Human proximal tubular cell responses to angiotensin II analyzed using DNA microarray

Modulation of the expression of connexins 37, 40, and 43 in endothelial cells in a culture

Connexin (Cx) 37, 40, and 43 are implicated in vascular function, specifically in the electrical coupling of endothelial cells and vascular smooth-muscle cells. In the present study, we investigated whether factors implicated in vascular dysfunction can modulate the gene expression of Cx37, Cx40, and Cx43 and whether this is associated with changes in endothelial layer barrier function in human microvascular endothelial cells (HMEC-1). First, HMEC-1 were subjected to stimuli for 4 and 8 h. We tested their responses to DETA-NONOate, H2O2, high glucose, and angiotensin II, none of which relevantly affected the transcription of the connexin genes. Next, we tested inflammatory factors IL-6, interferon gamma (IFNγ), and TNFα. IFNγ (10 ng/mL) consistently induced Cx40 expression at 4 and 8 h to 10-20-fold when corrected for the control. TNFα and IL-6 resulted in small but significant depressions of Cx37 expression at 4 h. Two JAK inhibitors, epigallocatechin-3-gallate (EGCG) (100-250 μM) and AG490 (100-250 μM), dose-dependently inhibited the induction of Cx40 expression by IFNγ. Subsequently, HMEC-1 were subjected to 10 ng/mL IFNγ for 60 h, and intercellular and transcellular impedance was monitored by electric cell-substrate impedance sensing (ECIS). In response to IFNγ, junctional-barrier impedance increased more than cellular-barrier impedance; this was prevented by AG490 (5 μM). In conclusion, IFNγ can strongly induce Cx40 expression and modify the barrier properties of the endothelial cell membrane through the JAK/STAT pathway. Moreover, the Cx37, Cx40, and Cx43 expression in endothelial cells is stable and, apart from IFNγ, not affected by a number of factors implicated in endothelial dysfunction and vascular diseases.

Salt, Angiotensin II, Superoxide, and Endothelial Function

Proper function of the vascular endothelium is essential for cardiovascular health, in large part due to its antiproliferative, antihypertrophic, and anti-inflammatory properties. Crucial to the protective role of the endothelium is the production and liberation of nitric oxide (NO), which not only acts as a potent vasodilator, but also reduces levels of reactive oxygen species, including superoxide anion (O2•-). Superoxide anion is highly injurious to the vasculature because it not only scavenges NO molecules, but has other damaging effects, including direct oxidative disruption of normal signaling mechanisms in the endothelium and vascular smooth muscle cells. The renin-angiotensin system plays a crucial role in the maintenance of normal blood pressure. This function is mediated via the peptide hormone angiotensin II (ANG II), which maintains normal blood volume by regulating Na+ excretion. However, elevation of ANG II above normal levels increases O2•- production, promotes oxidative stress and endothelial dysfunction, and plays a major role in multiple disease conditions. Elevated dietary salt intake also leads to oxidant stress and endothelial dysfunction, but these occur in the face of salt-induced ANG II suppression and reduced levels of circulating ANG II. While the effects of abnormally high levels of ANG II have been extensively studied, far less is known regarding the mechanisms of oxidant stress and endothelial dysfunction occurring in response to chronic exposure to abnormally low levels of ANG II. The current article focuses on the mechanisms and consequences of this less well understood relationship among salt, superoxide, and endothelial function.

Checkpoint kinase Chk2 controls renal Cyp27b1 expression, calcitriol formation, and calcium-phosphate metabolism

