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Basic Science Investigations |
Department of Nuclear Medicine, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea
| ABSTRACT |
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(IFN-
), and respiratory burst was generated by stimulation of primed cells with phorbol-12-myristate-13-acetate (PMA). Oxygen-intermediate generation was assessed by luminescence measurements after the addition of lucigenin. 18F-FDG uptake after 30 min of incubation was measured for unprimed control cells, primed cells, and PMA-stimulated cells. The role of protein kinases was investigated using respective inhibitors. Results: PMA stimulation of primed monocytes dramatically increased oxygen-intermediate generation, leading to a 42.2 ± 1.1 fold higher level of cumulative luminescence compared with unprimed control cells, whereas IFN-
priming alone resulted in low luminescence levels (13.9% ± 4.6% of PMA-stimulated cells). In contrast, priming alone was sufficient to augment monocytic 18F-FDG uptake to 273.3% ± 16.7% of control levels (P < 0.001), and it was not further increased by PMA stimulation. The tyrosine kinase inhibitor, genistein, and the specific protein kinase C inhibitor, staurosporine, completely abolished the priming-induced enhancement of 18F-FDG uptake and lowered uptake to control levels. Under the same conditions, wortmannin, a phosphatidylinositol 3 kinase (PI3 kinase)-specific inhibitor, and cycloheximide, a protein synthesis inhibitor, were associated with only minor reductions in the enhanced-uptake effect of priming. Conclusion: IFN-
priming alone, without stimulation of respiratory-burst activity, is sufficient to induce maximal augmentation of 18F-FDG uptake in monocytes. Furthermore, this metabolic effect appears to involve tyrosine kinases and the protein kinase C pathway but is independent of the PI3 kinase pathway.
Key Words: 18F-FDG PET monocytes interferon-
priming respiratory burst
| INTRODUCTION |
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Complete activation of monocytes results from 2 sequential steps: priming and a second triggering stimulus. The T cell-produced cytokine interferon-
(IFN-
) is considered the major priming signal, whereas the triggering signal includes various stimuli such as bacterial endotoxin, tumor necrosis factor-
, and chemical agents such as phorbol-12-myristate-13-acetate (PMA) (5). A remarkable trait of monocytes is their ability to enhance their oxidative metabolism in response to activating factors, leading to the production of superoxide ions, hydrogen peroxide, and hydrogen radicals referred to as respiratory burst. This response by monocytes is characterized by a dramatic increase in oxygen consumption and glucose metabolism (6). However, the precise conditions and machinery through which activated monocytes acquire augmented levels of 18F-FDG uptake are not clear. We thus investigated the effect of IFN-
priming and PMA stimulation of monocytes on respiratory-burst generation and 18F-FDG uptake. We further evaluated the role of potential signal transduction pathways by using their respective protein kinase inhibitors.
| MATERIALS AND METHODS |
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After the cells were incubated in a flat-bottomed 150-mm plastic dish at 37°C with 5% CO2 for 1 h, free-floating lymphocytes were collected and transferred to a separate dish. Monocytes attached to the plastic surface were collected with careful pipetting and washing with phosphate-buffered saline. The cell purity of the monocyte preparation was determined with flow cytometry and microscopic inspection after Wright staining.
Cell Priming and Respiratory-Burst Activation
For priming, 0.21 x 106 cells were transferred to culture tubes containing 500 µL of culture medium and treated with a human recombinant IFN-
(Sigma) concentration of 100 U/mL for 3 d. Respiratory-burst activity was induced by stimulating primed cells with 100 nmol of PMA (Sigma) per liter for 20 min. Development of respiratory burst was confirmed by chemiluminescent measurements of oxygen intermediates for 30 min immediately after the addition of a lucigenin (Sigma) concentration of 250 µmol/L. The luminometer was programmed to output raw data of relative light units every 30 s, and measurement of control tubes using this method generated only background numbers.
