A new compartmental method for the analysis of liver FDG kinetics in small animal models

A 5:2 intermittent fasting regimen ameliorates NASH and fibrosis and blunts HCC development via hepatic PPARα and PCK1

The role and molecular mechanisms of intermittent fasting (IF) in non-alcoholic steatohepatitis (NASH) and its transition to hepatocellular carcinoma (HCC) are unknown. Here, we identified that an IF 5:2 regimen prevents NASH development as well as ameliorates established NASH and fibrosis without affecting total calorie intake. Furthermore, the IF 5:2 regimen blunted NASH-HCC transition when applied therapeutically. The timing, length, and number of fasting cycles as well as the type of NASH diet were critical parameters determining the benefits of fasting. Combined proteome, transcriptome, and metabolome analyses identified that peroxisome-proliferator-activated receptor alpha (PPARα) and glucocorticoid-signaling-induced PCK1 act co-operatively as hepatic executors of the fasting response. In line with this, PPARα targets and PCK1 were reduced in human NASH. Notably, only fasting initiated during the active phase of mice robustly induced glucocorticoid signaling and free-fatty-acid-induced PPARα signaling. However, hepatocyte-specific glucocorticoid receptor deletion only partially abrogated the hepatic fasting response. In contrast, the combined knockdown of Ppara and Pck1 in vivo abolished the beneficial outcomes of fasting against inflammation and fibrosis. Moreover, overexpression of Pck1 alone or together with Ppara in vivo lowered hepatic triglycerides and steatosis. Our data support the notion that the IF 5:2 regimen is a promising intervention against NASH and subsequent liver cancer.

Advances and challenges in measuring hepatic glucose uptake with FDG PET: implications for diabetes research

The liver plays a crucial role in the control of glucose homeostasis and is therefore of great interest in the investigation of the development of type 2 diabetes. Hepatic glucose uptake (HGU) can be measured through positron emission tomography (PET) imaging with the tracer [18F]-2-fluoro-2-deoxy-D-glucose (FDG). HGU is dependent on many variables (e.g. plasma glucose, insulin and glucagon concentrations), and the metabolic state for HGU assessment should be chosen with care and coherence with the study question. In addition, as HGU is influenced by many factors, protocols and measurement conditions need to be standardised for reproducible results. This review provides insights into the protocols that are available for the measurement of HGU by FDG PET and discusses the current state of knowledge of HGU and its impairment in type 2 diabetes. Overall, a scanning modality that allows for the measurement of detailed kinetic information and influx rates (dynamic imaging) may be preferable to static imaging. The combination of FDG PET and insulin stimulation is crucial to measure tissue-specific insulin sensitivity. While the hyperinsulinaemic-euglycaemic clamp allows for standardised measurements under controlled blood glucose levels, some research questions might require a more physiological approach, such as oral glucose loading, with both advantages and complexities relating to fluctuations in blood glucose and insulin levels. The available approaches to address HGU hold great potential but await more systematic exploitation to improve our understanding of the mechanisms underlying metabolic diseases. Current findings from the investigation of HGU by FDG PET highlight the complex interplay between insulin resistance, hepatic glucose metabolism, NEFA levels and intrahepatic lipid accumulation in type 2 diabetes and obesity. Further research is needed to fully understand the underlying mechanisms and potential therapeutic targets for improving HGU in these conditions.

Novel approach using [18F]FTHA-PET and de novo synthesized VLDL for assessment of FFA metabolism in a rat model of diet induced NAFLD

BACKGROUND AND AIMS

The worldwide prevalence of Non-alcoholic Fatty Liver Disease (NAFLD) raises concerns about associated risk factors, such as obesity and type 2 Diabetes Mellitus, for leading causes of disability and death. Besides Magnetic Resonance Imaging (MRI) and Spectroscopy (MRS), functional imaging with Positron Emission Tomography (PET) could contribute to a deeper understanding of the pathophysiology of NAFLD. Here we describe a novel approach using the PET tracer [18F]FTHA, which is an analog of long-chain free fatty acids (FFA) and is taken up by tissues to enter mitochondria or to be incorporated into complex lipids for further export as very-low-density lipoprotein (VLDL).

METHODS

Male Sprague Dawley rats, after 6 weeks on a high-fat diet (HFD), were used as a model of diet induced NAFLD, while a standard diet (SD) served as a control group. Liver fat was estimated by MR spectroscopy at a 9.4 T system for phenotyping. To measure hepatic FFA uptake, rats underwent 60 min dynamic [18F]FTHA-PET scans after unrestricted access to food (HFD: n = 6; SD: n = 6) or overnight (≤16h) fasting (HFD: n = 6; SD: n = 5). FFA removal was assessed from incorporated 18F-residual in de novo synthesized VLDL out of plasma.