Checkpoint kinase 2 (Chk2) is the main effector kinase of ataxia telangiectasia mutated (ATM) and responsible for cell cycle regulation. ATM signaling has been shown to upregulate interferon-regulating factor-1 (IRF-1), a transcription factor also expressed in the kidney. Calcitriol (1,25 (OH)2D3), a major regulator of mineral metabolism, is generated by 25-hydroxyvitamin D 1α-hydroxylase in the kidney. Since 25-hydroxyvitamin D 1α-hydroxylase expression is enhanced by IRF-1, the present study explored the role of Chk2 for calcitriol formation and mineral metabolism. Chk2-deficient mice (chk2 (-/-)) were compared to wild-type mice (chk2 (+/+)). Transcript levels of renal 25-hydroxyvitamin D 1α-hydroxylase, Chk2, and IRF-1 were determined by RT-PCR; Klotho expression by Western blotting; bone density by μCT analysis; serum or plasma 1,25 (OH)2D3, PTH, and C-terminal FGF23 concentrations by immunoassays; and serum, fecal, and urinary calcium and phosphate concentrations by photometry. The renal expression of IRF-1 and 25-hydroxyvitamin D 1α-hydroxylase as well as serum 1,25 (OH)2D3 and FGF23 levels were significantly lower in chk2 (-/-) mice compared to chk2 (+/+) mice. Plasma PTH was not different between the genotypes. Renal calcium and phosphate excretion were significantly higher in chk2 (-/-) mice than in chk2 (+/+) mice despite hypophosphatemia and normocalcemia. Bone density was not different between the genotypes. We conclude that Chk2 regulates renal 25-hydroxyvitamin D 1α-hydroxylase expression thereby impacting on calcium and phosphate metabolism.

AT(1) receptor Gαq protein-independent signalling transcriptionally activates only a few genes directly, but robustly potentiates gene regulation from the β2-adrenergic receptor

The angiotensin II type 1 receptor (AT(1)R) is known to signal through heterotrimeric G proteins, and Gαq protein-independent signalling has only recently gained appreciation for profound impact on a diverse range of biological functions. β-Arrestins, among other central mediators of Gαq protein-independent signalling from the AT(1)R interact with transcriptional regulators and promote phosphorylation of nuclear proteins. However, the relative contribution of Gαq protein-independent signalling in AT(1)R mediated transcriptional regulation remains elusive. We here present a comprehensive comparative analysis of Gαq protein-dependent and -independent regulation of AT(1)R mediated gene expression. We found angiotensin II to regulate 212 genes, whereas Gαq-independent signalling obtained with the biased agonist, SII angiotensin II only regulated few genes. Interestingly, SII angiotensin II, like Ang II vastly potentiated β2-adrenergic receptor-stimulated gene expression. These novel findings indicate that the Gαq protein-independent signalling mainly modifies the transcriptional response governed by other signalling pathways, while direct induction of gene expression by the AT(1)R is dependent on classical Gαq protein activation.

Angiotensin II stimulates trafficking of NHE3, NaPi2, and associated proteins into the proximal tubule microvilli

Angiotensin II (ANG II) stimulates proximal tubule (PT) sodium and water reabsorption. We showed that treating rats acutely with the angiotensin-converting enzyme inhibitor captopril decreases PT salt and water reabsorption and provokes rapid redistribution of the Na(+)/H(+) exchanger isoform 3 (NHE3), Na(+)/Pi cotransporter 2 (NaPi2), and associated proteins out of the microvilli. The aim of the present study was to determine whether acute ANG II infusion increases the abundance of PT NHE3, NaPi2, and associated proteins in the microvilli available for reabsorbing NaCl. Male Sprague-Dawley rats were infused with a dose of captopril (12 microg/min for 20 min) that increased PT flow rate approximately 20% with no change in blood pressure (BP) or glomerular filtration rate (GFR). When ANG II (20 ng x kg(-1) x min(-1) for 20 min) was added to the captopril infusate, PT volume flow rate returned to baseline without changing BP or GFR. After captopril, NHE3 was localized to the base of the microvilli and NaPi2 to subapical cytoplasmic vesicles; after 20 min ANG II, both NHE3 and NaPi2 redistributed into the microvilli, assayed by confocal microscopy and density gradient fractionation. Additional PT proteins that redistributed into low-density microvilli-enriched membranes in response to ANG II included myosin VI, DPPIV, NHERF-1, ezrin, megalin, vacuolar H(+)-ATPase, aminopeptidase N, and clathrin. In summary, in response to 20 min ANG II in the absence of a change in BP or GFR, multiple proteins traffic into the PT brush-border microvilli where they likely contribute to the rapid increase in PT salt and water reabsorption.

