Heme Oxygenase-1 protects astroglia against manganese-induced oxidative injury by regulating mitochondrial quality control

Signaling Pathways Involved in Manganese-Induced Neurotoxicity

Manganese (Mn) is an essential trace element, but insufficient or excessive bodily amounts can induce neurotoxicity. Mn can directly increase neuronal insulin and activate insulin-like growth factor (IGF) receptors. As an important cofactor, Mn regulates signaling pathways involved in various enzymes. The IGF signaling pathway plays a protective role in the neurotoxicity of Mn, reducing apoptosis in neurons and motor deficits by regulating its downstream protein kinase B (Akt), mitogen-activated protein kinase (MAPK), and mammalian target of rapamycin (mTOR). In recent years, some new mechanisms related to neuroinflammation have been shown to also play an important role in Mn-induced neurotoxicity. For example, DNA-sensing receptor cyclic GMP-AMP synthase (cCAS) and its downstream signal efficient interferon gene stimulator (STING), NOD-like receptor family pyrin domain containing 3(NLRP3)-pro-caspase1, cleaves to the active form capase1 (CASP1), nuclear factor κB (NF-κB), sirtuin (SIRT), and Janus kinase (JAK) and signal transducers and activators of the transcription (STAT) signaling pathway. Moreover, autophagy, as an important downstream protein degradation pathway, determines the fate of neurons and is regulated by these upstream signals. Interestingly, the role of autophagy in Mn-induced neurotoxicity is bidirectional. This review summarizes the molecular signaling pathways of Mn-induced neurotoxicity, providing insight for further understanding of the mechanisms of Mn.

Mechanisms of manganese-induced neurotoxicity and the pursuit of neurotherapeutic strategies

Chronic exposure to elevated levels of manganese via occupational or environmental settings causes a neurological disorder known as manganism, resembling the symptoms of Parkinson's disease, such as motor deficits and cognitive impairment. Numerous studies have been conducted to characterize manganese's neurotoxicity mechanisms in search of effective therapeutics, including natural and synthetic compounds to treat manganese toxicity. Several potential molecular targets of manganese toxicity at the epigenetic and transcriptional levels have been identified recently, which may contribute to develop more precise and effective gene therapies. This review updates findings on manganese-induced neurotoxicity mechanisms on intracellular insults such as oxidative stress, inflammation, excitotoxicity, and mitophagy, as well as transcriptional dysregulations involving Yin Yang 1, RE1-silencing transcription factor, transcription factor EB, and nuclear factor erythroid 2-related factor 2 that could be targets of manganese neurotoxicity therapies. This review also features intracellular proteins such as PTEN-inducible kinase 1, parkin, sirtuins, leucine-rich repeat kinase 2, and α-synuclein, which are associated with manganese-induced dysregulation of autophagy/mitophagy. In addition, newer therapeutic approaches to treat manganese's neurotoxicity including natural and synthetic compounds modulating excitotoxicity, autophagy, and mitophagy, were reviewed. Taken together, in-depth mechanistic knowledge accompanied by advances in gene and drug delivery strategies will make significant progress in the development of reliable therapeutic interventions against manganese-induced neurotoxicity.

Loss of slc39a14 causes simultaneous manganese hypersensitivity and deficiency in zebrafish

Manganese neurotoxicity is a hallmark of hypermanganesemia with dystonia 2, an inherited manganese transporter defect caused by mutations in SLC39A14. To identify novel potential targets of manganese neurotoxicity, we performed transcriptome analysis of slc39a14-/- mutant zebrafish that were exposed to MnCl2. Differentially expressed genes mapped to the central nervous system and eye, and pathway analysis suggested that Ca2+ dyshomeostasis and activation of the unfolded protein response are key features of manganese neurotoxicity. Consistent with this interpretation, MnCl2 exposure led to decreased whole-animal Ca2+ levels, locomotor defects and changes in neuronal activity within the telencephalon and optic tectum. In accordance with reduced tectal activity, slc39a14-/- zebrafish showed changes in visual phototransduction gene expression, absence of visual background adaptation and a diminished optokinetic reflex. Finally, numerous differentially expressed genes in mutant larvae normalised upon MnCl2 treatment indicating that, in addition to neurotoxicity, manganese deficiency is present either subcellularly or in specific cells or tissues. Overall, we assembled a comprehensive set of genes that mediate manganese-systemic responses and found a highly correlated and modulated network associated with Ca2+ dyshomeostasis and cellular stress. This article has an associated First Person interview with the first author of the paper.

