Sphingosine kinase: a mediator of vital cellular functions

Ample evidence indicates that sphingosine-1-phosphate (SPP) can serve as an intracellular second messenger regulating calcium mobilization, and cell growth and survival. Moreover, the dynamic balance between levels of the sphingolipids metabolites, ceramide and SPP, and consequent regulation of opposing signaling pathways, is an important factor that determines whether a cell survives or dies. SPP has also recently been shown to be the ligand for the EDG-1 family of G protein-coupled receptors, which now includes EDG-1, -3, -5, -6, and -8. SPP is thus a lipid mediator that has novel dual actions signaling inside and outside of the cell. This review is focussed on sphingosine kinase, the enzyme that regulates levels of SPP and thus plays a critical role in diverse biological processes.     

Sphingosine kinase: a mediator of vital cellular functions

Ample evidence indicates that sphingosine-1-phosphate (SPP) can serve as an intracellular second messenger regulating calcium mobilization, and cell growth and survival. Moreover, the dynamic balance between levels of the sphingolipids metabolites, ceramide and SPP, and consequent regulation of opposing signaling pathways, is an important factor that determines whether a cell survives or dies. SPP has also recently been shown to be the ligand for the EDG-1 family of G protein-coupled receptors, which now includes EDG-1, -3, -5, -6, and -8. SPP is thus a lipid mediator that has novel dual actions signaling inside and outside of the cell. This review is focussed on sphingosine kinase, the enzyme that regulates levels of SPP and thus plays a critical role in diverse biological processes.     

Sphingosine kinase: Role in regulation of bioactive sphingolipid mediators in inflammation

Sphingolipids and their synthetic enzymes are emerging as important mediators in inflammatory responses and as regulators of immune cell functions. In particular, sphingosine kinase (SK) and its product sphingosine-1-phosphate (S1P) have been extensively implicated in these processes. SK catalyzes the phosphorylation of sphingosine to S1P and exists as two isoforms, SK1 and SK2. SK1 has been shown to be activated by cytokines including tumor necrosis factor-alpha (TNF-α) and interleukin1-β (IL1-β). The activation of SK1 in this pathway has been shown to be, at least in part, required for mediating TNF-α and IL1-β inflammatory responses in cells, including induction of cyclo-oxygenase 2 (COX2). In addition to their role in inflammatory signaling, SK and S1P have also been implicated in various immune cell functions including, mast cell degranulation, migration of neutrophils, and migration and maturation of lymphocytes. The involvement of sphingolipids and sphingolipid metabolizing enzymes in inflammatory signaling and immune cell functions has implicated these mediators in numerous inflammatory disease states as well. The contribution of these mediators, specifically SK1 and S1P, to inflammation and disease are discussed in this review.     

Sphingosine stimulates cellular proliferation via a protein kinase C-independent pathway

Sphingosine, a metabolite of membrane sphingolipids, is generally considered to be cytotoxic for a variety of cell types. However, we have found that sphingosine at low concentrations stimulates DNA synthesis and acts synergistically with known growth factors to induce proliferation of quiescent Swiss 3T3 fibroblasts. Structurally related analogs of sphingosine, such as N-stearoylsphingosine and other long chain aliphatic amines, had no mitogenic effects, suggesting that sphingosine did not induce nonspecific membrane perturbations. Sphingosine, which has been proposed to be a physiological inhibitor of protein kinase C, also markedly potentiates the mitogenic effect of the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA). Sphingosine still stimulates DNA synthesis in cells made protein kinase C deficient by prolonged treatment with phorbol ester. At mitogenic concentrations, sphingosine does not bind to protein kinase C as shown by its lack of effect on phorbol dibutyrate binding. Only at higher concentrations, in the cytotoxic range, was there a displacement of phorbol dibutyrate from its cellular-binding sites. In contrast to sphingosine, H-7, a known inhibitor of protein kinase C, inhibited the mitogenic response to TPA and the TPA-induced phosphorylation of the 80 kDa cellular substrate of protein kinase C. Our results suggest that sphingosine may play an important role as a positive regulator of cell growth acting in a fundamentally different, protein kinase C-independent pathway.     

