C/EBP homologous protein induces mesangial cell apoptosis under hyperglycemia

Altered DNA methylation of TRIM13 in diabetic nephropathy suppresses mesangial collagen synthesis by promoting ubiquitination of CHOP

Background

Mesangial collagen synthesis in renal glomeruli contributes to the pathogenesis of diabetic nephropathy (DN) which is one of the most serious complications of diabetes mellitus. However, the underlying mechanism of mesangial collagen synthesis is largely unknown.

Methods

The differential expression of CHOP and TRIM13 which is a well-defined E3 ubiquitin ligase was compared in renal biopsy samples from DN/normal renal tissues, in isolated glomeruli of diabetic/control mice, as well as in high glucose (HG) or TGF-β1-stimulated renal mesangial cells. Then the relationship between TRIM13 and CHOP was explored using the ubiquitination assay.

Findings

We found that the expression of TRIM13 was downregulated in renal biopsies, isolated glomeruli of diabetic mice, and HG/TGF-β1-stimulated renal mesangial cells, while the expression of CHOP was upregulated. An increased level of TRIM13 promoter methylation contributed to the deregulation of TRIM13 in renal glomeruli of DN. The ubiquitination assay confirmed that TRIM13 promoted ubiquitination and degradation of CHOP. Meanwhile, overexpressing TRIM13 attenuated DN-induced collagen synthesis and restored renal function in vitro and in vivo via downregulating CHOP.

Interpretation

Our findings demonstrated that overexpressed TRIM13 suppresses mesangial collagen synthesis in DN by promoting ubiquitination of CHOP, suggesting TRIM13 as a potential therapeutic target in treating DN.

Sublytic C5b-9 Induces Glomerular Mesangial Cell Apoptosis through the Cascade Pathway of MEKK2-p38 MAPK-IRF-1-TRADD-Caspase 8 in Rat Thy-1 Nephritis

The apoptosis of glomerular mesangial cells (GMCs) in the early phase of rat Thy-1 nephritis (Thy-1N), a model of human mesangioproliferative glomerulonephritis (MsPGN), is primarily triggered by sublytic C5b-9. However, the mechanism of GMC apoptosis induced by sublytic C5b-9 remains unclear. In this study, we demonstrate that expressions of TNFR1-associated death domain-containing protein (TRADD) and IFN regulatory factor-1 (IRF-1) were simultaneously upregulated in the renal tissue of Thy-1N rats (in vivo) and in GMCs under sublytic C5b-9 stimulation (in vitro). In vitro, TRADD was confirmed to be a downstream gene of IRF-1, because IRF-1 could bind to TRADD gene promoter to promote its transcription, leading to caspase 8 activation and GMC apoptosis. Increased phosphorylation of p38 MAPK was verified to contribute to IRF-1 and TRADD production and caspase 8 activation, as well as to GMC apoptosis induced by sublytic C5b-9. Furthermore, phosphorylation of MEK kinase 2 (MEKK2) mediated p38 MAPK activation. More importantly, three sites (Ser^153/164/239) of MEKK2 phosphorylation were identified and demonstrated to be necessary for p38 MAPK activation. In addition, silencing of renal MEKK2, IRF-1, and TRADD genes or inhibition of p38 MAPK activation in vivo had obvious inhibitory effects on GMC apoptosis, secondary proliferation, and urinary protein secretion in rats with Thy-1N. Collectively, these findings indicate that the cascade axis of MEKK2-p38 MAPK-IRF-1-TRADD-caspase 8 may play an important role in GMC apoptosis following exposure to sublytic C5b-9 in rat Thy-1N.

Asymmetric dimethylarginine (ADMA) treatment induces apoptosis in cultured rat mesangial cells via endoplasmic reticulum stress activation

Asymmetric dimethylarginine (ADMA), a high risk factor for endothelial dysfunction and cardiovascular disease (CVD), has been reported to promote cellular dysfunction via endoplasmic reticulum (ER) stress activation in various cells. Additionally, increased serum ADMA levels have been observed in incipient kidney diseases. Previously, we reported that activated ER stress is associated with mesangial cell apoptosis, observed mainly in overt nephropathy or chronic kidney disease (CKD). However, the effect of ADMA on mesangial cell apoptosis is unknown. Thus, we investigated the effects of ADMA on mesangial cell apoptosis and ER stress signaling. ADMA treatment increased caspase-3 activity and activated three branches of ER stress signaling (PERK, IRE1, and ATF6) that induce mesangial cell apoptosis. Pharmacological inhibitors of ER stress (inhibitors of PERK, IRE1, and S1P) attenuated ADMA-induced cleavage of caspase-3 and induced a decrease in the mitochondrial membrane potential. Furthermore, these inhibitors diminished the number of apoptotic cells induced by ADMA treatment. Taken together, our results indicated that ADMA treatment induces mesangial cell apoptosis via ER stress signaling. These results suggest that ADMA-induced mesangial cell apoptosis could contribute to the progression of overt nephropathy and CKD.

