Targeting Phospholipid Metabolism in Cancer

Abstract

All cancers tested so far display abnormal choline and ethanolamine phospholipid metabolism, which has been detected with numerous magnetic resonance spectroscopy (MRS) approaches in cells, animal models of cancer, as well as the tumors of cancer patients. Since the discovery of this metabolic hallmark of cancer, many studies have been performed to elucidate the molecular origins of deregulated choline metabolism, to identify targets for cancer treatment, and to develop MRS approaches that detect choline and ethanolamine compounds for clinical use in diagnosis and treatment monitoring. Several enzymes in choline, and recently also ethanolamine, phospholipid metabolism have been identified, and their evaluation has shown that they are involved in carcinogenesis and tumor progression. Several already established enzymes as well as a number of emerging enzymes in phospholipid metabolism can be used as treatment targets for anticancer therapy, either alone or in combination with other chemotherapeutic approaches. This review summarizes the current knowledge of established and relatively novel targets in phospholipid metabolism of cancer, covering choline kinase α, phosphatidylcholine-specific phospholipase D1, phosphatidylcholine-specific phospholipase C, sphingomyelinases, choline transporters, glycerophosphodiesterases, phosphatidylethanolamine N-methyltransferase, and ethanolamine kinase. These enzymes are discussed in terms of their roles in oncogenic transformation, tumor progression, and crucial cancer cell properties such as fast proliferation, migration, and invasion. Their potential as treatment targets are evaluated based on the current literature.

Keywords: cancer; choline; ethanolamine; metabolism; phospholipid; target.

Figure 1

Biochemical network of ethanolamine and choline phospholipid metabolism showing crosstalk between these two metabolic cycles. MR detectable metabolites are drawn in boxes. Red shapes show enzymes discussed in this review, and green shapes show other enzymes responsible for the indicated reactions, but not discussed in this review. Metabolite abbreviations: 1-acyl-GPE, 1-acyl-glycerophosphoethanolamine; 1-acyl-GPC, 1-acyl-glycerophosphocholine; PtdCho, phosphatidylcholine; PtdEth, phosphatidylethanolamine; SM, sphingomyelin. Enzyme abbreviations: CCT, phosphocholine cytidylyltransferase; ChK, choline kinase; CPT, diacylglycerol cholinephosphotransferase; CTL, choline transporter-like protein; EtT, ethanolamine transporter; ECT, phosphoethanolamine cytidylyltransferase; EPT, diacylglycerol ethanolaminephosphotransferase; ETNK, ethanolamine kinase; GPE-PDE, glycerophosphoethanolamine phosphodiesterase; GDPD6, glycerophosphodiester phosphodiesterase domain containing 6; lyso-PL, lysophospholipase; PEMT, phosphatidylethanolamine N-methyltransferase; PLA2, phospholipase A2; PLC, phospholipase C; PLD, phospholipase D.

Figure 2

Example of in vivo pulse-acquire (PA, top) and BINEPT (bottom) ³¹P MR spectra of a representative MCF-7 (left) and MDA-MB-231 (right) tumor. Lorentzian lines as fitted by the software jMRUI (http://www.jmrui.eu/) are shown below each MR spectrum. All phosphorylated metabolites are visible in the PA spectrum, whereas the BINEPT spectrum only contains signals from phospholipid metabolites with H-P-coupling such as phosphoethanolamine (PE), phosphocholine (PC), glycerophosphoethanolamine (GPE), and glycerophosphocholine (GPC). Note the broad, uneven baseline in the 0–5 ppm region of the PA spectra, where signals from mobile membrane phospholipids are resonating. The signal of β-nucleoside triphosphate (NTP) is formed by β-NTP only. The signal labeled α-NTP is an overlapping signal from α-NTP, α-nucleoside diphosphate (α-NDP), nicotinamide adenine diphosphate, and diphosphodiesters. The signal labeled γ-NTP is an overlapping signal from γ-NTP and β-NDP. Typically, β-NTP is the smallest peak of the three NTP signals; however, here, γ-NTP overlaps with a broad baseline signal that makes it appear smaller than β-NTP. Adapted from Wijnen et al. (44).

