Determinants of plasma levels of proglucagon and the metabolic impact of glucagon receptor signalling: a UK Biobank study

AIMS/HYPOTHESES

Glucagon and glucagon-like peptide-1 (GLP-1) are derived from the same precursor; proglucagon, and dual agonists of their receptors are currently being explored for the treatment of obesity and metabolic dysfunction-associated steatotic liver disease (MASLD). Elevated levels of endogenous glucagon (hyperglucagonaemia) have been linked with hyperglycaemia in individuals with type 2 diabetes but are also observed in individuals with obesity and MASLD. GLP-1 levels have been reported to be largely unaffected or even reduced in similar conditions. We investigated potential determinants of plasma proglucagon and associations of glucagon receptor signalling with metabolic diseases based on data from the UK Biobank.

METHODS

We used exome sequencing data from the UK Biobank for ~410,000 white participants to identify glucagon receptor variants and grouped them based on their known or predicted signalling. Data on plasma levels of proglucagon estimated using Olink technology were available for a subset of the cohort (~40,000). We determined associations of glucagon receptor variants and proglucagon with BMI, type 2 diabetes and liver fat (quantified by liver MRI) and performed survival analyses to investigate if elevated proglucagon predicts type 2 diabetes development.

RESULTS

Obesity, MASLD and type 2 diabetes were associated with elevated plasma levels of proglucagon independently of each other. Baseline proglucagon levels were associated with the risk of type 2 diabetes development over a 14 year follow-up period (HR 1.13; 95% CI 1.09, 1.17; n=1562; p=1.3×10-12). This association was of the same magnitude across strata of BMI. Carriers of glucagon receptor variants with reduced cAMP signalling had elevated levels of proglucagon (β 0.847; 95% CI 0.04, 1.66; n=17; p=0.04), and carriers of variants with a predicted frameshift mutation had higher levels of liver fat compared with the wild-type reference group (β 0.504; 95% CI 0.03, 0.98; n=11; p=0.04).

CONCLUSIONS/INTERPRETATION

Our findings support the suggestion that glucagon receptor signalling is involved in MASLD, that plasma levels of proglucagon are linked to the risk of type 2 diabetes development, and that proglucagon levels are influenced by genetic variation in the glucagon receptor, obesity, type 2 diabetes and MASLD. Determining the molecular signalling pathways downstream of glucagon receptor activation may guide the development of biased GLP-1/glucagon co-agonist with improved metabolic benefits.

DATA AVAILABILITY

All coding is available through https://github.com/nicwin98/UK-Biobank-GCG.

In vivo mapping of hemodynamic responses mediated by tubuloglomerular feedback in hypertensive kidneys

The kidney has a sophisticated vascular structure that performs the unique function of filtering blood and managing blood pressure. Tubuloglomerular feedback is an intra-nephron negative feedback mechanism stabilizing single-nephron blood flow, glomerular filtration rate, and tubular flow rate, which is exhibited as self-sustained oscillations in single-nephron blood flow. We report the application of multi-scale laser speckle imaging to monitor global blood flow changes across the kidney surface (low zoom) and local changes in individual microvessels (high zoom) in normotensive and spontaneously hypertensive rats in vivo. We reveal significant differences in the parameters of TGF-mediated hemodynamics and patterns of synchronization. Furthermore, systemic infusion of a glucagon-like-peptide-1 receptor agonist, a potential renoprotective agent, induces vasodilation in both groups but only alters the magnitude of the TGF in Sprague Dawleys, although the underlying mechanisms remain unclear.

Renoprotective effects of GLP-1 receptor agonists and SGLT-2 inhibitors - is renal hemodynamics the key point?

