Crosstalk between Mitochondrial Protein Import and Lipids

Mitochondria import about 1000 precursor proteins from the cytosol. The translocase of the outer membrane (TOM complex) forms the major entry site for precursor proteins. Subsequently, membrane-bound protein translocases sort the precursor proteins into the outer and inner membrane, the intermembrane space, and the matrix. The phospholipid composition of mitochondrial membranes is critical for protein import. Structural and biochemical data revealed that phospholipids affect the stability and activity of mitochondrial protein translocases. Integration of proteins into the target membrane involves rearrangement of phospholipids and distortion of the lipid bilayer. Phospholipids are present in the interface between subunits of protein translocases and affect the dynamic coupling of partner proteins. Phospholipids are required for full activity of the respiratory chain to generate membrane potential, which in turn drives protein import across and into the inner membrane. Finally, outer membrane protein translocases are closely linked to organellar contact sites that mediate lipid trafficking. Altogether, intensive crosstalk between mitochondrial protein import and lipid biogenesis controls mitochondrial biogenesis.     

Recent Advances in Understanding the Control of Secretory Proteins by the Unfolded Protein Response in Plants

The membrane transport system is built on the proper functioning of the endoplasmic reticulum (ER). The accumulation of unfolded proteins in the ER lumen (ER stress) disrupts ER homeostasis and disturbs the transport system. In response to ER stress, eukaryotic cells activate intracellular signaling (named the unfolded protein response, UPR), which contributes to the quality control of secretory proteins. On the other hand, the deleterious effects of UPR on plant health and growth characteristics have frequently been overlooked, due to limited information on this mechanism. However, recent studies have shed light on the molecular mechanism of plant UPR, and a number of its unique characteristics have been elucidated. This study briefly reviews the progress of understanding what is happening in plants under ER stress conditions.     

Carfilzomib Promotes the Unfolded Protein Response and Apoptosis in Cetuximab-Resistant Colorectal Cancer

Cetuximab is a common treatment option for patients with wild-type K-Ras colorectal carcinoma. However, patients often display intrinsic resistance or acquire resistance to cetuximab following treatment. Here we generate two human CRC cells with acquired resistance to cetuximab that are derived from cetuximab-sensitive parental cell lines. These cetuximab-resistant cells display greater in vitro proliferation, colony formation and migration, and in vivo tumour growth compared with their parental counterparts. To evaluate potential alternative therapeutics to cetuximab-acquired-resistant cells, we tested the efficacy of 38 current FDA-approved agents against our cetuximab-acquired-resistant clones. We identified carfilzomib, a selective proteosome inhibitor to be most effective against our cell lines. Carfilzomib displayed potent antiproliferative effects, induced the unfolded protein response as determined by enhanced CHOP expression and ATF6 activity, and enhanced apoptosis as determined by enhanced caspase-3/7 activity. Overall, our results indicate a potentially novel indication for carfilzomib: that of a potential alternative agent to treat cetuximab-resistant colorectal cancer.     

Mitochondrial Mechanisms in Septic Cardiomyopathy

Sepsis is the manifestation of the immune and inflammatory response to infection that may ultimately result in multi organ failure. Despite the therapeutic strategies that have been used up to now, sepsis and septic shock remain a leading cause of death in critically ill patients. Myocardial dysfunction is a well-described complication of severe sepsis, also referred to as septic cardiomyopathy, which may progress to right and left ventricular pump failure. Many substances and mechanisms seem to be involved in myocardial dysfunction in sepsis, including toxins, cytokines, nitric oxide, complement activation, apoptosis and energy metabolic derangements. Nevertheless, the precise underlying molecular mechanisms as well as their significance in the pathogenesis of septic cardiomyopathy remain incompletely understood. A well-investigated abnormality in septic cardiomyopathy is mitochondrial dysfunction, which likely contributes to cardiac dysfunction by causing myocardial energy depletion. A number of mechanisms have been proposed to cause mitochondrial dysfunction in septic cardiomyopathy, although it remains controversially discussed whether some mechanisms impair mitochondrial function or serve to restore mitochondrial function. The purpose of this review is to discuss mitochondrial mechanisms that may causally contribute to mitochondrial dysfunction and/or may represent adaptive responses to mitochondrial dysfunction in septic cardiomyopathy.     

