**4. Phospholipids metabolism in metabolic diseases**

Phospholipids are polar lipids as major component of membrane structure and some intracellular complex such as lipoproteins. Enzymes involved in the metabolism of phospholipids include phospholipase A2 (PLA2), phospholipase C (PLC), phospholipase D (PLD), and lysophospholipase D (autotoxin), and alterations of these enzymes have been found to be linked with metabolic diseases, such as MS and cancer. In addition, the intermediates or end products of phospholipid metabolism such as phosphatidic acid (PA), DAG, LPA, sphingosine-1-phoshate (S-1-P), and free fatty acid arichidonic acid (ARA), are also involved in the pathogenesis of metabolic diseases.

#### **4.1. Phospholipid metabolism in metabolic syndrome**

Phosphatidylcholine (PC) is the most abundant phospholipids in animal cells. Blocking Sadenosylmethionine (SAMe) or PC synthesis in C. elegans, mouse liver, and human cells have been found to cause elevated SREBP-1-dependent transcription and lipid droplet accumulation [4], suggesting nutritional or genetic conditions limiting SAMe or PC production may activate SREBP-1, and contribute to human metabolic disorders.

Phosphatidylethanolamine (PE) is another abundant phospholipid in mammals. PE and its downstream signaling events play an important role in the heart function, and alteration in the asymmetrical transbilayer distribution of PE in sarcolemmal membranes during ischemia causes sarcolemmal disruption [140]. Moreover, abnormalities in the molecular species profile of PE may contribute to membrane dysfunction and defective contractility of the diabetic heart [141, 142].

SREBPs may play critical roles in phospholipid homeostasis and lipotoxic cardiomyopathy. Dysregulated phospholipid signaling that alters SREBP activity has been reported to contribute to the progression of impaired heart function in flies and also act as a potential link to lipotoxic cardiac diseases in humans [143]. Thus the role of SREBPs in modulating heart function and its associated phospholipid signaling maybe a candidate target for future therapies for obesity- and diabetes- related cardiac dysfunction.

#### **4.2. Phospholipid metabolism in cancer**

196 Lipid Metabolism

FAS inhibitors have been developed [123[130], and the recent progress in developing FAS

Apart from FAS, other key lipogenic enzymes for de novo FA biosynthesis include ACL and ACC. While ACL produce the substrate acetyl-CoA from glycolytic product citrate, ACC activates the substrate to generate malanyl-CoA, the building block for fatty acid synthesis. Both ACC and ACL have been found to be over-expressed in many cancers such as breast, liver, lung, ovarian, prostate and leukemia cancers [132, 133]. Inhibition of either ACL or ACC induces growth arrest and apoptosis in several cancer cell lines [134-136]. The potential mechanism of ACL overexpression in tumorigenesis is through PI3K/AKT and MAPK signaling pathway [135, 137]. Yoon et al [138] found that the major mechanism of HER2 mediated induction of ACC alpha in breast cancer cells is translational regulated primarily through mTOR signaling pathway. While Mukherjee et al [117] found that LPA induced induction of ACC in ovarian cancer cells is through LPA2-Gq-PLC-AMPK signaling pathway. Many small molecule inhibitors for ACL and ACC have been developed as

Phospholipids are polar lipids as major component of membrane structure and some intracellular complex such as lipoproteins. Enzymes involved in the metabolism of phospholipids include phospholipase A2 (PLA2), phospholipase C (PLC), phospholipase D (PLD), and lysophospholipase D (autotoxin), and alterations of these enzymes have been found to be linked with metabolic diseases, such as MS and cancer. In addition, the intermediates or end products of phospholipid metabolism such as phosphatidic acid (PA), DAG, LPA, sphingosine-1-phoshate (S-1-P), and free fatty acid arichidonic acid (ARA), are

Phosphatidylcholine (PC) is the most abundant phospholipids in animal cells. Blocking Sadenosylmethionine (SAMe) or PC synthesis in C. elegans, mouse liver, and human cells have been found to cause elevated SREBP-1-dependent transcription and lipid droplet accumulation [4], suggesting nutritional or genetic conditions limiting SAMe or PC

Phosphatidylethanolamine (PE) is another abundant phospholipid in mammals. PE and its downstream signaling events play an important role in the heart function, and alteration in the asymmetrical transbilayer distribution of PE in sarcolemmal membranes during ischemia causes sarcolemmal disruption [140]. Moreover, abnormalities in the molecular species profile of PE may contribute to membrane dysfunction and defective contractility of

production may activate SREBP-1, and contribute to human metabolic disorders.

inhibitors as cancer drugs has been reviewed by Pandey et al [131].

*3.2.2. ATP-Citrate lyase and acetyl CoA carboxylase* 

potential therapeutic agents for cancer [133, 139].

