Regional cerebral L-[14C-methyl]methionine incorporation into proteins: evidence for methionine recycling in the rat brain

11C-methionine PET/CT findings in benign brain disease

¹¹C-methionine (MET) is one of the most commonly used positron emission tomography (PET) tracers for evaluation of malignant brain tumor, with MET-PET being a sensitive technique for visualization of primary and recurrent malignant brain tumors. However, previous reports have demonstrated MET uptake in lesions associated with benign brain diseases. These diseases usually show an increase in MET uptake similar to that of malignant tumors. This pitfall in MET-PET image interpretation is important not only for nuclear medicine professionals, but also for radiologists. In this review, we demonstrate the imaging characteristics of MET uptake in benign brain disease, and recommend physician interpretation of imaging findings and disease characteristics for optimal patient management. Benign uptake must be identified to prevent misdiagnosis and unnecessary surgical operations.

ERp29, a general endoplasmic reticulum marker, is highly expressed throughout the brain

ERp29 is a recently discovered resident of the endoplasmic reticulum (ER) that is abundant in brain and most other mammalian tissues. Investigations of nonneural secretory tissues have implicated ERp29 in a major role producing export proteins, but a molecular activity remains wanting for this functional orphan. Intriguingly, ERp29 appears to be heavily utilized in the cerebellum, a brain region not conventionally regarded as neurosecretory. To elucidate this functional quandary, we used immunochemical approaches to characterize the regional, cellular, and subcellular distributions of ERp29 in rat brain. Immunohistochemistry revealed ubiquitous expression in neuronal and nonneuronal cells, with a distinctive variation in somatic ERp29 levels. Highly expressing cells were found in diverse locations, implying that ERp29 is not biased towards the cerebellum functionally. Using immunolocalization data mined from the literature, a proteomic profile was developed to assess the functional significance of ERp29's characteristic expression pattern. Surprisingly, ERp29 correlated poorly with classical markers of neurosecretion, but strongly with a variety of major membrane proteins. Together with immunogold localization of ERp29 to somatic ER, these observations led to a novel hypothesis that ERp29 is involved primarily in production of endomembrane proteins rather than proteins destined for export. This study establishes ERp29 as a general ER marker for brain cells and provides a stimulating clue about ERp29's enigmatic function. ERp29 appears to have broad significance for neural pathophysiology, given its ubiquitous distribution and prominence in brain over classical ER residents like BiP and protein disulfide isomerase.

Transforming growth factor-alpha attenuates N-methyl-D-aspartic acid toxicity in cortical cultures by preventing protein synthesis inhibition through an Erk1/2-dependent mechanism

Transforming growth factor-alpha (TGF-alpha), a ligand of the epidermal growth factor receptor, reduces the infarct size after focal cerebral ischemia in rat, but the molecular basis underlying the protection is unknown. Excitotoxicity and global inhibition of translation are acknowledged to contribute significantly to the ischemic damage. Here we studied whether TGF-alpha can rescue neurons from excitotoxicity in vitro and how it affects calcium homeostasis, protein synthesis, and the associated Akt and extracellular signal-regulated kinase 1/2 (Erk1/2) intracellular signaling pathways in mixed neuron-glia cortical cultures. We found that 100 ng/ml TGF-alpha attenuated neuronal cell death induced by a 30-min exposure to 35 microM N-methyl-D-aspartic acid (NMDA) (as it reduced lactate dehydrogenase release, propidium iodide staining, and caspase-3 activation) and decreased the elevation of intracellular Ca2+ elicited by NMDA. TGF-alpha induced a prompt and sustained phosphorylation of Erk1/2 and prevented the loss of Akt-P induced by NMDA 3 h after exposure. The protective effect of TGF-alpha was completely prevented by PD 98059, an inhibitor of the Erk1/2 pathway. Studies of incorporation of [3H]leucine into proteins showed that NMDA decreased the rate of protein synthesis, and TGF-alpha attenuated this effect. TGF-alpha stimulated the phosphorylation of the eukaryotic initiation factor 4E (eIF4E) but did not affect eIF2 alpha, two proteins involved in translation regulation. PD 98059 abrogated the TGF-alpha effect on eIF4E. Our data demonstrate that TGF-alpha exerts a neuroprotective action against NMDA toxicity, in which Erk1/2 activation plays a key role, and suggest that the underlying mechanisms involve recovery of translation inhibition, mediated at least in part by eIF4E phosphorylation.

