Sphingolipids in mitochondria

Abstract

Sphingolipids are bioactive lipids found in cell membranes that exert a critical role in signal transduction. In recent years, it has become apparent that sphingolipids participate in growth, senescence, differentiation and apoptosis. The anabolism and catabolism of sphingolipids occur in discrete subcellular locations and consist of a strictly regulated and interconnected network, with ceramide as the central hub. Altered sphingolipid metabolism is linked to several human diseases. Hence, an advanced knowledge of how and where sphingolipids are metabolized is of paramount importance in order to understand the role of sphingolipids in cellular functions. In this review, we provide an overview of sphingolipid metabolism. We focus on the distinct pathways of ceramide synthesis, highlighting the mitochondrial ceramide generation, transport of ceramide to mitochondria and its role in the regulation of mitochondrial-mediated apoptosis, mitophagy and implications to disease. We will discuss unanswered questions and exciting future directions. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

Keywords: Cancer; Ceramide; Mitochondrial apoptosis; Mitophagy; Sphingolipid metabolism.

Figure 1. Chemical structures of sphingolipids

Sphingolipids are composed of a sphingosine or dihrydrosphingosine backbone (marked in blue). N-acylation (red) of the sphingoid backbone generates (dihydro)ceramide and derivatives. Complex sphingolipids are generated by the addition of head groups (black) to ceramide. Sphingomyelin contains a phosphocholine head group; glucosylceramide and galactosylceramide contain a single glucose or galactose molecule, respectively, whereas ceramide-1-phosphate consists of a phosphate head group linked to ceramide.

Figure 2. Sphingolipid metabolism

Ceramide is the centerpiece of the sphingolipid metabolism and can be synthesized by multiple pathways. Ceramide can be generated by the de novo pathway (orange box) from the condensation of serine and palmitoyl-CoA to generate 3-ketodihydrosphingosine, which is then reduced to dihydrosphingosine (also known as sphinganine) and further acylated by ceramide synthases (CerS). Ceramide can be hydrolyzed to sphingosine and then reacylated back to ceramide in the salvage/ recycling pathway or phosphorylated to sphingosine-1-phosphate by sphingosine kinase (SK) and exit the sphingolipid metabolism by the action of the sphingosine-1-phosphate lyase (SPL) (blue box). Ceramide can also be formed by the hydrolysis of more complex sphingolipids such as sphingomyelin (purple box), glucosylceramide (turquoise box) and galactosylceramide (black box). Serine palmitoyltransferase (SPT); 3-ketodihydrosphingosine reductase (KDHR); (dihydro)ceramide synthase (CerS); dihydroceramide desaturase (DES); ceramidase (CDase); sphingosine-1-phosphate phosphatases (SPP); sphingomyelinases (SMase); sphingomyelin synthase (SMS); glucosylceramide synthase (GCS); glucosylceramidase (GCase); ceramide galactosyltransferase (CGT); galactosylceramidase (GALC); ceramide kinase (CERK); ceramide-1-phosphate phosphatase (C1PP).

Figure 3. The extrinsic (receptor-mediated) and intrinsic (mitochondrial) pathways of apoptosis

The receptor-mediated (extrinsic) pathway of apoptosis is initiated when ligands of TNF-α family such as FAS and TNF-α bind their plasma membrane-bound death receptors, this binding leads to activation of caspase-8 via TNFR-associated death domain (TRADD) and FAS-associated death domain (FADD) proteins. TRADD is required for induction of apoptosis in response to TNF-α a binding to TNFR. TRADD and FADD activate effector caspases (caspase-3, 6 and 7) causing apoptosis. Moreover, caspase-8 generates the active truncated form of BID (tBID) that inhibits pro-survival BCL-2-like proteins (BCL-2) and engages in the mitochondrial pathway. However, this engagement is only apparent in certain cells such as liver cells (type 2 cells), while it is absent in type 1 cells, such as thymocytes. Several stimuli such as DNA damaging agents, oncogenes, and ER stress stimulate the mitochondrial (intrinsic) pathway via activation of BH3-only family of proteins that inhibit pro-survival BCL-2 like proteins. Such inhibition leads to activation of pro-apoptotic BAX and BAK and subsequent disruption of the mitochondrial membrane and the release of cytochrome c and SMAC (second mitochondria-derived activator of caspases). Cytochrome c activates caspase-9 via APAF1 (apoptotic protease-activating factor 1) that activates effector caspases, whereas SMAC blocks the caspase inhibitor protein XIAP (X-linked inhibitor of apoptosis protein).

Figure 4. Regulation of mitochondrial apoptosis by ceramide

Ceramide can regulate apoptosis via multiple mechanisms. Elevated ceramide levels controls MOMP, and subsequent cytochrome c release by BAX oligomerization and ceramide channel formation. In addition, ceramide-induced activation of PKC-ζ induces BAK oligomer formation and MOMP via AKT inhibition and JNK activation, whereas activation of PKC-δ leads to cytochrome c release from mitochondria to cytosol. On the other hand, dephosphorylation of SR protein or GSK-3β by ceramide-activated protein phosphatase PP1 and PP2A, respectively, results in activation of effectors caspases. Ceramide acts as activation of the protease cathepsin D, which in turn cleaves BID to its active form resulting in caspase activation.