IHG-1 must be localised to mitochondria to decrease Smad7 expression and amplify TGF-β1-induced fibrotic responses

RCAN1.4 mediates high glucose-induced matrix production by stimulating mitochondrial fission in mesangial cells

High glucose (HG)-induced mitochondrial dynamic changes and oxidative damage are closely related to the development and progression of diabetic kidney disease (DKD). Recent studies suggest that regulators of calcineurin 1 (RCAN1) is involved in the regulation of mitochondrial function in different cell types, so we investigate the role of RCAN1 in mitochondrial dynamics under HG ambience in rat glomerular mesangial cells (MCs). MCs subjected to HG exhibited an isoform-specific up-regulation of RCAN1.4 at both mRNA and protein levels. RCAN1.4 overexpression induced translocation of Dynamin related protein 1 (Drp1) to mitochondria, mitochondrial fragmentation and depolarization, accompanied by increased matrix production under normal glucose and HG ambience. In contrast, decreasing the expression of RCAN1.4 by siRNA inhibited HG-induced mitochondrial fragmentation and matrix protein up-regulation. Moreover, both mitochondrial fission inhibitor Mdivi-1 and Drp1 shRNA prevented RCAN1.4-induced fibronectin up-regulation, suggesting that RCAN1.4-induced matrix production is dependent on its modulation of mitochondrial fission. Although HG-induced RCAN1.4 up-regulation was achieved by activating calcineurin, RCAN1.4-mediated mitochondrial fragmentation and matrix production is independent of calcineurin activity. These results provide the first evidence for the HG-induced RCAN1.4 up-regulation involving increased mitochondrial fragmentation, leading to matrix protein up-regulation.

Pioglitazone Improves Mitochondrial Function in the Remnant Kidney and Protects against Renal Fibrosis in 5/6 Nephrectomized Rats

Pioglitazone is a type of peroxisome proliferator-activated receptor γ (PPARγ) agonist and has been demonstrated to be effective in chronic kidney diseases (CKD) treatment. However, the underlying mechanism involved in the renoprotection of pioglitazone has not been fully revealed. In the present study, the renoprotective mechanism of pioglitazone was investigated in 5/6 nephrectomized (Nx) rats and TGF-β1-exposed HK-2 cells. Pioglitazone attenuated renal injury and improved renal function, as examined by 24 h urinary protein, blood urea nitrogen and plasma creatinine in Nx rats. Renal fibrosis and enhanced expressions of profibrotic proteins TGF-β1, fibronectin and collagen I caused by Nx were significantly alleviated by pioglitazone. In addition, pioglitazone protected mitochondrial functions by stabilizing the mitochondrial membrane potential, inhibiting ROS generation, maintaining ATP production and the activities of complexes I and III, and preventing cytochrome C leakage from mitochondria. Pioglitazone also upregulated the expression levels of ATP synthase β, COX I and NDUFB8, which were downregulated in the kidney of Nx rats and TGF-β1-exposed HK-2 cells. Furthermore, pioglitazone increased fusion proteins Opa-1 and Mfn2 expressions and decreased fission protein Drp1 expression. The results imply that pioglitazone may exert the renoprotective effects through modulating mitochondrial electron transport chain and mitochondrial dynamics in CKD. Finally, these recoveries were completely or partly inhibited by GW9662, which suggests that these effects at least partly PPARγ dependent. This study provides evidence for the pharmacological mechanism of pioglitazone in the treatment of CKD.

