Yanggang Yuan

Mitochondrial dysfunction in the pathophysiology of renal diseases

Mitochondrial dysfunction has gained recognition as a contributing factor in many diseases. The kidney is a kind of organ with high energy demand, rich in mitochondria. As such, mitochondrial dysfunction in the kidney plays a critical role in the pathogenesis of kidney diseases. Despite the recognized importance mitochondria play in the pathogenesis of the diseases, there is limited understanding of various aspects of mitochondrial biology. This review examines the physiology and pathophysiology of mitochondria. It begins by discussing mitochondrial structure, mitochondrial DNA, mitochondrial reactive oxygen species production, mitochondrial dynamics, and mitophagy, before turning to inherited mitochondrial cytopathies in kidneys (inherited or sporadic mitochondrial DNA or nuclear DNA mutations in genes that affect mitochondrial function). Glomerular diseases, tubular defects, and other renal diseases are then discussed. Next, acquired mitochondrial dysfunction in kidney diseases is discussed, emphasizing the role of mitochondrial dysfunction in the pathogenesis of chronic kidney disease and acute kidney injury, as their prevalence is increasing. Finally, it summarizes the possible beneficial effects of mitochondrial-targeted therapeutic agents for treatment of mitochondrial dysfunction-mediated kidney injury-genetic therapies, antioxidants, thiazolidinediones, sirtuins, and resveratrol-as mitochondrial-based drugs may offer potential treatments for renal diseases.

Activation of peroxisome proliferator-activated receptor-γ coactivator 1α ameliorates mitochondrial dysfunction and protects podocytes from aldosterone-induced injury

Glomerular podocytes are highly specialized epithelial cells whose injury in glomerular diseases causes proteinuria. Since mitochondrial dysfunction is an early event in podocyte injury, we tested whether a major regulator of oxidative metabolism and mitochondrial function, the transcriptional coactivator peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α), affects podocyte damage. Aldosterone-induced injury decreased PGC-1α expression, and induced mitochondrial and podocyte damage in dose- and time-dependent manners. The suppression of endogenous PGC-1α by RNAi caused podocyte mitochondrial damage and apoptosis while its increase by infection with an adenoviral vector prevented aldosterone-induced mitochondrial malfunction and inhibited injury. Overexpression of the silent mating type information regulation 2 homolog 1, a gene upstream of PGC-1α, prevented aldosterone-induced mitochondrial damage and podocyte injury by upregulating PGC-1α at both the transcriptional and post-translational levels. Resveratrol, a SIRT1 activator, attenuated aldosterone-induced mitochondrial malfunction and podocyte injury in vitro and in aldosterone-infused mice in vivo. Hence, endogenous PGC-1α may be important for maintenance of mitochondrial function in podocytes under normal conditions. Activators of SIRT1, such as resveratol, may be therapeutically useful in glomerular diseases to promote and maintain PGC-1α expression and, consequently, podocyte integrity.

Mitochondrial Dysfunction Mediates Aldosterone-Induced Podocyte Damage: A Therapeutic Target of PPARγ

Aldosterone (Aldo) causes podocyte damage by an unknown mechanism. We examined the role of mitochondrial dysfunction (MtD) in Aldo-treated podocytes in vitro and in vivo. Exposure of podocytes to Aldo reduced nephrin expression dose dependently, accompanied by increased production of reactive oxygen species (ROS). The ROS generation and podocyte damage were abolished by the mitochondrial (mt) respiratory chain complex I inhibitor rotenone. Pronounced MtD, including reduced mt membrane potential, ATP levels, and mtDNA copy number were seen in Aldo-treated podocytes and in the glomeruli of Aldo-infused mice. The mineralocorticoid receptor antagonist eplerenone significantly inhibited Aldo-induced MtD. The MtD was associated with higher levels of ROS, reduction in the activity of complexes I, III, and IV, and expression of the peroxisome proliferator-activated receptor-γ (PPARγ) coactivator-1α and mt transcription factor A. Both the PPARγ agonist rosiglitazone and PPARγ overexpression protected against podocyte injury by preventing MtD and oxidative stress, as evidenced by reduced ROS production, by maintenance of mt morphology, by restoration of mtDNA copy number, by decrease in mt membrane potential loss, and by recovery of mt electron transport function. The protective effect of rosiglitazone was abrogated by the specific PPARγ small interference RNA, but not a control small interference RNA. We conclude that MtD is involved in Aldo-induced podocyte injury, and that the PPARγ agonist rosiglitazone may protect podocytes from this injury by improving mitochondrial function.