Comparison of MET-PET and FDG-PET for differentiation between benign lesions and malignant tumors of the lung

OBJECTIVE

We retrospectively assessed and compared the usefulness of 11C-methionine (MET)-PET with that of 18F-FDG-PET for the differentiation between benign lesions and malignant tumors of the lung.

METHODS

We examined 101 patients with a suspected lung tumor including 79 patients with primary lung cancer and 22 patients with benign lesions. One hundred and forty PET studies (46 studies with MET-PET and 94 studies with FDG-PET) were performed. Both MET-PET and FDG-PET were performed on 39 patients. The MET-PET was performed 15 minutes after the administration of 67-740 MBq of MET, and FDG-PET 45 minutes after the administration of 30-437 MBq of FDG. The results were then evaluated by the standardized uptake value (SUV).

RESULTS

The MET uptake in lung cancer was 3.69+/-1.22 (n = 37) which was significantly higher than that in benign lesions 1.81+/-1.04 (n = 9) (p < 0.001). The sensitivity, specificity and accuracy of MET-PET were 83.8%, 88.9% and 84.8%, respectively, when 2.66 of SUV was used as the cutoff value. The FDG uptake in lung cancer was 5.94+/-2.89 (n = 75) and was also significantly larger than that in benign lesions 2.46+/-1.01 (n = 19) (p < 0.001). The sensitivity, specificity and accuracy of FDG-PET were 81.3%, 78.9% and 80.9%, respectively (cutoff = 3.20). The MET uptake in the lesions correlated significantly with FDG uptake (r = 0.71, p < 0.001). According to an ROC analysis, the area under the curve for MET-PET (area = 0. 833) was higher than that for FDG-PET (area = 0.828), but the difference was not statistically significant. Furthermore, the combined use of MET-PET and FDG-PET did not improve the diagnostic ability.

CONCLUSIONS

In conclusion, both MET-PET and FDG-PET were considered to be equally useful for the differential diagnosis of lung tumors. Furthermore, MET uptake in lung lesions was found to correlate significantly with FDG uptake.

[11C]methionine positron emission tomography for the evaluation of pancreatic exocrine function in chronic pancreatitis

Positron emission tomography (PET) can be used for the quantitative analysis of amino acid metabolism. The aim of this study was to investigate whether pancreatic exocrine function can be evaluated by [11C]methionine PET in chronic pancreatitis (CP) patients. Dynamic PET scan of the pancreas and liver was performed in eight healthy subjects and seven patients with CP after intravenous (i.v.) injection of [11C]methionine. Simultaneously, duodenal juice was collected with the background of continuous i.v. administration of secretin (125 ng/kg/h). The radioactivity ratio of the pancreas to that of the liver (PLR) was calculated by regions of interest (ROI) analysis. Amylase output and bicarbonate concentration were measured in the duodenal aspirates. The PLR of CP patients was significantly lower than that of healthy subjects at all time points after methionine injection. The PLRs at 4.5 minutes (PLR4.5) after methionine injection were positively correlated with the amylase output, mean bicarbonate concentration, and volume of duodenal aspirates (R = 0.74, 0.69, 0.46). It is concluded that [11C]methionine PET would be a noninvasive method for the evaluation of exocrine pancreatic function, which may represent total amino acids uptake of viable acinar cells in the pancreas.