Profibrotic IHG-1 complexes with renal disease associated HSPA5 and TRAP1 in mitochondria

The highly conserved mitochondrial protein induced in high glucose-1 (IHG-1) functions to maintain mitochondrial quality and is associated with the development of fibrosis in diabetic nephropathy. Towards identifying novel approaches to treating diabetic kidney disease, IHG-1-protein-protein interactions were investigated using epitope-tagged immunoprecipitation analyses followed by mass spectrometry. Here we show that IHG-1 is solely expressed in mitochondria and localised to the inner mitochondrial membrane, the region where mitochondrial reactive oxygen species are generated. Chaperones HSPA5 and TRAP1 and cold shock protein YBX1 were identified as IHG-1 binding partners. All three proteins are important in the cellular response to oxidative stress and play important roles in mitochondrial transcription and DNA repair. Both redox imbalance and IHG-1 stimulate TGF-β signalling. IHG-1, HSPA5 and YBX1 all show increased expression in diabetic nephropathy, chronic kidney disease and in the Unilateral Ureteral Obstruction model of kidney fibrosis. Increased IHG-1 expression in UUO correlated with loss of TRAP1 expression. IHG-1 may target TRAP1 for degradation. When IHG-1 is no longer localised to mitochondria, it retains the ability to interact with the cold shock protein YBX1, facilitating anti-fibrotic actions in the nucleus. Targeting these proteins may offer alternative treatments for fibrotic kidney disease.

Liquiritigenin attenuates cardiac injury induced by high fructose-feeding through fibrosis and inflammation suppression

Diabetes combined with cardiomyopathy is considered as an essential complication, showing diastolic persistently and causing cardiac injury, which is linked to fibrosis progression and inflammation response. Fibrosis and inflammation response are two markers for cardiomyopathy. Liquiritigenin is a flavanone, isolated from Radix glycyrrhiza, which exhibits various biological properties, including anti-cancer and anti-inflammatory activities. Here, in our study, the protective effects and anti-inflammatory activity of liquiritigenin were explored in mice and cardiac muscle cells treated by fructose to reveal the possible mechanism by which liquiritigenin attenuates cardiac injury. The mice were separated into five groups. The diabetic model of mouse was established with 30% high fructose feeding. Liquiritigenin dramatically reduced the lipid accumulation induced by high fructose diet. Compared to mice only treated with high fructose, mice in the presence of liquiritigenin after fructose feeding developed less cardiac fibrosis with lower levels of alpha smooth muscle-actin (α-SMA), Collagen type I, Collagen type II, TGF-β1 and Procol1a1. Additionally, liquiritigenin markedly down-regulated inflammatory cytokines secretion and phosphorylated NF-κB via inhibiting IKKα/IκBα signaling pathway. Our results indicate that liquiritigenin has a protective role in high fructose feeding-triggered cardiac injury through fibrosis and inflammation response suppression by inactivating NF-κB signaling pathway. Thus, liquiritigenin may be a potential candidate for diabetes-associated cardiac injury.

Identification of distinct biological functions for four 3'-5' RNA polymerases

The superfamily of 3'-5' polymerases synthesize RNA in the opposite direction to all other DNA/RNA polymerases, and its members include eukaryotic tRNA(His) guanylyltransferase (Thg1), as well as Thg1-like proteins (TLPs) of unknown function that are broadly distributed, with family members in all three domains of life. Dictyostelium discoideum encodes one Thg1 and three TLPs (DdiTLP2, DdiTLP3 and DdiTLP4). Here, we demonstrate that depletion of each of the genes results in a significant growth defect, and that each protein catalyzes a unique biological reaction, taking advantage of specialized biochemical properties. DdiTLP2 catalyzes a mitochondria-specific tRNA(His) maturation reaction, which is distinct from the tRNA(His) maturation reaction typically catalyzed by Thg1 enzymes on cytosolic tRNA. DdiTLP3 catalyzes tRNA repair during mitochondrial tRNA 5'-editing in vivo and in vitro, establishing template-dependent 3'-5' polymerase activity of TLPs as a bona fide biological activity for the first time since its unexpected discovery more than a decade ago. DdiTLP4 is cytosolic and, surprisingly, catalyzes robust 3'-5' polymerase activity on non-tRNA substrates, strongly implying further roles for TLP 3'-5' polymerases in eukaryotes.

