Volume 401, Issue 9

Eukaryotic organisms have evolved complex and robust cellular stress response pathways to ensure maintenance of proteostasis and survival during fluctuating environmental conditions. Highly conserved stress response pathways can be triggered and coordinated at the cell-autonomous and cell-nonautonomous level by proteostasis transcription factors, including HSF1, SKN-1/NRF2, HIF1, and DAF-16/FOXO that combat proteotoxic stress caused by environmental challenges. While these transcription factors are often associated with a specific stress condition, they also direct “noncanonical” transcriptional programs that serve to integrate a multitude of physiological responses required for development, metabolism, and defense responses to pathogen infections. In this review, we outline the established function of these key proteostasis transcription factors at the cell-autonomous and cell-nonautonomous level and discuss a newly emerging stress responsive transcription factor, PQM-1, within the proteostasis network. We look beyond the canonical stress response roles of proteostasis transcription factors and highlight their function in integrating different physiological stimuli to maintain cytosolic organismal proteostasis.

The proper production, degradation, folding and activity of proteins, proteostasis, is essential for any cellular function. From single cell organisms to humans, selective pressures have led to the evolution of adaptive programs that ensure proteins are properly produced and disposed of when necessary. Environmental factors such as temperature, nutrient availability, pathogens as well as predators have greatly influenced the development of mechanisms such as the unfolded protein response, endoplasmic reticulum-associated protein degradation and autophagy, working together in concert to secure cellular proteostasis. In our modern society, the metabolic systems of the human body face the distinct challenge of changed diets, chronic overnutrition and sedentary lifestyles. Obesity and excess white adipose tissue accumulation are linked to a cluster of metabolic diseases and disturbed proteostasis is a common feature. Conversely, processes that promote energy expenditure such as exercise, shivering as well as non-shivering thermogenesis by brown adipose tissue (BAT) and beige adipocytes counteract metabolic dysfunction. Here we review the basic concepts of proteostasis in obesity-linked metabolic diseases and focus on adipocytes, which are critical regulators of mammalian energy metabolism.

Citrate synthase (CS), the rate-limiting enzyme in the tricarboxylic acid (TCA) cycle catalyzes the first step of the cycle, namely, the condensation of oxaloacetate and acetyl-CoA to produce citrate. The expression and enzymatic activity of CS are altered in cancers, but posttranslational modification (PTM) of CS and its regulation in tumorigenesis remain largely obscure. SIRT5 belongs to the nicotinamide adenine dinucleotide (NAD) + -dependent deacetylase sirtuin family and plays vital roles in multiple biological processes via modulating various substrates. Here, we show that SIRT5 interacts with CS and that SIRT5 desuccinylates CS at the evolutionarily conserved residues K393 and K395. Moreover, hypersuccinylation of CS at K393 and K395 dramatically reduces its enzymatic activity and suppresses colon cancer cell proliferation and migration. These results provide experimental evidence in support of a potential therapeutic approach for colon cancer.

Glycosylation is a very frequent post-translational modification in proteins, and the initiation of O- N -acetylgalactosamine (O-GalNAc) glycosylation has been recently described on relevant nuclear proteins. Here we evaluated the nuclear incorporation of a second sugar residue in the biosynthesis pathway of O-GalNAc glycans to yield the terminal core 1 glycan (C1G, Gal β 3GalNAc α Ser/Thr). Using confocal microscopy, enzymatic assay, affinity chromatography, and mass spectrometry, we analyzed intact cells, purified nuclei and soluble nucleoplasms to identify the essential factors for C1G biosynthesis in the cell nucleus. The enzyme C1GalT1 responsible for C1G synthesis was detected inside the nucleus, while catalytic activity of C1Gal-transferase was present in nucleoplasm and purified nuclei. In addition, C1G were detected in the nucleus inside of intact cells, and nuclear proteins exposing C1G were also identified. These evidences represent the first demonstration of core 1 O-GalNAc glycosylation of proteins in the human cell nucleus. These findings reveal a novel post-translational modification on nuclear proteins, with relevant repercussion in epigenetic and chemical biology areas.

Accumulating studies highlight the critical role of long non-coding RNAs (lncRNAs) in the development of various human cancers. Extracellular leucine rich repeat and fibronectin type III domain containing 1-antisense RNA 1 (ELFN1-AS1) was shown to be a newly found lncRNA that abnormally expressed in human tumors. However, till now the specific function of this lncRNA in esophageal cancer (ESCA) remains unknown. In this study, we discovered that higher ELFN1-AS1 expression indicated shorter patient survival in pan-cancer, including ESCA, using online The Cancer Genome Atlas (TCGA) tools. The lncRNA ELFN1-AS1 was significantly up-regulated in ESCA tissues and cell lines when compared with the counterparts. Down-regulation of ELFN1-AS1 restrained cell proliferation, migration, and invasion of ESCA in vitro . In addition, we found that the expression of microRNA-183-3p (miR-183-3p) and ELFN1-AS1 or glutamine-fructose-6-phosphate transaminase 1 (GFPT1) were inversely correlated in ESCA. Both ELFN1-AS1 and GFPT1 are direct targets of miR-183-3p in ESCA. The effects of ELFN1-AS1 knockdown on ESCA progression were partially rescued by inhibition of miR-183-3p or over-expression of GFPT1. In summary, the results of this study suggest that the lncRNA ELFN1-AS1 facilitates the progression of ESCA by acting as a competing endogenous RNA (ceRNA) to promote GFPT1 expression via sponging miR-183-3p.

