Preischemic targeting of HIF prolyl hydroxylation inhibits fibrosis associated with acute kidney injury

Acute kidney injury (AKI) due to ischemia is an important contributor to the progression of chronic kidney disease (CKD). Key mediators of cellular adaptation to hypoxia are oxygen-sensitive hypoxia-inducible factors (HIF), which are regulated by prolyl-4-hydroxylase domain (PHD)-containing dioxygenases. While activation of HIF protects from ischemic cell death, HIF has been shown to promote fibrosis in experimental models of CKD. The impact of HIF activation on AKI-induced fibrosis has not been defined. Here, we investigated the role of pharmacologic HIF activation in AKI-associated fibrosis and inflammation. We found that pharmacologic inhibition of HIF prolyl hydroxylation before AKI ameliorated fibrosis and prevented anemia, while inhibition of HIF prolyl hydroxylation in the early recovery phase of AKI did not affect short- or long-term clinical outcome. Therefore, preischemic targeting of the PHD/HIF pathway represents an effective therapeutic strategy for the prevention of CKD resulting from AKI, and it warrants further investigation in clinical trials.     

Targeting ferroptosis in acute kidney injury

Acute kidney injury (AKI) is a major public health problem with high incidence and mortality. As a form of programmed cell death (PCD), ferroptosis could be considered as a process of iron accumulation and enhanced lipid peroxidation. Recently, the fundamental roles of ferroptosis in AKI have attracted much attention. The network mechanism of ferroptosis in AKI and its roles in the AKI to chronic kidney disease (CKD) transition is complicated and multifactorial. Strategies targeting ferroptosis show great potential. Here, we review the research progress on ferroptosis and its participation in AKI. We hope that this work will provide clues for further studies of ferroptosis in AKI.Subject terms: Acute kidney injury, Cell death     

Augmenter of liver regeneration protects the kidney from ischaemia‐reperfusion injury in ferroptosis

Acute kidney injury (AKI) is a common and severe clinical condition with high morbidity and mortality. Ischaemia‐reperfusion (I/R) injury remains the major cause of AKI in the clinic. Ferroptosis is a recently discovered form of programmed cell death (PCD) that is characterized by iron‐dependent accumulation of reactive oxygen species (ROS). Compelling evidence has shown that renal tubular cell death involves ferroptosis, although the underlying mechanisms remain unclear. Augmenter of liver regeneration (ALR) is a widely distributed multifunctional protein that is expressed in many tissues. Our previous study demonstrated that ALR possesses an anti‐oxidant function. However, the modulatory mechanism of ALR remains unclear and warrants further investigation. Here, to elucidate the role of ALR in ferroptosis, ALR expression was inhibited using short hairpin RNA lentivirals (shRNA) in vitro model of I/R‐induced AKI. The results suggest that the level of ferroptosis is increased, particularly in the shRNA/ALR group, accompanied by increased ROS and mitochondrial damage. Furthermore, inhibition of system xc‐ with erastin aggravates ferroptosis, particularly silencing of the expression of ALR. Unexpectedly, we demonstrate a novel signalling pathway of ferroptosis. In summary, we show for the first time that silencing ALR aggravates ferroptosis in an in vitro model of I/R. Notably, we show that I/R induced kidney ferroptosis is mediated by ALR, which is linked to the glutathione‐glutathione peroxidase (GSH‐GPx) system.     

Prolyl hydroxylase domain enzymes and their role in cell signaling and cancer metabolism

The prolyl hydroxylase domain (PHD) enzymes regulate the stability of the hypoxia-inducible factor (HIF) in response to oxygen availability. During oxygen limitation, the inhibition of PHD permits the stabilization of HIF, allowing the cellular adaptation to hypoxia. This adaptation is especially important for solid tumors, which are often exposed to a hypoxic environment. However, and despite their original role as the oxygen sensors of the cell, PHD are currently known to display HIF-independent and hydroxylase-independent functions in the control of different cellular pathways, including mTOR pathway, NF-kB pathway, apoptosis and cellular metabolism. In this review, we summarize the recent advances in the regulation and functions of PHD in cancer signaling and cell metabolism.     