Roles of ERK and cPLA2 in the angiotensin II-mediated biphasic regulation of Na+-HCO3(-) transport

Regulation of renal proximal transport by angiotensin II (Ang II) is biphasic: low concentrations (picomolar to nanomolar) stimulate reabsorption, but higher concentrations (nanomolar to micromolar) inhibit reabsorption. Traditionally, the stimulatory effect has been attributed to activation of protein kinase C and/or a decrease in intracellular cAMP, whereas the inhibitory action has been attributed to the activation of phospholipase A2 (PLA2) and the subsequent release of arachidonic acid. The Ang II receptor subtype responsible for these effects and the intracellular signaling pathways involved are not completely understood. We isolated proximal tubules from wild-type, Ang II type 1A receptor (AT1A)-deficient, and group IVA cytosolic phospholipase A2 (cPLA2alpha)-deficient mice, and compared their responses to Ang II. In wild-type mice, we found that the stimulatory and inhibitory effects of Ang II on Na+-HCO3(-) cotransporter activity are both AT1-mediated but that ERK activation only plays a role in the former. The stimulatory effect of Ang II was also observed in AT1A-deficient mice, suggesting that this occurs through AT1B. In contrast, the inhibitory effects of Ang II appeared to be mediated by cPLA2alpha activation because high-concentration Ang II stimulated Na+-HCO3(-) cotransporter activity when cPLA2alpha activity was abrogated by pharmacological means or genetic knockout. Consistent with this observation, we found that activation of the cPLA2alpha/P450 pathway suppressed ERK activation. We conclude that Ang II activates ERK and cPLA2alpha in a concentration-dependent manner via AT1, and that the balance between ERK and cPLA2alpha activities determines the ultimate response to Ang II in intact proximal tubules.

Metabolic inhibition-induced transient Ca2+ increase depends on mitochondria in a human proximal renal cell line

During ischemia or hypoxia an increase in intracellular cytosolic Ca(2+) induces deleterious events but is also implicated in signaling processes triggered in such conditions. In MDCK cells (distal tubular origin), it was shown that mitochondria confer protection during metabolic inhibition (MI), by buffering the Ca(2+) overload via mitochondrial Na(+)-Ca(2+) exchanger (NCX). To further assess this process in cells of human origin, human cortical renal epithelial cells (proximal tubular origin) were subjected to MI and changes in cytosolic Ca(2+) ([Ca(2+)](i)), Na(+), and ATP concentrations were monitored. MI was accomplished with both antimycin A and 2-deoxyglucose and induced a 3.5-fold increase in [Ca(2+)](i), reaching 136.5 +/- 15.8 nM in the first 3.45 min. Subsequently [Ca(2+)](i) dropped and stabilized to 62.7 +/- 7.3 nM by 30 min. The first phase of the transient increase was La(3+) sensitive, not influenced by diltiazem, and abolished when mitochondria were deenergized with the protonophore carbonylcyanide p-trifluoromethoxyphenylhydrazone. The subsequent recovery phase was impaired in a Na(+)-free medium and weakened when the mitochondrial NCX was blocked with 7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one (CGP-37157). Thus Ca(2+) entry is likely mediated by store-operated Ca(2+) channels and depends on energized mitochondria, whereas [Ca(2+)](i) recovery relied partially on the activity of mitochondrial NCX. These results indicate a possible mitochondrial-mediated signaling process triggered by MI, support the hypothesis that mitochondrial NCX has an important role in the Ca(2+) clearance, and overall suggest that mitochondria play a preponderant role in the regulation of responses to MI in human renal epithelial cells.

Albumin overload induces adaptive responses in human proximal tubular cells through oxidative stress but not via angiotensin II type 1 receptor

Proteinuria is pathogenic to proximal tubular cells (PTC) and linked with progression to renal failure. The aim of this study was to determine the effects of human serum albumin (HSA) overload on the changes in gene and protein expression stimulated by oxidative stress in PTC and any interaction with ANG II that is pivotal in disease pathogenesis. Markers of oxidative stress, antioxidant defences, transcription factor activation, and the expression of stress-related genes were measured in human PTC (HK-2 cells) overloaded with either globulin-free fatty acid free (GF/FAF) HSA or globulin-free (GF) HSA. The effects of ANG II were also determined. HSA overload in HK-2 cells caused PTC hyperfunction, increased oxidative stress, and an upregulation of adaptive responses and stress-related genes. Some responses were common to both HSAs but others were unique to either HSA and unaffected by addition of ANG II or candesartan (a specific ANG II type 1 receptor blocker). ANG II also independently induced oxidative stress and upregulated other stress-related genes. HSA overload in HK-2 cells stimulated increased oxidative stress and upregulated changes in stress-related gene expression indicating new pathways of PTC injury that are not mediated via ANG II type 1 receptors.

