PKC stimulated by glucagon decreases UT-A1 urea transporter expression in rat IMCD

Incretin and glucagon receptor polypharmacology in chronic kidney disease

Chronic kidney disease is a debilitating condition associated with significant morbidity and mortality. In recent years, the kidney effects of incretin-based therapies, particularly glucagon-like peptide-1 receptor agonists (GLP-1RAs), have garnered substantial interest in the management of type 2 diabetes and obesity. This review delves into the intricate interactions between the kidney, GLP-1RAs, and glucagon, shedding light on their mechanisms of action and potential kidney benefits. Both GLP-1 and glucagon, known for their opposing roles in regulating glucose homeostasis, improve systemic risk factors affecting the kidney, including adiposity, inflammation, oxidative stress, and endothelial function. Additionally, these hormones and their pharmaceutical mimetics may have a direct impact on the kidney. Clinical studies have provided evidence that incretins, including those incorporating glucagon receptor agonism, are likely to exhibit improved kidney outcomes. Although further research is necessary, receptor polypharmacology holds promise for preserving kidney function through eliciting vasodilatory effects, influencing volume and electrolyte handling, and improving systemic risk factors.

Glucagon infusion alters the hyperpolarized 13 C-urea renal hemodynamic signature

Renal urea handling is central to the urine concentrating mechanism, and as such the ability to image urea transport in the kidney is an important potential imaging biomarker for renal functional assessment. Glucagon levels associated with changes in dietary protein intake have been shown to influence renal urea handling; however, the exact mechanism has still to be fully understood. Here we investigate renal function and osmolite distribution using [¹³ C,¹⁵ N] urea dynamics and ²³ Na distribution before and 60 min after glucagon infusion in six female rats. Glucagon infusion increased the renal [¹³ C,¹⁵ N] urea mean transit time by 14%, while no change was seen in the sodium distribution, glomerular filtration rate or oxygen consumption. This change is related to the well-known effect of increased urea excretion associated with glucagon infusion, independent of renal functional effects. This study demonstrates for the first time that hyperpolarized ¹³ C-urea enables monitoring of renal urinary excretion effects in vivo.

Glucagon actions on the kidney revisited: possible role in potassium homeostasis

It is now recognized that the metabolic disorders observed in diabetes are not, or not only due to the lack of insulin or insulin resistance, but also to elevated glucagon secretion. Accordingly, selective glucagon receptor antagonists are now proposed as a novel strategy for the treatment of diabetes. However, besides its metabolic actions, glucagon also influences kidney function. The glucagon receptor is expressed in the thick ascending limb, distal tubule, and collecting duct, and glucagon regulates the transepithelial transport of several solutes in these nephron segments. Moreover, it also influences solute transport in the proximal tubule, possibly by an indirect mechanism. This review summarizes the knowledge accumulated over the last 30 years about the influence of glucagon on the renal handling of electrolytes and urea. It also describes a possible novel role of glucagon in the short-term regulation of potassium homeostasis. Several original findings suggest that pancreatic α-cells may express a “potassium sensor” sensitive to changes in plasma K concentration and could respond by adapting glucagon secretion that, in turn, would regulate urinary K excretion. By their combined actions, glucagon and insulin, working in a combinatory mode, could ensure an independent regulation of both plasma glucose and plasma K concentrations. The results and hypotheses reviewed here suggest that the use of glucagon receptor antagonists for the treatment of diabetes should take into account their potential consequences on electrolyte handling by the kidney.

Expression of urea transporters and their regulation

UT-A and UT-B families of urea transporters consist of multiple isoforms that are subject to regulation of both acutely and by long-term measures. This chapter provides a brief overview of the expression of the urea transporter forms and their locations in the kidney. Rapid regulation of UT-A1 results from the combination of phosphorylation and membrane accumulation. Phosphorylation of UT-A1 has been linked to vasopressin and hyperosmolality, although through different kinases. Other acute influences on urea transporter activity are ubiquitination and glycosylation, both of which influence the membrane association of the urea transporter, again through different mechanisms. Long-term regulation of urea transport is most closely associated with the environment that the kidney experiences. Low-protein diets may influence the amount of urea transporter available. Conditions of osmotic diuresis, where urea concentrations are low, will prompt an increase in urea transporter abundance. Although adrenal steroids affect urea transporter abundance, conflicting reports make conclusions tenuous. Urea transporters are upregulated when P2Y2 purinergic receptors are decreased, suggesting a role for these receptors in UT regulation. Hypercalcemia and hypokalemia both cause urine concentration deficiencies. Urea transporter abundances are reduced in aging animals and animals with angiotensin-converting enzyme deficiencies. This chapter will provide information about both rapid and long-term regulation of urea transporters and provide an introduction into the literature.