Figure 3

Choline kinase α (ChKα) as target for anticancer treatment. (A) ChKα converts free choline to phosphocholine (PC). (B) Knockdown of ChKα resulted in a dramatic reduction of cellular PC as evident by the decreased PC signal in high-resolution ¹H MR spectrum obtained from water-soluble metabolites of MDA-MB-231 cells transfected with anti-ChKα siRNA compared to the spectrum from non-target siRNA control. (C) Stable knockdown of ChKα with a pSHAG-U6-shRNA vector significantly decreased MDA-MB-231 cell growth rate. Abbreviations: GPC, glycerophosphocholine. ***represents P < 0.001, average proliferation rates were calculated from five independent experiments. Adapted from Glunde et al. (69).

Figure 4

Phospholipase D1 activity in the tumor microenvironment is important for tumor growth and metastasis. (A) Representative picture showing that melanoma xenografts grown in Pld^−/− mice have limited growth volume compared to that growing in wild-type (WT) mice. Scale bar, 1 cm. (B) Melanoma xenografts growing in Pld^−/− mice have fewer blood vessels as compared to those grown in wild-type mice, as evident from H&E stains of the respective tumor sections. Arrowheads in panel (B) indicate blood vessels. Scale bar, 50 µm. (C) For melanoma xenografts, lung metastasis has been significantly lower in lungs of Pld^−/− mice than in lungs of wild-type mice. Adapted from Chen et al. (94).

Figure 5

Targeting acid sphingomyelinases (ASM) is an effective strategy to overcome drug-resistant cancer through lysosome destabilization-mediated cell kill. (A) Siramesine and Desipramine, two cationic amphiphilic drugs and inhibitors of ASM, cause lysosomal membrane permeabilization in MCF-7 cells. The numbers given underneath each image are the respective percentage of cells with lysosomal membrane permeabilization as indicated by cytosolic Alexa Fluor 488-dextran staining. Scale bar, 20 µm. (B) Cell death of transformed NIH 3T3-c-src^Y527F cells caused by siramesine can be rescued by adding 75 µM ASM from Bacillus cereus prior to starting drug treatment. Scale bar, 50 µm. (C) ASM inhibition reverts drug resistance of cancer cells. Siramesine and Desipramine treatment of multidrug-resistant (MDR) PC3-MDR or Du145-MDR greatly increased the percentage of cells undergoing apoptotic cell death as evident by condensed chromatin when co-treated with docetaxel. **represents P < 0.01, ***represents P < 0.001. Adapted from Petersen Nikolaj et al. (121).

Figure 6

Choline is an essential nutrient transported into cells by choline transporters. In some cancer types such as lung cancers, choline that is taken up into cells is converted to acetylcholine, serving as an autocrine or paracrine growth factor which stimulates cancer cell growth. Alternatively, choline is converted to phosphocholine and ultimately phosphatidylcholine, serving as building block to satisfy the increased proliferation rate of cancer cells and tumor growth. Abbreviations: ACh, acetylcholine; CCT, phosphocholine cytidylyltransferase; CDP-Cho, cytidine 5′-diphosphocholine; ChAT, choline acetyltransferase; ChKα, choline kinase α; Cho, choline; CPT, diacylglycerol cholinephosphotransferase; nAChR, nicotinic acetycholine receptor; mAChR, muscarinic acetycholine receptor; PC, phosphocholine; PtdCho, phosphatidylcholine.

Figure 7

GDPD5 and GDPD6, the two glycerophosphocholine phosphodiesterases (GPC-PDEs) reported to release choline from glycerophosphocholine (GPC), show potential anticancer effects. (A) GPC-PDE enzyme activity is defined as cleaving the choline moiety from GPC and thereby generating glycerol-3-phosphate. (B) Transient knockdown of GDPD5 or GDPD6 by siRNA treatment of MDA-MB-231 cells shows that GDPD6 silencing leads to a significant elevation of the GPC peak in high-resolution ¹H MR spectra from water-soluble metabolites, while GDPD5 silencing marginally increases GPC levels in this cell line. (C) GDPD5, but not GDPD6, siRNA shows cytotoxic effect in MCF-7 breast cancer cells indicated by reduced cell numbers following siRNA treatment of MCF-7 cells. (D) Both GDPD5 and GDPD6 silencing decreased MCF-7 cell migration detected by scratch assay. Abbreviations: PC, phosphocholine. *represents P < 0.05, **represents P < 0.01. Adapted from Cao et al. (163).