasmic reticulum (ER) and transmit information through the lipid bilayer to the cytoplasm to alert the cell of stress conditions within the ER lumen. That same lipid bilayer is the site of de novo synthesis of phospholipids and sphingolipids. Thus, it is no surprise that lipids are modulated by and are modulators of ER stress. Given that sphingolipids have both prosurvival and proapoptotic effects, they also exert opposing effects on life/death decisions in the face of prolonged ER stress detected by the UPR. In this review, we will focus on several recent studies that demonstrate how sphingolipids affect each arm of the UPR. We will also discuss the role of sphingolipids in the process of immunogenic cell death downstream of the protein kinase RNA-like endoplasmic reticulum kinase (PERK)/eukaryotic initiating factor 2α (eIF2α) arm of the UPR. Furthermore, we will discuss strategies to target the sphingolipid metabolic pathway that could potentially act synergistically with agents that induce ER stress as novel anticancer treatments. SIGNIFICANCE STATEMENT: This review provides the readers with a brief discussion of the sphingolipid metabolic pathway and the unfolded protein response. The primary focus of the review is the mechanism(s) by which sphingolipids modulate the endoplasmic reticulum (ER) stress response pathways and the critical role of sphingolipids in the process of immunogenic cell death associated with the ER stress response.

Mysterious sphingolipids: metabolic interrelationships at the center of pathophysiology

Metabolic pathways are complex and intertwined. Deficiencies in one or more enzymes in a given pathway are directly linked with genetic diseases, most of them having devastating manifestations. The metabolic pathways undertaken by sphingolipids are diverse and elaborate with ceramide species serving as the hubs of sphingolipid intermediary metabolism and function. Sphingolipids are bioactive lipids that serve a multitude of cellular functions. Being pleiotropic in function, deficiency or overproduction of certain sphingolipids is associated with many genetic and chronic diseases. In this up-to-date review article, we strive to gather recent scientific evidence about sphingolipid metabolism, its enzymes, and regulation. We shed light on the importance of sphingolipid metabolism in a variety of genetic diseases and in nervous and immune system ailments. This is a comprehensive review of the state of the field of sphingolipid biochemistry.

Toxoplasma ceramide synthases: Gene duplication, functional divergence, and roles in parasite fitness

Toxoplasma gondii is an obligate, intracellular apicomplexan protozoan parasite of both humans and animals that can cause fetal damage and abortion and severe disease in the immunosuppressed. Sphingolipids have indispensable functions as signaling molecules and are essential and ubiquitous components of eukaryotic membranes that are both synthesized and scavenged by the Apicomplexa. Ceramide is the precursor for all sphingolipids, and here we report the identification, localization and analyses of the Toxoplasma ceramide synthases TgCerS1 and TgCerS2. Interestingly, we observed that while TgCerS1 was a fully functional orthologue of the yeast ceramide synthase (Lag1p) capable of catalyzing the conversion of sphinganine to ceramide, in contrast TgCerS2 was catalytically inactive. Furthermore, genomic deletion of TgCerS1 using CRISPR/Cas-9 led to viable but slow-growing parasites indicating its importance but not indispensability. In contrast, genomic knock out of TgCerS2 was only accessible utilizing the rapamycin-inducible Cre recombinase system. Surprisingly, the results demonstrated that this "pseudo" ceramide synthase, TgCerS2, has a considerably greater role in parasite fitness than its catalytically active orthologue (TgCerS1). Phylogenetic analyses indicated that, as in humans and plants, the ceramide synthase isoforms found in Toxoplasma and other Apicomplexa may have arisen through gene duplication. However, in the Apicomplexa the duplicated copy is hypothesized to have subsequently evolved into a non-functional "pseudo" ceramide synthase. This arrangement is unique to the Apicomplexa and further illustrates the unusual biology that characterize these protozoan parasites.