Functional genetic screen for genes involved in senescence: role of Tid1, a homologue of the Drosophila tumor suppressor l(2)tid, in senescence and cell survival

A peptide derived from TID1S rescues frataxin deficiency and mitochondrial defects in FRDA cellular models

Friedreich's ataxia (FRDA), the most common recessive inherited ataxia, results from homozygous guanine-adenine-adenine (GAA) repeat expansions in intron 1 of the FXN gene, which leads to the deficiency of frataxin, a mitochondrial protein essential for iron-sulphur cluster synthesis. The study of frataxin protein regulation might yield new approaches for FRDA treatment. Here, we report tumorous imaginal disc 1 (TID1), a mitochondrial J-protein cochaperone, as a binding partner of frataxin that negatively controls frataxin protein levels. TID1 interacts with frataxin both in vivo in mouse cortex and in vitro in cortical neurons. Acute and subacute depletion of frataxin using RNA interference markedly increases TID1 protein levels in multiple cell types. In addition, TID1 overexpression significantly increases frataxin precursor but decreases intermediate and mature frataxin levels in HEK293 cells. In primary cultured human skin fibroblasts, overexpression of TID1S results in decreased levels of mature frataxin and increased fragmentation of mitochondria. This effect is mediated by the last 6 amino acids of TID1S as a peptide made from this sequence rescues frataxin deficiency and mitochondrial defects in FRDA patient-derived cells. Our findings show that TID1 negatively modulates frataxin levels, and thereby suggests a novel therapeutic target for treating FRDA.

Putting human Tid-1 in context: an insight into its role in the cell and in different disease states

Background

Tumorous imaginal disc 1 (hTid-1) or DnaJ homolog subfamily A member 3 (DNAJA3), is a part of the heat shock protein (Hsp) 40 family and is predominantly found to reside in the mitochondria. hTid-1 has two mRNA splicing variants, hTid-1S and hTid-1L of 40 and 43 kDa respectively in the cytosol which are later processed upon import into the mitochondrial matrix. hTid-1 protein is a part of the DnaJ family of proteins which are co-chaperones and specificity factors for DnaK proteins of the Hsp70 family, and bind to Hsp70, thereby activating its ATPase activity. hTid-1 has been found to be critical for a lot of important cellular processes such as proliferation, differentiation, growth, survival, senescence, apoptosis, and movement and plays key roles in the embryo and skeletal muscle development.

Main body

hTid-1 participates in several protein–protein interactions in the cell, which mediate different processes such as proteasomal degradation and autophagy of the interacting protein partners. hTid-1 also functions as a co-chaperone and participates in interactions with several different viral oncoproteins. hTid-1 also plays a critical role in different human diseases such as different cancers, cardiomyopathies, and neurodegenerative disorders.

Conclusion

This review article is the first of its kind presenting consolidated information on the research findings of hTid-1 to date. This review suggests that the current knowledge of the role of hTid-1 in disorders like cancers, cardiomyopathies, and neurodegenerative diseases can be correlated with the findings of its protein–protein interactions that can provide a deep insight into the pathways by which hTid-1 affects disease pathogenesis and it can be stated that hTid-1 may serve as an important therapeutic target for these disorders.

Graphical Abstract

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.

Mitochondrial HSP70 Chaperone System-The Influence of Post-Translational Modifications and Involvement in Human Diseases

Since their discovery, heat shock proteins (HSPs) have been identified in all domains of life, which demonstrates their importance and conserved functional role in maintaining protein homeostasis. Mitochondria possess several members of the major HSP sub-families that perform essential tasks for keeping the organelle in a fully functional and healthy state. In humans, the mitochondrial HSP70 chaperone system comprises a central molecular chaperone, mtHSP70 or mortalin (HSPA9), which is actively involved in stabilizing and importing nuclear gene products and in refolding mitochondrial precursor proteins, and three co-chaperones (HSP70-escort protein 1-HEP1, tumorous imaginal disc protein 1-TID-1, and Gro-P like protein E-GRPE), which regulate and accelerate its protein folding functions. In this review, we summarize the roles of mitochondrial molecular chaperones with particular focus on the human mtHsp70 and its co-chaperones, whose deregulated expression, mutations, and post-translational modifications are often considered to be the main cause of neurological disorders, genetic diseases, and malignant growth.