TLR4 Signaling and Heme Oxygenase-1/Carbon Monoxide Pathway Crosstalk Induces Resiliency of Myeloma Plasma Cells to Bortezomib Treatment

Relapse in multiple myeloma (MM) decreases therapy efficiency through unclear mechanisms of chemoresistance. Since our group previously demonstrated that heme oxygenase-1 (HO-1) and Toll-like receptor 4 (TLR4) are two signaling pathways protecting MM cells from the proteasome inhibitor bortezomib (BTZ), we here evaluated their cross-regulation by a pharmacological approach. We found that cell toxicity and mitochondrial depolarization by BTZ were increased upon inhibition of HO-1 and TLR4 by using tin protoporphyrin IX (SnPP) and TAK-242, respectively. Furthermore, the combination of TAK-242 and BTZ activated mitophagy and decreased the unfolded protein response (UPR) survival pathway in association with a downregulation in HO-1 expression. Notably, BTZ in combination with SnPP induced effects mirroring the treatment with TAK-242/BTZ, resulting in a blockade of TLR4 upregulation. Interestingly, treatment of cells with either hemin, an HO-1 inducer, or supplementation with carbon monoxide (CO), a by-product of HO-1 enzymatic activity, increased TLR4 expression. In conclusion, we showed that treatment of MM cells with BTZ triggers the TLR4/HO-1/CO axis, serving as a stress-responsive signal that leads to increased cell survival while protecting mitochondria against BTZ and ultimately promoting drug resistance.

Mitochondrial quality control in acute ischemic stroke

Mitochondria play a central role in the pathophysiological processes of acute ischemic stroke. Disruption of the cerebral blood flow during acute ischemic stroke interrupts oxygen and glucose delivery, leading to the dysfunction of mitochondrial oxidative phosphorylation and cellular bioenergetic stress. Cells can respond to such stress by activating mitochondrial quality control mechanisms, including the mitochondrial unfolded protein response, mitochondrial fission and fusion, mitophagy, mitochondrial biogenesis, and intercellular mitochondrial transfer. Collectively, these adaptive response strategies contribute to retaining the integrity and function of the mitochondrial network, thereby helping to recover the homeostasis of the neurovascular unit. In this review, we focus on mitochondrial quality control mechanisms occurring in acute ischemic stroke. A better understanding of how these regulatory pathways work in maintaining mitochondrial homeostasis will provide a rationale for developing innovative neuroprotectants when these mechanisms fail in acute ischemic stroke.

Critical Involvement of Glial Cells in Manganese Neurotoxicity

Over the years, most of the research concerning manganese exposure was restricted to the toxicity of neuronal cells. Manganese is an essential trace element that in high doses exerts neurotoxic effects. However, in the last two decades, efforts have shifted toward a more comprehensive approach that takes into account the involvement of glial cells in the development of neurotoxicity as a brain insult. Glial cells provide structural, trophic, and metabolic support to neurons. Nevertheless, these cells play an active role in adult neurogenesis, regulation of synaptogenesis, and synaptic plasticity. Disturbances in glial cell function can lead to neurological disorders, including neurodegenerative diseases. This review highlights the pivotal role that glial cells have in manganese-induced neurotoxicity as well as the most sounding mechanisms involved in the development of this phenomenon.

Mechanisms of Metal-Induced Mitochondrial Dysfunction in Neurological Disorders

Metals are actively involved in multiple catalytic physiological activities. However, metal overload may result in neurotoxicity as it increases formation of reactive oxygen species (ROS) and elevates oxidative stress in the nervous system. Mitochondria are a key target of metal-induced toxicity, given their role in energy production. As the brain consumes a large amount of energy, mitochondrial dysfunction and the subsequent decrease in levels of ATP may significantly disrupt brain function, resulting in neuronal cell death and ensuing neurological disorders. Here, we address contemporary studies on metal-induced mitochondrial dysfunction and its impact on the nervous system.

