Targeting DNAJB9, a novel ER luminal co-chaperone, to rescue ΔF508-CFTR

COPII cage assembly factor Sec13 integrates information flow regulating endomembrane function in response to human variation

How information flow is coordinated for managing transit of 1/3 of the genome through endomembrane pathways by the coat complex II (COPII) system in response to human variation remains an enigma. By examining the interactome of the COPII cage-assembly component Sec13, we show that it is simultaneously associated with multiple protein complexes that facilitate different features of a continuous program of chromatin organization, transcription, translation, trafficking, and degradation steps that are differentially sensitive to Sec13 levels. For the trafficking step, and unlike other COPII components, reduction of Sec13 expression decreased the ubiquitination and degradation of wild-type (WT) and F508del variant cargo protein cystic fibrosis transmembrane conductance regulator (CFTR) leading to a striking increase in fold stability suggesting that the events differentiating export from degradation are critically dependent on COPII cage assembly at the ER Golgi intermediate compartment (ERGIC) associated recycling and degradation step linked to COPI exchange. Given Sec13's multiple roles in protein complex assemblies that change in response to its expression, we suggest that Sec13 serves as an unanticipated master regulator coordinating information flow from the genome to the proteome to facilitate spatial covariant features initiating and maintaining design and function of membrane architecture in response to human variation.

CFTR Folding: From Structure and Proteostasis to Cystic Fibrosis Personalized Medicine

Cystic fibrosis (CF) is a lethal genetic disease caused by mutations in the chloride ion channel cystic fibrosis transmembrane conductance regulator (CFTR). Class-II mutants of CFTR lack intermolecular interactions important for CFTR structural stability and lead to misfolding. Misfolded CFTR is detected by a diverse suite of proteostasis factors that preferentially bind and route mutant CFTR toward premature degradation, resulting in reduced plasma membrane CFTR levels and impaired chloride ion conductance associated with CF. CF treatment has been vastly improved over the past decade by the availability of small molecules called correctors. Correctors directly bind CFTR, stabilize its structure by conferring thermodynamically favorable interactions that compensate for mutations, and thereby lead to downstream folding fidelity. However, each of over 100 Class-II CF causing mutations causes unique structural defects and shows a unique response to drug treatment, described as theratype. Understanding CFTR structural defects, the proteostasis factors evaluating those defects, and the stabilizing effects of CFTR correctors will illuminate a path toward personalized medicine for CF. Here, we review recent advances in our understanding of CFTR folding, focusing on structure, corrector binding sites, the mechanisms of proteostasis factors that evaluate CFTR, and the implications for CF personalized medicine.

Volleying plasma membrane proteins from birth to death: Role of J-domain proteins

The function, stability, and turnover of plasma membrane (PM) proteins are crucial for cellular homeostasis. Compared to soluble proteins, quality control of plasma membrane proteins is extremely challenging. Failure to meet the high quality control standards is detrimental to cellular and organismal health. J-domain proteins (JDPs) are among the most diverse group of chaperones that collaborate with other chaperones and protein degradation machinery to oversee cellular protein quality control (PQC). Although fragmented, the available literature from different models, including yeast, mammals, and plants, suggests that JDPs assist PM proteins with their synthesis, folding, and trafficking to their destination as well as their degradation, either through endocytic or proteasomal degradation pathways. Moreover, some JDPs interact directly with the membrane to regulate the stability and/or functionality of proteins at the PM. The deconvoluted picture emerging is that PM proteins are relayed from one JDP to another throughout their life cycle, further underscoring the versatility of the Hsp70:JDP machinery in the cell.

Multi-Faceted Roles of DNAJB Protein in Cancer Metastasis and Clinical Implications

Heat shock proteins (HSPs) are highly conserved molecular chaperones with diverse cellular activities, including protein folding, assembly or disassembly of protein complexes, and maturation process under diverse stress conditions. HSPs also play essential roles in tumorigenesis, metastasis, and therapeutic resistance across cancers. Among them, HSP40s are widely accepted as regulators of HSP70/HSP90 chaperones and an accumulating number of biological functions as molecular chaperones dependent or independent of either of these chaperones. Despite large numbers of HSP40s, little is known about their physiologic roles, specifically in cancer progression. This article summarizes the multi-faceted role of DNAJB proteins as one subclass of the HSP40 family in cancer development and metastasis. Regulation and deregulation of DNAJB proteins at transcriptional, post-transcriptional, and post-translational levels contribute to tumor progression, particularly cancer metastasis. Furthermore, understanding differences in function and regulating mechanism between DNAJB proteins offers a new perspective on tumorigenesis and metastasis to improve therapeutic opportunities for malignant diseases.