Identification of Piperidine-3-carboxamide Derivatives Inducing Senescence-like Phenotype with Antimelanoma Activities

This study evaluated the potential use of senescence-inducing small molecules in the treatment of melanoma. We screened commercially available small-molecule libraries with high-throughput screening and high-content screening image-based technology. Our findings showed an initial hit with the embedded N-arylpiperidine-3-carboxamide scaffold-induced senescence-like phenotypic changes in human melanoma A375 cells without serious cytotoxicity against normal cells. A focused library containing diversely modified analogues were constructed and examined to evaluate the structure-activity relationship of N-arylpiperidine-3-carboxamide derivatives starting from hit 1. This work identified a novel compound with remarkable antiproliferative activity in vitro and demonstrated the key structural moieties within.

Genes and pathways involved in senescence bypass identified by functional genetic screens

Cellular senescence is a state of stable and irreversible cell cycle arrest with active metabolism, that normal cells undergo after a finite number of divisions (Hayflick limit). Senescence can be triggered by intrinsic and/or extrinsic stimuli including telomere shortening at the end of a cell's lifespan (telomere-initiated senescence) and in response to oxidative, genotoxic or oncogenic stresses (stress-induced premature senescence). Several effector mechanisms have been proposed to explain senescence programmes in diploid cells, including the induction of DNA damage responses, a senescence-associated secretory phenotype and epigenetic changes. Senescent cells display senescence-associated-β-galactosidase activity and undergo chromatin remodeling resulting in heterochromatinisation. Senescence is established by the pRb and p53 tumour suppressor networks. Senescence has been detected in in vitro cellular settings and in premalignant, but not malignant lesions in mice and humans expressing mutant oncogenes. Despite oncogene-induced senescence, which is believed to be a cancer initiating barrier and other tumour suppressive mechanisms, benign cancers may still develop into malignancies by bypassing senescence. Here, we summarise the functional genetic screens that have identified genes, uncovered pathways and characterised mechanisms involved in senescence evasion. These include cell cycle regulators and tumour suppressor pathways, DNA damage response pathways, epigenetic regulators, SASP components and noncoding RNAs.

DNAJA3/Tid1 Is Required for Mitochondrial DNA Maintenance and Regulates Migration and Invasion of Human Gastric Cancer Cells

BACKGROUND

Gastric cancer is a common health issue. Deregulated cellular energetics is regarded as a cancer hallmark and mitochondrial dysfunction might contribute to cancer progression. Tid1, a mitochondrial co-chaperone, may play a role as a tumor suppressor in various cancers, but the role of Tid1 in gastric cancers remains under investigated.

METHODS

The clinical TCGA online database and immunohistochemical staining for Tid1 expression in tumor samples of gastric cancer patients were analyzed. Tid1 knockdown by siRNA was applied to investigate the role of Tid1 in gastric cancer cells.

RESULTS

Low Tid1 protein-expressing gastric cancer patients had a poorer prognosis and higher lymph node invasion than high Tid1-expressing patients. Knockdown of Tid1 did not increase cell proliferation, colony/tumor sphere formation, or chemotherapy resistance in gastric cancer cells. However, Tid1 knockdown increased cell migration and invasion. Moreover, Tid1 knockdown reduced the mtDNA copy number of gastric cancer cells. In addition, the Tid1-galectin-7-MMP-9 axis might be associated with Tid1 knockdown-induced cell migration and invasion of gastric cancer cells.

CONCLUSIONS

Tid1 is required for mtDNA maintenance and regulates migration and invasion of gastric cancer cells. Tid1 deletion may be a poor prognostic factor in gastric cancers and could be further investigated for development of gastric cancer treatments.

The one thousand and one chaperones of the NF-κB pathway

The NF-κB pathway represents a crucial signaling mechanism in sensing and integrating a multitude of environmental and intracellular stimuli and directing a coordinated response that from the cellular level may impact on the entire organism. A plethora of chaperone proteins work at multiple steps of the pathway, from membrane receptor activation to transcription factor binding to DNA. Indeed, chaperones are required to assist protein conformational changes, to assemble supramolecular complexes and to regulate protein ubiquitination, required for pathway activation. Some chaperones acquired a role as integral components of the signaling complexes, needed for signal progression. Here we describe the chaperones involved in the NF-κB pathway and their specific roles in the different contexts.