**4. Phospholipids metabolism in metabolic diseases** 

also involved in the pathogenesis of metabolic diseases.

the diabetic heart [141, 142].

**4.1. Phospholipid metabolism in metabolic syndrome** 

An aberrant choline phospholipid metabolism is another major hallmark of cancer cells. In deed alterations of choline phospholipid metabolism have been reported in ovarian cancer and also in breast cancer [144, 145]. Altered choline phospholipid metabolism in ovarian cancer has been found to be linked with the regulation of FAS. Because the drop in the level of PC (59%) was significantly correlated with a drop in *de novo* synthesized FA levels, PC was identified as a potential noninvasive magnetic resonance spectroscopy–detectable biomarker of FAS inhibition *in vivo* [146]. Phospholipids and their metabolism have been found to be involved in ovarian cancer in several forms, including LPA, PLA2, PLD, and autotoxin (ATX). Although aberrant phospholipid metabolism has been found in other cancers, the most detailed research work has been carried out using ovarian cancer as a model, so in this section we summarized the recent advances in the research of phospholipid metabolism and ovarian cancer.

#### *4.2.1. Lysophosphatidic acid*

The LPAs, with their various FA side chains, are the constituents of a growth-stimulating factor—ovarian cancer activating factor—that has been identified from ascites in patients with ovarian cancer [147]. As a bioactive compound, LPA works to induce cell proliferation or differentiation, prevents apoptosis induced by environmental stress or stimuli, induce platelet aggregation and smooth muscle contraction, and stimulate morphological changes, adhesion and migration of cells. It thus is involved in a broad range of biologic processes in a variety of cellular systems [148, 149]. As an established mitogen, LPA also promotes the invasiveness of hepatoma cells into monolayers of mesothelial cells, and stimulates proliferation of ovarian and breast cancer cell lines even in the absence of other growth promoters such as serum. Furthermore, LPA stimulates rapid neurite retraction and rounding of the cell body in serum-deprived neuroblastoma cells [150], and plays a critical role in regulation of gene expression in normal and neoplastic cells. It is a potent modulator of the expression of genes involved in inflammation, angiogenesis, and carcinogenesis such as interleukin [151-154], vascular endothelial growth factor (VEGF) [155], urokinase plasminogen activator [156], and cyclooxygenase-2 [157]. Thus LPA may contribute to cancer progression by triggering expression of those target genes, resulting in a more invasive and metastatic microenvironment for tumor cells [152, 158]. A significant increase in the expression of LPA receptors (LPA2 and LPA3) with VEGF was found by Fujita *et al.*  [159], who suggested that LPA receptors might be involved in VEGF expression mediated by LPA signals in human ovarian oncogenesis. The recent identification of metabolizing enzymes that mediate the degradation and production of LPA and the development of receptor selective-analogs has opened a potential new approach to the treatment of ovarian cancer [160]. LPA also stimulates VEGF expression independent of hypoxia-inducible factor 1 (H1F1) and promotes tumor angiogenesis by activation of c-Myc and Sp-1 transcription factors [161]. A very recent study shows that LPA induces de novo lipogenesis through LPA2-G12/13-SREBP-FAS, and LPA2-G(q)-AMPK-ACC signaling pathway.

Lipid Metabolism, Metabolic Syndrome, and Cancer 199

connection for integrin and PLD-mediated cancer metastasis [167]. A new mechanism has also been suggested for PLD and PA mediated carcinogenesis through Wnt/β-catenin

The ATX protein is a member of the ectonucleotide pyrophosphatase and phosphodiesterase family of enzymes, but unlike other members of this group, ATX possesses lysophospholipase D activity. This enzyme hydrolyzes lysophosphatidylcholine (LPC) to generate bioactive lipid LPA, which is an important signaling molecule regulates a variety of biological process through its receptors. ATX is essential for normal development and is implicated in various physiological processes. It also acts as a potent tumor growth factor and mitogen that is, associated with pathological conditions such as cancer, pain and fibrosis. Exogenous addition of VEGF-A to cultured cells induces ATX expression and secretion, resulting in increased extracellular LPA production [169]. This elevated LPA, acting through LPA4, modulates VEGF responsiveness by inducing VEGF receptor 2 expression. Downregulation of ATX secretion in SKOV3 cells significantly attenuates cell motility responses to VEGF, ATX, LPA, LPC [169]. Through their respective G protein– coupled receptors, LPC and LPA have both been reported to stimulate migration [170]. LPC was unable to stimulate the cellular migration by itself, ATX had to be present. Knocking down ATX secretion, or inhibiting its catalytic activity, blocked cellular migration by

preventing LPA production and the subsequent activation of LPA receptors [170].

would be helpful for health and prevention of both MS and cancer.