Mitochondrial dysfunction accounts for aldosterone-induced epithelial-to-mesenchymal transition of renal proximal tubular epithelial cells.

Epithelial-mesenchymal transition (EMT) plays a pivotal role in the pathogenesis of renal tubulointerstitial fibrosis. We previously demonstrated that aldosterone (Aldo)-induced EMT is dependent on mitochondrial-derived oxidative stress. This study investigated whether mitochondrial dysfunction (MtD) is involved in the pathogenesis of EMT and whether peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), a major regulator of oxidative metabolism and mitochondrial function, prevents EMT by improving MtD. Aldo decreased PGC-1α expression while increasing its acetylation and induced MtD, as evidenced by oxidative stress, mitochondrial membrane potential collapse, mitochondrial DNA damage, and mitochondrial complex activity reduction. Aldo time-dependently induced p66Shc phosphorylation and expression. Mineralocorticoid receptor antagonist eplerenone and p66Shc short interfering RNA prevented Aldo-induced MtD and EMT, as evidenced by downregulation of α-smooth muscle actin and upregulation of E-cadherin. Mitochondrial DNA depletion by ethidium bromide or mitochondrial transcription factor A inhibitory RNA (RNAi) induced MtD, further promoting EMT. RNAi-mediated suppression of PGC-1α induced MtD and EMT, whereas overexpression of PGC-1α prevented Aldo-induced MtD and inhibited EMT. Similarly, overexpression of silent mating type information regulation 2 homolog 1 (SIRT1), a gene upstream of PGC-1α, or the SIRT1 activator resveratrol restored Aldo-induced MtD and EMT by upregulating PGC-1α. These findings, which implicate a role for MtD in EMT and suggest that SIRT1 and PGC-1α coordinate to improve mitochondrial function and EMT, may guide us in therapeutic strategies for renal tubulointerstitial fibrosis.

Mitochondrial dysfunction is an early event in aldosterone-induced podocyte injury

We previously showed that mitochondrial dysfunction (MtD) is involved in an aldosterone (Aldo)-induced podocyte injury. Here, the potential role of MtD in the initiation of podocyte damage was investigated. We detected the dynamic changes of urinary protein, urinary F2-isoprostane and renal malondialdehyde levels, kidney ultrastructure morphology, mitochondrial DNA (mtDNA) copy number, mitochondrial membrane potential (ΔΨm), and nephrin and podocin expressions in Aldo-infused mice. Aldo infusion first induced renal oxidative stress, as evidenced by increased levels of urinary F2-isoprostane and renal malondialdehyde, and MtD, as demonstrated by reduced mtDNA, ΔΨm, and ATP production. Later, at 5 days after Aldo infusion, proteinuria and podocyte injury began to appear. In cultured podocytes, Aldo or hydrogen peroxide (H2O2) induced MtD after 2-8 h of treatment, whereas the podocyte damage, as shown by decreased nephrin and podocin expressions, occurred later after 12 h of treatment. Thus Aldo treatment both in vitro and in vivo indicated that MtD occurred before podocyte damage. Additionally, MtDNA depletion by ethidium bromide or mitochondrial transcription factor A (TFAM) RNAi induced MtD, further promoting podocyte damage. TFAM expression was found to be reduced in Aldo-infused mice and Aldo-treated podocytes. Adenoviral vector-mediated overexpression of TFAM prevented Aldo-induced MtD and protected against podocyte injury. Together, these findings support MtD as an early event in podocyte injury, and manipulation of TFAM may be a novel strategy for treatment of glomerular diseases such as podocytopathy.