Regulation of Translation, Translocation, and Degradation of Proteins at the Membrane of the Endoplasmic Reticulum

The endoplasmic reticulum (ER) of mammalian cells is the central organelle for the maturation and folding of transmembrane proteins and for proteins destined to be secreted into the extracellular space. The proper folding of target proteins is achieved and supervised by a complex endogenous chaperone machinery. BiP, a member of the Hsp70 protein family, is the central chaperone in the ER. The chaperoning activity of BiP is assisted by ER-resident DnaJ (ERdj) proteins due to their ability to stimulate the low, intrinsic ATPase activity of BiP. Besides their co-chaperoning activity, ERdj proteins also regulate and tightly control the translation, translocation, and degradation of proteins. Disturbances in the luminal homeostasis result in the accumulation of unfolded proteins, thereby eliciting a stress response, the so-called unfolded protein response (UPR). Accumulated proteins are either deleterious due to the functional loss of the respective protein and/or due to their deposition as intra- or extracellular protein aggregates. A variety of metabolic diseases are known to date, which are associated with the dysfunction of components of the chaperone machinery. In this review, we will delineate the impact of ERdj proteins in controlling protein synthesis and translocation under physiological and under stress conditions. A second aspect of this review is dedicated to the role of ERdj proteins in the ER-associated degradation pathway, by which unfolded or misfolded proteins are discharged from the ER. We will refer to some of the most prominent diseases known to be based on the dysfunction of ERdj proteins.

The function of the co-chaperone ERdj4 in diverse (patho-)physiological conditions

Accumulation of misfolded proteins in the endoplasmic reticulum (ER) induces a well-orchestrated cellular response to reduce the protein burden within the ER. This unfolded protein response (UPR) is controlled primarily by three transmembrane proteins, IRE1α, ATF6, and PERK, the activity of which is controlled by BiP, the ER-resident Hsp70 protein. Binding of BiP to co-chaperones via their highly conserved J-domains stimulates the intrinsic ATPase activity of BiP, thereby providing the energy necessary for (re-)folding of proteins, or for targeting of misfolded proteins to the degradation pathway, processes specified and controlled by the respective co-chaperone. In this review, our aim is to elucidate the function of the co-chaperone ERDJ4, also known as MDG1, MDJ7, or DNAJB9. Knockout and knockin experiments clearly point to the central role of ERDJ4 in controlling lipogenesis and protein synthesis by promoting degradation of SREBP1c and the assembly of the protein complex mTORC2. Accumulating data reveal that ERDJ4 controls epithelial-to-mesenchymal transition, a central process during embryogenesis, in wound healing, and tumor development. Overexpression of ERdj4 has been shown to improve engraftment of transplanted human stem cells, possibly due to its ability to promote cellular survival in stressed cells. High ERDJ4-plasma levels are specific for fibrillary glomerulonephritis and serve as a diagnostic marker. As outlined in this review, the functions of ERDJ4 are manifold, depending on the cellular (patho-) physiological state, the cellular protein repertoire, and the subcellular localization of ERDJ4.

Regulation of p53 and Cancer Signaling by Heat Shock Protein 40/J-Domain Protein Family Members

Heat shock proteins (HSPs) are molecular chaperones that assist diverse cellular activities including protein folding, intracellular transportation, assembly or disassembly of protein complexes, and stabilization or degradation of misfolded or aggregated proteins. HSP40, also known as J-domain proteins (JDPs), is the largest family with over fifty members and contains highly conserved J domains responsible for binding to HSP70 and stimulation of the ATPase activity as a co-chaperone. Tumor suppressor p53 (p53), the most frequently mutated gene in human cancers, is one of the proteins that functionally interact with HSP40/JDPs. The majority of p53 mutations are missense mutations, resulting in acquirement of unexpected oncogenic activities, referred to as gain of function (GOF), in addition to loss of the tumor suppressive function. Moreover, stability and levels of wild-type p53 (wtp53) and mutant p53 (mutp53) are crucial for their tumor suppressive and oncogenic activities, respectively. However, the regulatory mechanisms of wtp53 and mutp53 are not fully understood. Accumulating reports demonstrate regulation of wtp53 and mutp53 levels and/or activities by HSP40/JDPs. Here, we summarize updated knowledge related to the link of HSP40/JDPs with p53 and cancer signaling to improve our understanding of the regulation of tumor suppressive wtp53 and oncogenic mutp53 GOF activities.

