Background and aims
Non‐alcoholic fatty liver disease (NAFLD) is becoming increasingly prevalent and nutrition intervention remains the most important therapeutic approach for NAFLD. Our aim was to ...investigate whether low‐ (LP) or high‐protein (HP) diets are more effective in reducing liver fat and reversing NAFLD and which mechanisms are involved.
Methods
19 participants with morbid obesity undergoing bariatric surgery were randomized into two hypocaloric (1500‐1600 kcal/day) diet groups, a low protein (10E% protein) and a high protein (30E% protein), for three weeks prior to surgery. Intrahepatic lipid levels (IHL) and serum fibroblast growth factor 21 (FGF21) were measured before and after the dietary intervention. Autophagy flux, histology, mitochondrial activity and gene expression analyses were performed in liver samples collected during surgery.
Results
IHL levels decreased by 42.6% in the HP group, but were not significantly changed in the LP group despite similar weight loss. Hepatic autophagy flux and serum FGF21 increased by 66.7% and 42.2%, respectively, after 3 weeks in the LP group only. Expression levels of fat uptake and lipid biosynthesis genes were lower in the HP group compared with those in the LP group. RNA‐seq analysis revealed lower activity of inflammatory pathways upon HP diet. Hepatic mitochondrial activity and expression of β‐oxidation genes did not increase in the HP group.
Conclusions
HP diet more effectively reduces hepatic fat than LP diet despite of lower autophagy and FGF21. Our data suggest that liver fat reduction upon HP diets result primarily from suppression of fat uptake and lipid biosynthesis.
Lipid droplet functions beyond energy storage Welte, Michael A.; Gould, Alex P.
Biochimica and biophysica acta. Molecular and cell biology of lipids,
10/2017, Volume:
1862, Issue:
10
Journal Article
Peer reviewed
Open access
Lipid droplets are cytoplasmic organelles that store neutral lipids and are critically important for energy metabolism. Their function in energy storage is firmly established and increasingly well ...characterized. However, emerging evidence indicates that lipid droplets also play important and diverse roles in the cellular handling of lipids and proteins that may not be directly related to energy homeostasis. Lipid handling roles of droplets include the storage of hydrophobic vitamin and signaling precursors, and the management of endoplasmic reticulum and oxidative stress. Roles of lipid droplets in protein handling encompass functions in the maturation, storage, and turnover of cellular and viral polypeptides. Other potential roles of lipid droplets may be connected with their intracellular motility and, in some cases, their nuclear localization. This diversity highlights that lipid droplets are very adaptable organelles, performing different functions in different biological contexts. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.
•Lipid droplets have important functions beyond energy homeostasis.•They store vitamins, signaling precursors, and other hydrophobic molecules.•They mitigate some harmful effects of ER and oxidative stress.•They function in protein maturation, storage, and turnover.•They are motile and can exist in the nucleus.
Osteoarthritis (OA) is a chronic degenerative disease that affects the whole synovial joint. OA causes severe pain and disability that significantly affects the livelihood of an individual and incurs ...a huge socioeconomic burden. Current management strategies are limited to supporting functional improvement with physiotherapy and pain reduction as there are no drugs available that can reverse the progression of OA with only joint replacement surgery for late stage OA. OA is associated with advancing age and obesity, both of which compromise the functions of key endoplasmic reticulum (ER) molecular chaperones leading to improper protein folding and ER stress. Failure to restore protein homeostasis leads to increased cellular stress and eventually apoptotic cell death. Cartilage is avascular and is dependent on its constituent cells, chondrocytes, for extracellular matrix maintenance. Chondrocytes have limited proliferative capacity and their apoptosis eventually leads to extracellular matrix loss and cartilage degeneration. Recent studies on attenuating ER stress and chondrocytes apoptosis offer a credible strategy for reducing OA progression. The established roles of ER stress responses in OA have paved the way for targeted drug discovery studies aiming to mitigate ER stress and OA progression. In this review, in vitro, pre-clinical and clinical evidence of naturally-derived ER stress inhibitors for OA, the prospect and challenges in bringing these compounds to clinics are discussed.
•Autophagy and UPR interaction underlies ERS response outcomes.•ERS modulates mitochondrial biogenesis and function.•Ubiquitin ligases fine-tune the ERS response.
Cellular stress, induced by external ...or internal cues, activates several well-orchestrated processes aimed at either restoring cellular homeostasis or committing to cell death. Those processes include the unfolded protein response (UPR), autophagy, hypoxia, and mitochondrial function, which are part of the global endoplasmic reticulum (ER) stress (ERS) response. When one of the ERS elements is impaired, as often occurs under pathological conditions, overall cellular homeostasis may be perturbed. Further, activation of the UPR could trigger changes in mitochondrial function or autophagy, which could modulate the UPR, exemplifying crosstalk processes. Among the numerous factors that control the magnitude or duration of these processes are ubiquitin ligases, which govern overall cellular stress outcomes. Here we summarize crosstalk among the fundamental processes governing ERS responses.
