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Malignant transformation: The role of MRS
European Journal of Radiology 81S1 (2012) S66–S68
Contents lists available at SciVerse ScienceDirect
European Journal of Radiology journal homepage: www.elsevier.com/locate/ejrad
Malignant transformation: The role of MRS Lu Jianga , Jannie P. Wijnenb , Kristine Glundea,c, * a Division
of Cancer Imaging Research, Russell H. Morgan Department, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA of Radiology, University Medical Centre Utrecht, Utrecht, Netherlands c Sidney Kimmel Comprehensive Cancer Center, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA b Department
Magnetic Resonance Spectroscopy (MRS) has been instrumental in elucidating the metabolic changes that arise as a consequence of malignant transformation. The metabolic phenotype of cancers is characterized by high glucose uptake, high glycolytic activity and lactate production, low mitochondrial activity, low bioenergetic status, and abnormal phospholipid metabolism [1,2]. As shown in Fig. 1, noninvasive 1 H MRS is able to detect elevated lactate and increased total choline (tCho) levels in breast (and other) cancers, which primarily arise from oncogenic signaling pathways in cancer cells as indicated by green boxes in Fig. 1A. The increased levels of lactate and tCho provide noninvasive MR biomarkers of transformation, staging, and response to therapy. Enzymes in glucose and choline metabolism present targets for image-guided cancer therapy. 1. Glucose metabolism in cancer Tumors exhibit abnormal glucose metabolism. Cancer cells rapidly take up and utilize glucose, which is converted to lactate under aerobic conditions unlike in normal cells, in which glucose is predominantly utilized by the tricarboxylic acid (TCA) cycle. Otto Warburg first studied this phenomenon in the 1920’s, making it known as the Warburg effect [3]. As shown in Fig. 1A, cancer cells exhibit enhanced glucose uptake through upregulation of facilitative glucose transporters (GLUTs), primarily GLUT1, which have been attributed to malignant transformation [4]. Several glycolytic enzymes such as hexokinase (HK2), 6-phosphofructokinase (PFK1), and pyruvate kinase muscle isozyme 2 (PKM2) among others are upregulated in cancer [4]. Lactate dehydrogenase A (LDHA) is upregulated in cancers as well and converts pyruvate to lactate, thereby regenerating NAD+ from NADH to avoid NAD+ depletion as a result of glycolytic NAD+ consumption. Thus, the net result of upregulated aerobic glycolysis in cancer cells is the formation of large amounts of lactate, leading to abnormally high lactate concentrations [4]. The upregulation of these glycolytic enzymes in cancer is caused by oncogenes and oncogenic signaling pathways as indicated by the green lines in Fig. 1A. The PI3K/AKT/mTOR signaling pathway, which is frequently activated in cancer, regulates glycolytic activity by GLUT1 induction and HK2 activation [4]. Increased expression of the oncogenic transcription factor c-Myc induces lactate dehydrogenase A (LDHA), which converts pyruvate to lactate [5]. c-Myc also directly regulates other glycolytic genes such as GLUT1, HK2, PFK1 and enolase 1 (ENO1) [5]. Mutations in p53, which is a tumor suppressor gene that is frequently deregulated in cancers, * E-mail address: [email protected] (K. Glunde). 0720-048X/$ – see front matter © 2012 Elsevier Ireland Ltd. All rights reserved.