Molecular mechanisms of urea transport in health and disease

In the late 1980s, urea permeability measurements produced values that could not be explained by paracellular transport or lipid phase diffusion. The existence of urea transport proteins were thus proposed and less than a decade later, the first urea transporter was cloned. The family of urea transporters has two major subgroups, designated SLC14A1 (or UT-B) and Slc14A2 (or UT-A). UT-B and UT-A gene products are glycoproteins located in various extra-renal tissues however, a majority of the resulting isoforms are found in the kidney. The UT-B (Slc14A1) urea transporter was originally isolated from erythrocytes and two isoforms have been reported. In kidney, UT-B is located primarily in the descending vasa recta. The UT-A (Slc14A2) urea transporter yields six distinct isoforms, of which three are found chiefly in the kidney medulla. UT-A1 and UT-A3 are found in the inner medullary collecting duct (IMCD), while UT-A2 is located in the thin descending limb. These transporters are crucial to the kidney's ability to concentrate urine. The regulation of urea transporter activity in the IMCD involves acute modification through phosphorylation and subsequent movement to the plasma membrane. UT-A1 and UT-A3 accumulate in the plasma membrane in response to stimulation by vasopressin or hypertonicity. Long-term regulation of the urea transporters in the IMCD involves altering protein abundance in response to changes in hydration status, low protein diets, or adrenal steroids. Urea transporters have been studied using animal models of disease including diabetes mellitus, lithium intoxication, hypertension, and nephrotoxic drug responses. Exciting new genetically engineered mouse models are being developed to study these transporters.

New insights into urea and glucose handling by the kidney, and the urine concentrating mechanism

The mechanism by which urine is concentrated in the mammalian kidney remains incompletely understood. Urea is the dominant urinary osmole in most mammals and may be concentrated a 100-fold above its plasma level in humans and even more in rodents. Several facilitated urea transporters have been cloned. The phenotypes of mice with deletion of the transporters expressed in the kidney have challenged two previously well-accepted paradigms regarding urea and sodium handling in the renal medulla but have provided no alternative explanation for the accumulation of solutes that occurs in the inner medulla. In this review, we present evidence supporting the existence of an active urea secretion in the pars recta of the proximal tubule and explain how it changes our views regarding intrarenal urea handling and UT-A2 function. The transporter responsible for this secretion could be SGLT1, a sodium-glucose cotransporter that also transports urea. Glucagon may have a role in the regulation of this secretion. Further, we describe a possible transfer of osmotic energy from the outer to the inner medulla via an intrarenal Cori cycle converting glucose to lactate and back. Finally, we propose that an active urea transporter, expressed in the urothelium, may continuously reclaim urea that diffuses out of the ureter and bladder. These hypotheses are all based on published findings. They may not all be confirmed later on, but we hope they will stimulate further research in new directions.

Urea transport in the kidney

Urea transport proteins were initially proposed to exist in the kidney in the late 1980s when studies of urea permeability revealed values in excess of those predicted by simple lipid-phase diffusion and paracellular transport. Less than a decade later, the first urea transporter was cloned. Currently, the SLC14A family of urea transporters contains two major subgroups: SLC14A1, the UT-B urea transporter originally isolated from erythrocytes; and SLC14A2, the UT-A group with six distinct isoforms described to date. In the kidney, UT-A1 and UT-A3 are found in the inner medullary collecting duct; UT-A2 is located in the thin descending limb, and UT-B is located primarily in the descending vasa recta; all are glycoproteins. These transporters are crucial to the kidney's ability to concentrate urine. UT-A1 and UT-A3 are acutely regulated by vasopressin. UT-A1 has also been shown to be regulated by hypertonicity, angiotensin II, and oxytocin. Acute regulation of these transporters is through phosphorylation. Both UT-A1 and UT-A3 rapidly accumulate in the plasma membrane in response to stimulation by vasopressin or hypertonicity. Long-term regulation involves altering protein abundance in response to changes in hydration status, low protein diets, adrenal steroids, sustained diuresis, or antidiuresis. Urea transporters have been studied using animal models of disease including diabetes mellitus, lithium intoxication, hypertension, and nephrotoxic drug responses. Exciting new animal models are being developed to study these transporters and search for active urea transporters. Here we introduce urea and describe the current knowledge of the urea transporter proteins, their regulation, and their role in the kidney.

Protein kinase C regulates urea permeability in the rat inner medullary collecting duct

Hypertonicity increases urea transport independently of, as well as synergistically with, vasopressin in the inner medullary collect duct (IMCD). We previously showed that hypertonicity does not increase the level of cAMP in the IMCD, but it does increase the level of intracellular calcium. Since we also showed that hypertonicity increases both the phosphorylation and biotinylation of the urea transporters UT-A1 and UT-A3, this would suggest involvement of a calcium-dependent protein kinase in the regulation of urea transport in the inner medulla. In this study, we investigated whether protein kinase C (PKC), which is present in the IMCD, is a regulator of urea permeability. We tested the effect of PKC inhibitors and activators on urea permeability in the isolated, perfused rat terminal IMCD. Increasing osmolality from 290 to 690 mosmol/kgH(2)O significantly stimulated (doubled) urea permeability; it returned to control levels on inhibition of PKC with either 10 μM chelerythrine or 50 μM rottlerin. To determine the potential synergy between vasopressin and PKC, phorbol dibutyrate (PDBu) was used to stimulate PKC. Vasopressin stimulated urea permeability 247%. Although PDBu alone did not change basal urea permeability, in the presence of vasopressin, it significantly increased urea permeability an additional 92%. The vasopressin and PDBu-stimulated urea permeability was reduced to AVP alone levels by inhibition of PKC. We conclude that hypertonicity stimulates urea transport through a PKC-mediated phosphorylation. Whether PKC directly phosphorylates UT-A1 and/or UT-A3 or phosphorylates it as a consequence of a cascade of activations remains to be determined.