The Effect of X-Ray and Heavy Ions Radiations on Chemotherapy Refractory Tumor Cells

Purpose

The purpose of this study is to link both numeric and structural chromosomal aberrations to the effectiveness of radiotherapy in chemotherapy refractory tumor cells.

Materials and methods

Neuroblastoma (LAN-1) and 79HF6 glioblastoma cells derived from patients and their chemoresistant sublines were artificially cultured as neurospheres and irradiated by X-rays and heavy ions sources. All the cell lines were irradiated by Carbon-SIS with LET of 100 keV/μm. However, 79HF6 cells and LAN-1 cells were also irradiated by Carbon-UNILAC with LET of 168 keV/μm and Nickel ions with LET of 174 keV/μm, respectively. The effect of radiation on the survival and proliferation of cells was addressed by standard clonogenic assays. In order to analyze cell karyotype standard Giemsa staining, multicolor fluorescence in situ hybridization (mFISH) and multicolor banding (mBAND) techniques were applied.

Results

Relative biological effectiveness values of heavy ion beams relative to X-rays at the D₁₀ values were found between 2.3 and 2.6 with Carbon-SIS and Nickel for LAN-1 and between 2.5 and 3.4 with Carbon-SIS and Carbon-UNILAC for 79HF6 cells. Chemorefractory LAN-1^RETO cells were found more radioresistant than untreated LAN-1^WT cells. 79HF6^RETO glioblastoma cells were found more radiosensitive than cytostatic sensitive cells 79HF6^WT. Sphere formation assay showed that LAN-1^RETO cells were able to form spheres in serum-free culture, whereas 79HF6 cells could not. Most of 79HF6^WT cells revealed a number of 71–90 chromosomes, whereas 79HF6^RETO revealed a number of 52–83 chromosomes. The majority of LAN-1^WT cells revealed a number of 40–44 chromosomes. mFISH analysis showed some stable aberrations, especially on chromosome 10 as judged by the impossibility to label this region with specific probes. This was corroborated using mBAND analysis.

Conclusion

Heavy ion irradiation was more effective than X-ray in both cytostatic naive cancer and chemoresistant cell lines. LAN-1^RETO chemoresistant neuroblastoma cells were found to be more radioresistant than the cytostatic naive cells (LAN-1^WT), whereas this effect was not found in 79HF6 cells.

Tid1, the Mammalian Homologue of Drosophila Tumor Suppressor Tid56, Mediates Macroautophagy by Interacting with Beclin1-containing Autophagy Protein Complex

One of the fundamental functions of molecular chaperone proteins is to selectively conjugate cellular proteins, targeting them directly to lysosome. Some of chaperones, such as the stress-induced Hsp70, also play important roles in autophagosome-forming macroautophagy under various stress conditions. However, the role of their co-chaperones in autophagy regulation has not been well defined. We here show that Tid1, a DnaJ co-chaperone for Hsp70 and the mammalian homologue of the Drosophila tumor suppressor Tid56, is a key mediator of macroautophagy pathway. Ectopic expression of Tid1 induces autophagy by forming LC3+ autophagosome foci, whereas silencing Tid1 leads to drastic impairment of autophagy as induced by nutrient deprivation or rapamycin. In contrast, Hsp70 is dispensable for a role in nutrient deprivation-induced autophagy. The murine Tid1 can be replaced with human Tid1 in murine fibroblast cells for induction of autophagy. We further show that Tid1 increases autophagy flux by interacting with the Beclin1-PI3 kinase class III protein complex in response to autophagy inducing signal and that Tid1 is an essential mediator that connects IκB kinases to the Beclin1-containing autophagy protein complex. Together, these results reveal a crucial role of Tid1 as an evolutionarily conserved and essential mediator of canonical macroautophagy.