Possible role of mitochondrial injury in Caulis Aristolochia manshuriensis-induced chronic aristolochic acid nephropathy

CONTEXT

The proximal tubular epithelial cells (PTECs) are the primary target of aristolochic acids and especially vulnerable to mitochondrial injury from insults of toxic xenobiotics.

OBJECTIVES

This study aimed to investigate the possible role of mitochondrial injury in Caulis Aristolochia manshuriensis (CAM)-induced aristolochic acid nephropathy (AAN).

MATERIALS AND METHODS

Male Sprague-Dawley rats were gavaged with CAM extract every other week for 1, 4, 8 and 12 weeks, respectively.

RESULTS

The rats in the model group showed chronic AAN as evidenced by worsening kidney function evaluated by blood urea nitrogen, creatinine and proteinuria levels, and severe tubulointerstitial injury marked by massive tubular atrophy and interstitial fibrosis in kidney tissues. Moreover, overt apoptosis and impaired regeneration of PTECs were observed in AAN rats. Furthermore, the study revealed that mitochondria in PTECs were fragmented into small, punctuate suborganelles in AAN rats. Two mitochondrial respiratory chain proteins, mitochondrial DNA (mtDNA)-encoded cytochrome c oxidase subunit І (COX-І) and nuclear DNA-encoded nicotinamide adenine dinucleotide dehydrogenase (ubiquinone)-1β subcomplex 8 (NDUFβ8), were both down-regulated after one week of CAM treatment. However, with AAN progression, NDUFβ8 level restored, while COX-І level maintained low. Peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), master regulator of mitochondrial biogenesis, was significantly down-regulated at week 4 and week 8, but significantly up-regulated at week 12. In addition, mtDNA copy number reduced markedly along with AAN progression.

DISCUSSION AND CONCLUSION

A rat model of chronic AAN was successfully reproduced by gavage with CAM extract. Dynamic changes of mitochondrial injury induced by CAM might contribute to the AAN progression.

Reciprocal regulation of TGF-β and reactive oxygen species: A perverse cycle for fibrosis

Transforming growth factor beta (TGF-β) is the most potent pro-fibrogenic cytokine and its expression is increased in almost all of fibrotic diseases. Although signaling through Smad pathway is believed to play a central role in TGF-β's fibrogenesis, emerging evidence indicates that reactive oxygen species (ROS) modulate TGF-β's signaling through different pathways including Smad pathway. TGF-β1 increases ROS production and suppresses antioxidant enzymes, leading to a redox imbalance. ROS, in turn, induce/activate TGF-β1 and mediate many of TGF-β's fibrogenic effects, forming a vicious cycle (see graphic flow chart on the right). Here, we review the current knowledge on the feed-forward mechanisms between TGF-β1 and ROS in the development of fibrosis. Therapeutics targeting TGF-β-induced and ROS-dependent cellular signaling represents a novel approach in the treatment of fibrotic disorders.

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TGF-β1 is the most potent ubiquitous profibrogenic cytokine.

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TGF- β 1 induces redox imbalance by ↑ ROS production and ↓ anti-oxidant defense system

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Redox imbalance, in turn, activates latent TGF-β1 and induces TGF-β1 expression.

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Redox imbalance also mediates many of TGF-β1’s profibrogenic effects

IHG-1 increases mitochondrial fusion and bioenergetic function

Induced in high glucose-1 (IHG-1) is a conserved mitochondrial protein associated with diabetic nephropathy (DN) that amplifies profibrotic transforming growth factor (TGF)-β1 signaling and increases mitochondrial biogenesis. Here we report that inhibition of endogenous IHG-1 expression results in reduced mitochondrial respiratory capacity, ATP production, and mitochondrial fusion. Conversely, overexpression of IHG-1 leads to increased mitochondrial fusion and also protects cells from reactive oxygen species-induced apoptosis. IHG-1 forms complexes with known mediators of mitochondrial fusion-mitofusins (Mfns) 1 and 2-and enhances the GTP-binding capacity of Mfn2, suggesting that IHG-1 acts as a guanine nucleotide exchange factor. IHG-1 must be localized to mitochondria to interact with Mfn1 and Mfn2, and this interaction is necessary for increased IHG-1-mediated mitochondrial fusion. Together, these findings indicate that IHG-1 is a novel regulator of both mitochondrial dynamics and bioenergetic function and contributes to cell survival following oxidant stress. We propose that in diabetic kidney disease increased IHG-1 expression protects cell viability and enhances the actions of TGF-β, leading to renal proximal tubule dedifferentiation, an important event in the pathogenesis of this devastating condition.

