Roles of facilitative glucose transporter GLUT1 in [F]FDG positron emission tomography (PET) imaging of human diseases

The facilitative glucose transport protein GLUT1 has important roles in positron emission tomography (PET) imaging of human diseases. GLUT1 has widespread expression and catalyses the energyindependent facilitated diffusion of glucose down its concentration gradient across red blood cell membranes, blood-brain and blood-tissue barriers and membranes of some oragnelles. Import is usually the prevailing direction of transport for providing metabolic fuel, especially in proliferating cells. PET imaging using 2-deoxy-2-[F]fluoro-D-glucose ([F]FDG) measures the uptake of [F]FDG into cells and tissues as a marker of glucose transport and glycolytic activity. Diseases can alter glycolytic activity in localised regions of tissues or organs, which can be visualised using [F]FDG PET. Expression and/or activity levels of GLUT1 contribute to the pattern and intensity of [F]FDG. [F]FDG PET imaging is used in diagnosing and monitoring a range of human diseases and in analysing their response to treatments. Proliferating cancer cells display overexpression of GLUT1 and a vastly higher rate of glycolysis for satisfying their increased nutrient demands. Tumours therefore have significantly enhanced [F]FDG uptake compared with normal cells, so [F]FDG PET is routinely used in diagnosing and monitoring cancers. [F]FDG PET imaging of the brain allows identification of distinct patterns of hypometabolism and/or hypermetabolism associated with neurological disorders including Alzheimer’s disease, Parkinson’s disease, epilespsy, Journal of Diagnostic Imaging in Therapy. 2015; 2(1): 30-102 Patching http://dx.doi.org/10.17229/jdit.2015-0301-014 31 ISSN: 2057-3782 (Online) schizophrenia, multiple sclerosis and cerebral ischemia. Cardiovascular diseases, along with underlying conditions such as inflammation, sarcoidosis, atherosclerosis, and infections of implants and prosthetics are routinely assessed using [F]FDG PET. Diabetes alters the distribution of [F]FDG, which can affect diagnosis of other diseases. The effects of anti-diabetic drugs on glucose metabolism and activation of brown adipose tissue as a preventative measure or treatment for obesity and diabetes have been investigated using [F]FDG PET. GLUT1 itself is a potential therapeutic target for treatment of some diseases, which has also been investigated using [F]FDG PET.