Measurement of muscle protein synthesis by positron emission tomography with L-[methyl-11C]methionine: effects of transamination and transmethylation

BACKGROUND

Positron emission tomography with L-[methyl-11C]methionine provides a measure of regional protein synthesis rate (PSR) in skeletal muscle. However, the validity of the method depends on incorporation of methionine into protein with minimal transamination, transmethylation, or both. To test directly these assumptions, uptake of L-[methyl-14C]methionine in skeletal muscle was measured in control and cycloheximide-treated rats.

METHODS

Normal and cycloheximide-treated rats (n = 8/group) were injected with 50 microCi of L-[methyl-14C]methionine and arterial blood sampled over 90 minutes. After killing, thigh muscle was homogenized, centrifuged, and treated with trichloroacetic acid. PSR from circulating methionine was estimated from trichloroacetic acid-precipitable radioactivity, arterial time-activity curves, and plasma methionine concentrations.

RESULTS

In normal rats, approximately 70% of the tissue radioactivity was precipitated with trichloroacetic acid. In normal animals, PSR was 0.22 nmoles x min(-1) x g(-1), in excellent agreement with previous results. In the cycloheximide-treated group, PSR was 0.0032 nmoles x min(-1) x g(-1); approximately 98% reduction compared with controls.

CONCLUSION

These studies support the hypothesis that L-[methyl-11(14C]methionine accumulates in skeletal muscle as 11(14)C-labeled protein.

The Rudolf Schoenheimer Centenary Lecture. Isotopes in nutrition research

The present lecture begins with a brief overview of the professional and scientific journey taken by Rudolf Schoenheimer, before turning to a discussion of the power of isotopic tracers in nutrition research. Schoenheimer's remarkable contributions to the study of intermediary metabolism and the turnover of body constituents, based initially on compounds tagged with 2H and later with 15N, spanned a mere decade. It is difficult, however, to overestimate the enormous impact of Schoenheimer's research on the evolution of biological science. After a relative hiatus, following Schoenheimer's death in 1941, in the use of stable nuclides as tracers in metabolism and nutrition, especially in human subjects, there is now an expanded and exciting range of techniques, experimental protocols and stable-isotope tracer compounds that are helping to probe the dynamic aspects of the metabolism of the major energy-yielding substrates, amino acids and other N-containing compounds, vitamins and mineral elements in human subjects. Various aspects of the contemporary applications of these tracers in nutrition research are covered in the present lecture.

Quantitative measurement of cerebral protein synthesis in vivo: theory and methodological considerations

The true rate of cerebral protein synthesis can be calculated from the ratio of labeled proteins to integrated arterial plasma amino acid specific activity (SA) only when the fraction of amino acid precursor pool dilution is known. In the following, current experimental designs on the measurement of cerebral protein synthesis are discussed and compared to our own approach in which the determination of regional precursor pool dilution by recycled unlabeled leucine is combined with the quantitation of regional cerebral protein synthesis rates. For this purpose, a constant arterial plasma leucine SA level is maintained for 45 min by programmed intravenous infusion which is sufficient for complete equilibrium between tissue leucine pool SAs and plasma free leucine SA. In addition to the regional assessment of the precursor dilution factor, protein radioactivity can be determined in the same tissue sample or in parallel brain sections of the same animal by quantitative autoradiography. It is then possible to calculate the actual rate of protein synthesis using the correct fraction of precursor pool dilution. This renders our approach particularly suitable for the quantitative measurement of regional CPS under pathological conditions.

The heat shock stress response after brain lesions: induction of 72 kDa heat shock protein (cell types involved, axonal transport, transcriptional regulation) and protein synthesis inhibition