Effects of ACE inhibition on proximal tubule sodium transport

Angiotensin-converting enzyme (ACE) inhibitors such as captopril, which block ANG II formation, are commonly used for treatment of hypertension. There is substantial evidence that the proximal tubule (PT) is a primary target site for captopril but the molecular mechanisms for its action in PT are not well defined. The aim of this study was to determine the physiological and molecular changes in PT provoked by acute captopril treatment in the absence of changes in blood pressure or glomerular filtration rate (GFR). Captopril (infused at 12 μg/min for 20 min) did not change blood pressure or GFR but induced an immediate (<10 min) increase in PT flow measured with a nonobstructive optical method (to 117 ± 14% of baseline) along with a rapid diuresis from 2.1 ± 0.6 mg/min (baseline) to 3.7 ± 0.9 mg/min (captopril). Captopril also provoked a significant retraction of PT Na+/H+exchanger isoform 3 (NHE3), NHE regulatory factor (NHERF)-1, myosin-VI, and Na+-Picotransporter type 2 (NaPi2), but not ACE, out of apical microvillus-enriched membranes. Proteomic analysis with MALDI-TOF MS revealed an additional eight abundant membrane-associated proteins that redistributed out of the microvillus-enriched membrane during captopril treatment: megalin, myosin II-A, clathrin, aminopeptidase N, DPPIV, ezrin, moesin, and vacuolar H+-ATPase subunit β2. In summary, captopril can rapidly depress PT reabsorption in the absence of a change in GFR or BP and provokes the redistribution of a set of transporters and transporter-associated proteins that likely participate in the decrease in PT reabsorption and may also contribute to the blood pressure-lowering effect of ACE inhibitors.

An unexpected role for angiotensin II in the link between dietary salt and proximal reabsorption

We set out to confirm the long-held, but untested, assumption that dietary salt affects proximal reabsorption through reciprocal effects on the renin-angiotensin system in a way that facilitates salt homeostasis. Wistar rats were fed standard or high-salt diets for 7 days and then subjected to renal micropuncture for determination of single-nephron GFR (SNGFR) and proximal reabsorption. The tubuloglomerular feedback (TGF) system was used as a tool to manipulate SNGFR in order to distinguish primary changes in net proximal reabsorption (Jprox) from changes due to glomerulotubular balance. The influence of Ang II over Jprox was determined by the sensitivity of Jprox to the AT1 receptor antagonist, losartan. Plasma, whole kidneys, and fluid from midproximal tubules were assayed for Ang II content by radioimmunoassay. In rats on the standard diet, losartan reduced Jprox by 25% and reduced the maximum range of the TGF response by 50%. The high-salt diet suppressed plasma and whole-kidney Ang II levels. But the high-salt diet failed to reduce the impact of losartan on Jprox or the TGF response and actually caused tubular fluid Ang II content to increase. The persistent effect of Ang II on Jprox prevented a major rise in late proximal flow rate in response to the high-salt diet. These observations challenge the traditional model and indicate that the role of proximal tubular Ang II in salt-replete rats is to stabilize nephron function rather than to contribute to salt homeostasis.

Transcriptional response in the unaffected kidney after contralateral hydronephrosis or nephrectomy

BACKGROUND

Unilateral loss of kidney function is followed by compensatory contralateral growth. The early, genome-wide transcriptional response of the untouched kidney to unilateral ureteral obstruction (UUO) or unilateral nephrectomy is unknown.

METHODS

Twelve adult male Sprague-Dawley rats were subjected to UUO and twelve rats to unilateral nephrectomy. At time points 12, 24, and 72 hours after insult four rats each were sacrificed and the contralateral kidney harvested for genome-wide gene expression analysis, transcription factor analysis, and histomorphology.