Two novel treatments for diabetic kidney disease have emerged after decades with little progression. Both agents were developed for improved glycemic control in patients with type-2 diabetes. However, large clinical trials showed renoprotective effects beyond their ability to lower plasma glucose levels, body weight, and blood pressure. How this renal protection occurs is unknown. We will discuss their physiological effects, with special focus on the renal effects. We discuss how these drugs affect the function of the diabetic and non-diabetic kidneys to elucidate mechanisms by which the renoprotection could arise. Diabetic kidney disease affects the glomerular capillaries, which are usually protected by the renal autoregulatory mechanisms, the myogenic response, and the tubuloglomerular feedback mechanism. Animal models with reduced renal autoregulatory capacity develop chronic kidney disease. Despite different cellular targets, both drugs are suspected to affect renal hemodynamics through changes in the renal autoregulatory mechanisms. The glucagon-like peptide-1 receptor agonists exert a direct vasodilatory effect on the afferent arteriole positioned just before the glomerulus. Paradoxically, this effect is expected to increase glomerular capillary pressure, causing glomerular injury. In contrast, the sodium-glucose transporter-2 inhibitors are believed to activate the tubuloglomerular feedback mechanism to elicit vasoconstriction of the afferent arteriole. Because of their opposing effects on the renal afferent arterioles, it appears unlikely that their renoprotective effects can be explained by common effects of renal hemodynamics, but both drugs appear to add protection to the kidney beyond what can be obtained with classical treatment targeted at lowering blood glucose levels and blood pressure.

The assessment of cortical hemodynamic responses induced by tubuloglomerular feedback using in vivo imaging

The tubuloglomerular feedback (TGF) mechanism modulates renal hemodynamics and glomerular filtration rate in individual nephrons. Our study aimed to evaluate the TGF-induced vascular responses by inhibiting Na-K-2Cl co-transporters and sodium-glucose co-transporters in rats. We assessed cortical hemodynamics with high-resolution laser speckle contrast imaging, which enabled the evaluation of blood flow in individual microvessels and analysis of their dynamical patterns in the time-frequency domain. We demonstrated that a systemic administration of furosemide abolishes TGF-mediated hemodynamic responses. Furthermore, we showed that the local microcirculatory blood flow decreased, and the TGF-induced hemodynamic oscillations were sustained but weakened after inhibiting sodium-glucose co-transporters in Sprague-Dawley rats.

Glucagon receptor antagonism impairs and glucagon receptor agonism enhances triglycerides metabolism in mice

OBJECTIVE

Treatment with glucagon receptor antagonists (GRAs) reduces blood glucose but causes dyslipidemia and accumulation of fat in the liver. We investigated the acute and chronic effects of glucagon on lipid metabolism in mice.

METHODS

Chronic effects of glucagon receptor signaling on lipid metabolism were studied using oral lipid tolerance tests (OLTTs) in overnight fasted glucagon receptor knockout (Gcgr^-/-) mice, and in C57Bl/6JRj mice treated with a glucagon receptor antibody (GCGR Ab) or a long-acting glucagon analogue (GCGA) for eight weeks. Following treatment, liver tissue was harvested for RNA-sequencing and triglyceride measurements. Acute effects were studied in C57Bl/6JRj mice treated with a GRA or GCGA 1 h or immediately before OLTTs, respectively. Direct effects of glucagon on hepatic lipolysis were studied using isolated perfused mouse liver preparations. To investigate potential effects of GCGA and GRA on gastric emptying, paracetamol was, in separate experiments, administered immediately before OLTTs.

RESULTS

Plasma triglyceride concentrations increased 2-fold in Gcgr^-/- mice compared to their wild-type littermates during the OLTT (P = 0.001). Chronic treatment with GCGR Ab increased, whereas GCGA treatment decreased, plasma triglyceride concentrations during OLTTs (P < 0.05). Genes involved in lipid metabolism were upregulated upon GCGR Ab treatment while GCGA treatment had opposite effects. Acute GRA and GCGA treatment, respectively, increased (P = 0.02) and decreased (P = 0.003) plasma triglyceride concentrations during OLTTs. Glucagon stimulated hepatic lipolysis, evident by an increase in free fatty acid concentrations in the effluent from perfused mouse livers. In line with this, GCGR Ab treatment increased, while GCGA treatment decreased, liver triglyceride concentrations. The effects of glucagon appeared independent of changes in gastric emptying of paracetamol.