The roles of oxidative stress, endoplasmic reticulum stress, and autophagy in aldosterone/mineralocorticoid receptor-induced podocyte injury.

Podocytes play an important role in the pathogenesis and progression of glomerulosclerosis. Recent studies indicate that aldosterone/mineralocorticoid receptor (MR) is a major contributor of chronic kidney disease (CKD) progression. Aldosterone/MR induces glomerular podocyte injury, causing the disruption of the glomerular filtration barrier and proteinuria. The present study investigated the mechanisms by which aldosterone/MR mediated podocyte injury, focusing on the involvement of oxidative stress, endoplasmic reticulum (ER) stress, and autophagy. We observed that aldosterone/MR induced ER stress and podocyte injury both in vivo and in vitro. Blockade of ER stress significantly reduced aldosterone/MR-induced podocyte injury. In addition, we found that ER stress-induced podocyte injury was mediated by CCAAT/enhancer-binding protein (C/EBP) homologous protein (Chop). Interestingly, autophagy was also enhanced by aldosterone/MR. Pharmacological inhibition of autophagy resulted in increased apoptosis. Inhibition of ER stress significantly reduced aldosterone/MR-induced autophagy. In addition, the activation of ER stress increased the formation of autophagy, which protected podocytes from apoptosis. Moreover, we observed that the addition of ROS scavenger, N-acetyl cystein (NAC), blocked both ER stress and autophagy by aldosterone/MR. Collectively, these results suggest that oxidant stress-mediated aldosterone/MR-induced podocyte injury via activating ER stress, which then triggers both Chop-dependent apoptosis and autophagy to cope with the injury. These findings may guide us to therapeutic strategies for glomerular diseases.

A PPARγ agonist inhibits aldosterone-induced mesangial cell proliferation by blocking ROS-dependent EGFR intracellular signaling

Mesangial cell (MC) proliferation is a key feature in the pathogenesis of a number of renal diseases. Peroxisome proliferator-activated receptor-γ (PPARγ) has attracted considerable attention for its effects on stimulating cell differentiation and on inducing cell cycle arrest. We previously showed that aldosterone (Aldo) stimulates MC proliferation via the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway, which was dependent on reactive oxygen species (ROS)-mediated epithelial growth factor receptor (EGFR) transactivation (Huang S, Zhang A, Ding G, and Chen R. Am J Physiol Renal Physiol 296: F1323-F1333, 2009). In this study, we examined whether the PPARγ agonist rosiglitazone inhibited Aldo-induced MC proliferation by modulating ROS-dependent EGFR intracellular signaling. Rosiglitazone at 1-10 μM dose dependently inhibited Aldo-induced MC proliferation of cultured mouse MCs. The inhibitory effect was blocked by the PPARγ antagonist PD-68235, indicating that the rosiglitazone effect acted through PPARγ activation. Rosiglitazone also arrested Aldo-induced cell cycle progression and suppressed expression of cyclins D1 and A. Moreover, rosiglitazone dose dependently blocked Aldo-induced ROS production, EGFR phosphorylation, and PI3K/Akt activation. These results suggest that the PPARγ agonist rosiglitazone may inhibit Aldo-induced MC proliferation directly, by affecting ROS/EGFR/PI3K/Akt signaling pathways and cell cycle-regulatory proteins. PPARγ might be a novel therapeutic target against glomerular diseases.

