Arginase-II negatively regulates renal aquaporin-2 and water reabsorption

Vasopressin, protein metabolism, and water conservation

PURPOSE OF REVIEW

Highlight the mechanisms through which vasopressin and hypertonic stress regulate protein metabolism.

RECENT FINDINGS

Mammals have an 'aestivation-like' response in which hypertonic stress increases muscle catabolism and urea productionVasopressin can directly regulate ureagenesis in the liver and the kidneyIn humans chronic hypertonic stress is associated with premature aging, diabetes, cardiovascular disease, and premature mortality.

SUMMARY

There is an evolutionarily conserved 'aestivation-like' response in humans in which hypertonic stress results in activation of the vasopressin system, muscle catabolism, and ureagenesis in order to promote water conservation.

Aquaporin 2 in Cerebral Edema: Potential Prognostic Marker in Craniocerebral Injuries

Despite continuous medical advancements, traumatic brain injury (TBI) remains a leading cause of death and disability worldwide. Consequently, there is a pursuit for biomarkers that allow non-invasive monitoring of patients after cranial trauma, potentially improving clinical management and reducing complications and mortality. Aquaporins (AQPs), which are crucial for transmembrane water transport, may be significant in this context. This study included 48 patients, with 27 having acute (aSDH) and 21 having chronic subdural hematoma (cSDH). Blood plasma samples were collected from the participants at three intervals: the first sample before surgery, the second at 15 h, and the third at 30 h post-surgery. Plasma concentrations of AQP1, AQP2, AQP4, and AQP9 were determined using the sandwich ELISA technique. CT scans were performed on all patients pre- and post-surgery. Correlations between variables were examined using Spearman's nonparametric rank correlation coefficient. A strong correlation was found between aquaporin 2 levels and the volume of chronic subdural hematoma and midline shift. However, no significant link was found between aquaporin levels (AQP1, AQP2, AQP4, and AQP9) before and after surgery for acute subdural hematoma, nor for AQP1, AQP4, and AQP9 after surgery for chronic subdural hematoma. In the chronic SDH group, AQP2 plasma concentration negatively correlated with the midline shift measured before surgery (Spearman's ρ -0.54; p = 0.017) and positively with hematoma volume change between baseline and 30 h post-surgery (Spearman's ρ 0.627; p = 0.007). No statistically significant correlation was found between aquaporin plasma levels and hematoma volume for AQP1, AQP2, AQP4, and AQP9 in patients with acute SDH. There is a correlation between chronic subdural hematoma volume, measured radiologically, and serum AQP2 concentration, highlighting aquaporins' potential as clinical biomarkers.

Elevation of Arginase-II in Podocytes Contributes to Age-Associated Albuminuria in Male Mice

One of the manifestations of renal aging is podocyte dysfunction and loss, which are associated with proteinuria and glomerulosclerosis. Studies show a male bias in glomerular dysfunction and chronic kidney diseases, and the underlying mechanisms remain obscure. Recent studies demonstrate the role of an age-associated increase in arginase-II (Arg-II) in proximal tubules of both male and female mice. However, it is unclear whether Arg-II is also involved in aging glomeruli. The current study investigates the role of the sex-specific elevation of Arg-II in podocytes in age-associated increased albuminuria. Young (3-4 months) and old (20-22 months) male and female mice of wt and arginase-II knockout (arg-ii-/-) were used. Albuminuria was employed as a readout of glomerular function. Cellular localization and expression of Arg-II in glomeruli were analyzed using an immunofluorescence confocal microscope. A more pronounced age-associated increase in albuminuria was found in male than in female mice. An age-associated induction of Arg-II in glomeruli and podocytes (as demonstrated by co-localization of Arg-II with the podocyte marker synaptopodin) was also observed in males but not in females. Ablation of the arg-ii gene in mice significantly reduces age-associated albuminuria in males. Also, age-associated decreases in podocyte density and glomerulus hypertrophy are significantly prevented in male arg-ii-/- but not in female mice. However, age-associated glomerulosclerosis is not affected by arg-ii ablation in both sexes. These results demonstrate a role of Arg-II in sex-specific podocyte injury in aging. They may explain the sex-specific differences in the development of renal disease in humans during aging.

