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Chapter 7 Signaling Transduction and Metabolomics 75

iron. NADPH oxidase catalyzes the NADPH-dependent reduction form active enzyme polymers, which are depolymerized by the end
of oxygen into the superoxide anion. product of fatty acid synthesis: long-chain fatty acids. Growth factors
ROS cause cellular damage through oxidation and chemical modi- positively control ACC dephosphorylation. Catecholamines, on the
fications of proteins, lipids, and DNA. Nuclear and mitochondrial other hand, result in the phosphorylation and inhibition of ACC via
DNA can be oxidized, producing strand breaks. Intracellular levels of PKA. Fatty acids are synthesized in the cytoplasm by a multifunc-
ROS are regulated through different signaling transduction pathways. tional enzyme, fatty acid synthase (FAS). Two of these functional
Growth factor–mediated signaling increases ROS levels, for instance. domains are the acyl carrier protein and the condensing enzyme (CE).
Conversely, ROS also affect this signaling through modulation of After completion of the different rounds of synthesis, the palmityl
protein tyrosine phosphatases that contain cysteine-sensitive residues group is transferred to CoASH. In macrophages, lipopolysaccharide
that modulate their enzymatic activity and regulate the biologic (LPS) activates lipogenesis through activation of sterol regulatory
responses associated with this signaling. element–binding protein (SREBP), a key transcriptional mediator
ROS are particularly deleterious to hematopoietic stem cells of cholesterol and fatty acid synthesis.
because of their effect on genomic stability and survival. In phagocytic
cells (neutrophils, macrophages, or eosinophils), NADPH oxidase is
responsible for the oxidative burst that is triggered upon phagocytosis Fatty Acid Oxidation
of pathogens. Superoxide generated by NADPH oxidase is rapidly
converted to other ROS, which, in cooperation with pH-sensitive Fatty acids are “charged” before oxidation to form acyl-SCoA, a cyto-
proteases, are responsible for killing the microorganisms in the phago- plasmic reaction catalyzed by the enzyme fatty acyl-CoA synthetase.
some vacuole. Fatty acid β-oxidation, however, occurs in the mitochondrial matrix
Recently, gain-of-function mutations of isocitrate dehydrogenase and charged fatty acids must first be conjugated to carnitine in order
1 and 2 (IDH1 is cytoplasmic and is unrelated to the TCA cycle; to cross the mitochondrial membrane. This transport is carried out
IDH2 is the TCA mitochondrial form) have been found in 20% of by the carnitine acyltransferases I and II. These enzymes constitute a
acute leukemia patients. IDH1 and IDH2 are highly homologous rate-limiting step for β-oxidation of fatty acids and are allosterically
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but distinct (in structure and function) from the NAD -dependent regulated by malonyl CoA, allowing the cell to avoid a futile cycle
heterotrimeric IDH3 enzyme that is part of the TCA cycle produc- of fatty acid synthesis and breakdown. Inside the mitochondria,
ing NADH to the respiratory chain. The cellular function of the acyl-CoA undergoes a cycle of reactions removing acetyl-CoA from
NADP-dependent IDH1/2 enzymes is not clear but they are part the main chain. This acetyl-CoA is then processed through the TCA
of glucose, fatty acids, and glutamine metabolism, and contribute cycle.
to the maintenance of cellular reduction–oxidation balance. In
three identified mutations, the enzyme undergoes a change in its
normal physiologic catalytic reaction (i.e., oxidative decarboxylation Cholesterol
of isocitrate to produce α-ketoglutarate and CO 2 while converting
NAD[P] to NAD[P]H) and instead produces 2-hydroxyglutarate, Cholesterol is an important component of cellular membranes and a
which is now considered to be a protoncometabolite. The mechanism substrate for the production of steroid hormones. Free cholesterol is
appears to be linked to competition with α-ketoglutarate for the tightly controlled in cells through synthesis, storage, and transport.
active site of ketoglutarate-dependent dioxygenases, such as TET2, Excess cholesterol in cells is secreted through reverse cholesterol
which functions as a cytosine demethylase. transport or stored in the cytoplasm as cholesterol ester, produced
by Acy-CoA:cholesterol acyltransferase located in the endoplasmic
reticulum. Cholesterol is transported in the plasma by lipoproteins
Lipid Metabolism including chylomicrons and very low-density lipoprotein (VLDL).
The main sources of cellular cholesterol for hematopoietic cells are
Fatty acids and triglycerides (the storage form of fatty acids) consti- the cholesterol-rich lipoprotein, low-density lipoprotein (LDL), and
tute an energetic reserve in the body. Most of the cells are able to de novo synthesis from acetyl-CoA. The rate-limiting step for choles-
synthesize fatty acids, but there are essential fatty acids such as linoleic terol synthesis is catalyzed by HMG-CoA reductase, the direct target
acid, α-linoleic, and arachidonic acid that cannot be synthesized. of cholesterol-lowering statin drugs, and converts hydroxymethylglu-
Arachidonic acid is made from linoleic acid, and is the precursor for taryl CoA to mevalonic acid. Cellular cholesterol levels are sensed in
prostaglandins, thromboxanes, and leukotrienes that participate in the endoplasmic reticulum through the SREBP transcription factor,
different pathways such as the inflammatory response. Drugs that which directly controls most the enzymes in cholesterol synthesis as
block the enzyme cyclo-oxygenase and prostaglandin synthesis such well as LDL transport. Excess of LDL becomes oxidized and taken by
as acetaminophen, ibuprofen, and acetylsalicylate provide pain relief. macrophages, a main cause of atherosclerosis. The SREBP pathway
Fatty acids can directly mediate transcriptional responses, acting as is also important for T-cell activation under antigenic challenge, as
ligands for peroxisome proliferator-activated receptors, a family of its activation favors cholesterol synthesis and transport, which is used
nuclear hormone receptors. In addition, there are specific GPCR for membrane biogenesis and cell proliferation in the activated T cell.
receptors such as GPR40 and GPR120 activated by medium- or
long-chain fatty acids. GPR43 is activated by short-chain fatty acids
and is highly abundant in leukocytes. Amino Acid Metabolism

The major sources of amino acids derive from the diet or protein
Fatty Acid Synthesis breakdown. Nonessential amino acids are synthesized from carbon
skeletons using different metabolic pathways. Amino acids conjugated
In the mitochondrial matrix acetyl-CoA is generated from pyruvate to tRNA are used in protein synthesis; however, in excess they can
and is the precursor for fatty acid synthesis. Acetyl-CoA cannot be used for energy production. In addition, amino acids are neces-
cross the mitochondrial membrane; thus acetyl-CoA condenses with sary for the synthesis of other compounds. For example, tryptophan
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oxaloacetate (first reaction in the TCA cycle) to form citrate, and catabolism constitutes a route for de novo NAD synthesis in a
is exchanged into the cytoplasm through TCA translocases. Once pathway that is important in leukocytes for the replenishment of
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in the cytoplasm, citrate is converted to acetyl-CoA by ATP citrate NAD levels after oxidative stress. Interestingly, different metabolites
lyase. The rate-limiting reaction of fatty acid synthesis is the carbox- derived from tryptophan catabolism via the kynurenine pathway play
ylation of acetyl-CoA to form malonyl CoA, which is catalyzed by a role in immune tolerance. Plasma amino acids are transported in
acetyl-CoA carboxylase (ACC). Malonyl CoA is a potent inhibitor cells against a concentration gradient. Amino acid transporters are
of fatty acid oxidation. ACC is allosterically regulated by citrate to specific for neutral (small and larger), basic, and acidic amino acids.