can modulate glycolysis through various transcription factors [6]. The hypoxia-inducible factor 1 (HIF-1) transcription factor, which is activated in several cancers, stimulates LDHA, induces GLUTs and several glycolytic enzymes, and reduces pyruvate dehydrogenase kinase 1 (PDK1) activity [7]. Combined, these oncogenic signaling pathways enhance glucose transport, glycolytic flux, and lactate production, which can be detected by 1 H MRS as shown in Fig. 1B. Different MRS techniques have been used to study glucose metabolism in cancer in cell cultures, tissue samples, and extracts thereof, as well as in animal models and human subjects in vivo. Glucose consumption and lactate production are important parameters in cancer and have been studied extensively. Other glycolytic metabolites cannot be easily detected with 1 H MRS due to their low concentrations. Lactate is present in measurable concentrations in some cancers and has been measured in the clinic with 1 H MRS [8]. In clinical applications, 1 H MRS/I can be used in combination with MR imaging (MRI) for diagnostic purposes. Ex vivo analysis of biopsy tissue can be an alternative approach for measuring lactate concentrations. Sample collection, handling, storage, and preparation needs to be done carefully to avoid degradation of metabolites [4]. If all precautions are taken, the lactate concentration can be measured accurately ex vivo using high-resolution (HR) magic angle spinning (MAS) MRS [4]. Altered glucose metabolism in cancer has been studied with 13 C MRS for more than two decades by tracing the 13 C-label in metabolites produced from [1-13 C]-glucose or other isotopically enriched substrates in cancer cell lines, animal models of cancer, as well as in human subjects in vivo [4]. However, 13 C MRS is currently not employed in clinical diagnosis or management of cancer due to its low sensitivity, which may change with further development of MRS techniques, such as for example hyperpolarized 13 C MRS. The introduction of dynamic nuclear polarization (DNP) has revitalized 13 C MRS as hyperpolarized 13 C-labeled substrates increase the signal-to-noise ratio by up to 10,000-fold [9]. However, the time window for hyperpolarized 13 C MRSI is short and requires completion within a few minutes following tracer preparation. Hyperpolarized 13 C MRSI can be performed in small animals with voxel sizes in the range of 0.135 cm3 with a temporal resolution of around 5 seconds [9]. Hyperpolarized [1-13 C] pyruvate, among others, has been used in DNP MRS studies of preclinical models of cancer and has been suggested for cancer detection and therapy monitoring [4]. 2. Choline metabolism in cancer Choline phospholipid metabolism is activated in tumors and leads to increased phosphocholine (PCho) and tCho levels in cancer
L. Jiang et al. / European Journal of Radiology 81S1 (2012) S66–S68
A
Glucose
Choline
GLUT
CTs
PI3K
Glucose
Choline (Cho)
Akt
HK2
Glycerophosphocholine (GPC)
GPC-PDE
CHKD
PFK1
mTOR Phosphocholine (PCho)
PKM2
HIF-1
Pyruvate
CPT
c-Myc LDHA
Her2
Lactate (Lac)
TCA cycle
LPL
CCT
p53
PDH Acetyl-CoA
S67
PtdChoPLC
PtdChoPLD
Phosphatidylcholine
PtdChoPLA2
Ras Raf Erk
B
PCho PCho
GPC
GPC
Lac
Cho Cho PPM
PPM
3.5
3.0
3.24
2.5
3.20
2.0
1.5
1.0
Lac/Lipid-CH3tCho
PPM
3.5
Lipid-CH2-
3.0
2.5
2.0
1.5
1.0
Fig. 1. (A) Schematic showing the most important cancer-related cell signaling pathways (center) that impact upon enzymes in glucose metabolism (left) and enzymes in choline phospholipid metabolism (right). Arrows depict an activation in the direction of the arrow, and short vertical lines at the end of a line symbolize inhibition in the direction of vertical line. Abbreviations: acetyl-CoA, acetyl coenzyme A; AKT, v-akt murine thymoma viral oncogene homolog 1; CCT, CTP: phosphocholine cytidylyltransferase; Chk-a, choline kinase alpha; c-Myc, v-myc myelocytomatosis viral oncogene homolog; CPT, diacylglycerol; CT, choline transporter; CTP, cytidine-5 -triphosphate; Erk, extracellular signal-regulated kinases; GLUT, glucose transporter; GPC-PDE, glycerophosphocholine phosphodiesterase; HIF-1, hypoxia inducible factor 1; HK, hexokinase; LDH, lactate dehydrogenase; LPL, lysophospholipase; mTOR, mammalian target of rapamycin; PDH, pyruvate dehydrogenase; PFK1, phosphofructokinase 1; PI3K, phosphatidylinositol 3-kinase; PKM2, pyruvate kinase muscle isozyme 2; PtdCho-PLA2 , phosphatidylcholine-specific phospholipase A2 ; PtdCho-PLC, phosphatidylcholine-specific phospholipase C; PtdCho-PLD, phosphatidylcholine-specific phospholipase D; TCA, tricarboxylic acid. (B) High-resolution ex vivo 1 H MR spectra of triple-negative human MDA-MB231 breast cancer cell extracts (top) and in vivo 1 H MR spectra of the same cell line grown as orthotopic tumor (bottom). Assignments: Cho, free choline; GPC, glycerophosphocholine; Lac, lactate; Lipid-CH2 –, methylene groups of mobile lipids; Lipid-CH3 –, methyl groups of mobile lipids; PCho, phosphocholine; tCho, total choline-containing compounds (Cho + PC + GPC). The metabolites Cho, PCho, GPC, and Lac and their respective 1 H signals in the MR spectra are color-coded to identify the MR signals that arise from the corresponding metabolites.
