Jessica F. Briffa

Adipokines as a link between obesity and chronic kidney disease

Adipocytes secrete a number of bioactive adipokines that activate a variety of cell signaling pathways in central and peripheral tissues. Obesity is associated with the altered production of many adipokines and is linked to a number of pathologies. As an increase in body weight is directly associated with an increased risk for developing chronic kidney disease (CKD), there is significant interest in the link between obesity and renal dysfunction. Altered levels of the adipokines leptin, adiponectin, resistin, and visfatin can decrease the glomerular filtration rate and increase albuminuria, which are pathophysiological changes typical of CKD. Specifically, exposure of the glomerulus to altered adipokine levels can increase its permeability, fuse the podocytes, and cause mesangial cell hypertrophy, all of which alter the glomerular filtration rate. In addition, the adipokines leptin and adiponectin can act on tubular networks. Thus, adipokines can act on multiple cell types in the development of renal pathophysiology. Importantly, most studies have been performed using in vitro models, with future studies in vivo required to further elucidate the specific roles that adipokines play in the development and progression of CKD.

Leptin in pregnancy and development: a contributor to adulthood disease?

Emerging research has highlighted the importance of leptin in fetal growth and development independent of its essential role in the maintenance of hunger and satiety through the modulation of neuropeptide Y and proopiomelanocortin neurons. Alterations in maternal-placental-fetal leptin exchange may modify the development of the fetus and contribute to the increased risk of developing disease in adulthood. In addition, leptin also plays an important role in reproductive functions, with plasma leptin concentrations rising in pregnant women, peaking during the third trimester. Elevated plasma leptin concentrations occur at the completion of organogenesis, and research in animal models has demonstrated that leptin is involved in the development and maturation of a number of organs, including the heart, brain, kidneys, and pancreas. Elevated maternal plasma leptin is associated with maternal obesity, and reduced fetal plasma leptin is correlated with intrauterine growth restriction. Alterations in plasma leptin during development may be associated with an increased risk of developing a number of adulthood diseases, including cardiovascular, metabolic, and renal diseases via altered fetal development and organogenesis. Importantly, research has shown that leptin antagonism after birth significantly reduces maturation of numerous organs. Conversely, restoration of the leptin deficiency after birth in growth-restricted animals restores the offsprings body weight and improves organogenesis. Therefore, leptin appears to play a major role in organogenesis, which may adversely affect the risk of developing a number of diseases in adulthood. Therefore, greater understanding of the role of leptin during development may assist in the prevention and treatment of a number of disease states that occur in adulthood.

Cannabinoid Receptor 2 Expression in Human Proximal Tubule Cells is Regulated by Albumin Independent of ERK1/2 Signaling

Background: The cannabinoid receptor type 2 (CB2) is reduced in podocytes of animals and humans with Type 2 Diabetes Mellitus (T2DM), with activation of CB2 ameliorating albuminuria in animals. As albuminuria also is due to proximal tubule dysfunction, the aim of this study is to investigate tubular expression of CB2 under diabetic conditions in addition to the cell signaling pathways that underlie these changes. Methods: We characterized total CB2 protein in diabetic animals and in Human Kidney 2 (HK2) cells exposed to elevated albumin and glucose, the levels of CB2 mRNA and protein. We also used latrunculin to determine if internalization of albumin was required to regulate CB2 levels. Finally, we characterized the levels of active and total AKT, ERK1/2 and p38 in response to albumin. Results: There were no changes to CB2 expression in kidney lysate from diabetic rats. In HK2 cells, expression of CB2 was unaltered following exposure to high glucose. High albumin treatment alone and in combination with high glucose, resulted in a significant reduction in CB2 receptor mRNA expression at 6 and 18 hours. CB2 protein expression was reduced at 6 and 24 hours, in high albumin and in combination with high glucose. Internalization of albumin was required to regulate CB2 levels, and inhibition of ERK1/2, did not rescue the loss of CB2 in response to albumin. Conclusion: We have demonstrated that internalization of albumin is required to reduce CB2 mRNA and protein expression in proximal tubules in vitro. Consequently, altered expression of CB2 in both the podocytes and tubules may contribute to the albuminuria observed in T2DM.