Measurement of Cellular 18F-FDG Uptake
Monocytes were suspended in 500 µL of Hanks balanced salt solution and incubated with 370 kBq of 18F-FDG at 37°C in a CO2 incubator for 30 min. The cells were centrifuged and then washed twice with 500 µL of phosphate-buffered saline. Radioactive counts from the pellets were measured on a high-energy
-counter (Wallac). 18F-FDG uptake levels were corrected for protein content in each sample, as determined by the Bradford method. Results were expressed as percentage uptake relative to the mean radioactivity for unprimed control cells.
Protein Kinase Inhibition Experiments
To determine the relationship between the effect of IFN-
priming on 18F-FDG uptake and protein kinase activity, cells were treated with staurosporine (4 µmol/L), a specific protein kinase C (PKC) inhibitor; wortmannin (200 nmol/L), a specific phosphatidylinositol 3 kinase (PI3 kinase) inhibitor; genistein (500 µmol/L), a tyrosine kinase inhibitor; or cycloheximide (100 ng/mL), a protein synthesis inhibitor. The inhibitors were added to monocytes 1 h before the initiation of priming with IFN-
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Statistical Analysis
All experiments were performed with either duplicate or triplicate samples. The results were expressed as mean ± SD, and P values < 0.05 were considered significant.
| RESULTS |
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primed were low and reached only 13.9% ± 4.6% of the level for PMA-stimulated cells (P < 0.02; Fig. 1B).
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significantly increased 18F-FDG uptake to 273.3% ± 16.7% of the levels for unprimed control monocytes (P < 0.001). PMA stimulation of primed cells, however, did not further increase 18F-FDG uptake over the level for cells that were primed only (83.9% ± 11.5%, P = NS; Fig. 2). Cell number as assessed by the protein content of the samples showed no significant change after 3 d of treatment with IFN-
, compared with the number for unprimed control cells (108.4% ± 14.8%; P = NS). The protein content was slightly decreased for primed cells after stimulation with 100 nmol of PMA per liter, compared with the content for unstimulated cells (24.1% ± 0.6% reduction; P < 0.001).
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| DISCUSSION |
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priming alone, which led to only minor levels of oxygen-intermediate generation, was sufficient to achieve a comparable if not higher level of 18F-FDG uptake. This implies that augmented 18F-FDG uptake seen in activated monocytes is driven by the cellular events associated with the process of priming rather than those that occur during development of respiratory-burst activity. Our results further provide evidence that the increase in monocytic 18F-FDG uptake induced by IFN-
priming involves tyrosine kinases and the protein kinase C pathway but not the PI3 kinase pathway.
Priming is considered the in vitro analog of activation in vivo, in which hosts exposed to pathogens acquire resistance to subsequent infection through acquisition of aggressive phagocytes (1). IFN-
is a well-established priming agent for studying monocyte activation states (7,8). IFN-
-primed monocytes resemble macrophages activated in vivo and acquire a dramatic increase in responsiveness to activating agents, a critical trait for monocyte-mediated immune reactions (9). The molecular mechanisms regulating the priming effect of IFN-
are not fully explained. It is known that IFN-
bound to its cell-surface receptor activates the receptor-associated Janus tyrosine kinases, leading to the activation of the signal transducer and activator of transcription (STAT) pathway, which in turn induces the expression of various responsive genes (5). The wide variety of effects by IFN-
and the complex patterns of cell-specific gene regulation preclude illumination of the precise mechanism through which IFN-
priming increases 18F-FDG uptake in monocytes. Although glucose uptake has been related to proliferation in growth factor-stimulated macrophages (10), it appears from the lack of change in protein content after priming that the metabolic effect of IFN-
is not coupled to proliferative response pathways.
We selected 30 min as the incubation time for 18F-FDG uptake because this time frame has frequently been used in previous in vitro experiments measuring radiolabeled deoxyglucose uptake in monocytes. In a previous study, we observed that 18F-FDG uptake in freshly purified monocytes gradually increases with incubation time but only to a modest degree37% ± 3%from 20 to 80 min of incubation (4). Therefore, although a lack of data using longer durations of 18F-FDG uptake (as is commonly used for 18F-FDG PET studies of clinical infection) is a limitation of our study, we believe that this factor would not have significantly affected the results of our study.
