Sphingolipid metabolism in cancer signalling and therapy

Abstract

Sphingolipids, including the two central bioactive lipids ceramide and sphingosine-1-phosphate (S1P), have opposing roles in regulating cancer cell death and survival, respectively, and there have been exciting developments in understanding how sphingolipid metabolism and signalling regulate these processes in response to anticancer therapy. Recent studies have provided mechanistic details of the roles of sphingolipids and their downstream targets in the regulation of tumour growth and response to chemotherapy, radiotherapy and/or immunotherapy using innovative molecular, genetic and pharmacological tools to target sphingolipid signalling nodes in cancer cells. For example, structure-function-based studies have provided innovative opportunities to develop mechanism-based anticancer therapeutic strategies to restore anti-proliferative ceramide signalling and/or inhibit pro-survival S1P-S1P receptor (S1PR) signalling. This Review summarizes how ceramide-induced cellular stress mediates cancer cell death through various mechanisms involving the induction of apoptosis, necroptosis and/or mitophagy. Moreover, the metabolism of ceramide for S1P biosynthesis, which is mediated by sphingosine kinase 1 and 2, and its role in influencing cancer cell growth, drug resistance and tumour metastasis through S1PR-dependent or receptor-independent signalling are highlighted. Finally, studies targeting enzymes involved in sphingolipid metabolism and/or signalling and their clinical implications for improving cancer therapeutics are also presented.

Figure 1. Pathways of sphingolipid metabolism and key enzymes

Ceramide is the central molecule in sphingolipid metabolism and can be synthesized de novo (dark blue boxes) by the functions of serine palmitoyl-transferase (SPT), 3-ketosphinganine reductase, (dihydro)ceramide synthases (CERS1–6) and dihydroceramide desaturase (DES). Ceramide can also be generated by the hydrolysis of complex sphingolipids such as sphingomyelin (SM) by the action of sphingomyelinases (SMases) (purple box). Conversion of sphingosine to ceramide is catalysed by the salvage pathway (red boxes) via CERS1–6, the same enzymes involved in de novo ceramide synthesis. Ceramide is metabolized to generate sphingosine-1-phosphate (S1P) by the functions of ceramidases (CDases) and sphingosine kinase 1 (SPHK1) and SPHK2. S1P is further hydrolysed by S1P lyase to exit the sphingolipid metabolic cycle. The metabolism of ceramide to generate complex sphingolipids such as glycosphingolipids or gangliosides requires the synthesis of glucosylceramide (GlcCer) by glucosylceramide synthase (GCS) (green box), which is a precursor for the synthesis of complex sphingolipids (dotted arrows). Sphingomyelin synthase (SMS) enzymes convert ceramide to SM via insertion of a choline moiety into ceramide as a head group using phosphatidylcholine (PC) as a donor, yielding free diacylglycerol (DAG), and ceramide kinase (CERK) converts ceramide to ceramide-1-phosphate (C1P) (orange boxes). Ceramide is hydrolysed by CDases, including acid ceramidase (AC), to release free fatty acids and sphingosine. Ceramide can be synthesized by the hydrolysis of SM by SMases, including acid sphingomyelinase (ASMase; encoded by SMPD1). Mutations in enzymes involved in sphingolipid metabolism have been associated with various lysosomal storage disorders (grey boxes), including Farber, Gaucher, Niemann–Pick and Krabbe diseases. Patients with Gaucher disease have increased susceptibility to the development of cancer, such as myeloma. Mutations in the genes encoding SPT subunits, such as SPTLC1, are associated with hereditary sensory neuropathy type I (REF. 27). GCDase, galactosylceramidase; GlcCDase, glucosylceramidase; SGPP, S1P phosphatase.

Figure 2. Intracellular ceramide signalling in cancer cell death and tumour suppression