Arginase: Biological and Therapeutic Implications in Diabetes Mellitus and Its Complications

Arginase is a ubiquitous enzyme in the urea cycle (UC) that hydrolyzes L-arginine to urea and L-ornithine. Two mammalian arginase isoforms, arginase1 (ARG1) and arginase2 (ARG2), play a vital role in the regulation of β-cell functions, insulin resistance (IR), and vascular complications via modulating L-arginine metabolism, nitric oxide (NO) production, and inflammatory responses as well as oxidative stress. Basic and clinical studies reveal that abnormal alterations of arginase expression and activity are strongly associated with the onset and development of diabetes mellitus (DM) and its complications. As a result, targeting arginase may be a novel and promising approach for DM treatment. An increasing number of arginase inhibitors, including chemical and natural inhibitors, have been developed and shown to protect against the development of DM and its complications. In this review, we discuss the fundamental features of arginase. Next, the regulatory roles and underlying mechanisms of arginase in the pathogenesis and progression of DM and its complications are explored. Furthermore, we review the development and discuss the challenges of arginase inhibitors in treating DM and its related pathologies.

Role of Arginase-II in Podocyte Injury under Hypoxic Conditions

Hypoxia plays a crucial role in acute and chronic renal injury, which is attributable to renal tubular and glomerular cell damage. Some studies provide evidence that hypoxia-dependent upregulation of the mitochondrial enzyme arginase type-II (Arg-II) in tubular cells promotes renal tubular injury. It is, however, not known whether Arg-II is also expressed in glomerular cells, particularly podocytes under hypoxic conditions, contributing to hypoxia-induced podocyte injury. The effects of hypoxia on human podocyte cells (AB8/13) in cultures and on isolated kidneys from wild-type (wt) and arg-ii gene-deficient (arg-ii−/−) mice ex vivo, as well as on mice of the two genotypes in vivo, were investigated, respectively. We found that the Arg-II levels were enhanced in cultured podocytes in a time-dependent manner over 48 h, which was dependent on the stabilization of hypoxia-inducible factor 1α (HIF1α). Moreover, a hypoxia-induced derangement of cellular actin cytoskeletal fibers, a decrease in podocin, and an increase in mitochondrial ROS (mtROS) generation—as measured by MitoSOX—were inhibited by adenoviral-mediated arg-ii gene silencing. These effects of hypoxia on podocyte injury were mimicked by the HIFα stabilizing drug DMOG, which inhibits prolyl hydroxylases (PHD), the enzymes involved in HIFα degradation. The silencing of arg-ii prevented the detrimental effects of DMOG on podocytes. Furthermore, the inhibition of mtROS generation by rotenone—the inhibitor of respiration chain complex-I—recapitulated the protective effects of arg-ii silencing on podocytes under hypoxic conditions. Moreover, the ex vivo experiments with isolated kidney tissues and the in vivo experiments with mice exposed to hypoxic conditions showed increased Arg-II levels in podocytes and decreased podocyte markers regarding synaptopodin in wt mice but not in arg-ii−/− mice. While age-associated albuminuria was reduced in the arg-ii−/− mice, the hypoxia-induced increase in albuminuria was, however, not significantly affected in the arg-ii−/−. Our study demonstrates that Arg-II in podocytes promotes cell injury. Arg-ii ablation seems insufficient to protect mice in vivo against a hypoxia-induced increase in albuminuria, but it does reduce albuminuria in aging.

Arginase: An emerging and promising therapeutic target for cancer treatment

Arginase is a key hydrolase in the urea cycle that hydrolyses L-arginine to urea and L-ornithine. Increasing number of studies in recent years demonstrate that two mammalian arginase isoforms, arginase 1 (ARG1) and arginase 2 (ARG2), were aberrantly upregulated in various types of cancers, and played crucial roles in the regulation of tumor growth and metastasis through various mechanisms such as regulating L-arginine metabolism, influencing tumor immune microenvironment, etc. Thus, arginase receives increasing focus as an attractive target for cancer therapy. In this review, we provide a comprehensive overview of the physiological and biological roles of arginase in a variety of cancers, and shed light on the underlying mechanisms of arginase mediating cancer cells growth and development, as well as summarize the recent clinical research advances of targeting arginase for cancer therapy.