Interplay between Zn2+ Homeostasis and Mitochondrial Functions in Cardiovascular Diseases and Heart Ageing

Zinc plays an important role in cardiomyocytes, where it exists in bound and histochemically reactive labile Zn²⁺ forms. Although Zn²⁺ concentration is under tight control through several Zn²⁺-transporters, its concentration and intracellular distribution may vary during normal cardiac function and pathological conditions, when the protein levels and efficacy of Zn²⁺ transporters can lead to zinc re-distribution among organelles in cardiomyocytes. Such dysregulation of cellular Zn²⁺ homeostasis leads to mitochondrial and ER stresses, and interrupts normal ER/mitochondria cross-talk and mitophagy, which subsequently, result in increased ROS production and dysregulated metabolic function. Besides cardiac structural and functional defects, insufficient Zn²⁺ supply was associated with heart development abnormalities, induction and progression of cardiovascular diseases, resulting in accelerated cardiac ageing. In the present review, we summarize the recently identified connections between cellular and mitochondrial Zn²⁺ homeostasis, ER stress and mitophagy in heart development, excitation–contraction coupling, heart failure and ischemia/reperfusion injury. Additionally, we discuss the role of Zn²⁺ in accelerated heart ageing and ageing-associated rise of mitochondrial ROS and cardiomyocyte dysfunction.     

Molecular Hydrogen Enhances Proliferation of Cancer Cells That Exhibit Potent Mitochondrial Unfolded Protein Response

Molecular hydrogen ameliorates pathological states in a variety of human diseases, animal models, and cell models, but the effects of hydrogen on cancer have been rarely reported. In addition, the molecular mechanisms underlying the effects of hydrogen remain mostly unelucidated. We found that hydrogen enhances proliferation of four out of seven human cancer cell lines (the responders). The proliferation-promoting effects were not correlated with basal levels of cellular reactive oxygen species. Expression profiling of the seven cells showed that the responders have higher gene expression of mitochondrial electron transport chain (ETC) molecules than the non-responders. In addition, the responders have higher mitochondrial mass, higher mitochondrial superoxide, higher mitochondrial membrane potential, and higher mitochondrial spare respiratory capacity than the non-responders. In the responders, hydrogen provoked mitochondrial unfolded protein response (mtUPR). Suppression of cell proliferation by rotenone, an inhibitor of mitochondrial ETC complex I, was rescued by hydrogen in the responders. Hydrogen triggers mtUPR and induces cell proliferation in cancer cells that have high basal and spare mitochondrial ETC activities.     

Targeting the Unfolded Protein Response as a Disease-Modifying Pathway in Dementia

Dementia is a global medical and societal challenge; it has devastating personal, social and economic costs, which will increase rapidly as the world’s population ages. Despite this, there are no disease-modifying treatments for dementia; current therapy modestly improves symptoms but does not change the outcome. Therefore, new treatments are urgently needed—particularly any that can slow down the disease’s progression. Many of the neurodegenerative diseases that lead to dementia are characterised by common pathological responses to abnormal protein production and misfolding in brain cells, raising the possibility of the broad application of therapeutics that target these common processes. The unfolded protein response (UPR) is one such mechanism. The UPR is a highly conserved cellular stress response to abnormal protein folding and is widely dysregulated in neurodegenerative diseases. In this review, we describe the basic machinery of the UPR, as well as the evidence for its overactivation and pathogenicity in dementia, and for the marked neuroprotective effects of its therapeutic manipulation in murine models of these disorders. We discuss drugs identified as potential UPR-modifying therapeutic agents—in particular the licensed antidepressant trazodone—and we review epidemiological and trial data from their use in human populations. Finally, we explore future directions for investigating the potential benefit of using trazodone or similar UPR-modulating compounds for disease modification in patients with dementia.     