Drp1-dependent mitophagy protects against cisplatin-induced apoptosis of renal tubular epithelial cells by improving mitochondrial function

Cisplatin chemotherapy often causes acute kidney injury (AKI) in cancer patients. There is increasing evidence that mitochondrial dysfunction plays an important role in cisplatin-induced nephrotoxicity. Degradation of damaged mitochondria is carried out by mitophagy. Although mitophagy is considered of particular importance in protecting against AKI, little is known of the precise role of mitophagy and its molecular mechanisms during cisplatin-induced nephrotoxicity. Also, evidence that activation of mitophagy improved mitochondrial function is lacking. Furthermore, several evidences have shown that mitochondrial fission coordinates with mitophagy. The aim of this study was to investigate whether activation of mitophagy protects against mitochondrial dysfunction and renal proximal tubular cells injury during cisplatin treatment. The effect of mitochondrial fission on mitophagy was also investigated. In cultured human renal proximal tubular cells, we observed that 3-methyladenine, a pharmacological inhibitor of autophagy, blocked mitophagy and exacerbated cisplatin-induced mitochondrial dysfunction and cells injury. In contrast, autophagy activator rapamycin enhanced mitophagy and protected against the harmful effects of cisplatin on mitochondrial function and cells viability. Suppression of mitochondrial fission by knockdown of its main regulator dynamin-related protein-1 (Drp1) decreased cisplatin-induced mitophagy. Meanwhile, Drp1 suppression protected against cisplatin-induced cells injury by inhibiting mitochondrial dysfunction. Our results provide evidence that Drp1-depedent mitophagy has potential as renoprotective targets for the treatment of cisplatin-induced AKI.

Pink1/Parkin-mediated mitophagy play a protective role in cisplatin induced renal tubular epithelial cells injury

ABSTRACT Cisplatin often causes acute kidney injury (AKI) in the treatment of a wide variety of malignancies. Mitochondrial dysfunction is one of the main reasons for cisplatin nephrotoxicity. Previous study showed that Pink1 and Parkin play central roles in regulating the mitophagy, which is a key protective mechanism by specifically eliminating dysfunctional or damaged mitochondria. However, the mechanisms that modulate mitophagy in cisplatin induced nephrotoxicity remain to be elucidated. The purpose of this study was to investigate the effects of Pink1/Parkin pathway in mitophagy, mitochondrial dysfunction and renal proximal tubular cells injury during cisplatin treatment. In cultured human renal proximal tubular cells, we found that knockdown of Pink1/Parkin induced the aggravation of mitochondrial function, leading to the increase of cell injury through inhibition of mitophagy. Additionally, the overexpression of Pink1/Parkin protected against cisplatin‐induced mitochondrial dysfunction and cell injury by promoting mitophagy. Our results provide clear evidence that Pink1/Parkin‐dependent mitophagy has identified potential targets for the treatment of cisplatin‐induced AKI.

Association of Increased Serum Leptin with Ameliorated Anemia and Malnutrition in Stage 5 Chronic Kidney Disease Patients after Parathyroidectomy

Leptin is an adipokine that regulates various metabolism, but its association with secondary hyperparathyroidism (SHPT), a clinical manifestation of chronic kidney disease-mineral and bone disorder (CKD-MBD), remains obscure. Parathyroidectomy (PTX) is recommended for severe SHPT patients. Here, the associations between circulating leptin and clinical characteristics in CKD patients were investigated. Effects of PTX on leptin production were analyzed in vivo and in vitro. Controls and CKD patients had approximate serum leptin levels in that a larger proportion of CKD patients with body mass index (BMI) <23 kg/m2. Serum leptin was related to anemia, albumin, and bone metabolism disorders in CKD patients. Lower intact parathyroid hormone (PTH) was related with higher leptin in PTX patients group. Severe SHPT inhibited uremia-enhanced leptin production in 3T3-L1 adipocytes, which was attenuated after PTX. High levels of PTH were found to reduce Akt phosphorylation and leptin production in vitro but high levels of calcium and phosphorus were not. Successful PTX was found to improve anemia and malnutrition in severe SHPT patients, and this was correlated with increased circulating leptin levels via up-regulated Akt signaling in adipocytes. These findings indicated the therapeutic potential of leptin and related target pathway for improving survival and quality of life in CKD.