The HIF signaling pathway in osteoblasts directly modulates erythropoiesis through the production of EPO

Osteoblasts are an important component of the hematopoietic microenvironment in bone. However, the mechanisms by which osteoblasts control hematopoiesis remain unknown. We show that augmented HIF signaling in osteoprogenitors results in HSC niche expansion associated with selective expansion of the erythroid lineage. Increased red blood cell production occurred in an EPO-dependent manner with increased EPO expression in bone and suppressed EPO expression in the kidney. In contrast, inactivation of HIF in osteoprogenitors reduced EPO expression in bone. Importantly, augmented HIF activity in osteoprogenitors protected mice from stress-induced anemia. Pharmacologic or genetic inhibition of prolyl hydroxylases1/2/3 in osteoprogenitors elevated EPO expression in bone and increased hematocrit. These data reveal an unexpected role for osteoblasts in the production of EPO and modulation of erythropoiesis. Furthermore, these studies demonstrate a molecular role for osteoblastic PHD/VHL/HIF signaling that can be targeted to elevate both HSCs and erythroid progenitors in the local hematopoietic microenvironment.     

胞红蛋白在调控细胞铁死亡中的作用

近年来,以细胞内氧化还原平衡失调为重要诱因,具有铁依赖性和以脂质过氧化物堆积引起细胞膜损伤为主要特征的细胞铁死亡备受关注。越来越多的研究表明,细胞铁死亡在疾病发生及防治方面具有重要作用。胞红蛋白(cytoglobin,CYGB),又名星状细胞激活蛋白 (stellate cell activating protein, STAP),是一种珠蛋白,不仅能可逆地结合氧分子,储存和传递氧气,同时在其氨基酸序列中含2个半胱氨酸残基,可形成分子内部的二硫键,在感受细胞内氧化还原状态变动时,改变自身空间结构,引起生物活性及下游信号通路的变化。同时,CYGB还具有一氧化氮双加氧酶活性,能够清除过量一氧化氮与活性氧物质超氧阴离子反应生成的有毒ONOO^-,防止其对线粒体功能的破坏。而细胞内活氧物质和线粒体是影响细胞铁死亡的重要因素。因此,本综述主要围绕CYGB清除活性氧物质及调控一氧化氮代谢等的作用机制,并结合我们最近有关CYGB通过p53-YAP1轴调控细胞内脂质代谢的研究进行阐述,提出CYGB通过参与细胞铁死亡调控来行使功能,为心血管功能,肝纤维化及癌症发生等相关疾病的预防和治疗提供重要的理论依据。     

Cofilin1 oxidation links oxidative distress to mitochondrial demise and neuronal cell death

Many cell death pathways, including apoptosis, regulated necrosis, and ferroptosis, are relevant for neuronal cell death and share common mechanisms such as the formation of reactive oxygen species (ROS) and mitochondrial damage. Here, we present the role of the actin-regulating protein cofilin1 in regulating mitochondrial pathways in oxidative neuronal death. Cofilin1 deletion in neuronal HT22 cells exerted increased mitochondrial resilience, assessed by quantification of mitochondrial ROS production, mitochondrial membrane potential, and ATP levels. Further, cofilin1-deficient cells met their energy demand through enhanced glycolysis, whereas control cells were metabolically impaired when challenged by ferroptosis. Further, cofilin1 was confirmed as a key player in glutamate-mediated excitotoxicity and associated mitochondrial damage in primary cortical neurons. Using isolated mitochondria and recombinant cofilin1, we provide a further link to toxicity-related mitochondrial impairment mediated by oxidized cofilin1. Our data revealed that the detrimental impact of cofilin1 on mitochondria depends on the oxidation of cysteine residues at positions 139 and 147. Overall, our findings show that cofilin1 acts as a redox sensor in oxidative cell death pathways of ferroptosis, and also promotes glutamate excitotoxicity. Protective effects by cofilin1 inhibition are particularly attributed to preserved mitochondrial integrity and function. Thus, interfering with the oxidation and pathological activation of cofilin1 may offer an effective therapeutic strategy in neurodegenerative diseases.Subject terms: Apoptosis, Cell death in the nervous system, Neurodegeneration     

Inhibition of a prolyl hydroxylase domain (PHD) by substrate analog peptides

Oxygen dependent degradation of hypoxia-inducible factor (HIF)-1α is triggered with hydroxylation by proline hydroxylase domain 2 (PHD2) under normoxic conditions. Some of previously developed PHD2 inhibitors show a considerable potency against factor inhibiting HIF (FIH), the HIF asparagine hydroxylase. For specific inhibition of PHD2, we have synthesized peptides containing 556-575 residues of HIF-1α with modifications at the Pro-564 and examined their inhibitory effect against PHD2. Adopting fluorescence polarization-based assays, we evaluated inhibitory potency of the peptides and selected potent inhibitors. These PHD2 inhibitor peptides showed no significant potency against FIH, demonstrating their specific inhibitory effect on PHD2.     