RESULTS

Microarray studies revealed that the majority of differentially expressed transcripts were suppressed in UUO and unilateral nephrectomy compared to control kidneys. The function of these suppressed genes is predominantly growth inhibition and apoptosis suggesting a net pro-hypertrophic response. Insulin-like growth factor-2 (IGF-2)-binding protein was one of the few activated genes. We observed a distinctly different molecular signature between UUO and unilateral nephrectomy at the three time points investigated. The early response in UUO rats suggests a counterbalance to the nonfiltering kidney by activation of transport pathways such as the aquaporins. Unilateral nephrectomy kidneys, on the other hand, respond immediately to contralateral nephrectomy by activation of cell cycle regulators such as the cyclin family. Several genes with weakly defined function were found to be associated with either UUO or unilateral nephrectomy. Transcription factor analysis of the identified transcripts suggests common regulation at least of some of these genes. All kidneys showed normal histology.

CONCLUSION

Release of growth inhibition by nephrectomy leads to immediate cell cycle activation after unilateral nephrectomy, whereas UUO kidneys counterbalance filtration failure by activation of several transporters.

Transcriptome analysis and kidney research: Toward systems biology

An enormous amount of data has been generated in kidney research using transcriptome analysis techniques. In this review article, we first describe briefly the principles and major characteristics of several of these techniques. We then summarize the progress in kidney research that has been made by using transcriptome analysis, emphasizing the experience gained and the lessons learned. Several technical issues regarding DNA microarray are highlighted because of the rapidly increased use of this technology. It appears clear from this brief survey that transcriptome analysis is an effective and important tool for question-driven exploratory science. To further enhance the power of this and other high throughput, as well as conventional approaches, in future studies of the kidney, we propose a multidimensional systems biology paradigm that integrates investigation at multiple levels of biologic regulation toward the goal of achieving a global understanding of physiology and pathophysiology.

Megalin binds and internalizes angiotensin II

Megalin is an abundant membrane protein heavily involved in receptor-mediated endocytosis. The major functions of megalin in vivo remain incompletely defined as megalin typically faces specialized milieus such as glomerular filtrate, airways, epididymal fluid, thyroid colloid, and yolk sac fluid, which lack many of its known ligands. In the course of studies on ANG II internalization, we were surprised when only part of the uptake of labeled ANG II into immortalized yolk sac cells (BN-16 cells) was blocked by specific peptide inhibitors and direct competitors of the angiotensin type 1 receptor. This led us to test if megalin was a receptor for ANG II. Four lines of direct evidence demonstrate that megalin and, to a lesser extent, its chaperone protein cubilin are receptors for ANG II. First, in BN-16 cells anti-megalin and anti-cubilin antisera interfere with ANG II uptake. Second, also in BN-16 cells, pure ANG II competes for uptake of a known megalin ligand. Third, in proximal tubule cell brush-border membrane vesicles extracted from mice, anti-megalin antisera interfere with ANG II binding. Fourth, purified megalin binds ANG II directly in surface plasmon resonance experiments. The finding that megalin is a receptor for ANG II suggests a major new function for the megalin pathway in vivo. These results also indicate that ANG II internalization in some tissues is megalin dependent and that megalin may play a role in regulating proximal tubule ANG II levels.

Serial analysis of gene expression in mouse kidney following angiotensin II administration

As a new line of inquiry into the molecular mechanisms underlying pathophysiological processes associated with angiotensin (ANG II)-dependent hypertension, we applied the method of serial analysis of gene expression (SAGE) to examine genome-wide transcription changes in the kidneys of mice that developed hypertension in response to chronic ANG II administration. Mice were infused subcutaneously via osmotic minipumps with ANG II for 7 days, and systolic blood pressure was measured by tail-cuff plethysmography. Subsequently, mice were euthanized, and the total RNA isolated from the kidneys was used to construct SAGE libraries. Comparison of 11,447 SAGE tags from the hypertensive kidneys, representing 5,740 unique transcripts, and 11,273 tags from the control kidneys, corresponding to 5,619 different transcripts, identified genes that are significantly ( P < 0.05) down- or upregulated in the hypertensive kidney. Our assessment of the genome-wide influence of ANG II resulted in the detection of several novel genes and in a recognition of potential new roles for the previously characterized genes, thus providing new probes with which to further explore the ANG II effects in normal and disease states.