Post-translational modifications are at the apex of cellular communication and eventually regulate every aspect of life. The identification of new post-translational modifiers is opening alternative ...avenues in understanding fundamental cell biology processes and may ultimately provide novel therapeutic opportunities. The ubiquitin-fold modifier 1 (UFM1) is a post-translational modifier discovered a decade ago but its biological significance has remained mostly unknown. The field has recently witnessed an explosion of research uncovering the implications of the pathway to cellular homeostasis in living organisms. We overview recent advances in the function and regulation of the UFM1 pathway, and its implications for cell physiology and disease.
The known functions of the UFM1 pathway mostly encompass the control of the proteostasis network, highlighting the involvement of the endoplasmic reticulum stress response known as the unfolded protein response.Recent discoveries have revealed unexpected roles for the UFM1 pathway in DNA damage response, ribosome biology, autophagy, gene expression control, and cell differentiation.The recent discovery of UFM1 conjugation to ribosomal protein RPL26 represents a unique way for cells with high secretory activity to tune ribosome turnover.Study of preclinical models of multiple diseases (liver, brain, intestine, and heart injury) has identified UFM1 conjugation as an important protective pathway.Genetic alterations to the UFM1 pathway are linked to human diseases affecting the nervous system.
The secretory capacity of a cell is constantly challenged by physiological demands and pathological perturbations. To adjust and match the protein-folding capacity of the endoplasmic reticulum (ER) ...to changing secretory needs, cells employ a dynamic intracellular signaling pathway known as the unfolded protein response (UPR). Homeostatic activation of the UPR enforces adaptive programs that modulate and augment key aspects of the entire secretory pathway, whereas maladaptive UPR outputs trigger apoptosis. Here, we discuss recent advances into how the UPR integrates information about the intensity and duration of ER stress stimuli in order to control cell fate. These findings are timely and significant because they inform an evolving mechanistic understanding of a wide variety of human diseases, including diabetes mellitus, neurodegeneration, and cancer, thus opening up the potential for new therapeutic modalities to treat these diverse diseases.
Maintenance of endoplasmic reticulum (ER) proteostasis is controlled by a dynamic signaling network known as the unfolded protein response (UPR). IRE1α is an ER-located kinase and endoribonuclease that operates as a major ER stress transducer, mediating the establishment of adaptive and pro-apoptotic programs. Here, Hetz and Papa discuss recent advances of their understanding on how the UPR is regulated and its significance to a variety of different human diseases.
Flavokawain B (FKB), a natural kava chalcone, displays potent antitumor activity in various types of cancer. The mechanism of action, however, remains unclear. Here, we evaluated the efficacy of FKB ...in the treatment of human glioblastoma multiforme (GBM) as well as the molecular basis for its inhibitory effects in cancer. Approximately 60% of GBM cells became senescent after treatment with FKB as assessed in the senescence-associated (SA)-GLB1/SA-β-galactosidase assay. The cellular process of autophagy potentially contributed to the establishment of senescence. Transmission electron microscopy revealed the formation of autophagic vesicles under FKB treatment, and MAP1LC3B (microtubule associated protein 1 light chain 3 beta)-II was increased. Transfection of ATG5 or ATG7 small interfering RNAs (siRNAs) inhibited FKB-induced autophagy in U251 cells. Western blot revealed that molecular components of the endoplasmic reticulum stress pathway were activated, including ATF4 (activating transcription factor 4) and DDIT3 (DNA damage inducible transcript 3), while levels of TRIB3 (tribbles pseudokinase 3) increased. In addition, based on the phosphorylation status, the AKT-MTOR-RPS6KB1 pathway was inhibited, which induced autophagy in GBM cells. Inhibition of autophagy by autophagy inhibitors 3-methyladenine and chloroquine or knockdown of ATG5 or ATG7 caused FKB-treated U251 cells to switch from senescence to apoptosis. Finally, knockdown of ATG5 or treatment with chloroquine in combination with FKB, significantly inhibited tumor growth in vivo. Our results demonstrated that FKB induced protective autophagy through the ATF4-DDIT3-TRIB3-AKT-MTOR-RPS6KB1 signaling pathway in GBM cells, indicating that the combination treatment of FKB with autophagy inhibitors may potentially be an effective therapeutic strategy for GBM.