Oxidative Folding in the Mitochondrial Intermembrane Space in Human Health and Disease

Oxidative folding in the mitochondrial intermembrane space (IMS) is a key cellular event associated with the folding and import of a large and still undetermined number of proteins. This process is catalyzed by an oxidoreductase, Mia40 that is able to recognize substrates with apparently little or no homology. Following substrate oxidation, Mia40 is reduced and must be reoxidized by Erv1/Alr1 that consequently transfers the electrons to the mitochondrial respiratory chain. Although our understanding of the physiological relevance of this process is still limited, an increasing number of pathologies are being associated with the impairment of this pathway; especially because oxidative folding is fundamental for several of the proteins involved in defense against oxidative stress. Here we review these aspects and discuss recent findings suggesting that oxidative folding in the IMS is modulated by the redox state of the cell.     

Mitochondrial Unfolded Protein Responses in White Adipose Tissue: Lipoatrophy,Whole-Body Metabolism and Lifespan

The mitochondrial unfolded protein response (UPR^mt) is a stress response mediated by the expression of genes such as chaperones, proteases, and mitokines to maintain mitochondrial proteostasis. Certain genetically modified mice, which defect mitochondrial proteins specifically in adipocytes, developed atrophy of the white adipose tissue, resisted diet-induced obesity, and had altered whole-body metabolism. UPR^mt, which has beneficial functions for living organisms, is termed “mitohormesis”, but its specific characteristics and detailed regulatory mechanism have not been elucidated to date. In this review, we discuss the function of UPR^mt in adipose atrophy (lipoatrophy), whole-body metabolism, and lifespan based on the concept of mitohormesis.     

Autophagy,Unfolded Protein Response,and Neuropilin-1 Cross-Talk in SARS-CoV-2 Infection: What Can Be Learned from Other Coronaviruses

The COVID-19 pandemic is caused by the 2019–nCoV/SARS-CoV-2 virus. This severe acute respiratory syndrome is currently a global health emergency and needs much effort to generate an urgent practical treatment to reduce COVID-19 complications and mortality in humans. Viral infection activates various cellular responses in infected cells, including cellular stress responses such as unfolded protein response (UPR) and autophagy, following the inhibition of mTOR. Both UPR and autophagy mechanisms are involved in cellular and tissue homeostasis, apoptosis, innate immunity modulation, and clearance of pathogens such as viral particles. However, during an evolutionary arms race, viruses gain the ability to subvert autophagy and UPR for their benefit. SARS-CoV-2 can enter host cells through binding to cell surface receptors, including angiotensin-converting enzyme 2 (ACE2) and neuropilin-1 (NRP1). ACE2 blockage increases autophagy through mTOR inhibition, leading to gastrointestinal complications during SARS-CoV-2 virus infection. NRP1 is also regulated by the mTOR pathway. An increased NRP1 can enhance the susceptibility of immune system dendritic cells (DCs) to SARS-CoV-2 and induce cytokine storm, which is related to high COVID-19 mortality. Therefore, signaling pathways such as mTOR, UPR, and autophagy may be potential therapeutic targets for COVID-19. Hence, extensive investigations are required to confirm these potentials. Since there is currently no specific treatment for COVID-19 infection, we sought to review and discuss the important roles of autophagy, UPR, and mTOR mechanisms in the regulation of cellular responses to coronavirus infection to help identify new antiviral modalities against SARS-CoV-2 virus.     

Endoplasmic Reticulum Stress Signaling and the Pathogenesis of Hepatocarcinoma

Hepatocellular carcinoma (HCC), also known as hepatoma, is a primary malignancy of the liver and the third leading cause of cancer mortality globally. Although much attention has focused on HCC, its pathogenesis remains largely obscure. The endoplasmic reticulum (ER) is a cellular organelle important for regulating protein synthesis, folding, modification and trafficking, and lipid metabolism. ER stress occurs when ER homeostasis is disturbed by numerous environmental, physiological, and pathological challenges. In response to ER stress due to misfolded/unfolded protein accumulation, unfolded protein response (UPR) is activated to maintain ER function for cell survival or, in cases of excessively severe ER stress, initiation of apoptosis. The liver is especially susceptible to ER stress given its protein synthesis and detoxification functions. Experimental data suggest that ER stress and unfolded protein response are involved in HCC development, aggressiveness and response to treatment. Herein, we highlight recent findings and provide an overview of the evidence linking ER stress to the pathogenesis of HCC.     