Targeting cellular drivers and counter-regulators of hyperglycaemia- and transforming growth factor-β1-associated profibrotic responses in diabetic kidney disease

Diabetic kidney disease occurs in >30% of patients with type 2 diabetes mellitus and is characterized at source by a maladaptive response in the renal parenchyma to exposure to a glucotoxic-lipotoxic diabetic milieu that courses coincident with hypertension. The consequence of these maladaptive responses is progressive renal injury, which is exacerbated by the development of a chronic inflammatory infiltrate associated with the development of tubulointerstitial fibrosis. The evolution of tubulointerstitial fibrosis is correlated with the loss of functional renal mass and descent towards renal failure. Transforming growth factor-β1 (TGF-β1) is a recognized mediator of the profibrotic response of mesangial cells and renal tubular epithelial cells to hyperglycaemia. While euglycaemia remains the goal in the treatment of type 2 diabetes mellitus, the prevention, arrest and reversal of microvascular complications, such as diabetic kidney disease, may be assisted by pharmacological modulation of the effectors of glucotoxicity, such as TGF-β1. This review focuses on describing how, through reductionist in vitro experimentation focusing on TGF-β1-related responses to hyperglycaemia, we have identified induced in high glucose-1 (IHG-1), induced in high glucose-2 (IHG-2/Grem1) and the lipoxin-inducible microRNA let-7c as potential targets for harnessing new therapeutic approaches to limit the bioactivity of TGF-β1 in diabetic kidney disease.

SMAD7: a timer of tumor progression targeting TGF-β signaling

In the context of cancer, transforming growth factor β (TGF-β) is a cell growth suppressor; however, it is also a critical inducer of invasion and metastasis. SMAD is the important mediator of TGF-β signaling pathway, which includes receptor-regulated SMADs (R-SMADs), common-mediator SMADs (co-SMADs), and inhibitory SMADs (I-SMADs). I-SMADs block the activation of R-SMADs and co-SMADs and thus play important roles especially in the SMAD-dependent signaling. SMAD7 belongs to the I-SMADs. As an inhibitor of TGF-β signaling, SMAD7 is overexpressed in numerous cancer types and its abundance is positively correlated to the malignancy. Emerging evidence has revealed the switch-in-role of SMAD7 in cancer, from a TGF-β inhibiting protein at the early stages that facilitates proliferation to an enhancer of invasion at the late stages. This role change may be accompanied or elicited by the tumor microenvironment and/or somatic mutation. Hence, current knowledge suggests a tumor-favorable timer nature of SMAD7 in cancer progression. In this review, we summarized the advances and recent findings of SMAD7 and TGF-β signaling in cancer, followed by specific discussion on the possible factors that account for the functional changes of SMAD7.

Mitochondrial dysfunction and mitophagy: the beginning and end to diabetic nephropathy?