The cerebral stress response is examined following a variety of pathological conditions such as focal and global ischemia, administration of excitotoxins, and hyperthermia. Expression of 72 kDa heat shock protein (Hsp70) and hsp70 mRNA, the mechanism underlying induction of hsp70 mRNA involving activation of heat shock factor 1, and inhibition of cerebral protein synthesis are different aspects of the stress response considered here. The results are compared with those in the literature on induction, transcriptional regulation, expression, and cellular location of Hsp70, with a view to getting more insight into the function of the stress response in the injured brain. The present results illustrate that Hsp70 can be expressed in cells affected at various degrees following an insult that will either survive or dic as the brain lesion develops, depending on the severity of cell injury. This indicates that, under certain circumstances, synthesized Hsp70 might be necessary but not sufficient to ensure cell survival. Other situations involve uncoupling between synthesis of hsp70 mRNA and protein, probably due to very strict protein synthesis blockade, and often result in cell loss. Cells eventually will die if protein synthesis rates do not go back to normal after a period of protein synthesis inhibition. The stress response is a dynamic event that is switched on in neural cells sensitive to a brain insult. The stress response is, however, tricky, as affected cells seem to need it, have to deal transiently with it, but eventually be able to get rid of it, in order to survive. Putative therapeutic treatments can act either selectively, potentiating the synthesis of Hsp70 protein and recovery of protein synthesis, or preventing the stress response by deadening the insult severity.

Radiolabelled-tyrosine for the measurement of protein synthesis rate in vivo by positron emission tomography

The amino acid incorporation rate, generally described as protein synthesis rate or PSR, can be assessed in vivo using carboxylic-labelled amino acids such as L-[1-11C]tyrosine and PET. In animals, labelled tissue metabolites are below 4% of total tissue radioactivity and are therefore neglected in the model. Labelled plasma metabolites on the other hand rise continuously to 50% of total plasma radioactivity at 40 minutes. After correction of the total plasma radioactivity for the metabolite fraction, a Patlak analysis may be performed to calculate the PSR. A number of applications in the field of oncology were presented. The use of L-[1-11C]tyrosine in the study of metabolic disease was also discussed. It is concluded that the application of [11C]tyrosine-PET in a clinical setting is of interest for an increasing number of diseases.

Combined study of cerebral glucose metabolism and [11C]methionine accumulation in probable Alzheimer's disease using positron emission tomography

There is a characteristic decrease in glucose metabolism in associative frontal and temporo-parietal cortices of patients suffering from Alzheimer's disease (AD). The decrease in metabolism might result from local neuronal loss or from a decrease of synaptic activity. We measured in vivo [11C]methionine accumulation into proteins with positron emission tomography (PET) to assess cortical tissue loss in AD. Both global regional activity and compartmental analysis were used to express [11C]methionine accumulation into brain tissue. Glucose metabolism was measures with [18F]fluorodeoxyglucose and autoradiographic method. Combined studies were performed in 10 patients with probable AD, compared to age-matched healthy volunteers. There was a significant 45% decrease of temporo-parietal glucose metabolism in patients with AD, and frontal metabolism was lowered in most patients. Temporo-parietal metabolism correlated to dementia severity. [11C]methionine incorporation into temporo-parietal and frontal cortices was not significantly decreased in AD. There was no correlation with clinical symptoms. Data suggest that regional tissue loss, assessed by the decrease of [11C]methionine accumulation, is not sufficient to explain cortical glucose hypometabolism, which reflects, rather, reduced synaptic connectivity.

Analysis of L-[methyl-11C]methionine and metabolites in human plasma by an automated solid-phase extraction and a high-performance liquid chromatographic procedure

A fully automated method for separation of L-[methyl-11C]Methionine from metabolites in patient plasma was developed. L-[methyl-11C]Methionine was isolated from plasma by solid-phase extraction (SPE). The radioactivity retained on the SPE column was eluted and injected onto the HPLC system for separation of in vivo formed L-[methyl-11C]methionine radiolabeled metabolites. The yield through the isolation procedure and HPLC analysis was greater than 95% with a precision better than 5% (R.S.D.). The calculated rate of L-[methyl-11C]methionine transport into tumor tissue was markedly different with and without compensation for radiolabeled metabolites in patient plasma.