The Unfolded Protein Response Sensor PERK Mediates Stiffness-Dependent Adaptation in Glioblastoma Cells

Glioblastoma multiforme (GBM) is the most aggressive brain tumor in adults. In addition to genetic causes, the tumor microenvironment (TME), including stiffening of the extracellular matrix (ECM), is a main driver of GBM progression. Mechano-transduction and the unfolded protein response (UPR) are essential for tumor-cell adaptation to harsh TME conditions. Here, we studied the effect of a variable stiff ECM on the morphology and malignant properties of GBM stem cells (GSCs) and, moreover, examined the possible involvement of the UPR sensor PERK herein. For this, stiffness-tunable human blood plasma (HBP)/alginate hydrogels were generated to mimic ECM stiffening. GSCs showed stiffness-dependent adaptation characterized by elongated morphology, increased proliferation, and motility which was accompanied by F-Actin cytoskeletal remodeling. Interestingly, in PERK-deficient GSCs, stiffness adaptation was severely impaired, which was evidenced by low F-Actin levels, the absence of F-Actin remodeling, and decreased cell proliferation and migration. This impairment could be linked with Filamin-A (FLN-A) expression, a known interactor of PERK, which was strongly reduced in PERK-deficient GSCs. In conclusion, we identified a novel PERK/FLNA/F-Actin mechano-adaptive mechanism and found a new function for PERK in the cellular adaptation to ECM stiffening.     

Effects of Overexpression of Neurosecretory Protein GL-Precursor Gene on Glucose Homeostasis and Insulin Sensitivity in Mice

A high-fat diet (HFD) quickly induces obesity with insulin resistance and hyperglycemia. We previously reported that a novel hypothalamic small protein, named neurosecretory protein GL (NPGL), stimulates feeding and fat accumulation in mice. However, the effects of NPGL on insulin sensitivity and glucose homeostasis remain unknown. Hence, we subjected NPGL-precursor gene (Npgl)-overexpressing mice to the oral glucose tolerance test (OGTT) and intraperitoneal insulin tolerance test (IPITT) under normal chow (NC) and HFD conditions. Npgl overexpression promoted body mass gain and tended to increase food intake of NC-fed mice, whereas it had little effect on HFD-fed mice. The OGTT showed elevated blood glucose and insulin levels in Npgl-overexpressing NC-fed mice 15 min after glucose administration. Both the OGTT and IPITT demonstrated that Npgl overexpression decreased blood glucose levels in HFD-fed mice 60 min after glucose and insulin treatments. Notably, Npgl overexpression increased adipose tissue masses only in NC-fed mice, and it decreased blood glucose and insulin levels in HFD-fed mice at the experimental end point. It also increased the mRNA expression of galanin, one of the feeding and metabolic regulatory neuropeptides, in the hypothalamus of HFD-fed mice. Therefore, NPGL may alleviate HFD-induced hyperglycemia and insulin resistance in mice.     

Increasing Stress to Induce Apoptosis in Pancreatic Cancer via the Unfolded Protein Response (UPR)

High rates of cell proliferation and protein synthesis in pancreatic cancer are among many factors leading to endoplasmic reticulum (ER) stress. To restore cellular homeostasis, the unfolded protein response (UPR) activates as an adaptive mechanism through either the IRE1α, PERK, or ATF6 pathways to reduce the translational load and process unfolded proteins, thus enabling tumor cells to proliferate. Under severe and prolonged ER stress, however, the UPR may promote adaptation, senescence, or apoptosis under these same pathways if homeostasis is not restored. In this review, we present evidence that high levels of ER stress and UPR activation are present in pancreatic cancer. We detail the mechanisms by which compounds activate one or many of the three arms of the UPR and effectuate downstream apoptosis and examine available data on the pre-clinical and clinical-phase ER stress inducers with the potential for anti-tumor efficacy in pancreatic cancer. Finally, we hypothesize a potential new approach to targeting pancreatic cancer by increasing levels of ER stress and UPR activation to incite apoptotic cell death.     