As a combination of central obesity, dyslipidemia, and insulin resistance, MS is the central of world–wide prevalence of Type 2 Diabetes Mellitus (T2DM), cardiovascular diseases and inflammation. Current animal and clinical evidence strongly suggest that abnormal lipid metabolism is closely associated with onset of insulin resistance and cancer. Importantly, more and more evidence show that most of the components of the MS are linked in some way to the development of various cancers [171-173], although epidemiological studies linking the MS to cancer are highly required. Obesity and diabetes have been reported to be associated with breast, endometrial, colorectal, pancreatic, hepatic or renal cancer [174, 175]. The molecular links between MS and cancer are still unclear, but insulin/insulin-like growth factor (IGF) systems and associated intracellular signaling cascades may play an important role in mediating MS related cancers [173]. However, the mechanisms by which actually promote tumor cell growth in patients with MS need further investigation. Since lipids and their metabolites and metabolism pathways are related to metabolic diseases and cancer cell growth, we propose that lipids may link to MS and cancers and exploring the related molecules and understanding the underlying mechanisms will be helpful in developing potential therapies for both MS and cancer. Based on the discoveries of current research results, a diet with high amount of oleic acid and balanced ratio of omega-3/omega-6 PUFAs

signaling network [168].

*4.2.4. Autotaxin* 

**5. Summary** 

#### *4.2.2. Phospholipase A2*

The PLA2 enzyme has been implicated in the activation of cell migration and the production of LPA in ovarian carcinoma cells [162]. Autonomous replication and growth-factorstimulated proliferation of ovarian cancer cells are highly sensitive to inhibition of calciumindependent PLA2 (iPLA2), but are refractory to inhibition of cytosolic PLA2 [162]. Activation of iPLA2 plays a critical role in cell migration, which is involved in many important biologic processes such as development, the immunologic and inflammatory responses, and tumor biology [162]. When ovarian cancer cells were grown under growthfactor-independent conditions, suppression of iPLA2 activity led to an accumulation of cell populations in both the S and the G2/M-phases [163]. Supplementation with exogenous growth factors such as LPA and epidermal growth factor in culture released the S-phase arrest, but did not affect the G2/M arrest associated with inhibition of iPLA2. In addition to the prominent effect on the cell cycle, inhibition of iPLA2 also induced weak-to-modest increases in apoptosis [163]. Downregulation of iPLA2 �with lentivirus-mediated RNA interference targeting iPLA2 �expression inhibited cell proliferation in culture and decreased tumorigenicity of ovarian cancer cell lines in athymic nude mice [163]. Recently iPLA2 has been found to play a role in breast cancer metastasis as iPLA2 deficiency protects breast cancer from metastasis to the lung [164].

#### *4.2.3. Phospholipase D*

PLD, a family of signaling enzymes that most commonly responsible to generate most lipid second messenger phosphatidic acid (PA), is found in diverse organisms from bacteria to humans and functions in multiple cellular pathways. It has been increasingly recognized as a critical regulator of cell proliferation and tumorigenesis and the expression and activity of PLD are elevated in many different types of human cancers.

In ovarian cancer cells, PLD is involved in the formation of PA, which may be further converted to LPA by PLA2. It was suggested that PLD is also involved in cancer progression and metastasis and elevated PLD expression has been reported in various cancer tissues [165]. Moreover, PLD was found to stimulate cell protrusions in v-Src–transformed cells [166]. Furthermore, PLD activity was elevated by the integrin receptor signaling pathway in OVCAR-3 cells, and PLD blocking was found to inhibit integrin-mediated Rac translocation in, and the spreading and migration of, OVCAR-3 cells [167]. Thus, the PLD-PA-Rac pathway plays an important role in the metastasis of cancer cells, and might provide a connection for integrin and PLD-mediated cancer metastasis [167]. A new mechanism has also been suggested for PLD and PA mediated carcinogenesis through Wnt/β-catenin signaling network [168].

#### *4.2.4. Autotaxin*

198 Lipid Metabolism

*4.2.2. Phospholipase A2*

cancer from metastasis to the lung [164].

PLD are elevated in many different types of human cancers.