HO-1 Modulates Aerobic Glycolysis through LDH in Prostate Cancer Cells

Prostate cancer (PCa) is the second most diagnosed malignancy and the fifth leading cause of cancer associated death in men worldwide. Dysregulation of cellular energetics has become a hallmark of cancer, evidenced by numerous connections between signaling pathways that include oncoproteins and key metabolic enzymes. We previously showed that heme oxygenase 1 (HO-1), a cellular homeostatic regulator counteracting oxidative and inflammatory damage, exhibits anti-tumoral activity in PCa cells, inhibiting cell proliferation, migration, tumor growth and angiogenesis. The aim of this study was to assess the role of HO-1 on the metabolic signature of PCa. After HO-1 pharmacological induction with hemin, PC3 and C4-2B cells exhibited a significantly impaired cellular metabolic rate, reflected by glucose uptake, ATP production, lactate dehydrogenase (LDH) activity and extracellular lactate levels. Further, we undertook a bioinformatics approach to assess the clinical significance of LDHA, LDHB and HMOX1 in PCa, identifying that high LDHA or low LDHB expression was associated with reduced relapse free survival (RFS). Interestingly, the shortest RFS was observed for PCa patients with low HMOX1 and high LDHA, while an improved prognosis was observed for those with high HMOX1 and LDHB. Thus, HO-1 induction causes a shift in the cellular metabolic profile of PCa, leading to a less aggressive phenotype of the disease.

Transition metals and metal complexes in autophagy and diseases

Transition metals refer to the elements in the d and ds blocks of the periodic table. Since the success of cisplatin and auranofin, transition metal-based compounds have become a prospective source for drug development, particularly in cancer treatment. In recent years, extensive studies have shown that numerous transition metal-based compounds could modulate autophagy, promising a new therapeutic strategy for metal-related diseases and the design of metal-based agents. Copper, zinc, and manganese, which are common components in physiological pathways, play important roles in the progression of cancer, neurodegenerative diseases, and cardiovascular diseases. Furthermore, enrichment of copper, zinc, or manganese can regulate autophagy. Thus, we summarized the current advances in elucidating the mechanisms of some metals/metal-based compounds and their functions in autophagy regulation, which is conducive to explore the intricate roles of autophagy and exploit novel therapeutic drugs for human diseases.

HIF-1α ameliorates tubular injury in diabetic nephropathy via HO-1-mediated control of mitochondrial dynamics

Objectives

In diabetic nephropathy (DN), hypoxia‐inducible factor‐1α (HIF‐1α) activation in tubular cells plays an important protective role against kidney injury. The effects may occur via the target genes of HIF‐1α, such as haem oxygenase‐1 (HO‐1), but the exact mechanisms are incompletely understood.

Materials and methods

Mice with proximal tubule‐specific knockout of HIF‐1α (PT‐HIF‐1α^−/− mice) were generated, and diabetes was induced in these mice by streptozotocin (STZ) injection. In addition, to mimic a hypoxic state, cobaltous chloride (CoCl₂) was applied to HK‐2 cells.

Results

Our study first verified that conditional knockout of HIF‐1α worsened tubular injury in DN; additionally, aggravated kidney dysfunction, renal histopathological alterations, mitochondrial fragmentation, ROS accumulation and apoptosis were observed in diabetic PT‐HIF‐1α^−/− mice. In vitro study showed that compared to control group, HK‐2 cells cultured under hypoxic ambiance displayed increased mitochondrial fragmentation, ROS production, mitochondrial membrane potential loss and apoptosis. These increases were reversed by overexpression of HIF‐1α or treatment with a HO‐1 agonist. Importantly, cotreatment with a HIF‐1α inhibitor and a HO‐1 agonist rescued the HK‐2 cells from the negative impacts of the HIF‐1α inhibitor.

Conclusions

These data revealed that HIF‐1α exerted a protective effect against tubular injury in DN, which could be mediated via modulation of mitochondrial dynamics through HO‐1 upregulation.

Carbon Monoxide Attenuates High Salt-Induced Hypertension While Reducing Pro-inflammatory Cytokines and Oxidative Stress in the Paraventricular Nucleus