CONCLUSIONS

Glucagon receptor signaling regulates triglyceride metabolism, both chronically and acutely, in mice. These data expand glucagon´s biological role and implicate that intact glucagon signaling is important for lipid metabolism. Glucagon agonism may have beneficial effects on hepatic and peripheral triglyceride metabolism.

Opposing effects of chronic glucagon receptor agonism and antagonism on amino acids, hepatic gene expression, and alpha cells

The pancreatic hormone, glucagon, is known to regulate hepatic glucose production, but recent studies suggest that its regulation of hepatic amino metabolism is equally important. Here, we show that chronic glucagon receptor activation with a long-acting glucagon analog increases amino acid catabolism and ureagenesis and causes alpha cell hypoplasia in female mice. Conversely, chronic glucagon receptor inhibition with a glucagon receptor antibody decreases amino acid catabolism and ureagenesis and causes alpha cell hyperplasia and beta cell loss. These effects were associated with the transcriptional regulation of hepatic genes related to amino acid uptake and catabolism and by the non-transcriptional modulation of the rate-limiting ureagenesis enzyme, carbamoyl phosphate synthetase-1. Our results support the importance of glucagon receptor signaling for amino acid homeostasis and pancreatic islet integrity in mice and provide knowledge regarding the long-term consequences of chronic glucagon receptor agonism and antagonism.

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Glucagon receptor agonism increases amino acid catabolism and hepatic CPS-1 activity

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Glucagon receptor signaling regulates the number of pancreatic alpha cells

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Glucagon regulates the hepatic transcription of genes involved in amino acid metabolism

Multi-scale laser speckle contrast imaging of microcirculatory vasoreactivity

Laser speckle contrast imaging is a robust and versatile blood flow imaging tool in basic and clinical research for its relatively simple construction and ease of customization. One of its key features is the scalability of the imaged field of view. With minimal changes to the system or analysis, laser speckle contrast imaging allows for high-resolution blood flow imaging through cranial windows or low-resolution perfusion visualization of perfusion over large areas, e.g. in human skin. We further utilize this feature and introduce a multi-scale laser speckle contrast imaging system, which we apply to study vasoreactivity in renal microcirculation. We combine high resolution (small field of view) to segment blood flow in individual vessels with low resolution (large field of view) to monitor global blood flow changes across the renal surface. Furthermore, we compare their performance when analyzing blood flow dynamics potentially associated with a single nephron and show that the previously published approaches, based on low-zoom imaging alone, provide inaccurate results in such applications.

Glucagon-like peptide-1 receptors in the kidney: impact on renal autoregulation

Glucagon-like peptide-1 (GLP-1) and strategies based on this blood sugar-reducing and appetite-suppressing hormone are used to treat obesity and type 2 diabetes. However, the GLP-1 receptor (GLP-1R) is also present in the kidney, where it influences renal function. The effect of GLP-1 on the kidney varies between humans and rodents. The effect of GLP-1 on kidney function also seems to vary depending on its concentration and the physiological or pathological state of the kidney. In studies with rodents or humans, acute infusion of pharmacological doses of GLP-1 stimulates natriuresis and diuresis. However, the effect on the renal vasculature is less clear. In rodents, GLP-1 infusion increases renal plasma flow and glomerular filtration rate, suggesting renal vasodilation. In humans, only a subset of the study participants exhibits increased renal plasma flow and glomerular filtration rate. Differential status of kidney function and changes in renal vascular resistance of the preglomerular arterioles may account for the different responses of the human study participants. Because renal function in patients with type 2 diabetes is already at risk or compromised, understanding the effects of GLP-1R activation on kidney function in these patients is particularly important. This review examines the distribution of GLP-1R in the kidney and the effects elicited by GLP-1 or GLP-1R agonists. By integrating results from acute and chronic studies in healthy individuals and patients with type 2 diabetes along with those from rodent studies, we provide insight into how GLP-1R activation affects renal function and autoregulation.