GLUT facilitative transport proteins
Glucose homeostasis in the human body is maintained by the GLUT or solute carrier 2 (SLC2) family of facilitative transport proteins, which are members of the sugar porter sub-family of the large and widespread Major Facilitator Superfamily (MFS) of secondary transport proteins [1,2,3]. GLUT proteins catalyse the energy-independent facilitated diffusion of hydrophilic glucose molecules and other substrates down their concentration gradient across hydrophobic cell membranes. Import is usually the prevailing direction of transport in order to provide metabolic fuel, especially in proliferating cells ( Figure 1A). Fourteen GLUT isoforms (GLUT1-14) have been identified that are each comprised of ~ 500 amino acid residues. These share a high sequence similarity (19-65% identity, 39-81% homology) [4] and a number of structural features including twelve putative transmembrane-spanning α-helices arranged in two distinct N-and C-terminal domains of six helices, cytoplasmic N-and C-terminal ends, a large intracellular loop between helices 6 and 7 and a single-site of N-linked glycosylation on one of the extracellular loops. The different isoforms have different patterns of tissue-specific expression, cellular localisation, substrate specificity and kinetics, which can be altered under disease conditions. Details and physiologies of the fourteen GLUT isoforms have been reviewed extensively [5][6][7][8][9][10][11][12][13].
GLUT1 include galactose, mannose, and glucosamine and GLUT1 also transports the oxidized form of vitamin C, dehydroascorbic acid, in order to confer mitochondrial protection against oxidative injury [29]. The transport activity of GLUT1 is inhibited by a number of different compounds including cytochalasin B, forskolin, phloretin and other flavonoids, maltose and mercuric chloride, which all have low micromolar affinities [30][31][32][33][34] and these have been used in a range of experimental studies of GLUT1 sugar transport and function. The structure of GLUT1 is coloured with the N-terminus in blue and the C-terminus in red, which was drawn using PDB file 4PYP and PDB Protein Workshop 3.9 [35]. GLUT1 is highly overexpressed in many types of cancer cells [63] including brain [64], breast [65], cervical [66], colorectal [67], cutaneous [68], endometrial [69], esophageal [70], hepatic [71], lung [72], oral [73], ovarian [74], pancreatic [75], prostate [76] and renal [77]. Because cancer cells have an altered metabolism and an increased demand for nutrients they usually show an upregulation of GLUT1 in order to provide an enhanced uptake of glucose in correlation with a greater rate of glycolysis. This is accompanied by an increase in rate-limiting enzymes of the glycolytic pathway including hexokinase [78,79]. The ability of rapidly dividing tumour cells to break down glucose by glycolysis at a vastly higher rate than in normal tissues, even when ample oxygen is present, is known as the Warburg effect [80][81][82][83][84][85]. Under these 'aerobic glycolysis' conditions most glucose is converted to lactate rather than being metabolised through oxidative phosphorylation so a high rate of glucose uptake is required to sustain energy levels for tumour growth. The levels of GLUT1 expression and glucose uptake are therefore prognostic and diagnostic markers for the growth of tumours. Measuring uptake of the 18  By far the most common and successful radiolabelled compound used in PET imaging is [ 18 F]FDG, which is used in over 95% of PET procedures worldwide [86]. This compound was first synthesised by the direct electrophilic fluorination of 3,4,6-tri-O-acetyl-D-glucal with 18 F-fluorine gas ISSN: 2057-3782 (Online) [87] (Figure 3A), but this method and its variations have a relatively low radiochemical yield. The preferred method for synthesising [ 18 F]FDG in PET applications is nucleophilic substitution of the acetylated sugar derivative 1,3,4,6-tetra-O-acetyl-2-O-trifluoromethane-sulfonyl-β-D-mannopyranose by 18 F-fluoride ions using Kryptofix 2.2.2 TM as a catalyst followed by separation of reaction products and hydrolysis [88] ( Figure 3A). This method gives higher radiochemical yields (up to 60%) in a shorter time with modern automated synthesis modules producing [ 18 F]FDG in under half an hour.
The methods for synthesis of [ 18 F]FDG and associated quality control considerations have been reviewed [89][90][91]. The only difference in chemical structure between [ 18 F]FDG and glucose is a fluorine atom attached at carbon-2 instead of a hydroxyl group (Figure 1) Figure 3B).
[ 18 F]FDG-6-phosphate also cannot cross cell membranes so it becomes trapped and accumulates within the cell. As the 18 F label decays radioactively it is converted to 18 O -, which picks up a proton from a hydronium ion in the aqueous environment and the molecule becomes glucose-6-phosphate with non-radioactive 18 O at the 2-position, which is harmless ( Figure 3B). This 18 O-labelled glucose-6-phosphate can then be metabolised as normal. In PET studies, [ 18 F]FDG is therefore an excellent marker for the uptake of glucose into specific tissues and of their glycolytic state. A number of other radiofluorinated carbohydrates have also been used in PET studies [93].
In examining PET scans with [ 18 F]FDG it is important to be aware of the distribution of [ 18 F]FDG in a healthy individual before using them to recognise disease states. As would be expected, the highest levels of [ 18 F]FDG accumulation in a normal PET scan are in tissues with the highest expression of GLUT1 and the highest rates of glycolysis, which are principally the brain and cardiac tissue ( Figure 2). Normal individuals do not excrete glucose via the urinary system because it is freely filtered by glomeruli and rapidly reabsorbed by the nephron of the kidney. In contrast, [ 18 F]FDG is poorly reabsorbed after filtration and is excreted in large amounts in the urine [86]. Consequently, an intense [ 18 F]FDG activity is usually observed in the kidneys, ureters and bladder ( Figure 2). Lower levels of [ 18 F]FDG uptake can also be observed in a number of others tissues in a healthy individual depending on their physiological state [86,94]. This includes a low and diffuse activity in liver and spleen and variable activity in stomach and bowel smooth muscle. Uptake in skeletal muscle is dependent on levels of stress and/or physical activity, so a patient is usually rested prior to and following injection of [ 18  and/or of hexokinase is usually associated with a poor prognosis in many types of cancers [95,96].
There are of course a large number of other metabolic changes that occur in the microenvironment of a tumour [97,98], but these will not be considered here. inhibitors such as gefitinib and erlotinib [104]. A retrospective study of NSCLC cases has found a significant correlation between GLUT1 overexpression and KRAS mutations and the survival of patients with GLUT1 overexpression was significantly worse when compared to patients with normal expression of GLUT1. GLUT1 overexpression therefore correlates with this aggressive phenotype of lung cancer [105].