Carbon monoxide (CO) presents anti-inflammatory and antioxidant activities as a new gaseous neuromessenger produced by heme oxygenase-1 (HO-1) in the body. High salt-induced hypertension is relevant to the levels of pro-inflammatory cytokines (PICs) and oxidative stress in the hypothalamic paraventricular nucleus (PVN). We explored whether CO in PVN can attenuate high salt-induced hypertension by regulating PICs or oxidative stress. Male Dahl Salt-Sensitive rats were fed high-salt (8% NaCl) or normal-salt (0.3% NaCl) diet for 4 weeks. CORM-2, ZnPP IX, or vehicle was microinjected into bilateral PVN for 6 weeks. High-salt diet increased the levels of MAP, plasma norepinephrine (NE), reactive oxygen species (ROS), and the expressions of COX2, IL-1β, IL-6, NOX2, and NOX4 significantly in PVN (p < 0.05), but decreased the expressions of HO-1 and Cu/Zn-SOD in PVN (p < 0.05). Salt increased sympathetic activity as measured by circulating norepinephrine, and increased the ratio of basal RSNA to max RSNA, in part by decreasing max RSNA. PVN microinjection of CORM-2 decreased the levels of MAP, NE, RSNA, ROS and the expressions of COX2, IL-1β, IL-6, NOX2, NOX4 significantly in PVN of hypertensive rat (p < 0.05), but increased the expressions of HO-1 and Cu/Zn-SOD significantly (p < 0.05), which were all opposite to the effects of ZnPP IX microinjected in PVN (p < 0.05). We concluded that exogenous or endogenous CO attenuates high salt-induced hypertension by regulating PICs and oxidative stress in PVN.

CO-mediated cytoprotection is dependent on cell metabolism modulation

Carbon monoxide (CO) is a gasotransmitter endogenously produced by the activity of heme oxygenase, which is a stress-response enzyme. Endogenous CO or low concentrations of exogenous CO have been described to present several cytoprotective functions: anti-apoptosis, anti-inflammatory, vasomodulation, maintenance of homeostasis, stimulation of preconditioning and modulation of cell differentiation. The present review revises and discuss how CO regulates cell metabolism and how it is involved in the distinct cytoprotective roles of CO. The first found metabolic effect of CO was its increase on cellular ATP production, and since then much data have been generated. Mitochondria are the most described and studied cellular targets of CO. Mitochondria exposure to this gasotransmitter leads several consequences: ROS generation, stimulation of mitochondrial biogenesis, increased oxidative phosphorylation or mild uncoupling effect. Likewise, CO negatively regulates glycolysis and improves pentose phosphate pathway. More recently, CO has also been disclosed as a regulating molecule for metabolic diseases, such as obesity and diabetes with promising results.

HO-1 overexpression alleviates senescence by inducing autophagy via the mitochondrial route in human nucleus pulposus cells

Intervertebral disc degeneration (IDD) is closely associated with aging. Our previous studies have confirmed that heme oxygenase-1 (HO-1) can inhibit nucleus pulposus (NP) cell apoptosis. However, whether or not HO-1 is involved in NP cell senescence and autophagy is unclear. Our results indicated that HO-1 expression was reduced in IDD tissues and replicative senescent NP cells. HO-1 overexpression using a lentiviral vector reduced the NP cell senescence level, protected mitochondrial function, and promoted NP cell autophagy through the mitochondrial pathway. Autophagy inhibitor 3-MA pretreatment reversed the anti-senescent and protective effects on the mitochondrial function of HO-1, which promoted the degradation of the extracellular matrix (ECM) in the intervertebral disc. In vivo, HO-1 overexpression inhibited IDD and enhanced autophagy. In summary, these results suggested that HO-1 overexpression alleviates NP cell senescence by inducing autophagy via the mitochondrial route.

Preventive impacts of PAS-Na on the slow growth and activated inflammatory responses in Mn-exposed rats

BACKGROUND

Sodium para-aminosalicylic acid (PAS-Na), an anti-tuberculosis drug, has been demonstrated its function in facilitating the Mn elimination in manganism patients and Mn-exposed models in vivo and improving the symptoms of Mn poisoning. But whether it can improve the growth retardation and inflammatory responses induced by Mn have not been reported.

OBJECTIVES

This study was designed to investigate the preventive effects of PAS-Na on the development of retardation and inflammatory responses in Mn-exposed rats.