Sublytic C5b-9 triggers glomerular mesangial cell apoptosis via XAF1 gene activation mediated by p300-dependent IRF-1 acetylation

The apoptosis of glomerular mesangial cells (GMCs) in rat Thy-1 nephritis (Thy-1N), a model of human mesangioproliferative glomerulonephritis (MsPGN), is accompanied by sublytic C5b-9 deposition. However, the mechanism by which sublytic C5b-9 induces GMC apoptosis is unclear. In the present studies, the effect of X-linked inhibitor of apoptosis-associated factor 1 (XAF1) expression on GMC apoptosis and the role of p300 and interferon regulatory factor-1 (IRF-1) in mediating XAF1 gene activation were determined, both in the GMCs induced by sublytic C5b-9 (in vitro) and in the renal tissues of rats with Thy-1N (in vivo). The in vitro studies demonstrated that IRF-1-enhanced XAF1 gene activation and its regulation by p300-mediated IRF-1 acetylation were involved in GMC apoptosis induced by sublytic C5b-9. The element of IRF-1 binding to XAF1 promoter and two acetylated sites of IRF-1 protein were also revealed. In vivo, silence of p300, IRF-1 or XAF1 genes in the renal tissues diminished GMC apoptosis and secondary GMC proliferation as well as urinary protein secretion in Thy-1N rats. Together, these data implicate that sublytic C5b-9 induces the expression of both p300 and IRF-1, as well as p300-dependent IRF-1 acetylation that may contribute to XAF1 gene activation and subsequent GMC apoptosis in Thy-1N rats.

Liraglutide alleviates diabetic cardiomyopathy by blocking CHOP-triggered apoptosis via the inhibition of the IRE-α pathway

Clinically, diabetes mellitus is closely associated with and induces certain cardiovascular diseases. The aim of this study was to investigate endoplasmic reticulum (ER) stress-associated apoptosis of diabetic cardiomyopathy (DCM), and explore the protective mechanism of liraglutide. The DCM model was established with a high-fat diet and streptozotocin (STZ). Cardiac function was detected by echocardiogram examination and hematoxylin-eosin staining. ER stress unfolded protein response (UPR) hallmarks [inositol-requiring enzyme-α (IRE-α), p-Perk and ATF6] and transcription factors were detected with western blotting. Apoptosis inducers CHOP, c-Jun amino terminal kinase (JNK) and casapse-12 were also examined with western blotting. The results indicated that liraglutide is capable of improving cardiac function in DCM rats (P<0.05). IRE-α expression was significantly increased in the DCM group compared with the control group (P<0.05), and liraglutide is capable of decreasing IRE-α expression. X-box transcription factor-1 (XBP-1) was significantly spliced in the model group, and downregulated in the liraglutide-treated group. CHOP protein was upregulated in the DCM group, but inactivated by liraglutide treatment. In conclusion, liraglutide is capable of protecting DCM and blocking CHOP-mediated ER stress by inhibiting the IRE-α UPR pathway.

Impact of the 4G/5G polymorphism in the plasminogen activator inhibitor-1 gene on primary nephrotic syndrome

The aim of the present study was to investigate whether the four guanosines (4G)/five guanosines (5G) polymorphism in the gene coding for plasminogen activator inhibitor-1 (PAI-1) affects the clinical features of primary nephrotic syndrome (PNS). A cohort of 200 biopsy-diagnosed PNS patients was studied, with 40 healthy subjects as controls. The PAI-1 gene polymorphism was detected by polymerase chain reaction and DNA sequencing. Associations between the PAI-1 4G/5G polymorphism and clinical features and pathological types of PNS were analyzed. The results indicated that the PAI-1 genotype distribution is significantly different between patients with PNS and healthy controls, with significantly higher numbers of the 4G/4G genotype and lower numbers of the 5G5G genotype detected in PNS patients compared to controls (both P<0.05). The frequency of the 4G allele was also significantly higher in PNS patients compared to healthy controls (P<0.01). Among the different pathological types of PNS, IgA nephropathy (IgAN) and membranous nephropathy (MN) were associated with significantly increased frequencies of the 4G/4G and 4G/5G genotypes, as well as of the 4G allele. The increased 4G frequency was also detected in patients with minimal change disease (MCD). Significantly increased international normalized ratio (INR) and prolonged activated partial thromboplastin time (APTT) were observed in 4G/4G compared to 5G/5G PNS subjects. The response to steroids was not significantly different among the three genotypes. In conclusion, the 4G allele of the PAI-1 gene appears to be associated with PNS, especially in MN and IgAN patients. These findings suggest that specific targeting may be required for the treatment of PNS patients with the 4G/4G genotype.

Pro-apoptosis gene PUMA and cancer

The dysfunction of cell apoptosis signaling is involved in carcinogenesis. P53 up-regulated modulator of apoptosis (PUMA), a pro-apoptosis gene that has been found for a decade, encodes a protein that is one of Bcl-2 members and can induce apoptosis via the mitochondrial pathway. It is considered that mutation of the PUMA gene is not involved in carcinogenesis, because mutation of the PUMA gene has not been found in many types of tumors until now. The expression of PUMA protein is regulated transcriptionally via ER stress, p53, JNK, FOXO3a and E2F1 signaling or post-translationally by phosphorylation. Several studies have showed that the down-regulation of PUMA protein in cancer tissue is associated with carcinogenesis, lymph node metastasis and tumor prognosis, and that up-regulation of PUMA induces the inhibition of cancer cell proliferation. Increasing new findings on the precise role of PUMA in the regulation of cancer development provide new insights into the potential use of PUMA as a target for the prevention and treatment of cancer.