Cell senescence abrogates the therapeutic potential of human mesenchymal stem cells in the lethal endotoxemia model

Mesenchymal stem cells (MSCs) possess unique paracrine and immunosuppressive properties, which make them useful candidates for cellular therapy. Here, we address how cellular senescence influences the therapeutic potential of human MSCs (hMSCs). Senescence was induced in bone marrow-derived hMSC cultures with gamma irradiation. Control and senescent cells were tested for their immunoregulatory activity in vitro and in vivo, and an extensive molecular characterization of the phenotypic changes induced by senescence was performed. We also compared the gene expression profiles of senescent hMSCs with a collection of hMSCs used in an ongoing clinical study of Graft Versus Host disease (GVHD). Our results show that senescence induces extensive phenotypic changes in hMSCs and abrogates their protective activity in a murine model of LPS-induced lethal endotoxemia. Although senescent hMSCs retain an ability to regulate the inflammatory response on macrophages in vitro, and, in part retain their capacity to significantly inhibit lymphocyte proliferation, they have a severely impaired migratory capacity in response to proinflammatory signals, which is associated with an inhibition of the AP-1 pathway. Additionally, expression analysis identified PLEC, C8orf48, TRPC4, and ZNF14, as differentially regulated genes in senescent hMSCs that were similarly regulated in those hMSCs which failed to produce a therapeutic effect in a GVHD trial. All the observed phenotypic alterations were confirmed in replicative-senescent hMSCs. In conclusion, this study highlights important changes in the immunomodulatory phenotype of senescent hMSCs and provides candidate gene signatures which may be useful to evaluate the therapeutic potential of hMSCs used in future clinical studies.

DISP3 promotes proliferation and delays differentiation of neural progenitor cells

DISP3 (PTCHD2), a sterol-sensing domain-containing protein, is highly expressed in neural tissue but its role in neural differentiation is unknown. In the present study we used a multipotent cerebellar progenitor cell line, C17.2, to investigate the impact of DISP3 on the proliferation and differentiation of neural precursors. We found that ectopically expressed DISP3 promotes cell proliferation and alters expression of genes that are involved in tumorigenesis. Finally, the differentiation profile of DISP3-expressing cells was altered, as evidenced by delayed expression of neural specific markers and a reduced capacity to undergo neural differentiation.

The NFκB pathway: a therapeutic target in glioblastoma

Cancer initiating cells have been described to be the only cell population with tumorigenic capacity in glioblastoma multiforme, one of the most aggressive and untreatable cancers. Recent work from our group described that NFκB pathway was activated in glioblastoma initiating cells undergoing differentiation, and that blockade of this activation promoted senescence of differentiating cells. NFκB activation in cancer may be the result of either exposure to proinflammatory stimuli in the tumor microenvironment or upregulation of the signaling pathway by upstream regulators. Appropriate control of NFκB activity, which can be achieved by gene modification or pharmacological strategies, would provide a potential approach for the management of NFκB related tumors, including glioblastoma. Here, we summarize the current knowledge of the relevance of NFκB in cancer and its possible role as a target of therapeutic intervention.

Novel regulators of the systemic response to lipopolysaccharide

Our understanding of the role that host genetic factors play in the initiation and severity of infections caused by gram-negative bacteria is incomplete. To identify novel regulators of the host response to lipopolysaccharide (LPS), 11 inbred murine strains were challenged with LPS systemically. In addition to two strains lacking functional TLR4 (C3H/HeJ and C57BL/6J(TLR4-/-)), three murine strains with functional TLR4 (C57BL/6J, 129/SvImJ, and NZW/LacJ) were found to be relatively resistant to systemic LPS challenge; the other six strains were classified as sensitive. RNA from lung, liver, and spleen tissue was profiled on oligonucleotide microarrays to determine if unique transcripts differentiate susceptible and resistant strains. Gene expression analysis identified the Hedgehog signaling pathway and a number of transcription factors (TFs) involved in the response to LPS. RNA interference-mediated inhibition of six TFs (C/EBP, Cdx-2, E2F1, Hoxa4, Nhlh1, and Tead2) was found to diminish IL-6 and TNF-α production by murine macrophages. Mouse lines with targeted mutations were used to verify the involvement of two novel genes in innate immunity. Compared with wild-type control mice, mice deficient in the E2F1 transcription factor were found to have a reduced inflammatory response to systemic LPS, and mice heterozygote for Ptch, a gene involved in Hedgehog signaling, were found to be more responsive to systemic LPS. Our analysis of gene expression data identified novel pathways and transcription factors that regulate the host response to systemic LPS. Our results provide potential sepsis biomarkers and therapeutic targets that should be further investigated in human populations.