Stimulation history affects vasomotor responses in rat mesenteric arterioles

Resistance vessels regulate blood flow by continuously adjusting activity of the wall smooth muscle cells. These cells integrate a variety of stimuli from blood, endothelium, autonomic nerves, and surrounding tissues. Each stimulus elicits an intracellular signaling cascade that eventually influences activation of the contractile machinery. The characteristic time scale of each cascade and the sharing of specific reactions between cascades provide for complex behavior when a vessel receives multiple stimuli. Here, we apply sequential stimulation with invariant concentrations of vasoconstrictor (norepinephrine/methoxamine) and vasodilator (SNAP/carbacol) to rat mesenteric vessels in the wire myograph to show that (1) time elapsed between addition of two vasoactive drugs and (2) the sequence of addition may significantly affect final force development. Furthermore, force oscillations (vasomotion) often appear upon norepinephrine administration. Using computational modeling in combination with nitric oxide (NO) inhibition/NO addition experiments, we show that (3) amplitude and number of oscillating vessels increase over time, (4) the ability of NO to induce vasomotion depends on whether it is applied before or after norepinephrine, and (5) emergence of vasomotion depends on the prior dynamical state of the system; in simulations, this phenomenon appears as "hysteresis." These findings underscore the time-dependent nature of vascular tone generation which must be considered when evaluating the vasomotor effects of multiple, simultaneous stimuli in vitro or in vivo.

Liraglutide Reduces Both Atherosclerosis and Kidney Inflammation in Moderately Uremic LDLr-/- Mice

Chronic kidney disease (CKD) leads to uremia. CKD is characterized by a gradual increase in kidney fibrosis and loss of kidney function, which is associated with a progressive increase in risk of atherosclerosis and cardiovascular death. To prevent progression of both kidney fibrosis and atherosclerosis in uremic settings, insight into new treatment options with effects on both parameters is warranted. The GLP-1 analogue liraglutide improves glucose homeostasis, and is approved for treatment of type 2 diabetes. Animal studies suggest that GLP-1 also dampens inflammation and atherosclerosis. Our aim was to examine effects of liraglutide on kidney fibrosis and atherosclerosis in a mouse model of moderate uremia (5/6 nephrectomy (NX)). Uremic (n = 29) and sham-operated (n = 14) atherosclerosis-prone low density lipoprotein receptor knockout mice were treated with liraglutide (1000 μg/kg, s.c. once daily) or vehicle for 13 weeks. As expected, uremia increased aortic atherosclerosis. In the remnant kidneys from NX mice, flow cytometry revealed an increase in the number of monocyte-like cells (CD68⁺F4/80^-), CD4⁺, and CD8⁺ T-cells, suggesting that moderate uremia induced kidney inflammation. Furthermore, markers of fibrosis (i.e. Col1a1 and Col3a1) were upregulated, and histological examinations showed increased glomerular diameter in NX mice. Importantly, liraglutide treatment attenuated atherosclerosis (~40%, p < 0.05) and reduced kidney inflammation in NX mice. There was no effect of liraglutide on expression of fibrosis markers and/or kidney histology. This study suggests that liraglutide has beneficial effects in a mouse model of moderate uremia by reducing atherosclerosis and attenuating kidney inflammation.

Angiotensin II does not acutely regulate conduction velocity in rat atrial tissue

AIM

Atrial angiotensin II (Ang II) levels are increased in atrial fibrillation and are believed to be important in the pathogenesis of atrial arrhythmias. Ang II reduces intercellular coupling by inhibiting gap junctions (connexins) and may thereby increase the risk of reentry arrhythmia. The aim of the current study was to investigate the acute effect of Ang II on conduction velocity (CV) in atrial tissue from normal and chronically infarcted rats.

METHODS

Contractile force was measured and CV was determined from the conduction time between electrodes placed on the tissue preparation. Expression of AT1a and AT1b receptors was examined by real-time PCR.

RESULTS

Acute stimulation with Ang II did not affect CV in tissue from auricle or atrial free wall. A transient 6.5 ± 3.6% increase in resting tension was observed in atrial free wall preparations, indicating that receptors are present and functional in the free wall preparation. The difference between free wall and auricle was probably not caused by differences in receptor expression since equal amounts of AT1 mRNA were present. To test if myocardial infarction (MI) sensitizes the atrium to Ang II, free atrial wall from rats subjected to 4-5 weeks ventricular MI was examined. Although CV was significantly reduced by MI, no effect on CV of Ang II was seen.