Renal effects of chronic pharmacological manipulation of CB2 receptors in rats with diet-induced obesity.

In diabetic nephropathy agonism of CB2 receptors reduces albuminuria and podocyte loss; however, the role of CB2 receptors in obesity‐related nephropathy is unknown. The aim of this study was to determine the role of CB2 receptors in a model of diet‐induced obesity (DIO) and characterize the hallmark signs of renal damage in response to agonism (AM1241) and antagonism (AM630) of CB2 receptors.

Acute leptin exposure reduces megalin expression and upregulates TGFβ1 in cultured renal proximal tubule cells

Increased leptin concentrations observed in obesity can lead to proteinuria, suggesting that leptin may play a role in obesity-related kidney disease. Obesity reduces activation of AMP-activated protein kinase (AMPK) and increases transforming growth factor-β1 (TGF-β1) expression in the kidney, leading to albuminuria. Thus we investigated if elevated leptin altered AMPK and TGF-β1 signaling in proximal tubule cells (PTCs). In opossum kidney (OK) PTCs Western blot analysis demonstrated that leptin upregulates TGF-β1 secretion (0.50 µg/ml) and phosphorylated AMPKα (at 0.25, and 0.50 µg/ml), and downregulates megalin expression at all concentrations (0.05-0.50 µg/ml). Using the AMPK inhibitor, Compound C, leptin exposure regulated TGF-β1 expression and secretion in PTCs via an AMPK mediated pathway. In addition, elevated leptin exposure (0.50 µg/ml) reduced albumin handling in OK cells independently of megalin expression. This study demonstrates that leptin upregulates TGF-β1, reduces megalin, and reduces albumin handling in PTCs by an AMPK mediated pathway.

The role of maternal nutrition, metabolic function and the placenta in developmental programming of renal dysfunction

The intrauterine environment is critical for the development of the foetus. Barker and colleagues were the first to identify that adverse perturbations during foetal development are associated with an increased risk of developing diseases in adulthood, including cardiorenal disease. Specifically for the kidney, perturbations in utero can lead to nephron deficits and renal dysfunction by a number of mechanisms. Altered programming of nephron number is associated with an increased risk of developing kidney disease via glomerular hypertrophy and reduced vasodilative capacity of the renal blood vessels; both of which would contribute to hypertension in adulthood, with males being more susceptible to disease outcomes. Additionally, alterations in the renin‐angiotensin system (RAS) such as an upregulation or downregulation of specific receptors, depending on the nature of the insult, have also been implicated in the development of renal dysfunction. Sex‐specific differences in the expression of the RAS during late gestation and in the early postnatal environment have also been identified. Extensive research has demonstrated that both uteroplacental insufficiency and maternal malnutrition alter renal development in utero. Equally, exposure to maternal diabetes and maternal obesity during development are also associated with an increased risk of developing renal disease, however, the mechanism behind this association is poorly understood. Therefore, identifying the link between an adverse intrauterine environment and the programmed kidney disease risk in adulthood may facilitate the development of strategies to alleviate the epidemics of cardiorenal disease worldwide, in addition to understanding why males are more susceptible to adult‐onset cardiovascular diseases.

Short term exposure to elevated levels of leptin reduces proximal tubule cell metabolic activity

Leptin plays a pathophysiological role in the kidney, however, its acute effects on the proximal tubule cells (PTCs) are unknown. In opossum kidney (OK) cells in vitro, Western blot analysis identified that exposure to leptin increases the phosphorylation of the mitogen-activated protein kinase (MAPK) p44/42 and the mammalian target of rapamycin (mTOR). Importantly leptin (0.05, 0.10, 0.25 and 0.50 μg/ml) significantly reduced the metabolic activity of PTCs, and significantly decreased protein content per cell. Investigation of the role of p44/42 and mTOR on metabolic activity and protein content per cell, demonstrated that in the presence of MAPK inhibitor U0126 and mTOR inhibitor Ku-63794, that the mTOR pathway is responsible for the reduction in PTC metabolic activity in response to leptin. However, p44/42 and mTOR play no role the reduced protein content per cell in OKs exposed to leptin. Therefore, leptin modulates metabolic activity in PTCs via an mTOR regulated pathway.