The observed augmentation in 18F-FDG uptake independent of respiratory-burst generation appears to diverge from the well-known metabolic effect of the latter. PMA stimulation of IFN-
-primed monocytes directly activates reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, which results in a major release of superoxide anions. Such respiratory-burst activation imposes an acute metabolic demand to fuel the NADPH oxidase activity, and this demand is met by an increased use of both internal energy stores and exogenous metabolites such as glucose (11). A previous study on monocytic RAW cells confirmed an increase in 2-deoxyglucose uptake associated with high superoxide production after PMA stimulation (6). Our results also confirm the ability of PMA to stimulate respiratory-burst activity accompanied by an increase in monocytic 18F-FDG uptake, although a comparable metabolic response was found with priming alone. It may thus be that glucose uptake rates are increased in advance during the priming phase so that the high-energy demand during respiratory-burst generation can later be met. Similar to our findings, dissociation between an increase in glucose uptake and respiratory-burst activity has recently been observed for human circulating neutrophils. Priming of neutrophils with tumor necrosis factor-
caused an increase in deoxyglucose uptake identical in magnitude to that caused by N-formyl-methionyl-leucyl-phenylalanine-stimulated respiratory-burst activation, and the events were temporally dissociated (12).
The mechanisms that govern cellular glucose metabolism are diverse, and multiple signaling cascades might be involved in its regulation in different cells. In cultured skeletal muscle cells, tumor necrosis factor-
and IFN-
have been shown to stimulate glucose transport and glucose transporter 1 protein levels, presumably through signaling pathways that induce nitric oxide production (13). We investigated the role of potential signaling cascade pathways on the regulation of monocytic glucose uptake by evaluating the effect of protein kinase inhibitors. Genistein is an inhibitor of tyrosine kinases and is used in experiments to terminate IFN-
signaling by deactivation of STAT1 through inhibition of Janus tyrosine kinase activity (14). Staurosporine is a potent inhibitor of protein kinase C (15), which has been implicated in the regulation of glucose transport in phagocytes (16). As a result, both genistein and staurosporine completely inhibited the augmented 18F-FDG uptake response in monocytes treated with IFN-
. These findings are similar to previous observations on the effect of these inhibitors on deoxyglucose uptake in monocytic RAW cells (6) and in human neutrophils (17).
We also investigated the PI3 kinase pathway, which is essential for generation of respiratory-burst activity (18) and is the major pathway through which insulin signals an increase of glucose use. IFN-
induces the phosphorylation of the serine/threonine kinase Akt through PI3 kinase in human monocytes, and abrogation of this activation by PI3 kinase inhibitors has been shown to prevent some of the IFN-
-induced effects such as monocyte adhesion (19). However, we found that wortmannin, a PI3 kinase inhibitor (20), had little effect on 18F-FDG uptake in primed monocytes. Previous studies have also demonstrated results that are consistent with our findings (6,12,17). Taken together, the results of our inhibition study suggest that IFN-
-primed monocytes might circumvent a requirement for PI3 kinase in enhancing 18F-FDG uptake by activation of other pathways that involve tyrosine kinases or protein kinase C.
| CONCLUSION |
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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For correspondence or reprints contact: Kyung-Han Lee, MD, Department of Nuclear Medicine, Samsung Medical Center, 50 Ilwondong, Kangnamgu, Seoul, Korea.
E-mail: khleenm{at}samsung.co.kr
| REFERENCES |
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. Annu Rev Immunol. 1997;15:749795.[Medline]
and LPS overcome glucocorticoid inhibition of priming for superoxide release in human monocytes. J Immunol. 1989;142:39853992.[Abstract]
. Biochem J. 1987;248:281283.[Medline]
Jak-STAT signaling during macrophage activation. Nature Immunol. 2002;3:859866.[Medline]
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