Molecular Epidemiology of Mitochondrial Cardiomyopathy: A Search Among Mitochondrial and Nuclear Genes

Mitochondrial Cardiomyopathy (MCM) is a common manifestation of multi-organ Mitochondrial Diseases (MDs), occasionally present in non-syndromic cases. Diagnosis of MCM is complex because of wide clinical and genetic heterogeneity and requires medical, laboratory, and neuroimaging investigations. Currently, the molecular screening for MCM is fundamental part of MDs management and allows achieving the definitive diagnosis. In this article, we review the current genetic knowledge associated with MDs, focusing on diagnosis of MCM and MDs showing cardiac involvement. We searched for publications on mitochondrial and nuclear genes involved in MCM, mainly focusing on genetic screening based on targeted gene panels for the molecular diagnosis of the MCM, by using Next Generation Sequencing. Here we report twelve case reports, four case-control studies, eleven retrospective studies, and two prospective studies, for a total of twenty-nine papers concerning the evaluation of cardiac manifestations in mitochondrial diseases. From the analysis of published causal mutations, we identified 130 genes to be associated with mitochondrial heart diseases. A large proportion of these genes (34.3%) encode for key proteins involved in the oxidative phosphorylation system (OXPHOS), either as directly OXPHOS subunits (22.8%), and as OXPHOS assembly factors (11.5%). Mutations in several mitochondrial tRNA genes have been also reported in multi-organ or isolated MCM (15.3%). This review highlights the main disease-genes, identified by extensive genetic analysis, which could be included as target genes in next generation panels for the molecular diagnosis of patients with clinical suspect of mitochondrial cardiomyopathies.     

A Pyrazolate Osmium(VI) Nitride Exhibits Anticancer Activity through Modulating Protein Homeostasis in HepG2 Cells

Interest in the third-row transition metal osmium and its compounds as potential anticancer agents has grown in recent years. Here, we synthesized the osmium(VI) nitrido complex Na[Os^VI(N)(tpm)₂] (tpm = [5-(Thien-2-yl)-1H-pyrazol-3-yl]methanol), which exhibited a greater inhibitory effect on the cell viabilities of the cervical, ovarian, and breast cancer cell lines compared with cisplatin. Proteomics analysis revealed that Na[Os^VI(N)(tpm)₂] modulates the expression of protein-transportation-associated, DNA-metabolism-associated, and oxidative-stress-associated proteins in HepG2 cells. Perturbation of protein expression activity by the complex in cancer cells affects the functions of the mitochondria, resulting in high levels of cellular oxidative stress and low rates of cell survival. Moreover, it caused G2/M phase cell cycle arrest and caspase-mediated apoptosis of HepG2 cells. This study reveals a new high-valent osmium complex as an anticancer agent candidate modulating protein homeostasis.     

The Role of Mitochondrial Abnormalities in Diabetic Cardiomyopathy

Diabetic cardiomyopathy (DCM) is defined as the presence in diabetic patients of abnormal cardiac structure and performance (such as left ventricular hypertrophy, fibrosis, and arrhythmia) in the absence of other cardiac risk factors (such as hypertension or coronary artery disease). Although the pathogenesis of DCM remains unclear currently, mitochondrial structural and functional dysfunctions are recognised as a central player in the DCM development. In this review, we focus on the role of mitochondrial dynamics, biogenesis and mitophagy, Ca²⁺ metabolism and bioenergetics in the DCM development and progression. Based on the crucial role of mitochondria in DCM, application of mitochondria-targeting therapies could be effective strategies to slow down the progression of the disease.