Sphingosine kinase-dependent regulation of pro-resolving lipid mediators in Alzheimer's disease

The majority of peripheral and central nervous system disorders are related to hyperactive inflammatory responses, leading to irreversible and persistent cellular defects, functional impairments, and behavioral deficits. Advances in our understanding of these disorders have revealed the disruption of inflammation resolution pathways due to abrogated responses by specialized pro-resolving lipid mediators (SPMs). SPMs comprise a class of bioactive lipids and cell signaling molecules that function to resolve inflammation, pain, and function in host defense and tissue remodeling. Their cellular and systemic levels during physiology and pathology are regulated by sphingosine kinases (especially SphK1) that act by monitoring cyclooxygenase-2 (COX2), a potent inhibitor of SPMs production. This review presents the current understanding of the convergent mechanisms shared by bioactive lipids with SphK1 and COX2 in the etiology of chronic inflammatory disorders, focusing on neuroinflammation, as well as describes the translational directions of this trilogy for the treatment of Alzheimer's disease.     

The c-jun kinase/stress-activated pathway: regulation, function and role in human disease

c-Jun N-terminal kinases (JNKs), also referred to as stress-activated kinases (SAPKs), were initially characterized by their activation in response to cell stress such as UV irradiation. JNK/SAPKs have since been characterized to be involved in proliferation, apoptosis, motility, metabolism and DNA repair. Dysregulated JNK signaling is now believed to contribute to many diseases involving neurodegeneration, chronic inflammation, birth defects, cancer and ischemia/reperfusion injury. In this review, we present our current understanding of JNK regulation and their involvement in homeostasis and dysregulation in human disease.     

Role of protein kinase C in cellular regulation

Protein kinase C (PKC) consists of a family of closely related enzymes ubiquitously present in animal tissues. These enzymes respond to second messengers, Ca2+, diacylglycerol and arachidonic acid, to express their activities at membrane locations. Numerous hormones, neurotransmitters, growth factors and antigens are believed to transmit their signals by activation of a variety of phospholipases to generate these messengers. The various PKC isozymes, which exhibit distinct biochemical characteristics and unique cellular and subcellular localizations, may be differentially stimulated depending on the duration and strength of these messengers. Activation of PKC has been linked to the regulation of cell surface receptors, ion channels, secretion, gene expression, and neuronal plasticity and toxicity. The mechanisms of action of PKC in the regulation of these cellular functions are not entirely clear. Further study to identify the target substrates relevant to the various cellular functions is essential to define the functional diversity of this enzyme family.     

Sphingosine kinase and sphingosine 1-phosphate in asthma

Sphingolipids are amphiphatic molecules ubiquitously expressed in all eukaryotic cell membranes. Initially characterized as structural components of cell membranes, sphingolipids have emerged as sources of important signalling molecules over the past decade. Sphingolipid metabolites, such as ceramide and S1P (sphingosine 1-phosphate), have been demonstrated to have roles as potent bioactive messengers involved in cell differentiation, proliferation, apoptosis, migration and angiogenesis. The importance of SphK (sphingosine kinase) and S1P in inflammation has been demonstrated extensively. The prevalence of asthma is increasing in many developed nations. Consequently, there is an urgent need for the development of new agents for the treatment of asthma, especially for patients who respond poorly to conventional therapy. Recent studies have demonstrated the important role of SphK and S1P in the development of asthma by regulating pro-inflammatory responses. These novel pathways represent exciting potential therapeutic targets in the treatment of asthma and are described in the present review.     

Protein kinase C: properties and possible role in cellular division and differentiation

Protein kinase C was first described some eight years ago. Recent results indicate that this kinase may have a crucial role in signal transduction for substances involved in cellular differentiation and division. Protein kinase C is activated by attachment to plasma membranes, in the presence of calcium and diacylglycerol. The activator is produced in the membrane following the signal-induced breakdown of phosphoinositides. Tumor promoters, such as phorbol ester, can substitute for diacylglycerol. The recent findings that: tyrosine kinases might be involved in the phosphoinositide turnover and, phosphorylation of growth factor receptors by protein kinase C regulates some of their functions, indicate more and more clearly that this kinase is involved in the control of cell growth division and differentiation. Purification procedures, properties and mechanisms of regulation will be summarized and discussed.     

