Evaluation of renal glucose uptake with [18F]FDG-PET: Methodological advancements and metabolic outcomes

Design and application of synthetic 17B-HSD13 substrates reveals preserved catalytic activity of protective human variants

Several hydroxysteroid dehydrogenase 17-beta 13 variants have previously been identified as protective against metabolic dysfunction-associated steatohepatitis (MASH) fibrosis, ballooning and inflammation, and as such this target holds significant therapeutic potential. However, over 5 years later, the function of 17B-HSD13 remains unknown. Structure-aided design enables the development of potent and selective sulfonamide-based 17B-HSD13 inhibitors. In order to probe their inhibitory potency in endogenous expression systems like primary human hepatocytes, inhibitors are transformed into synthetic surrogate substrates with distinct selectivity advantages over substrates previously published. Their application to cells endogenously expressing 17B-HSD13 enables quantitative measures of enzymatic inhibition in primary human hepatocytes which has never been reported to date. Application to multiple cellular systems expressing the protective human variants reveals that the most prevalent IsoD variant maintains NAD-dependent catalytic activity towards some but not all substrates, contradicting reports that the truncation results in loss-of-function.

Unraveling Diabetic Kidney Disease: The Roles of Mitochondrial Dysfunction and Immunometabolism

Mitochondria are essential for cellular energy production and are implicated in numerous diseases, including diabetic kidney disease (DKD). Current evidence indicates that mitochondrial dysfunction results in alterations in several metabolic pathways within kidney cells, thereby contributing to the progression of DKD. Furthermore, mitochondrial dysfunction can engender an inflammatory milieu, leading to the activation and recruitment of immune cells to the kidney tissue, potentially perturbing intrarenal metabolism. In addition, this inflammatory microenvironment has the potential to modify immune cell metabolism, which may further accentuate the immune-mediated kidney injury. This understanding has led to the emerging field of immunometabolism, which views DKD as not just a metabolic disorder caused by hyperglycemia but also one with significant immune contributions. Targeting mitochondrial function and immunometabolism may offer protective effects for the kidneys, complementing current therapies and potentially mitigating the risk of DKD progression. This comprehensive review examines the impact of mitochondrial dysfunction and the potential role of immunometabolism in DKD. We also discuss tools for investigating these mechanisms and propose avenues for integrating this research with existing therapies. These insights underscore the modulation of mitochondrial function and immunometabolism as a critical strategy for decelerating DKD progression.

Quantitative assessment of kidney split function and mean transit time in healthy patients using dynamic 18F-FDG PET/MRI studies with denoising and deconvolution methods making use of Legendre polynomials

PURPOSE

Our objective was to assess a deconvolution and denoising technique based on Legendre polynomials compared to matrix deconvolution on dynamic 18F-FDG renography of healthy patients.

METHOD

The study was carried out and compared to the data of 24 healthy patients from a published study who underwent examinations with 99mTc-MAG3 planar scintigraphy and 18F-FDG PET/MRI. Due to corruption issues in some data used in the published article, post-publication measurements were provided. We have been warned that post-publication data were treated differently. The smoothing method switched from Bezier to Savitzky-Golay and the deconvolution from matrix-based (with Tikhonov Regularization) to Richardson-Lucy. A comparison of the split function and mean transit times of the published and post-publication data against our method based on Legendre polynomials was performed.

RESULTS

For split function, we only observed a good agreement between the processing methods for the 99mTc-MAG3 and the post-published data. No correlation was found between the split functions obtained on the 99mTc-MAG3 and the 18F-FDG, contrary to the published study. However, all calculated split function values for 18F-FDG and 99mTc-MAG3 were within the established normal range. For the mean transit time, the correlation was moderate with published data and very good with the post-publication measurements for both 99mTc-MAG3 and 18F-FDG. Bias of the Bland-Altman analysis of the mean transit times for 99mTc-MAG3 versus 18F-FDG was 1.1 min (SD 1.7 min) for the published data, - 0.11 min (SD 1.9 min) for the post-publication results and .05 min (SD 1.9 min) for our method.

CONCLUSIONS

The processing methods used in the original publication and in the post-publication work were quite complex and required adaptation of the fitting parameters for each individual and each type of examination. Our method did not require any specific adjustment; the same unmodified and fully automated algorithm was successfully applied to all data.

Renal Cortical Glucose Uptake Is Decreased in Insulin Resistance and Correlates Inversely With Serum Free-fatty Acids

CONTEXT

Studies on human renal metabolism are scanty. Nowadays, functional imaging allows the characterization of renal metabolism in a noninvasive manner. We have recently demonstrated that fluorodeoxyglucose F18 (18F FDG) positron emission tomography can be used to analyze renal glucose uptake (GU) rates, and that the renal cortex is an insulin-sensitive tissue.

OBJECTIVE

To confirm that renal GU is decreased in people with obesity and to test whether circulating metabolites are related to renal GU.