Diabetic nephropathy (DN) is a progressive microvascular complication arising from diabetes. Within the kidney, the glomeruli, tubules, vessels and interstitium are disrupted, ultimately impairing renal function and leading to end-stage renal disease (ESRD). Current pharmacological therapies used in individuals with DN do not prevent the inevitable progression to ESRD; therefore, new targets of therapy are urgently required. Studies from animal models indicate that disturbances in mitochondrial homeostasis are central to the pathogenesis of DN. Since renal proximal tubule cells rely on oxidative phosphorylation to provide adequate ATP for tubular reabsorption, an impairment of mitochondrial bioenergetics can result in renal functional decline. Defects at the level of the electron transport chain have long been established in DN, promoting electron leakage and formation of superoxide radicals, mediating microinflammation and contributing to the renal lesion. More recent studies suggest that mitochondrial-associated proteins may be directly involved in the pathogenesis of tubulointerstitial fibrosis and glomerulosclerosis. An accumulation of fragmented mitochondria are found in the renal cortex in both humans and animals with DN, suggesting that in tandem with a shift in dynamics, mitochondrial clearance mechanisms may be impaired. The process of mitophagy is the selective targeting of damaged or dysfunctional mitochondria to autophagosomes for degradation through the autophagy pathway. The current review explores the concept that an impairment in the mitophagy system leads to the accelerated progression of renal pathology. A better understanding of the cellular and molecular events that govern mitophagy and dynamics in DN may lead to improved therapeutic strategies.

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.

A potential wound healing-promoting peptide from frog skin

Cutaneous wound healing is a dynamic, complex, and well-organized process that requires the orchestration of many different cell types and cellular processes. Transforming growth factor β1 is an important factor that plays a key role during wound healing. Amphibian skin has been proven to possess excellent wound healing ability, whilst no bioactive substrate related to it has ever been identified. Here, a potential wound healing-promoting peptide (AH90, ATAWDFGPHGLLPIRPIRIRPLCG) was identified from the frog skin of Odorrana grahami. It showed potential wound healing-promoting activity in a murine model with full thickness dermal wound. AH90 promoted release of transforming growth factor β1 through activation of nuclear factor-κB and c-Jun NH2-terminal kinase mitogen-activated protein kinases signaling pathways, while inhibitors of nuclear factor-κB and c-Jun NH2-terminal kinase inhibited the process. In addition, the effects of AH90 on Smads family proteins, key regulators in transforming growth factor β1 signaling pathways, could also be inhibited by transforming growth factor β1 antibody. Altogether, this indicated that AH90 promoted wound healing by inducing the release of transforming growth factor β1. This current study may facilitate the understanding of effective factors involved in the wound repair of amphibians and the underlying mechanisms as well. Considering its favorable traits as a small peptide that greatly promoting generation of endogenous wound healing agents (transforming growth factor β1) without mitogenic effects, AH90 might be an excellent template for the future development of novel wound-healing agents.

Improved mitochondrial function underlies the protective effect of pirfenidone against tubulointerstitial fibrosis in 5/6 nephrectomized rats

Dysfunctional mitochondria participate in the progression of chronic kidney disease (CKD). Pirfenidone is a newly identified anti-fibrotic drug. However, its mechanism remains unclear. Mitochondrial dysfunction is an early event that occurs prior to the onset of renal fibrosis. In this context, we investigated the protective effect of pirfenidone on mitochondria and its relevance to apoptosis and oxidative stress in renal proximal tubular cells. A remnant kidney rat model was established. Human renal proximal tubular epithelial cells (HK2) using rotenone, a mitochondrial respiratory chain complex Ι inhibitor were further investigated in vitro to examine the mitochondrial protective effect of pirfenidone. Pirfenidone protected mitochondrial structures and functions by stabilizing the mitochondrial membrane potential, maintaining ATP production and improving the mitochondrial DNA (mtDNA) copy number. Pirfenidone decreased tubular cell apoptosis by inhibiting the mitochondrial apoptotic signaling pathway. Pirfenidone also reduced oxidative stress by enhancing manganese superoxide dismutase (Mn-SOD) and inhibiting intracellular reactive oxygen species (ROS) generation, which suggested that the anti-oxidant effects occurred at least partially via the mitochondrial pathway. Pirfenidone may be effective prior to the onset of renal fibrosis because this drug exerts its anti-fibrotic effect by protection of mitochondria in renal proximal tubular cells.