cells, detected by 1 H MRS (Fig. 1B) [10]. As shown in Fig. 1A, enhanced choline transport into cancer cells is a dominant cause for the ‘cholinic phenotype’ [10]. Increased choline kinase alpha (CHKa) expression and activity has been detected in multiple cancers including breast cancer, and is directly associated with increased cancer cell proliferation and increased malignancy [10]. Activation of CTP: phosphocholine cytidylyltransferase (CCT), phosphatidylcholine (PtdCho)-specific phospholipases C, D, and A2 , and glycerophosphocholine phosphodiesterase (GPC-PDE) [11] have also been reported in some cancers, including breast cancer [10]. The result of the concerted activation of these enzymes is the elevation of PCho and tCho.
As shown in Fig. 1A, the activation of these choline-metabolizing enzymes in cancer is caused by the same oncogenes and oncogenic signaling pathways that upregulate glycolytic enzymes in cancer. The PI3K/AKT/mTOR signaling pathway positively regulates CHKa [4]. PI3K inhibition results in a PCho and tCho decrease as monitored by MRS [4,10]. HIF-1 positively regulates CHKa along with cellular PCho and tumor tCho levels as detected by MRS [4,10]. The Her2 oncogene in breast cancer significantly increases PCho and tCho, and inhibition of PtdCho-PLC downregulates Her2 [4,10]. Ras oncogene transformation can inhibit CCT, decrease PtdChoPLC, activate PtdCho-PLD, and increase CHKa [4,10]. Inhibition of extracellular signal-regulated kinases (ERK), which are frequently
S68
L. Jiang et al. / European Journal of Radiology 81S1 (2012) S66–S68
deregulated in several types of cancer, can result in a significant drop in the PCho levels of cancer cells [4,10]. The connection between oncogenic signaling pathways and choline metabolism enable the use of the tCho and PCho signals as surrogate markers for molecular therapies that target these oncogenic signaling pathways [4,10]. Elevated tCho levels have been reported in several ongoing multi-center trials that are currently evaluating 1 H MRS/I for cancer detection in breast, brain, and prostate cancer [4,10]. In breast cancer, increased tCho has been associated with cancer aggressiveness [4,10]. In breast and ovarian cancers, malignant transformation leads to a ‘switch’ of low PCho and high glycerophosphocholine (GPC) in nonmalignant cells to high PCho and low GPC in malignant cells, which can be assessed by 1 H HR MAS MRS ex vivo [4]. Choline metabolism can be studied with 31 P MRS because PCho and GPC contain 31 P atoms with a better spectral separation than PCho and GPC have in 1 H MRS. However, additional signals from phosphoethanolamine (PEth) and glyerophosphoethanolamine (GPE) are detected in 31 P MRS, which can overlap with PCho and GPC, respectively. Although 1 H MRS has a higher sensitivity and is more popular on clinical scanners than 31 P MRS, the utility of in vivo 31 P MRS in detecting human cancers is currently being investigated. The availability of clinical 7 T scanners now allows for separating PCho and PEth, which can be achieved within 20 min in three dimensions over the entire breast with pixel volumes of 10 mL [12]. At lower fields, e.g. 3T, 1 H to 31 P polarization transfer methods can be applied to eliminate homonuclear J-coupling effects that attenuate 31 P signals from PEth, PCho, GPE, and GPC, which can achieve a more than twofold increase of SNR compared to direct 31 P MRS methods [13]. 1 H-decoupled 31 P HR MAS MRS has also been used to investigate choline metabolism in cancer [12]. In addition to diagnosis and staging, MRS of choline signals has shown promising results in monitoring the treatment with conventional chemotherapeutic agents, which results in a decrease of tCho in responding tumors in preclinical models and in
human tumors [4]. In addition, as novel targeted therapies become available, MRS of tCho and PCho can provide biomarkers of treatment response. Competing interests: The authors have no conflicts of interest to report. References 1. Glunde K, Serkova NJ. Therapeutic targets and biomarkers identified in cancer choline phospholipid metabolism. Pharmacogenomics 2006;7:1109–23. 2. Costello LC, Franklin RB. ‘Why do tumour cells glycolyse?’: from glycolysis through citrate to lipogenesis. Mol Cell Biochem 2005;280:1–8. ¨ 3. Warburg O. Uber den Stoffwechsel der Carcinomzelle. Naturwissenschaften 1924;12:1132–37. 4. Glunde K, Jiang L, Moestue SA, et al. MRS and MRSI guidance in molecular medicine: targeting and monitoring of choline and glucose metabolism in cancer. NMR Biomed 2011;24:673–90. 5. Dang CV, Le A, Gao P. MYC-induced cancer cell energy metabolism and therapeutic opportunities. Clin Cancer Res 2009;15:6479–83. 