Angiotensin II differentially regulates Morg1 expression in kidney cells

Shear Stress-Induced Alteration of Epithelial Organization in Human Renal Tubular Cells

Tubular epithelial cells in the kidney are continuously exposed to urinary fluid shear stress (FSS) generated by urine movement and recent in vitro studies suggest that changes of FSS could contribute to kidney injury. However it is unclear whether FSS alters the epithelial characteristics of the renal tubule. Here, we evaluated in vitro and in vivo the influence of FSS on epithelial characteristics of renal proximal tubular cells taking the organization of junctional complexes and the presence of the primary cilium as markers of epithelial phenotype. Human tubular cells (HK-2) were subjected to FSS (0.5 Pa) for 48h. Control cells were maintained under static conditions. Markers of tight junctions (Claudin-2, ZO-1), Par polarity complex (Pard6), adherens junctions (E-Cadherin, β-Catenin) and the primary cilium (α-acetylated Tubulin) were analysed by quantitative PCR, Western blot or immunocytochemistry. In response to FSS, Claudin-2 disappeared and ZO-1 displayed punctuated and discontinuous staining in the plasma membrane. Expression of Pard6 was also decreased. Moreover, E-Cadherin abundance was decreased, while its major repressors Snail1 and Snail2 were overexpressed, and β-Catenin staining was disrupted along the cell periphery. Finally, FSS subjected-cells exhibited disappeared primary cilium. Results were confirmed in vivo in a uninephrectomy (8 months) mouse model where increased FSS induced by adaptive hyperfiltration in remnant kidney was accompanied by both decreased epithelial gene expression including ZO-1, E-cadherin and β-Catenin and disappearance of tubular cilia. In conclusion, these results show that proximal tubular cells lose an important number of their epithelial characteristics after long term exposure to FSS both in vitro and in vivo. Thus, the changes in urinary FSS associated with nephropathies should be considered as potential insults for tubular cells leading to disorganization of the tubular epithelium.

The role of hypoxia and Morg1 in renal injury

BACKGROUND

Renal hypoxia is known to play an important role in the pathophysiology of acute renal injury as well as in chronic kidney diseases. The mediators of hypoxia are the transcription factors HIF (hypoxia-inducible factors), that are highly regulated. Under normoxic conditions constitutively expressed HIF-α subunits are hydroxylated by prolyl hydroxylases (PHD1, PHD2, and PHD3) and subsequently degraded by proteasomes.

MATERIALS AND METHODS

This narrative review is based on the material searched for and obtained via PubMed and MEDLINE up to January 2015.

RESULTS

The MAPK organizer 1 (Morg1) has been identified to act as a scaffold protein of PHD3 and suppression of Morg1 leads to the stabilization of HIF-α, which forms in the absence of oxygen a heterodimer with HIF-β, translocates to the nucleus and promotes the transcription of HIF target genes.

CONCLUSIONS

This review summarizes the current knowledge regarding the role of hypoxia, HIF signalling, and Morg1 in acute and chronic renal injury.

Cross-talk between angiotensin-II and toll-like receptor 4 triggers a synergetic inflammatory response in rat mesangial cells under high glucose conditions

Toll-like Receptor 4 (TLR4) may play an important role in the pathogenesis of diabetic nephropathy (DN). In this study, We observed the TLR4 signal and the release of inflammation factors after angiotensin II (Ang II) stimulation in rat mesangial cells (MCs) under high glucose conditions, this revealed the innate immune mechanism of injury by Ang II in DN. Our data showed that TLR4 and MyD88 were up-regulated significantly in high glucose and AngII-induced MCs; meanwhile, NF-κB as well as MCP-1, IL-6 were also highly expressed. In cells that were transfected with TLR4 SiRNA，the parameters were greatly inhibited; similar effects were detected in cells that were treated with Irbesartan. We concluded that Ang II synergized with high glucose in the release of pro-inflammatory factors mainly through the upregulation of TLR4 signaling in MCs, Cross-talk between Ang II and TLR4 contributed to the MC inflammatory injury under high glucose conditions.