The N-terminal 81-aa fragment is critical for UT-A1 urea transporter bioactivity

The serine protease, furin, is involved in the activation of a number of proteins most notably epithelial sodium channels (ENaC). The urea transporter UT-A1, located in the kidney inner medullary collecting duct (IMCD), is important for urine concentrating ability. UT-A1's amino acid sequence has a consensus furin cleavage site (RSKR) in the N-terminal region. Despite the putative cleavage site, we find that UT-A1, either from the cytosolic or cell surface pool, is not cleaved by furin in CHO cells. This result was further confirmed by an inability of furin to cleave in vitro an (35)S-labeled UT-A1 or the 126 N-terminal UT-A1 fragment. Functionally, mutation of the furin site (R78A, R81A) does not affect UT-A1 urea transport activity. However, deletion of the 81-aa N-terminal portion does not affect UT-A1 cell surface trafficking, but seriously impair UT-A1 urea transport activity. Our results indicate that UT-A1 maturation and activation does not require furin-dependent cleavage. The N-terminal 81-aa fragment is required for proper UT-A1 urea transport activity, but its effect is not through changing UT-A1 membrane trafficking.

Phosphoproteomic profiling reveals vasopressin-regulated phosphorylation sites in collecting duct

Protein phosphorylation is an important component of vasopressin signaling in the renal collecting duct, but the database of known phosphoproteins is incomplete. We used tandem mass spectrometry to identify vasopressin-regulated phosphorylation events in isolated rat inner medullary collecting duct (IMCD) suspensions. Using multiple search algorithms to identify the phosphopeptides from spectral data, we expanded the size of the existing collecting duct phosphoproteome database from 367 to 1187 entries. Label-free quantification in vasopressin- and vehicle-treated samples detected a significant change in the phosphorylation of 29 of 530 quantified phosphopeptides. The targets include important structural, regulatory, and transporter proteins. The vasopressin-regulated sites included two known sites (Ser-486 and Ser-499) present in the urea channel UT-A1 and one previously unknown site (Ser-84) on vasopressin-sensitive urea channels UT-A1 and UT-A3. In vitro assays using synthetic peptides showed that purified protein kinase A (PKA) could phosphorylate all three sites, and immunoblotting confirmed the PKA dependence of Ser-84 and Ser-486 phosphorylation. These results expand the known list of collecting duct phosphoproteins and highlight the utility of targeted phosphoproteomic approaches.

Aquaporin 2 expression increased by glucagon in normal rat inner medullary collecting ducts

It is well known that Glucagon (Gl) is released after a high protein diet and participates in water excretion by the kidney, principally after a protein meal. To study this effect in in vitro perfused inner medullary collecting ducts (IMCD), the osmotic water permeability (Pf; mum/s) at 37 degrees C and pH 7.4 in normal rat IMCDs (n = 36) perfused with Ringer/HCO(3) was determined. Gl (10(-7) M) in absence of Vasopressin (AVP) enhanced the Pf from 4.38 +/- 1.40 to 11.16 +/- 1.44 microm/s (P < 0.01). Adding 10(-8), 10(-7), and 10(-6) M Gl, the Pf responded in a dose-dependent manner. The protein kinase A inhibitor H8 blocked the Gl effect. The specific Gl inhibitor, des-His(1)-[Glu(9)] glucagon (10(-7) M), blocked the Gl-stimulated Pf but not the AVP-stimulated Pf. There occurred a partial additional effect between Gl and AVP. The cAMP level was enhanced from the control 1.24 +/- 0.39 to 59.70 +/- 15.18 fm/mg prot after Gl 10(-7) M in an IMCD cell suspension. The immunoblotting studies indicated an increase in AQP2 protein abundance of 27% (cont 100.0 +/- 3.9 vs. Gl 127.53; P = 0.0035) in membrane fractions extracted from IMCD tubule suspension, incubated with 10(-6) M Gl. Our data showed that 1) Gl increased water absorption in a dose-dependent manner; 2) the anti-Gl blocked the action of Gl but not the action of AVP; 3) Gl stimulated the cAMP generation; 4) Gl increased the AQP2 water channel protein expression, leading us to conclude that Gl controls water absorption by utilizing a Gl receptor, rather than a AVP receptor, increasing the AQP2 protein expression.