6. Matoba S, Kang JG, Patino WD, et al. p53 regulates mitochondrial respiration. Science 2006;312:1650–3. 7. Semenza GL, Roth PH, Fang HM, et al. Transcriptional regulation of genes encoding glycolytic enzymes by hypoxia-inducible factor 1. J Biol Chem 1994;269:23757–63. 8. Gillies RJ, Morse DL. In vivo magnetic resonance spectroscopy in cancer. Annu Rev Biomed Eng 2005;7:287–326. 9. Hu S, Lustig M, Balakrishnan A, et al. 3D compressed sensing for highly accelerated hyperpolarized (13)C MRSI with in vivo applications to transgenic mouse models of cancer. Magn Reson Med 2010;63:312–21. 10. Glunde K, Bhujwalla ZM, Ronen SM. Choline metabolism in malignant transformation. Nat Rev Cancer 2011;11:835–48. 11. Cao MD, Dopkens M, Krishnamachary B, et al. Glycerophosphodiester phosphodiesterase domain containing 5 (GDPD5) expression correlates with malignant choline phospholipid metabolite profiles in human breast cancer. NMR Biomed 2012 Jan 23 [Epub ahead of print]. 12. Klomp DW, van de Bank BL, Raaijmakers A, et al. 31P MRSI and 1H MRS at 7 T: initial results in human breast cancer. NMR Biomed 2011;24:1337–42. 13. Wijnen JP, Scheenen TW, Klomp DW, et al. 31P magnetic resonance spectroscopic imaging with polarisation transfer of phosphomono- and diesters at 3 T in the human brain: relation with age and spatial differences. NMR Biomed 2010;23: 968–76.
Contents lists available at SciVerse ScienceDirect
European Journal of Radiology journal homepage: www.elsevier.com/locate/ejrad
Malignant transformation: The role of MRS Lu Jianga , Jannie P. Wijnenb , Kristine Glundea,c, * a Division
of Cancer Imaging Research, Russell H. Morgan Department, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA of Radiology, University Medical Centre Utrecht, Utrecht, Netherlands c Sidney Kimmel Comprehensive Cancer Center, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA b Department
Magnetic Resonance Spectroscopy (MRS) has been instrumental in elucidating the metabolic changes that arise as a consequence of malignant transformation. The metabolic phenotype of cancers is characterized by high glucose uptake, high glycolytic activity and lactate production, low mitochondrial activity, low bioenergetic status, and abnormal phospholipid metabolism [1,2]. As shown in Fig. 1, noninvasive 1 H MRS is able to detect elevated lactate and increased total choline (tCho) levels in breast (and other) cancers, which primarily arise from oncogenic signaling pathways in cancer cells as indicated by green boxes in Fig. 1A. The increased levels of lactate and tCho provide noninvasive MR biomarkers of transformation, staging, and response to therapy. Enzymes in glucose and choline metabolism present targets for image-guided cancer therapy. 1. Glucose metabolism in cancer Tumors exhibit abnormal glucose metabolism. Cancer cells rapidly take up and utilize glucose, which is converted to lactate under aerobic conditions unlike in normal cells, in which glucose is predominantly utilized by the tricarboxylic acid (TCA) cycle. Otto Warburg first studied this phenomenon in the 1920’s, making it known as the Warburg effect [3]. As shown in Fig. 1A, cancer cells exhibit enhanced glucose uptake through upregulation of facilitative glucose transporters (GLUTs), primarily GLUT1, which have been attributed to malignant transformation [4]. Several glycolytic enzymes such as hexokinase (HK2), 6-phosphofructokinase (PFK1), and pyruvate kinase muscle isozyme 2 (PKM2) among others are upregulated in cancer [4]. Lactate dehydrogenase A (LDHA) is upregulated in cancers as well and converts pyruvate to lactate, thereby regenerating NAD+ from NADH to avoid NAD+ depletion as a result of glycolytic NAD+ consumption. Thus, the net result of upregulated aerobic glycolysis in cancer cells is the formation of large amounts of lactate, leading to abnormally high lactate concentrations [4]. The upregulation of these glycolytic enzymes in cancer is caused by oncogenes and oncogenic signaling pathways as indicated by the green lines in Fig. 1A. The PI3K/AKT/mTOR signaling pathway, which is frequently activated in cancer, regulates glycolytic activity by GLUT1 induction and HK2 activation [4]. Increased expression of the oncogenic transcription factor c-Myc induces lactate dehydrogenase A (LDHA), which converts pyruvate to lactate [5]. c-Myc also directly regulates other glycolytic genes such as GLUT1, HK2, PFK1 and enolase 1 (ENO1) [5]. Mutations in p53, which is a tumor suppressor gene that is frequently deregulated in cancers, * E-mail address: [email protected] (K. Glunde). 0720-048X/$ – see front matter © 2012 Elsevier Ireland Ltd. All rights reserved.