Colorectal Cancer (CRC) therapy confronts challenges as chemoresistance and side effects. Therefore, drugs with antitumor properties that downmodulate aggressiveness mediators are required. Studies have shown the relevance of Low Molecular Weight Protein Tyrosine Phosphatase (LMWPTP), Protein Tyrosine Phosphatase 1B (PTP1B), and Transforming Growth Factor β (TGF β ) in mediating proliferation, chemoresistance, and metastasis. In this study, we aimed to investigate the responsiveness of colorectal cancer lines (HT29 and HCT116) towards Vemurafenib and whether this treatment could modulate these aggressiveness mediators. Cytotoxicity Assays (MTT and Trypan Exclusion Test) were performed to evaluate the viability of HT29 and HCT116 cells treated with Vemurafenib. Western blotting was performed to analyze the amount and/or the activity of mediators (LMWPTP, PTP1B, TGF β , SMAD3), and the immunoprecipitation was performed to evaluate LMWPTP activity. This study brought up novel aspects of Vemurafenib action in colorectal cancer, which can decrease the activity of protein tyrosine phosphatases (LMWPTP and PTP1B) and the TGF β pathway, making them important in the CRC aggressiveness. By downmodulating colorectal cancer hallmarks, Vemurafenib appears as an interesting candidate for CRC therapeutic protocols.

Fibrosis is a phenomenon in which parenchyma is replaced with fibrous tissue. Persistent inflammation accompanied by dysregulation of cytokine production and repeated cycles of inflammation-associated tissue-repair induces fibrosis in various organs including the liver, lung, and kidney. In idiopathic pulmonary fibrosis, production of interleukin (IL)-6 and osteopontin (OPN) are dysregulated. Fibrosis leads to qualitative rather than quantitative changes of fibroblasts at the sites of tissue repair, and this leads to enlargement of fibrotic foci. These fibroblasts are immunohistochemically positive for OPN; however, the effect of overexpressed OPN in fibroblasts is not fully understood yet. In this study, we investigated the effect of OPN on IL-6 secretion and on migration and proliferation of fibroblasts. Lung fibroblasts overexpressing exogenous OPN showed that OPN was linked to the enhancement of cell migration through increased IL-6 secretion via the extracellular signal-regulated kinase (ERK) pathway. These results suggest that OPN may exert its pro-fibrotic functions, such as enhancement of fibroblasts migration by cooperating with chemoattractant IL-6, and may be involved in enlargement of fibrotic foci.

Lipocalins, small extracellular hydrophobic molecule carriers, can be internalized by a variety of different cells. However, to date receptors have only been identified for human lipocalins. Here, we specifically investigated uptake mechanisms for lipocalins ß-lactoglobulin and Fel d 4 in HeLa and Chinese hamster ovary (CHO) cells. We provide evidence that cell surface heparan sulphate proteoglycan is essential for internalization of these lipocalins. In HeLa cells, lipocalin uptake was inhibited by competition with soluble heparin, enzymatic digestion of cellular heparan sulphate by heparinase and inhibition of its biosynthesis by sodium chlorate. Biochemical studies by heparin affinity chromatography and colocalization studies further supported a role of heparan sulphate proteoglycan in lipocalin uptake. Finally, lipocalin uptake was blocked in CHO mutant cells defective in glycosaminoglycan biosynthesis whereas in wild-type cells it was clearly detectable. Thus, cell surface heparan sulphate proteoglycan represents a novel component absolutely participating in the cellular uptake of some lipocalins.

Treatment of different cell lines with progesterone receptor membrane component 1 (PGRMC1) antagonist AG-205 rapidly induces the formation of large vesicular structures that likely represent endosomes. Crispr/Cas9 was used to target the PGRMC1 and progesterone receptor membrane component 2 ( PGRMC2 ) genes in CHO-K1 and HeLa. Unexpectedly, deficiency in one of these or both genes did not inhibit the formation of enlarged vesicles by AG-205, demonstrating additional molecular target(s) of this compound besides PGRMC1. Thus, AG-205 cannot be regarded as a PGRMC1-specific antagonist. However, provided that its currently unknown target(s) will be identified, AG-205 may serve as a new reagent to study endosomal trafficking.