Hypoxia Induces Renal Epithelial Injury and Activates Fibrotic Signaling Through Up-Regulation of Arginase-II

The ureohydrolase, type-II arginase (Arg-II), is a mitochondrial enzyme metabolizing L-arginine into urea and L-ornithine and is highly expressed in renal proximal tubular cells (PTC) and upregulated by renal ischemia. Recent studies reported contradictory results on the role of Arg-II in renal injury. The aim of our study is to investigate the function of Arg-II in renal epithelial cell damage under hypoxic conditions. Human renal epithelial cell line HK2 was cultured under hypoxic conditions for 12–48 h. Moreover, ex vivo experiments with isolated kidneys from wild-type (WT) and genetic Arg-II deficient mice (Arg-II^–/–) were conducted under normoxic and hypoxic conditions. The results show that hypoxia upregulates Arg-II expression in HK2 cells, which is inhibited by silencing both hypoxia-inducible factors (HIFs) HIF1α and HIF2α. Treatment of the cells with dimethyloxaloylglycine (DMOG) to stabilize HIFα also enhances Arg-II. Interestingly, hypoxia or DMOG upregulates transforming growth factor β1 (TGFβ1) levels and collagens Iα1, which is prevented by Arg-II silencing, while TGFβ1-induced collagen Iα1 expression is not affected by Arg-II silencing. Inhibition of mitochondrial complex-I by rotenone abolishes hypoxia-induced reactive oxygen species (mtROS) and TGFβ1 elevation in the cells. Ex vivo experiments show elevated Arg-II and TGFβ1 expression and the injury marker NGAL in the WT mouse kidneys under hypoxic conditions, which is prevented in the Arg-II^–/– mice. Taking together, the results demonstrate that hypoxia activates renal epithelial HIFs-Arg-II-mtROS-TGFβ1-cascade, participating in hypoxia-associated renal injury and fibrosis.

AQP2 as a target of lycopene protects against atrazine-induced renal ionic homeostasis disturbance

Atrazine (ATR), a ubiquitous environmental contaminant in water and soil, causes environmental nephrosis. To reveal the toxic effect of ATR on the kidney and the potential chemical nephroprotective effect of lycopene (LYC), Kun-Ming mice of specific pathogen-free (SPF) grade were treated with LYC (5 mg kg-1) and/or ATR (50 mg kg-1 or 200 mg kg-1) for 21 days. The degree of renal injury was evaluated by measuring the ion concentration, ATPase activities and the mRNA expressions/levels of associated ATPase subunits. In addition, the expression of renal aquaporins (AQPs) was analyzed. The results showed that the renal tubular epithelial cells of ATR-exposed mice were swollen, the glomeruli were significantly atrophied, and the ion concentrations were obviously changed. The activity of Na+-K+-ATPase and the transcription of its subunits were downregulated. The activity of Ca2+-Mg2+-ATPase and the transcription of its subunits were upregulated. The expression of AQPs, especially the critical AQP2, was affected. Notably, ATR-induced nephrotoxicity was significantly improved by LYC supplementation. Therefore, LYC could protect the kidney against ATR-induced nephrotoxicity via maintaining ionic homeostasis, reversing the changes in ATPase activity and controlling the expression of AQPs on the cell membrane. These results suggested that AQP2 was a target of LYC and protected against ATR-induced renal ionic homeostasis disturbance.

Renal tubular arginase-2 participates in the formation of the corticomedullary urea gradient and attenuates kidney damage in ischemia-reperfusion injury in mice

AIM

Arginase 2 (ARG2) is a mitochondrial enzyme that catalyses hydrolysis of l-arginine into urea and l-ornithine. In the kidney, ARG2 is localized to the S3 segment of the proximal tubule. It has been shown that expression and activity of this enzyme are upregulated in a variety of renal pathologies, including ischemia-reperfusion (IR) injury. However, the (patho)physiological role of ARG2 in the renal tubule remains largely unknown.

METHODS

We addressed this question in mice with conditional knockout of Arg2 in renal tubular cells (Arg2^lox/lox /Pax8-rtTA/LC1 or, cKO mice).

RESULTS

We demonstrate that cKO mice exhibit impaired urea concentration and osmolality gradients along the corticomedullary axis. In a model of unilateral ischemia-reperfusion injury (UIRI) with an intact contralateral kidney, ischemia followed by 24 hours of reperfusion resulted in significantly more pronounced histological damage in ischemic kidneys from cKO mice compared to control and sham-operated mice. In parallel, UIRI-subjected cKO mice exhibited a broad range of renal functional abnormalities, including albuminuria and aminoaciduria. Fourteen days after UIRI, the cKO mice exhibited complex phenotype characterized by significantly lower body weight, increased plasma levels of early predictive markers of kidney disease progression (asymmetric dimethylarginine and symmetric dimethylarginine), impaired mitochondrial function in the ischemic kidney but no difference in kidney fibrosis as compared to control mice.

CONCLUSION

Collectively, these results establish the role of ARG2 in the formation of corticomedullary urea and osmolality gradients and suggest that this enzyme attenuates kidney damage in ischemia-reperfusion injury.