Prolyl Hydroxylase 3 (PHD3) Modulates Catabolic Effects of Tumor Necrosis Factor-α (TNF-α) on Cells of the Nucleus Pulposus through Co-activation of Nuclear Factor κB (NF-κB)/p65 Signaling

Recent studies suggest a differential role of prolyl hydroxylase (PHD) isoforms in controlling hypoxia-inducible factor (HIF)-α degradation and activity in nucleus pulposus (NP) cells. However, the regulation and function of PHDs under inflammatory conditions that characterize disc disease are not yet known. Here, we show that in NP cells, TNF-α and IL-1β induce PHD3 expression through NF-κB. Lentiviral delivery of Sh-p65 and Sh-IKKβ confirms that cytokine-mediated PHD3 expression is NF-κB-dependent. It is noteworthy that although both cytokines induce HIF activity, mechanistic studies using Sh-HIF-1α and PHD3 promoter/enhancer constructs harboring well characterized hypoxia response element (HRE) show lack of HIF involvement in cytokine-mediated PHD3 expression. Loss-of-function studies clearly indicate that PHD3 serves as a co-activator of NF-κB signaling activity in NP cells; PHD3 interacts with, and co-localizes with, p65. We observed that when PHD3 is silenced, there is a significant decrease in TNF-α-induced expression of catabolic markers that include ADAMTS5, syndecan4, MMP13, and COX2, and at the same time, there is restoration of aggrecan and collagen type II expression. It is noteworthy that hydroxylase function of PHDs is not required for mediating cytokine-dependent gene expression. These findings show that by enhancing the activity of inflammatory cytokines, PHD3 may serve a critical role in degenerative disc disease.     

Lipid storage and lipophagy regulates ferroptosis

The synthesis, storage, and degradation of lipids are highly regulated processes. Impaired lipid metabolism is implicated in inflammation and cell death. Although ferroptosis is a recently described form of regulated cell death driven by lipid peroxidation, the impact of lipid droplets on ferroptosis remains unidentified. Here, we demonstrate that lipophagy, the autophagic degradation of intracellular lipid droplets, promotes RSL3-induced ferroptotic cell death in hepatocytes. Lipid droplet accumulation is increased at the early stage but decreased at the late stage of ferroptosis in mouse or human hepatocytes. Importantly, either genetically enhancing TPD52-dependent lipid storage or blocking ATG5-and RAB7A-dependent lipid degradation prevents RSL3-induced lipid peroxidation and subsequent ferroptosis in vitro and in vivo. These studies support an antioxidant role for lipid droplets in cell death and suggest novel strategies for the inhibition of ferroptosis by targeting the lipophagy pathway.     

The role of VDAC in cell death: Friend or foe?

As the voltage-dependent anion channel (VDAC) forms the interface between mitochondria and the cytosol, its importance in metabolism is well understood. However, research on VDAC's role in cell death is a rapidly growing field, unfortunately with much confusing and contradictory results. The fact that VDAC plays a role in outer mitochondrial membrane permeabilization is undeniable, however, the mechanisms behind this remain very poorly understood. In this review, we will summarize the studies that show evidence of VDAC playing a role in cell death. To begin, we will discuss the evidence for and against VDAC's involvement in mitochondrial permeability transition (MPT) and attempt to clarify that VDAC is not an essential component of the MPT pore (MPTP). Next, we will evaluate the remaining literature on VDAC in cell death which can be divided into three models: proapoptotic agents escaping through VDAC, VDAC homo- or hetero-oligomerization, or VDAC closure resulting in outer mitochondrial membrane permeabilization through an unknown pathway. We will then discuss the growing list of modulators of VDAC activity that have been associated with induction/protection against cell death. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.     

Double-edge sword roles of iron in driving energy production versus instigating ferroptosis

Iron is vital for many physiological functions, including energy production, and dysregulated iron homeostasis underlies a number of pathologies. Ferroptosis is a recently recognized form of regulated cell death that is characterized by iron dependency and lipid peroxidation, and this process has been reported to be involved in multiple diseases. The mechanisms underlying ferroptosis are complex, and involve both well-described pathways (including the iron-induced Fenton reaction, impaired antioxidant capacity, and mitochondrial dysfunction) and novel interactions linked to cellular energy production. In this review, we examine the contribution of iron to diverse metabolic activities and their relationship to ferroptosis. There is an emphasis on the role of iron in driving energy production and its link to ferroptosis under both physiological and pathological conditions. In conclusion, excess reactive oxygen species production driven by disordered iron metabolism, which induces Fenton reaction and/or impairs mitochondrial function and energy metabolism, is a key inducer of ferroptosis.Subject terms: Cell biology, Biochemistry