Breast cancer
Breast cancer has the second highest number of diagnosed malignancies worldwide, although this does not correlate with the number of mortalities since treatments are relatively successful [99]. Alongside other screening techniques, [ 18 F]FDG PET imaging plays an important role in the diagnosis of different types and stages of breast cancers and during their treatment [106][107][108][109][110]. An example of using [ 18 F]FDG PET to diagnose a rare case of breast cancer in a lactating woman is shown in Figure 5.   [112]. An investigation of the relevance of single-nucleotide polymorphisms (SNPs) in the GLUT1 gene with respect to uptake of [ 18 F]FDG and tumour aggressiveness in breast cancer revealed a significant role of the XbaI G>T polymorphism [113]. The GLUT1 XbaI G>T SNP, which represents a G-to-T transversion in intron 2 of GLUT1, may therefore be a prognostic factor for the aggressiveness of the phenotype in breast cancers. A significant association between the GLUT1 XbaI G>T SNP and genetic susceptibility to nephropathy in type 1 diabetes has also been identified [114]. The monoclonal antibody trastuzumab used in the treatment of some breast cancers targets HER2 receptors, which are overexpressed in 20-30% of breast cancers.

Colorectal cancer
Colorectal or bowel cancer has the third highest number of diagnosed malignancies worldwide [99].  [116][117][118]. In contrast, a correlation between accumulation of [ 18 F]FDG and expression of proliferative cellular nuclear antigen (PCNA) has not been observed suggesting that the overexpression of GLUT1 is associated with the hypoxic environment in tumours rather than with tumour growth [119]. Indeed, GLUT1 has been specifically highlighted as an important molecular marker for the degree of hypoxia experienced by tumours in colorectal cancer patients [120]. A retrospective analysis of colorectal tumours showed that accumulation of [ 18 F]FDG was higher in the presence of KRAS/BRAF gene mutations and this was positively correlated with GLUT1 expression but not with hexokinase II expression [121]. KRAS gene mutations occur in 30-40% of colorectal cancers and are associated with resistance to anti-epidermal growth factor receptor therapy and with a poorer likelihood of survival [122]. In colorectal cancer cells with KRAS mutations the knockdown of GLUT1 produces a significant decrease in accumulation of [ 18 F]FDG.
Also, hypoxic induction of HIF-1α is higher in KRAS-mutant cells than in wild-type cells and elevated  [124,125]. An enhanced expression of GLUT1 is found in prostate carcinoma cells, which includes a novel co-localisation of GLUT1 with a Golgi marker. This GLUT1 Golgi association may supply glucose to the Golgi for by-product incorporation into the prostatic secretory fluid [76]. For prostate cancer patients treated with radical prostatectomy a significant number will endure a recurrence of the disease. An immunohistochemical study of prostate cancer tissue revealed that expression of GLUT1 correlates significantly with a shorter time to biochemical recurrence and accumulation of prolyl-4-hydroxylases 1 is also a significant marker for a worse prognosis [126]. Whilst early-stage prostate cancer is confined to the prostate and responds to androgens, a later stage more aggressive metastasised cancer is associated with loss of androgen responsiveness. Androgen responsive and non-responsive prostate cancer cells have different glycolytic metabolism profiles, including a higher lactate production by the latter [127].
The flavanoids genistein, phloretin, apigenin, and daidzein have different effects on reducing GLUT1 expression and glucose uptake in androgen responsive versus non-responsive prostate cancer cells and therefore different effects on reducing cell growth [128].