RESULTS

MRS of the liver confirmed the presence of NAFLD (>5.6% fat). Under non-fasting conditions, hepatic [18F]FTHA uptake was significantly increased in NAFLD: SUVmean (p = 0.03) within [0; 60] min interval, SUVmean (p = 0.01) and SUVmax (p = 0.03) within [30; 60] min interval. SUVs for hepatic uptake under fasting conditions were not significantly different between the groups. Analysis of FFA removal demonstrated elevated values of 18F-residue in the VLDL plasma fraction of the healthy group compared to the NAFLD (p = 0.0569).

CONCLUSION

Our novel approach for assessing FFA metabolism using [18F]FTHA demonstrated differences in the hepatic FFA uptake and FFA incorporation into VLDL between healthy and NAFLD rats. [18F]FTHA-PET could be used to study metabolic disturbances involved in the progression of NAFLD.

Mathematical Models for FDG Kinetics in Cancer: A Review

Compartmental analysis is the mathematical framework for the modelling of tracer kinetics in dynamical Positron Emission Tomography. This paper provides a review of how compartmental models are constructed and numerically optimized. Specific focus is given on the identifiability and sensitivity issues and on the impact of complex physiological conditions on the mathematical properties of the models.

Positron emission tomography and single photon emission computed tomography imaging of tertiary lymphoid structures during the development of lupus nephritis

Lymphoid neogenesis occurs in tissues targeted by chronic inflammatory processes, such as infection and autoimmunity. In systemic lupus erythematosus (SLE), such structures develop within the kidneys of lupus-prone mice ((NZBXNZW)F1) and are observed in kidney biopsies taken from SLE patients with lupus nephritis (LN). The purpose of this prospective longitudinal animal study was to detect early kidney changes and tertiary lymphoid structures (TLS) using in vivo imaging. Positron emission tomography (PET) by tail vein injection of 18-F-fluoro-2-deoxy-D-glucose (¹⁸F-FDG)(PET/FDG) combined with computed tomography (CT) for anatomical localization and single photon emission computed tomography (SPECT) by intraperitoneal injection of ^99mTC labeled Albumin Nanocoll (^99mTC-Nanocoll) were performed on different disease stages of NZB/W mice (n = 40) and on aged matched control mice (BALB/c) (n = 20). By using one-way ANOVA analyses, we compared two different compartmental models for the quantitative measure of ¹⁸F-FDG uptake within the kidneys. Using a new five-compartment model, we observed that glomerular filtration of ^18FFDG in lupus-prone mice decreased significantly by disease progression measured by anti-dsDNA Ab production and before onset of proteinuria. We could not visualize TLS within the kidneys, but we were able to visualize pancreatic TLS using ^99mTC Nanocoll SPECT. Based on our findings, we conclude that the five-compartment model can be used to measure changes of FDG uptake within the kidney. However, new optimal PET/SPECT tracer administration sites together with more specific tracers in combination with magnetic resonance imaging (MRI) may make it possible to detect formation of TLS and LN before clinical manifestations.

The role of endoplasmic reticulum in in vivo cancer FDG kinetics

A recent result obtained by means of an in vitro experiment with cancer cultured cells has configured the endoplasmic reticulum as the preferential site for the accumulation of 2-deoxy-2-[¹⁸F]fluoro-D-glucose (FDG). Such a result is coherent with cell biochemistry and is made more significant by the fact that the reticular accumulation rate of FDG is dependent upon extracellular glucose availability. The objective of the present paper is to confirm in vivo the result obtained in vitro concerning the crucial role played by the endoplasmic reticulum in FDG cancer metabolism. This study utilizes data acquired by means of a Positron Emission Tomography scanner for small animals in the case of CT26 models of cancer tissues. The recorded concentration images are interpreted within the framework of a three-compartment model for FDG kinetics, which explicitly assumes that the endoplasmic reticulum is the dephosphorylation site for FDG in cancer cells. The numerical reduction of the compartmental model is performed by means of a regularized Gauss-Newton algorithm for numerical optimization. This analysis shows that the proposed three-compartment model equals the performance of a standard Sokoloff’s two-compartment system in fitting the data. However, it provides estimates of some of the parameters, such as the phosphorylation rate of FDG, more consistent with prior biochemical information. These results are made more solid from a computational viewpoint by proving the identifiability and by performing a sensitivity analysis of the proposed compartment model.