DESIGN, SETTING AND PARTICIPANTS

Eighteen people with obesity and 18 nonobese controls were studied with [18F]FDG positron emission tomography during insulin clamp. Renal scans were obtained ∼60 minutes after [18F]FDG injection. Renal GU was measured using fractional uptake rate and after correcting for residual intratubular [18F]FDG. Circulating metabolites were measured using high-throughput proton nuclear magnetic resonance metabolomics.

RESULTS

Cortical GU was higher in healthy nonobese controls compared with people with obesity (4.7 [3.4-5.6] vs 3.1 [2.2-4.3], P = .004, respectively), and it associated positively with the degree of insulin sensitivity (M value) (r = 0.42, P = .01). Moreover, cortical GU was inversely associated with circulating β-OH-butyrate (r = -0.58, P = .009), acetoacetate (r = -0.48, P = .008), citrate (r = -0.44, P = .01), and free fatty acids (r = -0.68, P < .0001), even when accounting for the M value. On the contrary, medullary GU was not associated with any clinical parameters.

CONCLUSION

These data confirm differences in renal cortical GU between people with obesity and healthy nonobese controls. Moreover, the negative correlations between renal cortex GU and free fatty acids, ketone bodies, and citrate are suggestive of substrate competition in the renal cortex.

Correlation between Kidney Uptake at [18F]FDG PET/CT and Renal Function

Different insights into the connection between kidney [18F]fluorodesoxyglucose ([18F]FDG) uptake at positron emission tomography/computed tomography (PET/CT) and renal function have been proposed in the past. The aim of this study was therefore to assess the presence of a correlation between these two parameters. Kidney uptakes were assessed and compared to the creatinine (Cr) values and estimated glomerular filtration rate (EGFR) among different classes of renal functional impairment or kidney status. A total of 339 patients and 385 different PET/CT scans were included in this study. Significant correlations between kidney uptakes and renal function parameters were reported in most of the groups studied, with the exception of patients with Cr < 1.2 mg/dL and subjects with a kidney transplantation. Strong concordance in the assessment of renal parenchymal uptakes between the different readers was reported. To conclude, strong correlations for renal [18F]FDG uptake with Cr levels and the EGFR were reported, with the exception of the group of patients with a Cr value < 1.2 mg/dL and for the group with a kidney transplantation.

Renal Perfusion, Oxygenation and Metabolism: The Role of Imaging

Thanks to technical advances in the field of medical imaging, it is now possible to study key features of renal anatomy and physiology, but so far poorly explored due to the inherent difficulties in studying both the metabolism and vasculature of the human kidney. In this narrative review, we provide an overview of recent research findings on renal perfusion, oxygenation, and substrate uptake. Most studies evaluating renal perfusion with positron emission tomography (PET) have been performed in healthy controls, and specific target populations like obese individuals or patients with renovascular disease and chronic kidney disease (CKD) have rarely been assessed. Functional magnetic resonance (fMRI) has also been used to study renal perfusion in CKD patients, and recent studies have addressed the kidney hemodynamic effects of therapeutic agents such as glucagon-like receptor agonists (GLP-1RA) and sodium-glucose co-transporter 2 inhibitors (SGLT2-i) in an attempt to characterise the mechanisms leading to their nephroprotective effects. The few available studies on renal substrate uptake are discussed. In the near future, these imaging modalities will hopefully become widely available with researchers more acquainted with them, gaining insights into the complex renal pathophysiology in acute and chronic diseases.

The Role of Vitamin D in Health and Disease: A Narrative Review on the Mechanisms Linking Vitamin D with Disease and the Effects of Supplementation

Vitamin D insufficiency or deficiency (VDD) is a very prevalent condition in the general population. Vitamin D is necessary for optimal bone mineralization, but apart from the bone effects, preclinical and observational studies have suggested that vitamin D may have pleiotropic actions, whereas VDD has been linked to several diseases and higher all-cause mortality. Thus, supplementing vitamin D has been considered a safe and inexpensive approach to generate better health outcomes-and especially so in frail populations. Whereas it is generally accepted that prescribing of vitamin D in VDD subjects has demonstrable health benefits, most randomized clinical trials, although with design constraints, assessing the effects of vitamin D supplementation on a variety of diseases have failed to demonstrate any positive effects of vitamin D supplementation. In this narrative review, we first describe mechanisms through which vitamin D may exert an important role in the pathophysiology of the discussed disorder, and then provide studies that have addressed the impact of VDD and of vitamin D supplementation on each disorder, focusing especially on randomized clinical trials and meta-analyses. Despite there already being vast literature on the pleiotropic actions of vitamin D, future research approaches that consider and circumvent the inherent difficulties in studying the effects of vitamin D supplementation on health outcomes are needed to assess the potential beneficial effects of vitamin D. The evaluation of the whole vitamin D endocrine system, rather than only of 25-hydroxyvitamin D levels before and after treatment, use of adequate and physiologic vitamin D dosing, grouping based on the achieved vitamin D levels rather than the amount of vitamin D supplementation subjects may receive, and sufficiently long follow-up are some of the aspects that need to be carefully considered in future studies.