Thyroid cancer
Thyroid cancers are routinely diagnosed and monitored using [ 18 F]FDG PET imaging [129,130]. An example of enhanced accumulation of [ 18 F]FDG into a thyroid is shown in Figure 6, in this case demonstrating how use of [ 18 F]FDG PET has also reduced the number of unnecessary hemithyroidectomies for thyroid nodules that otherwise have inconclusive cytologic results [131]. An investigation of glucose transporter expression in thyroid carcinomas with different grades of malignancy revealed that overexpression of GLUT1 on the cell membrane of thyroid neoplasms is closely related to tumours at a more aggressive stage. It was therefore proposed that measurement of identifying patients at the highest risk [132]. This correlation has been confirmed more recently in a study that demonstrated an increase in GLUT1 expression and [ 18 F]FDG uptake with escalating dedifferentiation/aggressiveness of thyroid carcinoma types in the order: differentiated thyroid carcinoma (DTC) → poorly differentiated thyroid carcinoma (PDTC) → anaplastic thyroid carcinoma (ATC) [133]. The PTEN tumour suppressor is a phosphatase that antagonises the PI3k/Akt signalling pathway and is the second most mutated gene in human cancer [134].

Esophageal cancer
Esophageal cancer has one of the worst levels of prognosis since it is often identified at a relatively late stage [137]. a representation of tissue hypoxia rather than glucose transport in gastric cancers [142]. An overview of PET imaging in gastric cancer has been prepared by Kamimura and Masayuki [143]. Cervical cancer is the third most common cancer in women worldwide and is a significant cause of mortality in developing countries [144].  [152]. [ 18 F]FDG PET imaging also has a role in the detection and characterisation of brain tumours [153], but the brain is not usually included in routine whole body PET scans for cancers.
Because GLUT1 feeds the essential nutrient glucose into glycolysis and it is essentially the first rate limiting step in glycolytic activity, the expression and/or activity of GLUT1 in tumours is therefore a therapeutic target in cancer therapy. Furthermore, the amplified expression of GLUT1 could also be used for the targeted transport of anticancer compounds into tumours [159,[162][163][164]. Such approaches have to be specific for GLUT1 in cancer cells and not have adverse effects on glycolytic activity in normal cells. Cancer cells treated with WZB117 had decreased levels of GLUT1 expression, intracellular ATP and glycolytic enzymes resulting in a lowered rate of glycolysis and cellular growth [169].
Interestingly, the addition of exogenous ATP rescued the growth of WZB117-treated cancer cells, suggesting that the reduction of intracellular ATP plays an important role in the anticancer effect of WZB117 [159,169]. WZB117 also inhibits the self-renewal and tumor-initiating capacity of cancer stem cells in vitro and administration into an in vivo system resulted in inhibition of tumour initiation after implantation of cancer stem cells with no significant adverse effects on the host animals [170].
The naturally occurring polyphenol resveratrol (3,5,4'-trihydroxy-trans-stilbene) (Figure 8) interacts directly with GLUT1 and inhibits the transport of hexoses across the cell membrane. It is proposed that resveratrol binds at an endofacial site on GLUT1 and that the demonstrated inhibition is distinct from the effect of resveratrol on the intracellular phosphorylation/accumulation of glucose [171]. Resveratrol has shown some promise for the prevention or treatment of a number of cancers, but in vivo observations are still inconsistent [172]. In a study to investigate the effect of resveratrol on cancer cell glucose metabolism and the associated role of reactive oxygen species in the response, treatment with resveratrol resulted in a significant decrease in [ 18 F]FDG uptake. This was attributed to a reduction in glycolysis rate and GLUT1 expression [173]. imaging of the brain [184].