can modulate glycolysis through various transcription factors [6]. The hypoxia-inducible factor 1 (HIF-1) transcription factor, which is activated in several cancers, stimulates LDHA, induces GLUTs and several glycolytic enzymes, and reduces pyruvate dehydrogenase kinase 1 (PDK1) activity [7]. Combined, these oncogenic signaling pathways enhance glucose transport, glycolytic flux, and lactate production, which can be detected by 1 H MRS as shown in Fig. 1B. Different MRS techniques have been used to study glucose metabolism in cancer in cell cultures, tissue samples, and extracts thereof, as well as in animal models and human subjects in vivo. Glucose consumption and lactate production are important parameters in cancer and have been studied extensively. Other glycolytic metabolites cannot be easily detected with 1 H MRS due to their low concentrations. Lactate is present in measurable concentrations in some cancers and has been measured in the clinic with 1 H MRS [8]. In clinical applications, 1 H MRS/I can be used in combination with MR imaging (MRI) for diagnostic purposes. Ex vivo analysis of biopsy tissue can be an alternative approach for measuring lactate concentrations. Sample collection, handling, storage, and preparation needs to be done carefully to avoid degradation of metabolites [4]. If all precautions are taken, the lactate concentration can be measured accurately ex vivo using high-resolution (HR) magic angle spinning (MAS) MRS [4]. Altered glucose metabolism in cancer has been studied with 13 C MRS for more than two decades by tracing the 13 C-label in metabolites produced from [1-13 C]-glucose or other isotopically enriched substrates in cancer cell lines, animal models of cancer, as well as in human subjects in vivo [4]. However, 13 C MRS is currently not employed in clinical diagnosis or management of cancer due to its low sensitivity, which may change with further development of MRS techniques, such as for example hyperpolarized 13 C MRS. The introduction of dynamic nuclear polarization (DNP) has revitalized 13 C MRS as hyperpolarized 13 C-labeled substrates increase the signal-to-noise ratio by up to 10,000-fold [9]. However, the time window for hyperpolarized 13 C MRSI is short and requires completion within a few minutes following tracer preparation. Hyperpolarized 13 C MRSI can be performed in small animals with voxel sizes in the range of 0.135 cm3 with a temporal resolution of around 5 seconds [9]. Hyperpolarized [1-13 C] pyruvate, among others, has been used in DNP MRS studies of preclinical models of cancer and has been suggested for cancer detection and therapy monitoring [4]. 2. Choline metabolism in cancer Choline phospholipid metabolism is activated in tumors and leads to increased phosphocholine (PCho) and tCho levels in cancer
L. Jiang et al. / European Journal of Radiology 81S1 (2012) S66–S68
A
Glucose
Choline
GLUT
CTs
PI3K
Glucose
Choline (Cho)
Akt
HK2
Glycerophosphocholine (GPC)
