Sphingolipids as cell fate regulators in lung development and disease

Abstract

Sphingolipids are a diverse class of signaling molecules implicated in many important aspects of cellular biology, including growth, differentiation, apoptosis, and autophagy. Autophagy and apoptosis are fundamental physiological processes essential for the maintenance of cellular and tissue homeostasis. There is great interest into the investigation of sphingolipids and their roles in regulating these key physiological processes as well as the manifestation of several disease states. With what is known to date, the entire scope of sphingolipid signaling is too broad, and a single review would hardly scratch the surface. Therefore, this review attempts to highlight the significance of sphingolipids in determining cell fate (e.g. apoptosis, autophagy, cell survival) in the context of the healthy lung, as well as various respiratory diseases including acute lung injury, acute respiratory distress syndrome, bronchopulmonary dysplasia, asthma, chronic obstructive pulmonary disease, emphysema, and cystic fibrosis. We present an overview of the latest findings related to sphingolipids and their metabolites, provide a short introduction to autophagy and apoptosis, and then briefly highlight the regulatory roles of sphingolipid metabolites in switching between cell survival and cell death. Finally, we describe functions of sphingolipids in autophagy and apoptosis in lung homeostasis, especially in the context of the aforementioned diseases.

Fig. 1

Overview of sphingolipid metabolism and their major functions. Refer to text for further details. CoA coenzyme A, S1P sphingosine-1-phosphate, SMase sphingomyelinase, SphK sphingosine kinase, SPT serine palmitoyltransferase

Fig. 2

Schematic representation of key events in the apoptotic pathway and regulation of apoptosis by sphingolipids. There are two main apoptotic pathways. A Extrinsic pathway is triggered when cell death ligands (e.g., FasL, APO-2L, TRAIL, TNF) bind to their respective death-receptors (e.g., Fas, DR4, DR5, TNF-R1) and initiates pro-caspase-8 activation by recruiting FADD. Activation of caspase-8 results in cleavage of effector caspases, such as caspase-3,-6,-7, which are involved in the core apoptosis pathway. Furthermore, caspase-8 can truncate BID (tBID), which later induces the intrinsic pathway. B The intrinsic pathway can be directly initiated by a variety of stress signals. Stress signals initiate DNA damage and p53 phosphorylation, which leads to the up-regulation of BH3 only proteins and consequently results in mitochondrial translocation and oligomerization of BAX/BAK, followed by MOMP. Mitochondrial damage leads to cytochrome c release into the cytoplasm. Cytosolic cytochrome c binds to the pro-apoptotic factor Apaf-1 (in the presence of dATP) to form an apoptosome. Apoptosomes then activate caspase-9, which later leads to the activation of caspases-3 and-7, and subsequently to nuclear fragmentation and also chromatin condensation. Sphingolipids have been shown to modulate apoptosis at multiple steps of the process. Sphingolipids may directly affect mitochondria, a strategic center in the control of apoptosis. Ceramide forms channels in mitochondrial outer membranes and promotes the release of cytochrome c for caspase-9 activation. Ceramide channel formation has also shown to be inhibited by dihydroceramide. Furthermore, ceramide generates reactive oxygen species (ROS) via inhibition of mitochondrial complex III. The apoptotic action of ceramide could also be mediated by the recruitment and activation of pro-apoptotic Bax at the mitochondria through the PP2A-dependent dephosphorylation of Bax and formation of mitochondrial ceramide-rich macrodomains (MCRMs). aSMase-released ceramide binds directly to lysosomal protease cathepsin D, leading to cathepsin D activation, resulting in cleavage of the BH3-only protein BID and induction of the mitochondrial pathway of apoptosis. Sphingosine has been shown to downregulate expression of anti-apoptotic proteins, Bcl-2 and Bcl-xL, to enhance apoptosis. While ceramide-mediated activation of pro-apoptotic protein, BAD, promotes apoptosis, S1P suppresses apoptosis via BAD inactivation

Fig. 3

A schematic overview of autophagy machinery and its regulation by sphingolipids. A Autophagy induction and nucleation of phagophore membranes (pre-autophagosomal structures): in nutrient rich conditions (insulin, growth factors and amino acids), the mTORC1 kinase associates with the ULK1 complex to inhibit the initiation of autophagy. However, under growth factor deprivation or nutrient starvation, energy sensor AMPK activates the ULK1 complex by directly phosphorylating ULK1 and by suppression of mTORC1 activity through phosphorylation, and initiates vesicle nucleation. Phosphorylated and active ULK1 also promotes phosphorylation of Atg13 and FIP200, and dissociates from mTORC1. PI3K-III and VPS34 together with ATG14, AMBRA1, VPS15, and Beclin-1 form a protein complex (PI3K-III complex) and initiates phagophore formation. Autophagosome formation and maturation: Two ubiquitin-like proteins, Atg12 and LC3, are involved in double-membrane vesicle (autophagosome) formation, elongation, and closure. Atg12 is conjugated with Atg5 by Atg7 and Atg10, which then form a complex with Atg16. This complex works with Atg7 and Atg3 to conjugate LC3-I with the polar head of phosphatidylethanolamine (PE) to produce LC3-II, which is specifically located on autophagosome structures. Autophagosomes are sequentially fused with lysosomes to form autolysosomes. In the autolysosome, the autophagosomal cargoes are digested by lysosomal hydrolases and the contents are released for metabolic recycling. B Sphingolipids have direct effects on different stages of the autophagy pathway. At the initial step, ceramide may stimulate autophagy via PI3K-1/Akt activation which suppresses the inhibitory effects of mTORC1 on autophagy. Acid sphingomyelinase-derived ceramide also increases autophagy via reducing mTORC1 activity during amino acid deprivation in a PP2A-dependent manner. C2-ceramide-induced decrease of mitochondrial membrane potential upregulates BNIP3 expression which ultimately leads to induction of autophagy through dissociation of Beclin-1 from Bcl-2, Bcl-xL, and Mcl-1 in a competitive manner. Similarly, ceramide-mediated activation of JNK disrupts the inhibitory Beclin-1:Bcl-2 complex through direct phosphorylation of Bcl-2. Furthermore, ceramide-mediated activation of transcription factor c-Jun may increase autophagy activation via upregulation of Beclin-1 and LC3 expression. Ceramide may also activate calpain which subsequently cleaves Atg5 to generate a protein fragment that leads to suppression of autophagy and apoptosis induction. Mitochondrial ceramide has been shown to mediate mitophagy through the direct interaction between ceramide and LC3-II