Alzheimer's disease
Alzheimer's disease is the most common form of dementia, accounting for an estimated 60-80% of dementia cases [185], which starts with impairment of memory followed by multiple domains of cognitive dysfunction. [ 18 F]FDG PET is a widely accepted clinical tool for the examination of pathophysiological changes associated with Alzheimer's disease, especially in early stage diagnosis [186][187][188][189][190][191]. [ 18 F]FDG PET measures the cerebral metabolic glucose utilization rate (CMRglc), which is is a standard marker of synaptic activity, neuronal function and neuronal metabolic activity [191,192].
Alzheimer's disease is characterised in [ 18 F]FDG PET images by a distinct pattern of hypometabolism in the regions of parietotemporal association cortices, posterior cingulate, and precuneus at early stages of the disease, which spreads to the frontal association cortices in moderate to severe stages [189].  [193,194]. synaptic dysfunction in the neurons of affected regions prior to cell death and detectable atrophy [190,195]. As may be expected, the regions with a decrease in glucose consumption have a direct correlation with a decrease in GLUT1 expression and a downregulation of HIF-1α [196][197][198] and it is considered that reduction in glucose transport is a causative effect of hypometabolism in dementias. supranuclear palsy (PSP) and multiple system atrophy (MSA) have some common symptoms but different pathophysiology in the cortical and subcortical structures of the brain. In addition to other differences, IPD has normal or increased glucose metabolism in the striatum but hypometabolism in temporoparietal regions, PSP has bilateral striatal and frontal hypometabolism, whilst MSA has hypometabolism in striatal, brainstem, and cerebellar regions [176,[199][200][201].

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The atypical parkinsonian syndromes of PSP and MSA have a much poorer long-term prognosis than IPD so an accurate and early differential diagnosis is important. Spatial covariance analysis has been applied to in PSP has been further characterised by decreased accumulation in the upper brainstem and medial prefrontal cortex as well as in medial thalamus, caudate nuclei, anterior cingulated area and superior frontal cortex, whilst the MSA pattern has decreases in putamen and cerebellum [202]. Other computer-aided diagnosis methods have been used to assist in the differentiation of parkinsonian syndromes from [ 18 F]FDG PET images [203][204][205]. Association between motor, cognitive and emotional dysfunction with distinct patterns of cerebral metabolic changes has also been identified in PD from [ 18 F]FDG PET images [206]. Although GLUT1 clearly plays a role in the uptake of [ 18 F]FDG in PET imaging of parkinsonian disorders, very little work has been performed to investigate expression and/or activity of GLUT1 in direct relation to these. One study using a mouse model of PD showed no changes in localisation or density of GLUT1 despite the impairment in glucose metabolism [207].  developed. The tool operates on distributed metabolic changes across the whole brain to diagnose and lateralise epileptogenic sites and can work both independently and alongside expert analysis [217]. As already mentioned in the Introduction, defects in GLUT1 are increasingly being recognised as the cause of some genetic generalised epilepsies including early-onset absence epilepsy [58,59] and familial idiopathic generalized epilepsy [60].

Schizophrenia
In individuals with schizophrenia, cerebral glucose metabolism and pathophysiology have been investigated using [ 18 F]FDG PET with varying results reported under both medicated and unmedicated conditions. It is considered that significant heterogeneity in the patterns of schizophrenia make investigations of its origin and mechanisms a major challenge [218]. For example, one study in unmedicated patients showed enhanced glucose metabolism in cerebral white matter, specifically in the frontal white matter, corpus callosum, superior longitudinal fasciculus and white matter core of the temporal lobe. This was accompanied by hypometabolism in grey matter, specifically in the frontal and temporal lobes, caudate nucleus, cingulate gyrus, and mediodorsal nucleus of the thalamus [219].
[ 18 F]FDG PET has also been used to investigate the effects of antipsychotic drugs [220][221][222][223], cannabis use [224,225] and auditory verbal hallucinations [226,227] on the pattern of cerebral glucose metabolism in schizophrenia and to differentiate it from the pattern in bipolar disorder [228]. A hypothesis for pathogenesis of schizophrenia based on impaired neuronal glucose uptake by GLUT1 and GLUT3, either in expression levels or functional capacity, has been presented [229].