LncRNA-URHC Functions as ceRNA to Regulate DNAJB9 Expression by Competitively Binding to miR-5007-3p in Hepatocellular Carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) is often diagnosed at a late stage, when the prognosis is poor. The regulation of long noncoding RNAs (lncRNAs) plays a crucial role in HCC. However, the precise regulatory mechanisms of lncRNA signaling in HCC remain largely unknown. Our study aims to investigate the underlying mechanisms of lncRNA (upregulated in hepatocellular carcinoma) URHC in HCC.

OBJECTIVE

To study the in vivo and in vitro localization and biological effects of URHC on liver cancer cells. Through bioinformatics analysis, dual-luciferase reporter gene analysis and rescue experiments revealed the possible mechanism of URHC.

METHODS

RT-qPCR, fluorescence in situ hybridization (FISH) staining, EdU, colony formation, and tumor xenograft experiments were used to identify localized and biological effects of URHC on HCC cells in vitro and in vivo. The bioinformatics analysis, dual-luciferase reporter assay, and rescue experiments revealed the potential mechanism of URHC.

RESULTS

URHC silencing may inhibit the HCC cells' proliferation in vitro and in vivo. We found that URHC was mainly localized in the cytoplasm. The expression of miR-5007-3p was negatively regulated by URHC. And miR-5007-3p could reverse the effect of URHC in HCC cells. The expression of DNAJB9 was negatively regulated by miR-5007-3p but positively regulated by URHC. These suggestive of lncRNA-URHC positively regulated the level of DNAJB9 by sponging miR-5007-3p.

CONCLUSION

Together, our study elucidated the role of URHC as a miRNA sponge in HCC and shed new light on lncRNA-directed diagnostics and therapeutics in HCC.

DNAJB9 suppresses the metastasis of triple-negative breast cancer by promoting FBXO45-mediated degradation of ZEB1

DNAJB9, a member of the heat shock protein 40 family, acts as a multifunctional player involved in the maintenance of their client proteins and cellular homeostasis. However, the mechanistic action of DNAJB9 in human malignancies is yet to be fully understood. In this study, we found that ectopic restoration of DNAJB9 inhibits the migration, invasion, in vivo metastasis, and lung colonization of triple-negative breast cancer (TNBC) cells. Mechanistically, DNAJB9 stabilizes FBXO45 protein by suppressing self-ubiquitination and reduces the abundance of ZEB1 by Lys48-linked polyubiquitination to inhibit the epithelial-mesenchymal transition (EMT) and metastasis. Clinically, the reduction of DNAJB9 expression, concomitant with decreased FBXO45 abundance in breast cancer tissues, correlates with poorer clinical outcomes of patients with breast cancer. Taken together, our results provide a novel insight into the metastasis of TNBC and define a promising therapeutic strategy for cancers with overactive ZEB1 by regulating the DNAJB9-FBXO45 signaling axis.

Hepatic DNAJB9 Drives Anabolic Biasing to Reduce Steatosis and Obesity

Nutrients stimulate the anabolic synthesis of proteins and lipids, but selective insulin resistance in obesity biases the anabolic program toward lipogenesis. Here, we report the identification of a DNAJB9-driven program that favors protein synthesis and energy production over lipid accumulation. We show there are two pools of DNAJB9 cochaperone. DNAJB9 in the ER lumen promotes the degradation of the lipogenic transcription factor SREBP1c through ERAD, whereas its counterpart on the ER membrane promotes the assembly of mTORC2 in the cytosol and stimulates the synthesis of proteins and ATP. The expression of Dnajb9 is induced by nutrients and downregulated in the obese mouse liver. Restoration of hepatic DNAJB9 expression effectively improves insulin sensitivity, restores protein synthesis, and suppresses food intake, accompanied by reduced hepatic steatosis and adiposity in multiple mouse models of obesity. Therefore, targeting the anabolic balance may provide a unique opportunity to tackle obesity and diabetes.