Antibody effector mechanisms in myasthenia gravis-pathogenesis at the neuromuscular junction

Myasthenia gravis (MG) is an autoimmune disorder caused by autoantibodies that are either directed to the muscle nicotinic acetylcholine receptor (AChR) or to the muscle-specific tyrosine kinase (MuSK). These autoantibodies define two distinct subforms of the disease-AChR-MG and MuSK-MG. Both AChR and MuSK are expressed on the postsynaptic membrane of the neuromuscular junction (NMJ), which is a highly specialized region of the muscle dedicated to receive and process signals from the motor nerve. Autoantibody binding to proteins of the postsynaptic membrane leads to impaired neuromuscular transmission and muscle weakness. Pro-inflammatory antibodies of the human IgG1 and IgG3 subclass modulate the AChR, cause complement activation, and attract lymphocytes; together acting to decrease levels of the AChR and AChR-associated proteins and to reduce postsynaptic folding. In patients with anti-MuSK antibodies, there is no evidence of loss of junctional folds and no apparent loss of AChR density. Anti-MuSK antibodies are predominantly of the IgG4 isotype, which functionally differs from other IgG subclasses in its anti-inflammatory activity. Moreover, IgG4 undergoes a posttranslational modification termed Fab arm exchange that prevents cross-linking of antigens. These findings suggest that MuSK-MG may be different in etiological and pathological mechanisms from AChR-MG. The effector functions of IgG subclasses on synapse structure and function are discussed in this review.

Activation of Erk by sonic hedgehog independent of canonical hedgehog signalling

Hedgehog (Hh) signalling is mediated through the Patched-1 (Ptch1) receptor. Hh-binding to Ptch1 blocks the inhibitory effects of Ptch1 on the activity of the transmembrane protein, Smoothened (Smo), resulting induction of target genes by the Gli-family of transcription factors. We demonstrate here that Hh-binding to Ptch1 stimulates activation of Erk1/2. This activation is insensitive to the small molecule Smo antagonists and occurs in a cell line that does not express Smo. Specifically, the C-terminus of Ptch1 harbours motifs encoding Class I and II SH3-binding sites. SH3-domain binding activity was verified using GST-c-src(SH3), -Grb2(SH3) and -p85beta(SH3) fusion-proteins. Ectopically expressed Grb2 or p85beta could also be co-immunoprecipitated with the Ptch1 C-terminus. Addition of Shh to serum-starved human mammary epithelial cells and Shh Light II fibroblasts stimulated phosphorylation of Erk1/2. Erk1/2 activation was observed in cells where Smo activity had been inhibited using cyclopamine and in the breast epithelial cell line, MCF10A, that does not express Smo. These data reveal novel binding activities for the C-terminal region of Ptch1 and define a signalling pathway stimulated by the Hh-ligands operating independently of pathways requiring Smo.

How to become immortal: let MEFs count the ways

Understanding the molecular mechanisms and biological consequences of genetic changes occurring during bypass of cellular senescence spans a broad area of medical research from the cancer field to regenerative medicine. Senescence escape and immortalisation have been intensively studied in murine embryonic fibroblasts as a model system, and are known to occur when the p53/ARF tumour suppressor pathway is disrupted. We showed recently that murine fibroblasts with a humanised p53 gene (Hupki cells, from a human p53 knock-in mouse model) first senesce, and then become immortalised in the same way as their homologues with normal murine p53. In both cell types, immortalised cultures frequently sustain either a p53 gene mutation matching a human tumour mutation and resulting in loss of p53 transcriptional transactivation, or a biallelic deletion at the p19/ARF locus. Whilst these genetic events were not unexpected, we were surprised to find that a significant proportion of immortalised cell cultures apparently had neither a p53 mutation nor loss of p19/ARF. Here we consider various routes to p53/ARF disruption in senescence bypass, and dysfunction of other tumour suppressor networks that may contribute to release from tenacious cell cycle arrest in senescent cultures.