Measurement of muscle protein synthesis by positron emission tomography with L-[methyl-11C]methionine

Positron emission tomography (PET) with L-[methyl-11C]methionine was explored as an in vivo, noninvasive, quantitative method for measuring the protein synthesis rate (PSR) in paraspinal and hind limb muscles of anesthetized dogs. Approximately 25 mCi (1 Ci = 37 GBq) of L-[methyl-11C]methionine was injected intravenously, and serial images and arterial blood samples were acquired over 90 min. Data analysis was performed by fitting tissue- and metabolite-corrected arterial blood time-activity curves to a three-compartment model and assuming insignificant transamination and transmethylation in this tissue. PSR was calculated from fitted parameter values and plasma methionine concentrations. PSRs measured by PET were compared with arterio-venous (A-V) difference measurements across the hind limb during primed constant infusion (5-6 h) of L-[1-13C, methyl-2H3]methionine. Results of PET measurements demonstrated similar PSRs for paraspinal and hind limb muscles: 0.172 +/- 0.062 vs. 0.208 +/- 0.048 nmol-1.min-1.(g of muscle)-1 (P = not significant). PSR determined by the stable isotope technique was 0.27 +/- 0.050 nmol-1.min-1.(g of leg tissue)-1 (P < 0.07 from PET) and indicated that the contribution of transmethylation to total hind limb methionine utilization was approximately 10%. High levels of L-[methyl-11C]methionine utilization by bone marrow were observed. We conclude that muscle PSR can be measured in vivo by PET and that this approach offers promise for application in human metabolic studies.

Amino acid uptake in ischemically compromised brain tissue

BACKGROUND AND PURPOSE

Multitracer positron emission tomography (PET) was used to investigate local amino acid accumulation in brain tissue surrounding focal ischemia.

METHODS

PET using 15O-labeled oxygen and water for measuring cerebral metabolic rate of oxygen (CMRO2) and cerebral blood flow (CBF), C15O for determination of blood volume (CBV) and calculation of oxygen extraction fraction, and L-[11C]methylmethionine (11C-MET) for the assessment of amino acid accumulation was applied in 14 patients (mean age, 52 +/- 9.1 years) with acute ischemic hemispheric stroke. Two multitracer PET studies were completed, the first 8 to 24 hours after onset of neurological symptoms and the follow-up study 14 +/- 1 days after the ischemic attack. Functional changes were compared with morphological damage on cranial CT or MRI. Three-dimensional matching and volume of interest evaluation procedures were used to study 11C-MET accumulation in relation to various physiological variables in infarcted and noninfarcted tissue.

RESULTS

Compared with contralateral mirror regions, initially increased regional 11C-MET uptake (21.2 +/- 10.9%, P < .001) was found in patchy areas in the immediate vicinity of infarction as well as in distant areas within the same hemisphere. In those areas, regional CBF (-11.4 +/- 21.2%, P < .01) and oxygen extraction fraction (2.8 +/- 29.1%, P = NS) were highly variable, and regional CMRO2 was preserved or slightly reduced (-12.4 +/- 16.0%, P < .001). CBF data comprised severely ischemic as well as high values (14.6 to 64.2 mL/100 g per minute). Cranial CT and coregistered MRI in five patients demonstrated preserved morphology. In all peri-infarct areas (n = 62), the 11C-MET uptake showed a positive correlation with delta CMRO2 as the relative improvement of ipsilateral CMRO2 between the two PET studies (r = .378, P < .01). Particularly in areas with increased oxygen extraction fraction (n = 42), the 11C-MET uptake showed a mild correlation with CMRO2 at follow-up measurement (r = .31, P < .05). In all peri-infarct areas, 11C-MET uptake showed a negative correlation with oxygen extraction fraction (r = -.672, P < .001) and a positive correlation with CBF (r = .4, P = .001). In all infarcted and peri-infarct areas, normalized initial 11C-MET uptake was positively correlated with CMRO2 at follow-up (r = .603, P < .001).