Multiple sclerosis
Multiple sclerosis (MS) is the most common neurological disorder diagnosed in young adults. This is an autoimmune disease characterised by loss of motor and sensory function resulting from immunemediated inflammation, demyelination and subsequent axonal damage [230]. [ 18 F]FDG PET analysis has revealed both widespread and regional cerebral glucose hyometabolism in MS patients [231,232] and that the global cortical cerebral metabolism decreases in correlation with disease progression [233]. Some regions of glucose hypermetabolism are also evident and thought to be compensatory effects [234,235] with autonomic nervous system and motor dysfunctions [236]. Also, MS patients sometimes have asymmetries in comparative strengths of leg muscles accompanied by walking difficulties and this is in correlation with glucose uptake according to [ 18 F]FDG PET [237]. The exact origins and mechanisms of MS pathogenesis are yet to be unravelled, but the perturbed glucose metabolism has been considered as a cause as well as a consequence in MS [238]. An absence of the β2-adrenergic receptor (β2AR) in astrocytes occurs in MS patients, and since β2AR promotes glucose uptake through GLUT1 and accelerates glucose metabolism, a downregulation of β2AR activity may accelerate the development of MS [239]. β2AR and/or GLUT1 are therefore potential therapeutic targets for upregulation in the prevention or treatment of MS.

Cerebral ischemia
Cerebral ischemia and its response to potential therapies has been investigated using [ 18 F]FDG PET with the large majority of studies performed in animal or in vitro models due the nature of strokes.
Under conditions of oxygen deprivation in living brain slices, which is consistent with acute cerebral ischemia, a hyperaccumulation of [ 18 F]FDG was demonstrated especially in the hippocampus and thalamus. The enhanced glucose metabolism was associated with an increased glutamate efflux after hypoxia and anoxia and glucose metabolism was also increased by the addition of glutamate and attenuated by an N-methyl-D-aspartate (NMDA) receptor antagonist. It was therefore considered that activation of NMDA receptors by glutamate during acute cerebral ischemia might be responsible for the hyperutilisation of glucose in the hippocampus and thalamus [240]. Tissue regions of interest in cerebral ischemia include the ischemic core, the border that progresses to infarction (recruited tissue) and the border that recovers with early reperfusion (recoverable tissue). [ 18 F]FDG PET studies in rat models have shown that in the ischemic core glucose consumption is severely depressed due to irreversible cellular injury, whilst it is maintained or increased in the penumbral regions during ischemia. Early after reperfusion, glucose consumption is severely reduced even though glucose and oxygen are available, but this glycolytic depression is not always related to subsequent development of brain infarction [241][242][243]. product scutellarin [246], the herbal medicine Danhong [247] and after transplantation of induced pluripotent stem cells [248]. In rat brain, GLUT1 overexpression occurs rapidly and widely in microvessels and parenchyma following global cerebral ischemia, which may be associated with an immediate early-gene form of response to cellular stress [249], and cerebral hypoxia-ischemia leads to overexpression of GLUT1 in both damaged and undamaged hemispheres during both early and late stages in the recovery period [250,251]. Diabetic conditions combined with cerebral ischemia produced an even higher overexpression of GLUT1, although expression tended to decrease with increased blood glucose levels. It was therefore considered that in the treatment of diabetic patients with cerebral ischemia, blood glucose control should not be too strict, otherwise the up-regulation of GLUT1 induced by ischemia may not meet the requirements of energy metabolism the in cells [252].
The upregulation of cerebral GLUT1 (and GLUT3) is considered as a potential preventative neuroprotive therapy for ischemia [253]. Because hyperglycemia is an indicator of severe stroke and this promotes further ischemia in the brain, cerebral GLUTs are also considered as a therapeutic target for post ischemic stroke treatments [254].