Multi-faceted role of HSP40 in cancer

HSP40 (DNAJ) is an understudied family of co-chaperones. The human genome codes for over 41 members of HSP40 family that reside at distinct intracellular locations. Despite their large numbers, little is known about their physiologic roles. Recent research has revealed involvement of some of the DNAJ family members in various types of cancers. In this article we summarize the information about the involvement of human DNAJ family members in various aspects of cancer biology. Furthermore we discuss the potential role of the J domain of DNAJ proteins in cancer biology.

Ability to acquire drug resistance arises early during the tumorigenesis process

Resistance to chemotherapy is one of the principal causes of cancer mortality and is generally considered a late event in tumor progression. Although cellular models of drug resistance have been useful in identifying the molecules responsible for conferring drug resistance, most of these cellular models are derived from cell lines isolated from patients at a late stage in cancer progression. To ask at which stage in the tumorigenic progression does the cell gain the ability to acquire drug resistance, we generated a series of pre-tumorigenic and tumorigenic cells from human embryonic skin fibroblasts by introducing, sequentially, the catalytic subunit of telomerase, SV40 large T and small T oncoproteins, and an oncogenic form of ras. We show that the ability to acquire multidrug resistance (MDR) can arise before the malignant transformation stage. The minimal set of changes necessary to obtain pre-tumorigenic drug-resistant cells is expression of telomerase and inactivation of p53 and pRb. Thus, the pathways inactivated during tumorigenesis also confer the ability to acquire drug resistance. Microarray and functional studies of drug-resistant pre-tumorigenic cells indicate that the drug efflux pump P-glycoprotein is responsible for the MDR phenotype in this pre-tumorigenic cell model.

Promotion of Hras-induced squamous carcinomas by a polymorphic variant of the Patched gene in FVB mice

Mice of the C57BL/6 strain are resistant to the development of skin squamous carcinomas (SCCs) induced by an activated Ras oncogene, whereas FVB/N mice are highly susceptible. The genetic basis of this difference in phenotype is unknown. Here we show that susceptibility to SCC is under the control of a carboxy-terminal polymorphism in the mouse Ptch gene. F1 hybrids between C57BL/6 and FVB/N strains ((B6FVB)F1) are resistant to Ras-induced SCCs, but resistance can be overcome either by elimination of the C57BL/6 Ptch allele (Ptch(B6)) or by overexpression of the FVB/N Ptch allele (Ptch(FVB)) in the epidermis of K5Hras-transgenic (B6FVB)F1 hybrid mice. The human Patched (PTCH) gene is a classical tumour suppressor gene for basal cell carcinomas and medulloblastomas, the loss of which causes increased signalling through the Sonic Hedgehog (SHH) pathway. SCCs that develop in PtchB6+/- mice do not lose the wild-type Ptch gene or show evidence of increased SHH signalling. Although Ptch(FVB) overexpression can promote SCC formation, continued expression is not required for tumour maintenance, suggesting a role at an early stage of tumour cell lineage commitment. The Ptch polymorphism affects Hras-induced apoptosis, and binding to Tid1, the mouse homologue of the Drosophila l(2)tid tumour suppressor gene. We propose that Ptch occupies a critical niche in determining basal or squamous cell lineage, and that both tumour types can arise from the same target cell depending on carcinogen exposure and host genetic background.

Proteasome response to interferon-gamma is altered in senescent human fibroblasts

We have investigated immunoproteasomes in human fibroblasts during replicative senescence. Unlike levels of constitutive proteasome catalytic subunits and 26S proteasome regulatory subunits, levels of immunosubunits did not decrease dramatically in senescent cells. However, the induction of immunosubunits by interferon-gamma (IFN-gamma) was lost in senescent cells. In contrast, levels of the 11S proteasome regulator, PA28, were increased by IFN-gamma even in senescent cells, and both immunosubunits and PA28 increased with the reversible growth arrest in confluent cell cultures. The results highlight differences in the mechanisms of regulation of immunoproteasomes compared to constitutive proteasomes and in the irreversible growth arrest of senescent cells compared to reversible contact-induced growth arrest.