CONCLUSIONS

Focal increases of 11C-MET uptake seen in this study were generally mild. They might be seen in the core of ischemia, indicating breakdown of the blood-brain barrier with poor tissue prognosis, but they also frequently occurred during or after ischemic compromise in surviving brain tissue surrounding focal cerebral infarction, perhaps representing alterations of amino acid transport or protein synthesis in brain tissue with a favorable prognosis.

Restoration of brain protein synthesis in mature and aged rats by a DA agonist, piribedil

Brain ageing affects numerous cerebral metabolic pathways such as cerebral glucose consumption or protein synthesis rate. The pharmacological effect of a mixed D1-D2 dopaminergic agonist, piribedil, on this last metabolism is reported. Cerebral Protein Synthesis Rate (CPSR) was measured by the [35S]L-methionine autoradiographic procedure in 38 main brain regions of 11 and 26-month-old Wistar rats after a 2-month treatment per os at 9 and 30 mg/kg/day with piribedil. Mean decrease of CPSR was -21% during the 15-month ageing we followed, with important local variations. Mean CPSR increased with the two treatments, +25% in mature and +35% in aged rats. Treatments restored CPSR of aged rats to the exact mature subjects levels in quite all the brain regions. No dose-effect or asymetrical modification was statistically revealed for the two treatments. Metabolic increases involved particularly central brain gray structures, especially some DA-targeted brain nuclei concerned with behaviour and learning. This effect argued for a general metabotrophic effect of D1-D2 dopamine stimulation of the brain. The original pattern of local ageing of brain protein synthesis in rat was also incidentally reported. This was the first direct report of a wide and effective metabolic activation of CPSR in the brain during ageing by a curative dopaminergic agonist treatment.

Evaluation of radioselenium labeled selenomethionine, a potential tracer for brain protein synthesis by PET

The blood-brain transfer, protein incorporation and metabolism of L-[75Se]selenomethionine (SeMet) of relatively high specific activity (> 400 GBq mmol) were studied in male Wistar rats. The highest uptake was found in the pancreas, followed by the tumor, kidney, liver, brain and muscle. In addition, plasma and brain samples of rats were analyzed for labeled fractions of free SeMet, metabolites, and SeMet bound to t-RNA and proteins. For example, free SeMet represented more than 80% of brain radioactivity at 1.5 min while it was less than 15% at 360 min. A concomitant increase was observed for protein bound SeMet in brain. A three-compartment model was applied to calculate the blood-brain transfer constant (K1 (0.15 +/- 0.070 mL g-1 min-1) and the rate constant of SeMet incorporation into proteins (k3 = 0.026 +/- 0.008 min-1). The apparent incorporation of methionine into proteins was estimated to be about 0.73 nmol g-1 min-1. From the studies it is concluded that the use of L-[75Se]selenomethionine may be appropriate to measure brain protein incorporation in humans with PET.

A comparative study on protein incorporation of L-[methyl-3H]methionine, L-[1-14C]leucine and L-2-[18F]fluorotyrosine in tumor bearing mice

Brain and tumor uptake of L-[methyl-3H]methionine (3H-Met), L-[1-14C]leucine (14C-Leu) and L-2-[18F]fluorotyrosine (18F-Tyr) and their incorporation into the acid-precipitable fraction (APF) were investigated in mice bearing FM3A-P0 and the higher metastatic FM3A-P15A mammary carcinomas in order to compare the potential of positron emitting analogs for measuring protein synthesis rates (PSR) in brain and tumors by positron emission tomography (PET). The total uptake of the 3H-Met by the brain was higher than those of two other amino acids. On the other hand, the incorporation rates into the APF in the brain and tumors were significantly faster with 14C-Leu compared to the others. Most of the 14C in the APF was identified in labeled proteins; however, considerable amounts of 3H were recovered in the lipid and RNA fractions, and 18F was also detected in the lipids. Among the positron emitting analogs of three amino acids, 11C-labeled Leu could be the most suitable for measuring PSR by PET. For all the amino acids, the total uptake and the APF both expressed as the % injected dose seemed slightly higher in the FM3A-P15A than in the FM3A-P0, although the differences did not reach statistical significance.