Introduction
Cardiovascular diseases include all those of the heart and blood vessels such as coronary heart disease, congenital heart disease, peripheral arterial disease and stroke. These can be linked to each other and to underlying conditions such as atherosclerosis, cardiac sarcoidosis, inflammation and infection. inflammatory cells early in the infection process before more serious morphologic damages occur [276][277][278][279][280][281][282]. An [ 18 F]FDG PET-CT image from a patient with a cardiac device infection is shown in Figure 11, which reveals enhanced [ 18 F]FDG uptake on both the generator and pacemaker lead [274]. Myocarditis is most often due to infection by common viruses or a hypersensitivity response to medications [283]. [ 18 F]FDG PET has been used in the detection of myocarditis, for example due to infection by Epstein Barr virus [284,285]. Pericarditis may be caused by bacterial, viral or fungal infection or can present post-infarction (within 24 hours of a heart attack), or weeks to months after a heart attack (Dressler's syndrome). [ 18 F]FDG PET has been used for the visualisation of pericarditis [286,287], which includes applications in the diagnosis of postmeningococcal pericarditis [288], chemotherapy-induced pericarditis [289] and focal pericarditis in a huge heart [290]. [

Cardiac sarcoidosis
Sarcoidosis is a condition involving abnormal collections of inflammatory cells or granulomas that can form nodules in multiple organs. The cause of sarcoidosis is still not fully understood, but it appears to be triggered by infectious or environmental agents that act as antigens. These antigens are thought to trigger helper inducer T cells into forming the granulomas. At the early inflammation stage, the granuloma lesions contain mononuclear phagocytes and CD4 positive T cells with a T helper Type 1 response, secreting interleukin-2 and interferon-. At the later fibroblastic stage, there is a shift to a T helper type 2 response which produces anti-inflammatory effects and results in tissue scarring [307,308]. Sarcoidosis can affect any region of the heart, but most often the myocardium, and lead to other cardiac disorders such as heart block, ventricular arrhythmias, congestive heart failure, pulmonary hypertension and ventricular aneurysms. Hence, cardiac sarcoidosis has a poor prognosis so an early and accurate diagnosis is important for achieving any successful outcome. Due to the nonspecific clinical symptoms of the disease, erratic myocardial involvement and uncertainty of diagnostic tests the detection and management of cardiac sarcoidosis is challenging [309,310]. [ 18 F]FDG PET is emerging as a useful tool alongside other imaging techniques for the examination of cardiac sarcoidosis [311][312][313][314][315][316], an example is shown in Figure 12. The variable physiological uptake of

Atherosclerosis
Atherosclerotic cardiovascular diseases, including coronary heart disease, myocardial infarction and stroke, are the most common cause of death and disability in the developed world. The progressive inflammation and potential rupture of atherosclerotic plaques in arterial walls is due to infiltration by macrophages. Rupture can lead to the formation of a blood clot, which may block the artery resulting in a heart attack or it can be carried downstream causing a stroke. Because glucose uptake and metabolism by macrophages is significantly higher than in other plaque cells, [ 18 Figure 13. A non-radioactive assay of atherosclerotic plaque inflammation in a mouse model has been developed based on mass spectrometry detection of trapped FDG-6-phosphate and of cholesterol.
FDG-6-phosphate was accumulated in atherosclerotic lesions from arteries and anti-atherosclerotic effects were seen following treatment with the liver X receptor agonist T0901317 [336]. FDM also restricted binding of anti-mannose receptor antibody to macrophages by approximately 35%, so mannose receptors may be an additional target for imaging of plaque inflammation [337]. A novel system for dual-modality imaging of atherosclerotic plaques using the glucose probes [ 18 F]FDG and fluorescent 6-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-6-deoxyglucose (6-NBDG) has been developed. The system allowed detection of both [ 18 F]FDG and 6-NBDG taken up by mouse atherosclerotic plaques and demonstrates 6-NBDG as a promising fluorescent probe for detection of macrophage-rich atherosclerotic plaques [338]. diabetes. The effects of the surgery included a significant and stable increase in glucose uptake into skeletal and cardiac muscle along with lowered blood levels of both insulin and glucose, consistent with an improvement in glucose tolerance [343]. The insulin resistance that is a causal factor in pre-