In response to cellular stress, the increased generation of ceramide (Cer) via (dihydro)ceramide synthases (CERS1–6) and sphingomyelinases (SMases) results in cancer cell death and tumour suppression, which is regulated by various mechanisms involving both direct and indirect targets of ceramide. Ceramide accumulation in the mitochondrial membrane induces BAX recruitment to the mitochondria, resulting in mitochondrial outer membrane permeabilization (MOMP), caspase activation and apoptosis in response to radiation^–. In addition, regulation of ceramide transport from late endosomal organelles by lysosomal-associated transmembrane protein 4B (LAPTM4B) stabilizes lysosomal membranes, leading to ceramide-mediated caspase 3 activation and apoptosis induced by chemotherapeutic drugs. Activation of serine/ threonine-protein phosphatase 2A (PP2A) is mediated by direct binding between phosphatase 2A inhibitor I2PP2A and ceramide or FTY720, a sphingosine analogue drug, leading to receptor-interacting serine/threonine-protein kinase 1 (RIPK1)-dependent necroptosis in lung cancer cells. Cellular stress in response to chemotherapy results in C18 ceramide trafficking to the outer mitochondrial membrane, which then directly binds the lipidated form of microtubule-associated protein 1 light chain 3β (LC3B-II) to recruit autophagosomes to mediate lethal mitophagy by the activation of dynamin-related protein 1 (DRP1) and mitochondrial fission^,. Autophagy in cancer cells has been shown to be induced by accumulation of dihydroceramide and/or ceramide in the endoplasmic reticulum (ER) of glioma cells in response to tetrahydrocannabinol, a psychotropic cannabinoid. Accumulation of dihydroceramide and/or ceramide in the ER leads to permeabilization of the autophagosomal and lysosomal membranes, resulting in cathepsin release and apoptotic cell death. Moreover, hydrolysis of sphingomyelin (SM) to generate ceramide via acid sphingomyelinase (ASMase) is important to promote the elongation and maturation of autophagosomal membranes via recruitment of autophagy-related protein 9A (ATG9A), suggesting a novel role for ceramide in the maturation of early autophagosomal membranes.

Figure 3. Oncogenic S1P–S1PR1-5 signalling

a | Sphingosine kinase 1 (SPHK1)-generated sphingosine-1-phosphate (S1P) engages with G protein-coupled S1P receptors (S1PR1–5) to elicit oncogenic signalling. S1P is secreted from cancer cells by ATP-binding cassette sub-family C member 1 (ABCC1), ATP-binding cassette sub-family G member 2 (ABCG2) or protein spinster homologue 2 (SPNS2), the latter of which is active selectively in endothelial cells, leading to autocrine or paracrine signalling. S1PR signalling inhibits apoptosis, induces cell proliferation and/or migration and increases drug resistance via inhibition of BAX–caspase 3 signalling, induction of survival autophagy and/or inhibition of serine/threonine-protein phosphatase 2A (PP2A)^,,. b | Circulating S1P increases tumour metastasis^–. For example, SPNS2-dependent S1P secretion from endothelial cells attenuates cytotoxic T cell function, possibly by influencing S1PR functions on immune cells and/or cancer cells, and therefore promotes tumour metastasis^,. Moreover, secretion of SPHK1-generated S1P from host lymphoid or vascular endothelial cells activates S1PR2 signalling in tumour cells, which inhibits expression of breast cancer metastasis-suppressor 1 (BRMS1), a master suppressor of metastasis, leading to increased metastasis. Mice deficient in SPNS2 or SPHK1 exhibited reduced lung colonization and metastasis regardless of tumour levels of S1P, suggesting a role for systemic S1P in the communication between host immune cells and cancer cells to increase tumour metastasis. Inhibition of systemic S1P (using sonepcizumab; also known as Sphingomab) or S1PR2 in cancer cells (using JTE013) and/or S1PR1 in cancer cells (using FTY720) should attenuate metastasis. SPH, sphingosine.

Figure 4. Receptor-independent intracellular S1P signalling

Sphingosine kinase 1 (SPHK1)-generated sphingosine-1-phosphate (S1P) binds TNF receptor-associated factor 2 (TRAF2) to induce polyubiquitylation of receptor-interacting serine/threonine-protein kinase 1 (RIPK1), which then indirectly mediates nuclear factor-κB (NF-κB) activation. S1P also directly associates with peroxisome proliferator-activated receptor-γ (PPARγ), which then mediates the recruitment of PPARγ co-activator 1β (PGC1β) to induce PPARγ-dependent gene expression and neo-angiogenesis. Generation of S1P by SPHK2, which is localized in the nuclear membrane, binds to histone deacetylase 1 (HDAC1) and HDAC2 and inhibits their activity at the nuclear membrane to induce epigenetic regulation of expression ofCDKN1A, which encodes p21, and FOS, which encodes proto-oncogene FOS. SPHK2-generated S1P also binds prohibitin 2 (PHB2) on the mitochondrial membrane to induce cytochromec oxidase (complex IV) activity and mitochondrial respiration. Generation of S1P in the nuclear envelope by SPHK2 results in S1P–telomerase reverse transcriptase (TERT) binding at the nuclear membrane, which then protects telomerase from E3 ubiquitin-protein ligase makorin 1 (MKRN1)-dependent ubiquitylation and degradation, therefore stabilizing telomerase and attenuating senescence induction. This process is mediated by mimicking of TERT phosphorylation upon S1P–TERT binding, and SPHK2 knockdown, genetic loss or pharmacological inhibition using ABC294640 results in rapid telomerase degradation, leading to accelerated senescence and tumour suppression. SPH, sphingosine; Ub, ubiquitin.