CONCLUSION

Ang II does not acutely regulate CV in tissue preparations from the free wall of the left atria or the left auricle. Although ventricular MI reduces CV, this does not sensitize the atria to Ang II.

Hormonal regulation of mannan-binding lectin synthesis in hepatocytes

Activation of the complement system via the plasma protein mannan-binding lectin (MBL) provides a first line of defence against infections. The plasma level of MBL is, in part, determined genetically, but may also be influenced by different hormones in vivo. Here we study the hormonal regulation of MBL synthesis from the human hepatocyte cell line HuH-7. Cells were exposed to medium with growth hormone (GH), hydrocortisone, insulin-like growth factor (IGF)-1, insulin, interleukin (IL)-6 or thyroid hormones (T3 or T4). After 3 days the concentration of MBL in the culture supernatants was determined and the amount of mRNA for MBL was measured, relative to mRNA for beta2 microglobulin. GH, IL-6, T3 and T4 significantly increased MBL synthesis in a dose-dependent manner, while hydrocortisone, insulin and IGF-1 had no effect. T3 caused a fourfold increase at 1 nM of T3 (P < 0.001) and at 100 nM of T3 the production was increased more than eightfold. The effect of T4 was less potent, reaching an eightfold increase at 1 microM of T4 (P < 0.001). GH augmented the production of MBL threefold at a concentration of 100 ng/ml (P = 0.018) with no further effect up to 10 microg/ml, whereas IL-6 caused only a very weak increase in MBL production. MBL mRNA levels were stable during the first 24 h of T3 stimulation but increased significantly between 24 and 48 h. The results suggest that MBL synthesis in humans may be increased by thyroid hormone and GH, whereas it does not exhibit a classical IL-6-dependent response.

Abnormal growth plate function in pigs carrying a dominant mutation in type X collagen

We have identified a naturally occurring, dominant mutation that causes dwarfism in domestic pigs (Sus scrofa). With a positional candidate gene approach, the dwarf phenotype was shown to be a result of a single amino acid change, G590R, in the alpha1 (X) chain of type X collagen. Type X collagen is a homotrimer of alpha1(X) chains encoded by the COL10A1 gene, which is expressed in hypertrophic chondrocytes during the process of endochondral ossification. An amino acid substitution at the equivalent position in human type X collagen, G595E, has previously been shown to cause Schmid metaphyseal chondrodysplasia (SMCD), which is a relatively mild skeletal disorder associated with dwarfism and growth plate abnormality. Consistent with the clinical phenotype of SMCD patients, radiological and histological examination of the dwarf pigs revealed metaphyseal chondrodysplasia in the long bones. Yeast-based, two-hybrid protein interaction studies and in vitro assembly experiments demonstrated that the amino acid substitution interfered with the ability of the mutated collagen molecules to engage in trimerization. This work establishes that the chondrodysplastic dwarf pigs by genetic, biochemical, radiological and histological criteria provide a valid animal model of SMCD.

Iron stores in female blood donors evaluated by serum ferritin

Iron stores were evaluated by serum ferritin determinations in 948 menstruating and 141 non-menstruating female blood donors. Blood donation was associated with a decrease in ferritin. First-time donors (n = 163) had a geometric mean ferritin of 24 micrograms/l and multiple-time donors a value of 19 micrograms/l (p less than 0.01). In the donating population 31.5% had ferritin values less than 15 micrograms/l (i.e. depleted iron stores). Menstruating donors had lower mean serum ferritin than non-menstruating donors (p less than 0.001), and a higher frequency of ferritin values less than 15 micrograms/l (p less than 0.05). There was no relationship between ferritin levels and the number of pregnancies. The frequency of donations was more predictive of ferritin levels than the number of donations. Mean ferritin displayed a moderate fall up to the 2nd donation, and was hereafter relatively constant, whereas an increase in donation frequency was accompanied by a significant decrease in ferritin. Female donors, especially when phlebotomised greater than or equal to 3 times per year, should have their iron status checked at appropriate intervals by measurement of serum ferritin and should be advised regular iron supplementation.