*4.2.3. Phospholipase D* 

by LPA signals in human ovarian oncogenesis. The recent identification of metabolizing enzymes that mediate the degradation and production of LPA and the development of receptor selective-analogs has opened a potential new approach to the treatment of ovarian cancer [160]. LPA also stimulates VEGF expression independent of hypoxia-inducible factor 1 (H1F1) and promotes tumor angiogenesis by activation of c-Myc and Sp-1 transcription factors [161]. A very recent study shows that LPA induces de novo lipogenesis through

The PLA2 enzyme has been implicated in the activation of cell migration and the production of LPA in ovarian carcinoma cells [162]. Autonomous replication and growth-factorstimulated proliferation of ovarian cancer cells are highly sensitive to inhibition of calciumindependent PLA2 (iPLA2), but are refractory to inhibition of cytosolic PLA2 [162]. Activation of iPLA2 plays a critical role in cell migration, which is involved in many important biologic processes such as development, the immunologic and inflammatory responses, and tumor biology [162]. When ovarian cancer cells were grown under growthfactor-independent conditions, suppression of iPLA2 activity led to an accumulation of cell populations in both the S and the G2/M-phases [163]. Supplementation with exogenous growth factors such as LPA and epidermal growth factor in culture released the S-phase arrest, but did not affect the G2/M arrest associated with inhibition of iPLA2. In addition to the prominent effect on the cell cycle, inhibition of iPLA2 also induced weak-to-modest increases in apoptosis [163]. Downregulation of iPLA2 �with lentivirus-mediated RNA interference targeting iPLA2 �expression inhibited cell proliferation in culture and decreased tumorigenicity of ovarian cancer cell lines in athymic nude mice [163]. Recently iPLA2 has been found to play a role in breast cancer metastasis as iPLA2 deficiency protects breast

PLD, a family of signaling enzymes that most commonly responsible to generate most lipid second messenger phosphatidic acid (PA), is found in diverse organisms from bacteria to humans and functions in multiple cellular pathways. It has been increasingly recognized as a critical regulator of cell proliferation and tumorigenesis and the expression and activity of

In ovarian cancer cells, PLD is involved in the formation of PA, which may be further converted to LPA by PLA2. It was suggested that PLD is also involved in cancer progression and metastasis and elevated PLD expression has been reported in various cancer tissues [165]. Moreover, PLD was found to stimulate cell protrusions in v-Src–transformed cells [166]. Furthermore, PLD activity was elevated by the integrin receptor signaling pathway in OVCAR-3 cells, and PLD blocking was found to inhibit integrin-mediated Rac translocation in, and the spreading and migration of, OVCAR-3 cells [167]. Thus, the PLD-PA-Rac pathway plays an important role in the metastasis of cancer cells, and might provide a

LPA2-G12/13-SREBP-FAS, and LPA2-G(q)-AMPK-ACC signaling pathway.

The ATX protein is a member of the ectonucleotide pyrophosphatase and phosphodiesterase family of enzymes, but unlike other members of this group, ATX possesses lysophospholipase D activity. This enzyme hydrolyzes lysophosphatidylcholine (LPC) to generate bioactive lipid LPA, which is an important signaling molecule regulates a variety of biological process through its receptors. ATX is essential for normal development and is implicated in various physiological processes. It also acts as a potent tumor growth factor and mitogen that is, associated with pathological conditions such as cancer, pain and fibrosis. Exogenous addition of VEGF-A to cultured cells induces ATX expression and secretion, resulting in increased extracellular LPA production [169]. This elevated LPA, acting through LPA4, modulates VEGF responsiveness by inducing VEGF receptor 2 expression. Downregulation of ATX secretion in SKOV3 cells significantly attenuates cell motility responses to VEGF, ATX, LPA, LPC [169]. Through their respective G protein– coupled receptors, LPC and LPA have both been reported to stimulate migration [170]. LPC was unable to stimulate the cellular migration by itself, ATX had to be present. Knocking down ATX secretion, or inhibiting its catalytic activity, blocked cellular migration by preventing LPA production and the subsequent activation of LPA receptors [170].

#### **5. Summary**

As a combination of central obesity, dyslipidemia, and insulin resistance, MS is the central of world–wide prevalence of Type 2 Diabetes Mellitus (T2DM), cardiovascular diseases and inflammation. Current animal and clinical evidence strongly suggest that abnormal lipid metabolism is closely associated with onset of insulin resistance and cancer. Importantly, more and more evidence show that most of the components of the MS are linked in some way to the development of various cancers [171-173], although epidemiological studies linking the MS to cancer are highly required. Obesity and diabetes have been reported to be associated with breast, endometrial, colorectal, pancreatic, hepatic or renal cancer [174, 175]. The molecular links between MS and cancer are still unclear, but insulin/insulin-like growth factor (IGF) systems and associated intracellular signaling cascades may play an important role in mediating MS related cancers [173]. However, the mechanisms by which actually promote tumor cell growth in patients with MS need further investigation. Since lipids and their metabolites and metabolism pathways are related to metabolic diseases and cancer cell growth, we propose that lipids may link to MS and cancers and exploring the related molecules and understanding the underlying mechanisms will be helpful in developing potential therapies for both MS and cancer. Based on the discoveries of current research results, a diet with high amount of oleic acid and balanced ratio of omega-3/omega-6 PUFAs would be helpful for health and prevention of both MS and cancer.

#### 200 Lipid Metabolism