Effects of diabetes on measuring [ 18 F]FDG uptake in the diagnosis of other diseases
A concern for performing [ 18 Figure 15). Treatment with a β3-adrenergic receptor (β3-AR) agonist reversed this trend to produce a significant increase in BAT [ 18 F]FDG uptake in both obese and diabetic conditions, which was accompanied by significantly decreased blood glucose levels. Treatment with the thyroid hormone levothyroxine also increased BAT [ 18 F]FDG uptake under obese conditions but not under diabetic conditions. Activation of BAT may therefore be a useful strategy in the treatment of obesity and diabetes [369]. This possibility has been confirmed by a recent [ 18 F]FDG PET-CT study demonstrating activation of human BAT by the β3-AR agonist mirabegron ( Figure 15) [370]. A new signalling pathway has also recently been revealed for β3-AR stimulation of GLUT1-mediated glucose uptake in BAT cells [371]. In addition to β3-AR stimulation of cAMP-mediated increases in GLUT1 transcription and de novo synthesis of GLUT1, there is also β3-AR stimulation of mTOR complex 2, which itself stimulates translocation of newly synthesized GLUT1 to the plasma membrane. Both parts are essential for β3-AR-stimulated glucose uptake and this is independent of the classical PI3K/Akt pathway [371].

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Normal Obese Diabetes Placebo β3-AR agonist Figure 15. Effects of obesity and diabetes on brown adipose tissue and its activation by β3-adrenergic receptor agonists.

Further roles of GLUT1 in diabetes and therapy
Renal tubular glucose reabsorption is mediated by sugar transporters including GLUT1. Glucose transport in the diabetic kidney is upregulated and has been implicated in the pathogenesis of progressive diabetic nephropathy [372]. Hyperglycemia, hypertension and activation of the reninangiotensin system are thought to be important in the development of the disease and expression levels of GLUT1 are elevated under conditions of hypertension and diabetic nephropathy [373,374].
Polymorphisms of the GLUT1 gene are associated with susceptibility to diabetic nephropathy in both type 1 and type 2 diabetes [372,[375][376][377]. Skeletal muscle GLUT1 expression and basal leg glucose uptake are reduced in type 2 diabetes [378]. The anti-ischemic drug mildronate, which suppresses fatty acid metabolism and increases glucose utilisation in myocardium, has also been shown to normalise the diabetes-induced upregulated expression levels of GLUT1 in kidneys, heart, muscle and liver [379]. Red blood cell glucose transport in patients with type 2 diabetes is decreased whilst the abundance of GLUT1 is apparently unchanged and its affinity for binding cytochalasin B is increased [380]. In contrast, adolescents with type 1 diabetes display reduced levels of GLUT1 in red blood cells and this has been considered as a contributing factor to the perturbed cognition in adolescents with type 1 diabetes [381]. The suppression of GLUT1 has been considered as a strategy for preventing diabetic complications, especially of diabetic retinopathy. GLUT1 is the sole glucose transporter between blood and retina so it is an obvious target for reducing the high glucose levels in retina under conditions of diabetic retinopathy. In an animal model of diabetes, treatment with the GLUT1 inhibitors forskolin or genistein significantly reduced retinal glucose to the same levels as in nondiabetics. Forskolin prevented early biomarkers of diabetic retinopathy, including elevation of superoxide radicals, increased expression of the chaperone β2 crystallin and increased expression of vascular endothelial growth factor [382]. GLUT1 is therefore a potential therapeutic target for the prevention of diabetic retinopathy.

Conclusions
This review has highlighted the important roles that facilitative transport protein GLUT1 plays in positron emission tomography (PET) imaging of human diseases using 2-deoxy-2- are associated with GLUT1 deficiency syndrome and some epileptic disorders and single nucleotide polymorphisms in the GLUT1 gene are associated with a genetic susceptibility to some cancers and to diabetic nephropathy. GLUT1 itself is a potential therapeutic target for the treatment of some human diseases. Inhibition of GLUT1 expression and/or activity is a potential strategy for cancer therapy or GLUT1 could be used for the targeted transport of anticancer compounds into tumours. Upregulation of GLUT1 expression is a potential approach for the prevention or treatment of some neurological disorders including multiple sclerosis and cerebral ischemia. GLUT1 is also a potential therapeutic target for the suppression of some diabetic complications and obesity. Development of such therapies targeting GLUT1 will benefit from analysis by [ 18 F]FDG PET imaging.

Conflicts of Interest
The author reports no conflicts of interest.

Funding
This work was supported by the EU EDICT consortium (contract 201924