Abbreviations: 3-MA: 3-methyladenine; 4-PBA: 4-phenylbutyrate; AKT: AKT serine/threonine kinase; ATF4: activating transcription factor 4; ATG: autophagy related; CASP3: caspase 3; CCK-8: cell counting kit-8; CDKN1A: cyclin-dependent kinase inhibitor 1A; CQ: chloroquine; DDIT3: DNA damage inducible transcript 3; DMEM: Dulbecco's modified Eagle's medium; EIF2A: eukaryotic translation initiation factor 2A; EIF2AK3: eukaryotic translation initiation factor 2 alpha kinase 3; ER: endoplasmic reticulum; FKB: flavokawain B; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GBM: glioblastoma multiforme; GFP: green fluorescent protein; HSPA5: heat shock protein family A (Hsp70) member 5; MAP1LC3B: microtubule associated protein 1 light chain 3 beta; MTOR: mechanistic target of rapamycin kinase; PARP1: poly(ADP-ribose) polymerase; 1RPS6KB1: ribosomal protein S6 kinase B1; SA-GLB1: senescence-associated galactosidase beta 1; siRNA: short interfering RNA; SQSTM1: sequestosome 1; TEM: transmission electron microscopy; TRIB3: tribbles pseudokinase 3; TUNEL: deoxynucleotidyl transferase-mediated dUTP nick-end labeling
Through oxidative phosphorylation, mitochondria play a central role in energy production and are an important production source of reactive oxygen species (ROS). Not surprisingly, viruses have ...evolved to exploit this organelle in order to support their infection cycle. Beyond its role in the cellular antiviral response, induction of oxidative stress has emerged as a common strategy employed by many viruses to promote their replication. Here, we review the key molecular mechanisms employed by viruses to interact with mitochondria and induce oxidative stress. Furthermore, we discuss how viruses benefit from increased ROS levels, how they control ROS production to maintain a favorable redox environment, and how they cope with ROS-mediated cell death.
Many viruses manipulate mitochondrial respiratory and apoptotic functions to ensure a successful intracellular life cycle.Viral interactions with mitochondrial membranes and other mitochondria-associated components lead to increased production of reactive oxygen species (ROS).Virus-induced mitochondrial ROS (mtROS) benefit viral replication through modulation of host pathways and covalent changes in viral components.Viruses control the oxidative status of the host cell during the course of infection.Manipulation of mtROS-induced apoptosis (intrinsic apoptosis) represents an important viral strategy for optimal intracellular viral replication and timely release of viral progeny.Antioxidant treatments have emerged as a promising antiviral strategy, although further understanding of the complex interplay between viruses and the cellular redox response is needed.
The pathological mechanisms underlying increased outflow resistance at the trabecular meshwork (TM) that is responsible for elevating intraocular pressure (IOP) have not been fully delineated. Recent ...studies have shown that progressive accumulation of misfolded proteins and induction of endoplasmic reticulum (ER) stress is associated with the pathophysiology of glaucomatous TM damage and IOP elevation. We have shown that known causes of human glaucoma, including expression of mutant myocilin or dexamethasone treatment induce abnormal protein accumulation and ER stress in the TM in vitro and in vivo models. To cope up with abnormal protein accumulation, TM cells activate a cytoprotective pathway of unfolded protein response (UPR). However, chronic ER stress can lead to TM dysfunction and IOP elevation. Using cell culture, mouse models, and human postmortem tissues as well as genetic and pharmacological manipulations, we have analyzed ER stress and UPR mediators in the glaucomatous TM damage and IOP elevation. In this chapter, we have described a detailed protocol for the analysis of protein misfolding and ER stress in TM cells and tissues and its association with glaucomatous TM damage and IOP elevation.
In cartilage, chondrocytes are responsible for the biogenesis and maintenance of the extracellular matrix (ECM) composed of proteins, glycoproteins and proteoglycans. Various cellular stresses, such ...as hypoxia, nutrient deprivation, oxidative stress or the accumulation of advanced glycation end products (AGEs) during aging, but also translational errors or mutations in cartilage components or chaperone proteins affect the synthesis and secretion of ECM proteins, causing protein aggregates to accumulate in the endoplasmic reticulum (ER). This condition, referred to as ER stress, interferes with cartilage cell homeostasis and initiates the unfolded protein response (UPR), a rescue mechanism to regain cell viability and function. Chronic or irreversible ER stress, however, triggers UPR-initiated cell death. Due to unresolved ER stress in chondrocytes, diseases of the skeletal system, such as chondrodysplasias, arise. ER stress has also been identified as a contributing factor to the pathogenesis of cartilage degeneration processes such as osteoarthritis (OA). This review provides current knowledge about the biogenesis of ECM components in chondrocytes, describes possible causes for the impairment of involved processes and focuses on the ER stress-induced cell death in articular cartilage during OA. Targeting of the ER stress itself or intervention in UPR signaling to reduce death of chondrocytes may be promising for future osteoarthritis therapy.
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