Insulin-independent stimulation of skeletal muscle glucose uptake by low-dose abscisic acid via AMPK activation

Abscisic acid (ABA) is a plant hormone active also in mammals where it regulates, at nanomolar concentrations, blood glucose homeostasis. Here we investigated the mechanism through which low-dose ABA controls glycemia and glucose fate. ABA stimulated uptake of the fluorescent glucose analog 2-NBDG by L6, and of [¹⁸F]-deoxy-glucose (FDG) by mouse skeletal muscle, in the absence of insulin, and both effects were abrogated by the specific AMPK inhibitor dorsomorphin. In L6, incubation with ABA increased phosphorylation of AMPK and upregulated PGC-1α expression. LANCL2 silencing reduced all these ABA-induced effects. In vivo, low-dose oral ABA stimulated glucose uptake and storage in the skeletal muscle of rats undergoing an oral glucose load, as detected by micro-PET. Chronic treatment with ABA significantly improved the AUC of glycemia and muscle glycogen content in CD1 mice exposed to a high-glucose diet. Finally, both acute and chronic ABA treatment of hypoinsulinemic TRPM2^-/- mice ameliorated the glycemia profile and increased muscle glycogen storage. Altogether, these results suggest that low-dose oral ABA might be beneficial for pre-diabetic and diabetic subjects by increasing insulin-independent skeletal muscle glucose disposal through an AMPK-mediated mechanism.

FDG uptake tracks the oxidative damage in diabetic skeletal muscle: An experimental study

OBJECTIVES

The present study aims to verify the relationship between glucose consumption and uptake of ¹⁸F-2-deoxy-glucose (FDG) in the skeletal muscle (SM) of experimental models of streptozotocin-induced diabetes mellitus (STZ-DM).

METHODS

The study included 36 Balb/c mice. Two weeks after intraperitoneal administration of saline (control group, n = 18) or 150 mg streptozotocin (STZ-DM group, n = 18), the two cohorts were submitted to an oral glucose tolerance test and were further subdivided into three groups (n = 6 each): untreated and treated with metformin (MTF) at low or high doses (10 or 750 mg/kg daily, respectively). Two weeks thereafter, all mice were submitted to dynamic micro-positron emission tomography (PET) imaging after prolonged fasting. After sacrifice, enzymatic pathways and response to oxidative stress were evaluated in harvested SM.

RESULTS

On PET imaging, the FDG uptake rate in hindlimb SM was significantly lower in nondiabetic mice as compared with STZ-DM-untreated mice. MTF had no significant effect on SM FDG uptake in untreated mice; however, its high dose induced a significant decrease in STZ-DM animals. Upon conventional analysis, the SM standard uptake value was higher in STZ-DM mice, while MTF was virtually ineffective in either control or STZ-DM models. This metabolic reprogramming was not explained by any change in cytosolic glucose metabolism. By contrast, it closely agreed with the catalytic function of hexose-6P-dehydrogenase (H6PD; i.e., the trigger of a specific pentose phosphate pathway selectively located within the endoplasmic reticulum). In agreement with this role, the H6PD enzymatic response to both STZ-DM and MTF matched the activation of the NADPH-dependent antioxidant responses to the increased generation of reactive oxygen species caused by chronic hyperglycemia. Ex vivo analysis of tracer kinetics confirmed that the enhanced SM avidity for FDG occurred despite a significant reduction in glucose consumption, while it was associated with increased radioactivity transfer to the endoplasmic reticulum.

CONCLUSIONS

These data challenge the current dogma linking FDG uptake to the glycolytic rate. They instead introduce a new model considering a strict link between the uptake of this glucose analog, H6PD reticular activity, and oxidative damage in diabetes, at least under fasting condition.

Structural and practical identifiability of dual-input kinetic modeling in dynamic PET of liver inflammation

Dynamic 18F-FDG PET with tracer kinetic modeling has the potential to noninvasively evaluate human liver inflammation using the FDG blood-to-tissue transport rate K 1. Accurate kinetic modeling of dynamic liver PET data and K 1 quantification requires the knowledge of dual-blood input function from the hepatic artery and portal vein. While the arterial input function can be derived from the aortic region on dynamic PET images, it is difficult to extract the portal vein input function accurately from PET images. The optimization-derived dual-input kinetic modeling approach has been proposed to overcome this problem by jointly estimating the portal vein input function and FDG tracer kinetics from time activity curve fitting. In this paper, we further characterize the model properties by analyzing the structural identifiability of the model parameters using the Laplace transform and practical identifiability using computer simulation based on fourteen patient datasets. The theoretical analysis has indicated that all the kinetic parameters of the dual-input kinetic model are structurally identifiable, though subject to local solutions. The computer simulation results have shown that FDG K 1 can be estimated reliably in the whole-liver region of interest with reasonable bias, standard deviation, and high correlation between estimated and original values, indicating of practical identifiability of K 1. The result has also demonstrated the correlation between K 1 and histological liver inflammation scores is reliable. FDG K 1 quantification by the optimization-derived dual-input kinetic model is promising for assessing liver inflammation.