Maternal obesity in females born small: Pregnancy complications and offspring disease risk

Obesity is a major public health crisis, with 1.6 billion adults worldwide being classified as overweight or obese in 2014. Therefore, it is not surprising that the number of women who are overweight or obese at the time of conception is increasing. Obesity during pregnancy is associated with the development of gestational diabetes and preeclampsia. The developmental origins of health and disease hypothesis proposes that perturbations during critical stages of development can result in adverse fetal changes that leads to an increased risk of developing diseases in adulthood. Of particular concern, children born to obese mothers are at a greater risk of developing cardiometabolic disease. One subset of the population who are predisposed to developing obesity are children born small for gestational age, which occurs in 10% of pregnancies worldwide. Epidemiological studies report that these growth-restricted children have an increased susceptibility to type 2 diabetes, obesity, and hypertension. Importantly during pregnancy, growth-restricted females have a higher risk of developing cardiometabolic disease, indicating that they may have an exacerbated phenotype if they are also overweight or obese. Thus, the development of early pregnancy interventions targeted to obese mothers may prevent their children from developing cardiometabolic disease in adulthood.

Sgk-1 is a Positive Regulator of Constitutive Albumin Uptake in Renal Proximal Tubule Cells

Background/Aims: Receptor-mediated endocytosis of albumin by the renal proximal tubule requires a number of proteins including megalin/cubilin, sodium/hydrogen exchanger isoform 3 (NHE3) and ClC-5, as well as the PSD-95/Dlg/Zo-1 (PDZ) scaffold sodium/hydrogen exchanger regulatory factor 2 (NHERF2). Despite members from the AGC kinase family, v-Akt Murine Thymoma Viral Oncogene (Akt or Protein Kinase B) and Serum/Glucocorticoid regulated Kinase 1 (Sgk-1) regulating a number of essential proteins in the albumin handling pathway, their role in uptake is largely unknown. Methods: Opossum kidney (OK) cells were exposed to Texas-Red albumin, in the presence of silencing constructs against Sgk-1, Akt and NHERF2, in addition to the NHE3 inhibitor 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) and NHE3 activator dexamethasone. Target protein was also measured by Western blot analysis in OK cells following exposure to dexamethasone and albumin. Results: Silencing Sgk-1 or overexpression of a dominant negative mutant (DN-Sgk-1) led to a significant reduction of albumin endocytosis compared to control. Conversely, over-expression of wildtype (WT) or constitutively active (CA) Sgk-1 significantly increased uptake. Previous reports have shown Sgk-1 can activate NHE3 through an interaction mediated by NHERF2. We found that silencing both Sgk-1 and NHERF2 demonstrated no additive effect on uptake, suggesting signaling via similar endpoints. Treatment with dexamethasone increased Sgk-1 protein levels and increased albumin endocytosis in OK cells. Interestingly, silencing Akt also lead to a reduction in albumin endocytosis, however in cells silenced for both Sgk-1 and Akt, the additive change in albumin uptake demonstrated that these proteins may act via separate pathways. Conclusions: We have characterized a Sgk-dependent pathway that regulates albumin uptake in the proximal tubule which also includes NHE3 and NHERF2. These data provide further insights into this essential tubular process.

Uteroplacental insufficiency in rats induces renal apoptosis and delays nephrogenesis completion

Uteroplacental insufficiency in rats reduces nephron endowment, leptin concentrations and programmes cardiorenal disease in offspring. Cross‐fostering growth‐restricted (Restricted) offspring onto a mother with normal lactation restores leptin concentrations and nephron endowment. This study aimed to determine whether the reduced nephron endowment in Restricted offspring is due to delayed glomerular formation and dysregulation of renal genes regulating branching morphogenesis, apoptosis or leptin signalling. Furthermore, we aimed to investigate whether cross‐fostering Restricted offspring onto Control mothers could improve glomerular maturation and restore renal gene abundance.