Sphingosine kinase and sphingosine-1-phosphate in liver pathobiology

Over 20?years ago, sphingosine-1-phosphate (S1P) was discovered to be a bioactive signaling molecule. Subsequent studies later identified two related kinases, sphingosine kinase 1 and 2, which are responsible for the phosphorylation of sphingosine to S1P. Many stimuli increase sphingosine kinase activity and S1P production and secretion. Outside the cell, S1P can bind to and activate five S1P-specific G protein-coupled receptors (S1PR1–5) to regulate many important cellular and physiological processes in an autocrine or paracrine manner. S1P is found in high concentrations in the blood where it functions to control vascular integrity and trafficking of lymphocytes. Obesity increases blood S1P levels in humans and mice. With the world wide increase in obesity linked to consumption of high-fat, high-sugar diets, S1P is emerging as an accomplice in liver pathobiology, including acute liver failure, metabolic syndrome, control of blood lipid and glucose homeostasis, nonalcoholic fatty liver disease, and liver fibrosis. Here, we review recent research on the importance of sphingosine kinases, S1P, and S1PRs in liver pathobiology, with a focus on exciting insights for new therapeutic modalities that target S1P signaling axes for a variety of liver diseases.     

Mitochondrial glutathione: Features,regulation and role in disease

Background

Mitochondria are the powerhouse of mammalian cells and the main source of reactive oxygen species (ROS) associated with oxygen consumption. In addition, they also play a strategic role in controlling the fate of cells through regulation of death pathways. Mitochondrial ROS production fulfills a signaling role through regulation of redox pathways, but also contributes to mitochondrial damage in a number of pathological states.

Scope of review

Mitochondria are exposed to the constant generation of oxidant species, and yet the organelle remains functional due to the existence of an armamentarium of antioxidant defense systems aimed to repair oxidative damage, of which mitochondrial glutathione (mGSH) is of particular relevance. Thus, the aim of the review is to cover the regulation of mGSH and its role in disease.

Major conclusions

Cumulating evidence over recent years has demonstrated the essential role for mGSH in mitochondrial physiology and disease. Despite its high concentration in the mitochondrial matrix, mitochondria lack the enzymes to synthesize GSH de novo, so that mGSH originates from cytosolic GSH via transport through specific mitochondrial carriers, which exhibit sensitivity to membrane dynamics. Depletion of mGSH sensitizes cells to stimuli leading to oxidative stress such as TNF, hypoxia or amyloid β-peptide, thereby contributing to disease pathogenesis.

General significance

Understanding the regulation of mGSH may provide novel insights to disease pathogenesis and toxicity and the opportunity to design therapeutic targets of intervention in cell death susceptibility and disease. This article is part of a Special Issue entitled Cellular functions of glutathione.     

Evolution and regulation of cellular periodic processes: a role for paralogues

Several cyclic processes take place within a single organism. For example, the cell cycle is coordinated with the 24 h diurnal rhythm in animals and plants, and with the 40 min ultradian rhythm in budding yeast. To examine the evolution of periodic gene expression during these processes, we performed the first systematic comparison in three organisms (Homo sapiens, Arabidopsis thaliana and Saccharomyces cerevisiae) by using public microarray data. We observed that although diurnal‐regulated and ultradian‐regulated genes are not generally cell‐cycle‐regulated, they tend to have cell‐cycle‐regulated paralogues. Thus, diverged temporal expression of paralogues seems to facilitate cellular orchestration under different periodic stimuli. Lineage‐specific functional repertoires of periodic‐associated paralogues imply that this mode of regulation might have evolved independently in several organisms.     