Kinetic Analysis of Hepatic Metabolism Using Hyperpolarized Dihydroxyacetone

Hyperpolarized carbon-13 magnetic resonance (HP-MR) is a new metabolic imaging method the does not use ionizing radiation. Due to the inherent chemical specificity of MR, not only tracer uptake but also downstream metabolism of the agent is detected in a straightforward manner. HP [2-¹³C] dihydroxyacetone (DHA) is a promising new agent that directly interrogates hepatic glucose metabolism. DHA has three metabolic fates in the liver: glucose production, glycerol production and potential inclusion into triglycerides, and oxidation in the tricarboxylic acid cycle. Each pathway is regulated by flux through multiple enzymes. Using Duhamel's formula, the kinetics of DHA metabolism is modeled, resulting in estimates of specific reaction rate constants. The multiple enzymatic steps that control DHA metabolism make more simplified methods for extracting kinetic data less than satisfactory. The described modeling paradigm effectively identifies changes in metabolism between gluconeogenic and glycogenolytic models of hepatic function.

Dynamic PET of human liver inflammation: impact of kinetic modeling with optimization-derived dual-blood input function

The hallmark of nonalcoholic steatohepatitis is hepatocellular inflammation and injury in the setting of hepatic steatosis. Recent work has indicated that dynamic 18F-FDG PET with kinetic modeling has the potential to assess hepatic inflammation noninvasively, while static FDG-PET is less promising. Because the liver has dual blood supplies, kinetic modeling of dynamic liver PET data is challenging in human studies. This paper aims to identify the optimal dual-input kinetic modeling approach for dynamic FDG-PET of human liver inflammation. Fourteen patients with nonalcoholic fatty liver disease were included. Each patient underwent 1 h dynamic FDG-PET/CT scan and had liver biopsy within six weeks. Three models were tested for kinetic analysis: the traditional two-tissue compartmental model with an image-derived single-blood input function (SBIF), a model with population-based dual-blood input function (DBIF), and a new model with optimization-derived DBIF through a joint estimation framework. The three models were compared using Akaike information criterion (AIC), F test and histopathologic inflammation score. Results showed that the optimization-derived DBIF model improved liver time activity curve fitting and achieved lower AIC values and higher F values than the SBIF and population-based DBIF models in all patients. The optimization-derived model significantly increased FDG K1 estimates by 101% and 27% as compared with traditional SBIF and population-based DBIF. K1 by the optimization-derived model was significantly associated with histopathologic grades of liver inflammation while the other two models did not provide a statistical significance. In conclusion, modeling of DBIF is critical for dynamic liver FDG-PET kinetic analysis in human studies. The optimization-derived DBIF model is more appropriate than SBIF and population-based DBIF for dynamic FDG-PET of liver inflammation.

Searching for novel PET radiotracers: imaging cardiac perfusion, metabolism and inflammation

Advances in medical imaging technology have led to an increased demand for radiopharmaceuticals for early and accurate diagnosis of cardiac function and diseased states. Myocardial perfusion, metabolism, and hypoxia positron emission tomography (PET) imaging radiotracers for detection of cardiac disease lack specificity for targeting inflammation that can be an early indicator of cardiac disease. Inflammation can occur at all stages of cardiac disease and currently, 18F-fluorodeoxyglucose (FDG), a glucose analog, is the standard for detecting myocardial inflammation. 18F-FDG has many ideal characteristics of a radiotracer but lacks the ability to differentiate between glucose uptake in normal cardiomyocytes and inflammatory cells. Developing a PET radiotracer that differentiates not only between inflammatory cells and normal cardiomyocytes, but between types of immune cells involved in inflammation would be ideal. This article reviews current PET radiotracers used in cardiac imaging, their limitations, and potential radiotracer candidates for imaging cardiac inflammation in early stages of development of acute and chronic cardiac diseases. The select radiotracers reviewed have been tested in animals and/or show potential to be developed as a radiotracer for the detection of cardiac inflammation by targeting the enzymatic activities or subpopulations of macrophages that are recruited to an injured or infected site.