Cytotoxic Screening and Transcriptomics Reveal Insights into the Molecular Mechanisms of Trihexyl Phosphate-Triggered Hepatotoxicity

Mounting evidence shows that organophosphate flame retardants (OPFRs), especially aryl- and halogenated-OPFRs, exert various adverse health effects on living organisms. This study evaluated the hepatotoxic effect of trihexyl phosphate (THP) as a long-chain alkyl-OPFR on human hepatocyte cells (LO2) and mouse hepatocyte cells (AML12) by performing screening of cytotoxicity in vitro. In combination with transcriptomic analysis, toxicological mechanisms in vitro were further investigated. Results showed that THP triggered hepatotoxicity in vitro by altering four signaling pathways: endoplasmic reticulum (ER) stress, apoptosis, cell cycle, and the glycolysis signaling pathway. Exposure of LO2 and AML12 liver cells to THP (25 μg/mL) significantly induced ER stress-mediated apoptosis and cell cycle arrest. Meanwhile, downregulation of glycolysis caused the blockage of energy metabolism. Furthermore, the high performance liquid chromatography-quadrupole time-of-flight mass spectrometry (HPLC-Q-TOF-MS/MS) revealed that much of THP was absorbed into the cells and displayed stability in the two liver cell lines. In vivo assays using a mouse model demonstrated that exposure to THP at 400 mg/kg induced the ballooning degeneration of hepatocytes in liver tissue, whereas exposure to THP at 800 mg/kg caused acute liver injury with high alanine aminotransferase levels. This study provides novel insights into the impact of THP on hepatotoxicity in vitro and in vivo and uncovers the underlying toxicological mechanisms, which may serve as a guide for further ecological risk assessment and reasonable application of alkyl-OPFRs.

Protein expression pattern of the molecular chaperone Mdg1/ERdj4 during embryonic development

The vertebrate-specific co-chaperone Mdg1/ERdj4, which is localized in the endoplasmic reticulum, controls the folding and degradation of proteins. We characterized its protein pattern during chick embryonic development. During early development, Mdg1/ERdj4 protein is present in mesenchymal and epithelial cells. In mesenchymal cells, it has a salt and pepper pattern. In contrast, during epithelial tissue differentiation, Mdg1/ERdj4 marks the basal and/or apical compartment of epithelial linings. The distinct protein pattern in epithelial tissue might point to its role in organizing and maintaining the epithelial structure. This could be achieved, e.g. by controlling folding and secretion of membrane-bound receptors or by inhibiting the IRE1α-Xbp1s-SNAI1/2-induced mesenchymalization. High Mdg1/ERdj4 protein levels are maintained in tissue with sustained secretory activity as in ependymal cells or enterocytes, substantiating its important role for secretion. We conclude that the transient elevation of Mdg1/ERdj4 protein levels controls the differentiation of epithelial linings while constitutive high levels are closely linked to secretory activity.

Control of Protein Homeostasis in the Early Secretory Pathway: Current Status and Challenges

: Discrimination between properly folded proteins and those that do not reach this state is necessary for cells to achieve functionality. Eukaryotic cells have evolved several mechanisms to ensure secretory protein quality control, which allows efficiency and fidelity in protein production. Among the actors involved in such process, both endoplasmic reticulum (ER) and the Golgi complex play prominent roles in protein synthesis, biogenesis and secretion. ER and Golgi functions ensure that only properly folded proteins are allowed to flow through the secretory pathway while improperly folded proteins have to be eliminated to not impinge on cellular functions. Thus, complex quality control and degradation machineries are crucial to prevent the toxic accumulation of improperly folded proteins. However, in some instances, improperly folded proteins can escape the quality control systems thereby contributing to several human diseases. Herein, we summarize how the early secretory pathways copes with the accumulation of improperly folded proteins, and how insufficient handling can cause the development of several human diseases. Finally, we detail the genetic and pharmacologic approaches that could be used as potential therapeutic tools to treat these diseases.