GPC-PDE
CHKD
PFK1
mTOR Phosphocholine (PCho)
PKM2
HIF-1
Pyruvate
CPT
c-Myc LDHA
Her2
Lactate (Lac)
TCA cycle
LPL
CCT
p53
PDH Acetyl-CoA
S67
PtdChoPLC
PtdChoPLD
Phosphatidylcholine
PtdChoPLA2
Ras Raf Erk
B
PCho PCho
GPC
GPC
Lac
Cho Cho PPM
PPM
3.5
3.0
3.24
2.5
3.20
2.0
1.5
1.0
Lac/Lipid-CH3tCho
PPM
3.5
Lipid-CH2-
3.0
2.5
2.0
1.5
1.0
Fig. 1. (A) Schematic showing the most important cancer-related cell signaling pathways (center) that impact upon enzymes in glucose metabolism (left) and enzymes in choline phospholipid metabolism (right). Arrows depict an activation in the direction of the arrow, and short vertical lines at the end of a line symbolize inhibition in the direction of vertical line. Abbreviations: acetyl-CoA, acetyl coenzyme A; AKT, v-akt murine thymoma viral oncogene homolog 1; CCT, CTP: phosphocholine cytidylyltransferase; Chk-a, choline kinase alpha; c-Myc, v-myc myelocytomatosis viral oncogene homolog; CPT, diacylglycerol; CT, choline transporter; CTP, cytidine-5 -triphosphate; Erk, extracellular signal-regulated kinases; GLUT, glucose transporter; GPC-PDE, glycerophosphocholine phosphodiesterase; HIF-1, hypoxia inducible factor 1; HK, hexokinase; LDH, lactate dehydrogenase; LPL, lysophospholipase; mTOR, mammalian target of rapamycin; PDH, pyruvate dehydrogenase; PFK1, phosphofructokinase 1; PI3K, phosphatidylinositol 3-kinase; PKM2, pyruvate kinase muscle isozyme 2; PtdCho-PLA2 , phosphatidylcholine-specific phospholipase A2 ; PtdCho-PLC, phosphatidylcholine-specific phospholipase C; PtdCho-PLD, phosphatidylcholine-specific phospholipase D; TCA, tricarboxylic acid. (B) High-resolution ex vivo 1 H MR spectra of triple-negative human MDA-MB231 breast cancer cell extracts (top) and in vivo 1 H MR spectra of the same cell line grown as orthotopic tumor (bottom). Assignments: Cho, free choline; GPC, glycerophosphocholine; Lac, lactate; Lipid-CH2 –, methylene groups of mobile lipids; Lipid-CH3 –, methyl groups of mobile lipids; PCho, phosphocholine; tCho, total choline-containing compounds (Cho + PC + GPC). The metabolites Cho, PCho, GPC, and Lac and their respective 1 H signals in the MR spectra are color-coded to identify the MR signals that arise from the corresponding metabolites.
cells, detected by 1 H MRS (Fig. 1B) [10]. As shown in Fig. 1A, enhanced choline transport into cancer cells is a dominant cause for the ‘cholinic phenotype’ [10]. Increased choline kinase alpha (CHKa) expression and activity has been detected in multiple cancers including breast cancer, and is directly associated with increased cancer cell proliferation and increased malignancy [10]. Activation of CTP: phosphocholine cytidylyltransferase (CCT), phosphatidylcholine (PtdCho)-specific phospholipases C, D, and A2 , and glycerophosphocholine phosphodiesterase (GPC-PDE) [11] have also been reported in some cancers, including breast cancer [10]. The result of the concerted activation of these enzymes is the elevation of PCho and tCho.