IGF2: epigenetic regulation and role in development and disease

Insulin-like growth factor II (IGF2) is perhaps the most intricately regulated of all growth factors characterized to date. Its gene is imprinted – only one allele is active, depending on parental origin – and this pattern of expression is maintained epigenetically in almost all tissues. IGF2 activity is further controlled through differential expression of receptors and IGF-binding proteins (IGFBPs) that determine protein availability. This complex and multifaceted regulation emphasizes the importance of accurate IGF2 expression and activity. This review will examine the regulation of the IGF2 gene and what it has revealed about the phenomenon of imprinting, which is frequently disrupted in cancer. IGF2 protein function will be discussed, along with diseases that involve IGF2 overexpression. Roles for IGF2 in sonic hedgehog (Shh) signaling and angiogenesis will also be explored.     

On the regulation of cellular energetics in health and disease

Very recent experimental data, obtained by using the permeabilized cell technique or tissue homogenates for investigation of the mechanisms of regulation of respiration in the cells in vivo, are shortly summarized. In these studies, surprisingly high values of apparent Km for ADP, exceeding that for isolated mitochondria in vitro by more than order of magnitude, were recorded for heart, slow twitch skeletal muscle, hepatocytes, brain tissue homogenates but not for fast twitch skeletal muscle. Mitochondrial swelling in the hypo-osmotic medium resulted in the sharp decrease of the value of Km for ADP in correlation with the degree of rupture of mitochondrial outer membrane, as determined by the cytochrome c test. Very similar effect was observed when trypsin was used for treatment of skinned fibers, permeabilized cells or homogenates. It is concluded that, in many but not all types of cells, the permeability of the mitochondrial outer membrane for ADP is controlled by some cytoplasmic protein factor(s). Since colchicine and taxol were not found to change high values of the apparent Km for ADP, the participation of microtubular system seems to be excluded in this kind of control of respiration but studies of the roles of other cytoskeletal structures seem to be of high interest.In acute ischemia we observed rapid increase of the permeability of the mitochondrial outer membrane for ADP due to mitochondrial swelling and concomitant loss of creatine control of respiration as a result of dissociation of creatine kinase from the inner mitochondrial membrane. The extent of these damages was decreased by use of proper procedures of myocardial protection showing that outer mitochondrial membrane permeability and creatine control of respiration are valuable indices of myocardial preservation. In contrast to acute ischemia, chronic hypoxia seems to improve the cardiac cell energetics as seen from better postischemic recovery of phosphocreatine, and phosphocreatine overshoot after inotropic stimulation.In general, adaptational possibilities and pathophysiological changes in the mitochondrial outer membrane system point to the central role such a system may play in regulation of cellular energetics in vivo.     

(Na,K)-pump: cellular role and regulation in nonexcitable cells

The (Na,K)-pump develops and maintains ionic gradients that are of fundamental importance for proper function of most animal cells. These gradients are utilized in the form of ionic leak pathways by a number of special and general cell processes (e.g., nerve conduction, nutrient transport, pH regulation). As the sodium gradient in particular energizes many vital cell processes, alterations in cell activity will often be manifest as changes in sodium entry. The (Na,K)-pump rate varies accordingly, in order to maintain balance between Na entry and exit thereby maintaining the potential energy of the cell. Acute changes in sodium influx are balanced by increases in activity of existing pump units, with only a small change in intracellular sodium concentration. This is possible because intracellular is normally poised on the steep limb of the concentration versus activity curve for the (Na,K)-pump, at a point well below maximal activity, allowing large increases in (Na,K)-pump rate with only small changes in sodium concentration. If the increase in sodium influx is prolonged, it appears that the cell responds by synthesizing new pumps, allowing intracellular sodium concentration to return to its original values. Though increases in (Na,K)-pump activity must be accompanied by increases in potassium leak rates, in the experiments we have presented, there does not appear to be direct functional coupling between (Na,K)-pump and the K leak pathways. In these situations the matching of active influx and passive efflux of K short-term appears to occur by mechanisms not directly related to (Na,K)-pump activation.