METHODS

Male Sprague Dawley (SD) rats (8 weeks old, weighing 180 ± 20 g) were randomly divided into normal control group and Mn-exposed group in the 4 weeks experiment observation and normal control group, Mn-exposed group, PAS-Na preventive group and PAS-Na control group in the 8 weeks experiment observation. The Mn-exposed group received an intraperitoneal injection (i.p.) of 15 mg/kg MnCl₂ and the normal control group i.p. physiological Saline in the same volume once a day for 4 or 8 weeks, 5 days per week. The PAS-Na preventive group i.p. 15 mg/kg MnCl₂ along with back subcutaneous (s.c.) injection of 240 mg/kg PAS-Na once a day for 8 weeks, 5 days per week. PAS-Na control group received s.c. injection of 240 mg/kg PAS-Na along with i.p. injection of saline once daily. The body weight was determined once a week until the end of the experiment. The manganese contents in the blood were detected by graphite furnace atomic absorption spectrometry. The inflammatory factor levels (TNF-α, IL-1β, IL-6, and PGE2) in the blood were detected by using enzyme-linked immunosorbent assay (Elisa) and each organ taking from rats were weighed and recorded.

RESULTS

Mn exposure significantly suppressed the growth in rats and increased heart, liver, spleen and kidney coefficients as compared with the control group. The whole blood Mn level and serum levels of IL-1β, IL-6, PGE2, and TNF-α in sub-chronic Mn-exposure group were markedly higher than those in the control group. However, preventive treatment with PAS-Na obviously reduced the whole blood Mn level, the spleen and liver coefficients of the Mn-exposed rats. And serum levels of IL-1β and TNF-α were significantly reduced by 33.9% and 14.7% respectively in PAS-Na prevention group.

CONCLUSIONS

PAS-Na could improve the growth retardation and alleviate inflammatory responses in Mn-exposed rats.

Toxicity of blue led light and A2E is associated to mitochondrial dynamics impairment in ARPE-19 cells: implications for age-related macular degeneration

Age-related macular degeneration (AMD) is a multifactorial retinal disease characterized by a progressive loss of central vision. Retinal pigment epithelium (RPE) degeneration is a critical event in AMD. It has been associated to A2E accumulation, which sensitizes RPE to blue light photodamage. Mitochondrial quality control mechanisms have evolved to ensure mitochondrial integrity and preserve cellular homeostasis. Particularly, mitochondrial dynamics involve the regulation of mitochondrial fission and fusion to preserve a healthy mitochondrial network. The present study aims to clarify the cellular and molecular mechanisms underlying photodamage-induced RPE cell death with particular focus on the involvement of defective mitochondrial dynamics. Light-emitting diodes irradiation (445 ± 18 nm; 4.43 mW/cm²) significantly reduced the viability of both unloaded and A2E-loaded human ARPE-19 cells and increased reactive oxygen species production. A2E along with blue light, triggered apoptosis measured by MC540/PI-flow cytometry and activated caspase-3. Blue light induced mitochondrial fusion/fission imbalance towards mitochondrial fragmentation in both non-loaded and A2E-loaded cells which correlated with the deregulation of mitochondria-shaping proteins level (OPA1, DRP1 and OMA1). To our knowledge, this is the first work reporting that photodamage causes mitochondrial dynamics deregulation in RPE cells. This process could possibly contribute to AMD pathology. Our findings suggest that the regulation of mitochondrial dynamics may be a valuable strategy for treating retinal degeneration diseases, such as AMD.

Targeting of the respiratory chain by toxicants: beyond the toxicities to mitochondrial morphology

The mitochondrion is an important subcellular target of environmental toxicants. With environmental stress, a series of toxic effects on mitochondria are induced, which originate from the dynamic changes of mitochondrial fusion and fission, structure/membrane damage, and respiratory chain dysfunction. The toxic effects of various toxicants on mitochondrial morphology and intact membranes, and their determination of cell fate, have already been broadly studied and reported on. However, their effects on the integrity and function of the mitochondrial respiratory chain (RC) remain incompletely understood. Recently, Fan et al. and Yu et al. approached this topic by closely examining the mitochondrial toxicities, including the effect on the respiratory chain, induced by organic arsenical chemical 2-methoxy-4-(((4-(oxoarsanyl)phenyl)imino)methyl)phenol and thiourea gold(i) complexes (AuTuCl). Obviously, toxicant-induced dysfunction of the respiratory chain can hinder ATP production, and may elevate ROS generation. The increased ROS can further damage mtDNA, and consequently leads to inactivation of some RC protein-encoding mtDNA, generating a vicious circle of amplifying mitochondrial damage. We hope that these studies focused on RC structure and activity will broaden our view of mitochondrial toxicology and draw forth more profound mechanistic studies on the respiratory chain toxicity of environmental toxicants and their application in risk assessment.