Tid1 Isoforms Are Mitochondrial DnaJ-like Chaperones with Unique Carboxyl Termini That Determine Cytosolic Fate

Tid1 is a human homolog of bacterial DnaJ and the Drosophila tumor suppressor Tid56 that has two alternatively spliced isoforms, Tid1-long and -short (Tid1-L and -S), which differ only at their carboxyl termini. Although Tid1 proteins localize overwhelmingly to mitochondria, published data demonstrate principally nonmitochondrial protein interactions and activities. This study was undertaken to determine whether Tid1 proteins function as mitochondrial DnaJ-like chaperones and to resolve the paradox of how proteins targeted primarily to mitochondria function in nonmitochondrial pathways. Here we demonstrate that Tid1 isoforms exhibit a conserved mitochondrial DnaJ-like function substituting for the yeast mitochondrial DnaJ-like protein Mdj1p. Like Mdj1p, Tid1 localizes to human mitochondrial nucleoids, which are large protein complexes bound to mitochondrial DNA. Unlike other DnaJs, Tid1-L and -S form heterocomplexes; both unassembled and complexed Tid1 are observed in human cells. Results demonstrate that Tid1-L has a longer residency time in the cytosol prior to mitochondrial import as compared with Tid1-S; Tid1-L is also significantly more stable in the cytosol than Tid1-S, which is rapidly degraded. The longer cytosolic residency time and the half-life of Tid1-L are explained by its interaction with cytosolic Hsc70 and potential protein substrates such as the STAT1 and STAT3 transcription factors. We show that the unique carboxyl terminus of Tid1-L is required for interaction with Hsc70 and STAT1 and -3. We propose that the association of Tid1 with chaperones and/or protein substrates in the cytosol provides a mechanism for the alternate fates and functions of Tid1 in mitochondrial and nonmitochondrial pathways.

Tid1 is a Smad-binding protein that can modulate Smad7 activity in developing embryos

In a search for binding partners to Smad8, we identified the chicken homologue of the mammalian Tid1 protein (cTid1), which is a regulator of apoptosis related to the Drosophila tumour suppressor Tid56. The cTid1 coding sequence is highly conserved with mammalian Tid1, including the DnaJ domain that interacts with Hsp70 (heat-shock protein 70). The cTid1 gene is widely expressed with transcripts enriched in the developing blood islands of the embryonic-yolk sac. We show that cTid1 can bind to other members of the Smad family and that highest binding activity occurs with the negative regulatory Smad7, through the conserved MH2 domain. This interaction can have functional relevance in vivo, since co-expression of Tid1 blocks the dorsalizing and BMP (bone morphogenetic protein)-dependent regulatory activity of Smad7 in developing Xenopus embryos. The finding that these proteins can interact suggests the potential for linking two important cell survival/apoptosis pathways.

Human Tumorous Imaginal Disc 1 (TID1) Associates with Trk Receptor Tyrosine Kinases and Regulates Neurite Outgrowth in nnr5-TrkA Cells

The human tumorous imaginal disc 1 (TID1) proteins including TID1(L) and TID1(S), members of the DnaJ domain protein family, are involved in multiple intracellular signaling pathways such as apoptosis induction, cell proliferation, and survival. Here we report that TID1 associates with the Trk receptor tyrosine kinases and regulates nerve growth factor (NGF)-induced neurite outgrowth in PC12-derived nnr5 cells. Binding assays and transfection studies showed that the carboxyl-terminal end of TID1 (residues 224-429) bound to Trk at the activation loop (Tyr(P)(683)-Tyr(684)(P)(684) in rat TrkA) and that TID1 was tyrosine phosphorylated by Trk both in yeast and in transfected cells. Moreover endogenous TID1 was also tyrosine phosphorylated by and co-immunoprecipitated with Trk in neurotrophin-stimulated primary rat hippocampal neurons. Overexpression studies showed that both TID1(L) and TID1(S) significantly facilitated NGF-induced neurite outgrowth in TrkA-expressing nnr5 cells possibly through a mechanism involving increased activation of mitogen-activated protein kinase. Consistently knockdown of endogenous TID1, mediated with specific short hairpin RNA, significantly reduced NGF-induced neurite growth in nnr5-TrkA cells. These data provide the first evidence that TID1 is a novel intracellular adaptor that interacts with the Trk receptor tyrosine kinases in an activity-dependent manner to facilitate Trk-dependent intracellular signaling.