Morg1 heterozygous deficiency ameliorates hypoxia-induced acute renal injury

Acute kidney injury is a common complication of critically ill patients and may occur as a result of various factors and coexisting previous illnesses. Some pathophysiological responses seen in critical illness can be similar to the human physiological response to extreme environmental challenges, such as hypoxia from reduced oxygen availability at high altitudes (systemic hypoxia). Due to oxygen deficiency, mammalian cells activate the transcriptional factor hypoxia-inducible factor (HIF); its degradation is regulated by prolyl hydroxylase 3 (PHD3) in interaction with the scaffold protein MAPK organizer 1 (Morg1). While homozygous Morg1(-/-) mice are embryonically lethal, the kidneys of heterozygous Morg1(+/-) mice reveal elevated HIF protein levels and increased serum erythropoietin compared with wild-type Morg1(+/+) mice. In this study, we exposed wild-type and Morg1(+/-) mice to 10% oxygen in a hypoxic chamber for 3 days. This reduced oxygen concentration leads to a deterioration of renal function, an increase in renal inflammation, and significantly more tubular damage and apoptosis in the kidneys of wild-type (Morg1(+/+)) mice. In sharp contrast, Morg1(+/-) kidneys were protected against systemic hypoxia. They show significantly less renal lesions, reduced or no inflammation, and less tubular damage and apoptosis. Thus short-term systemic and subsequently renal hypoxia which may occur in many patients in the intensive care unit induces in wild-type mice renal injury, which is ameliorated by Morg1 deficiency. Our findings suggest that therapeutical manipulation of Morg1 may be an interesting novel target to prevent hypoxia-associated renal damage.

Advanced glycated end-products affect HIF-transcriptional activity in renal cells

Advanced glycated end-products (AGEs) are ligands of the receptor for AGEs and increase in diabetic disease. MAPK organizer 1 (Morg1) via its binding partner prolyl-hydroxylase domain (PHD)-3 presumably plays a role in the regulation of hypoxia-inducible factor (HIF)-1α and HIF-2α transcriptional activation. The purpose of this study was to analyze the influence of AGEs on Morg1 expression and its correlation to PHD3 activity and HIF-transcriptional activity in various renal cell types. The addition of glycated BSA (AGE-BSA) significantly up-regulated Morg1 mRNA levels in murine mesangial cells and down-regulated it in murine proximal tubular cells and differentiated podocytes. These effects were reversible when the cells were preincubated with a receptor for α-AGE antibody. AGE-BSA treatment induced a relocalization of the Morg1 cellular distribution compared with nonglycated control-BSA. Analysis of PHD3 activity demonstrated an elevated PHD3 enzymatic activity in murine mesangial cells but an inhibition in murine proximal tubular cells and podocytes after the addition of AGE-BSA. HIF-transcriptional activity was also affected by AGE-BSA treatment. Reporter gene assays and EMSAs showed that AGEs regulate HIF- transcriptional activity under nonhypoxic conditions in a cell type-specific manner. In proximal tubular cells, AGE-BSA stimulation elevated mainly HIF-1α transcriptional activity and to a lesser extent HIF-2α. We also detected an increased expression of the HIF-1α and the HIF-2α proteins in kidneys from Morg1 heterozygous (HZ) placebo mice compared with the Morg1 wild-type (WT) placebo-treated mice, and the HIF-1α protein expression in the Morg1 HZ streptozotocin-treated mice was significantly higher than the WT streptozotocin-treated mice. Analysis of isolated mesangial cells from Morg1 HZ (±) and WT mice showed an inhibited PHD3 activity and an increased HIF-transcriptional activity in cells with only one Morg1 allele. These findings are important for a better understanding of the molecular mechanisms of diabetic nephropathy.