As shown in Fig. 1A, the activation of these choline-metabolizing enzymes in cancer is caused by the same oncogenes and oncogenic signaling pathways that upregulate glycolytic enzymes in cancer. The PI3K/AKT/mTOR signaling pathway positively regulates CHKa [4]. PI3K inhibition results in a PCho and tCho decrease as monitored by MRS [4,10]. HIF-1 positively regulates CHKa along with cellular PCho and tumor tCho levels as detected by MRS [4,10]. The Her2 oncogene in breast cancer significantly increases PCho and tCho, and inhibition of PtdCho-PLC downregulates Her2 [4,10]. Ras oncogene transformation can inhibit CCT, decrease PtdChoPLC, activate PtdCho-PLD, and increase CHKa [4,10]. Inhibition of extracellular signal-regulated kinases (ERK), which are frequently
S68
L. Jiang et al. / European Journal of Radiology 81S1 (2012) S66–S68
deregulated in several types of cancer, can result in a significant drop in the PCho levels of cancer cells [4,10]. The connection between oncogenic signaling pathways and choline metabolism enable the use of the tCho and PCho signals as surrogate markers for molecular therapies that target these oncogenic signaling pathways [4,10]. Elevated tCho levels have been reported in several ongoing multi-center trials that are currently evaluating 1 H MRS/I for cancer detection in breast, brain, and prostate cancer [4,10]. In breast cancer, increased tCho has been associated with cancer aggressiveness [4,10]. In breast and ovarian cancers, malignant transformation leads to a ‘switch’ of low PCho and high glycerophosphocholine (GPC) in nonmalignant cells to high PCho and low GPC in malignant cells, which can be assessed by 1 H HR MAS MRS ex vivo [4]. Choline metabolism can be studied with 31 P MRS because PCho and GPC contain 31 P atoms with a better spectral separation than PCho and GPC have in 1 H MRS. However, additional signals from phosphoethanolamine (PEth) and glyerophosphoethanolamine (GPE) are detected in 31 P MRS, which can overlap with PCho and GPC, respectively. Although 1 H MRS has a higher sensitivity and is more popular on clinical scanners than 31 P MRS, the utility of in vivo 31 P MRS in detecting human cancers is currently being investigated. The availability of clinical 7 T scanners now allows for separating PCho and PEth, which can be achieved within 20 min in three dimensions over the entire breast with pixel volumes of 10 mL [12]. At lower fields, e.g. 3T, 1 H to 31 P polarization transfer methods can be applied to eliminate homonuclear J-coupling effects that attenuate 31 P signals from PEth, PCho, GPE, and GPC, which can achieve a more than twofold increase of SNR compared to direct 31 P MRS methods [13]. 1 H-decoupled 31 P HR MAS MRS has also been used to investigate choline metabolism in cancer [12]. In addition to diagnosis and staging, MRS of choline signals has shown promising results in monitoring the treatment with conventional chemotherapeutic agents, which results in a decrease of tCho in responding tumors in preclinical models and in
human tumors [4]. In addition, as novel targeted therapies become available, MRS of tCho and PCho can provide biomarkers of treatment response. Competing interests: The authors have no conflicts of interest to report. References 1. Glunde K, Serkova NJ. Therapeutic targets and biomarkers identified in cancer choline phospholipid metabolism. Pharmacogenomics 2006;7:1109–23. 2. Costello LC, Franklin RB. ‘Why do tumour cells glycolyse?’: from glycolysis through citrate to lipogenesis. Mol Cell Biochem 2005;280:1–8. ¨ 3. Warburg O. Uber den Stoffwechsel der Carcinomzelle. Naturwissenschaften 1924;12:1132–37. 4. Glunde K, Jiang L, Moestue SA, et al. MRS and MRSI guidance in molecular medicine: targeting and monitoring of choline and glucose metabolism in cancer. NMR Biomed 2011;24:673–90. 5. Dang CV, Le A, Gao P. MYC-induced cancer cell energy metabolism and therapeutic opportunities. Clin Cancer Res 2009;15:6479–83. 6. Matoba S, Kang JG, Patino WD, et al. p53 regulates mitochondrial respiration. Science 2006;312:1650–3. 7. Semenza GL, Roth PH, Fang HM, et al. Transcriptional regulation of genes encoding glycolytic enzymes by hypoxia-inducible factor 1. J Biol Chem 1994;269:23757–63. 8. Gillies RJ, Morse DL. In vivo magnetic resonance spectroscopy in cancer. Annu Rev Biomed Eng 2005;7:287–326. 9. Hu S, Lustig M, Balakrishnan A, et al. 3D compressed sensing for highly accelerated hyperpolarized (13)C MRSI with in vivo applications to transgenic mouse models of cancer. Magn Reson Med 2010;63:312–21. 10. Glunde K, Bhujwalla ZM, Ronen SM. Choline metabolism in malignant transformation. Nat Rev Cancer 2011;11:835–48. 11. Cao MD, Dopkens M, Krishnamachary B, et al. Glycerophosphodiester phosphodiesterase domain containing 5 (GDPD5) expression correlates with malignant choline phospholipid metabolite profiles in human breast cancer. NMR Biomed 2012 Jan 23 [Epub ahead of print]. 12. Klomp DW, van de Bank BL, Raaijmakers A, et al. 31P MRSI and 1H MRS at 7 T: initial results in human breast cancer. NMR Biomed 2011;24:1337–42. 13. Wijnen JP, Scheenen TW, Klomp DW, et al. 31P magnetic resonance spectroscopic imaging with polarisation transfer of phosphomono- and diesters at 3 T in the human brain: relation with age and spatial differences. NMR Biomed 2010;23: 968–76.