Induction of novel Grp75 isoforms by 2-deoxyglucose in human and murine fibroblasts

Aged gastrocnemius muscle of mice positively responds to a late onset adapted physical training

Introduction: A regular physical training is known to contribute to preserve muscle mass and strength, maintaining structure and function of neural and vascular compartments and preventing muscle insulin resistance and inflammation. However, physical activity is progressively reduced during aging causing mobility limitations and poor quality of life. Although physical exercise for rehabilitation purposes (e.g., after fractures or cardiovascular events) or simply aiming to counteract the development of sarcopenia is frequently advised by physicians, nevertheless few data are available on the targets and the global effects on the muscle organ of adapted exercise especially if started at old age. Methods: To contribute answering this question for medical translational purposes, the proteomic profile of the gastrocnemius muscle was analyzed in 24-month-old mice undergoing adapted physical training on a treadmill for 12 weeks or kept under a sedentary lifestyle condition. Proteomic data were implemented by morphological and morphometrical ultrastructural evaluations. Results and Discussion: Data demonstrate that muscles can respond to adapted physical training started at old age, positively modulating their morphology and the proteomic profile fostering protective and saving mechanisms either involving the extracellular compartment as well as muscle cell components and pathways (i.e., mitochondrial processes, cytoplasmic translation pathways, chaperone-dependent protein refolding, regulation of skeletal muscle contraction). Therefore, this study provides important insights on the targets of adapted physical training, which can be regarded as suitable benchmarks for future in vivo studies further exploring the effects of this type of physical activity by functional/metabolic approaches.

Mortalin: Protein partners, biological impacts, pathological roles, and therapeutic opportunities

Mortalin (GRP75, HSPA9A), a heat shock protein (HSP), regulates a wide range of cellular processes, including cell survival, growth, and metabolism. The regulatory functions of mortalin are mediated through a diverse set of protein partners associated with different cellular compartments, which allows mortalin to perform critical functions under physiological conditions, including mitochondrial protein quality control. However, alteration of mortalin's activities, its abnormal subcellular compartmentalization, and its protein partners turn mortalin into a disease-driving protein in different pathological conditions, including cancers. Here, mortalin's contributions to tumorigenic pathways are explained. Pathology information based on mortalin's RNA expression extracted from The Cancer Genome Atlas (TCGA) transcriptomic database indicates that mortalin has an independent prognostic value in common tumors, including lung, breast, and colorectal cancer (CRC). Subsequently, the binding partners of mortalin reported in different cellular models, from yeast to mammalian cells, and its regulation by post-translational modifications are discussed. Finally, we focus on colorectal cancer and discuss how mortalin and its tumorigenic downstream protein targets are regulated by a ubiquitin-like protein through the 26S proteasomal degradation machinery. A broader understanding of the function of mortalin and its positive and negative regulation in the formation and progression of human diseases, particularly cancer, is essential for developing new strategies to treat a diverse set of human diseases critically associated with dysregulated mortalin.

Identification of a new member of Mortaparib class of inhibitors that target mortalin and PARP1

Mortalin, a heat shock family protein enriched in cancer cells, is known to inactivate tumor suppressor protein p53. Abrogation of mortalin-p53 interaction and reactivation of p53 has been shown to trigger growth arrest/apoptosis in cancer cells and hence, suggested to be useful in cancer therapy. In this premise, we earlier screened a chemical library to identify potential disruptors of mortalin-p53 interaction, and reported two novel synthetic small molecules (5-[1-(4-methoxyphenyl) (1,2,3,4-tetraazol-5-yl)]-4-phenylpyrimidine-2-ylamine) and (4-[(1E)-2-(2-phenylindol-3-yl)-1-azavinyl]-1,2,4-triazole) called Mortaparib and Mortaparib^Plus, respectively. These compounds were shown to possess anticancer activity that was mediated through targeting mortalin and PARP1 proteins, essential for cancer cell survival and proliferation. Here, we report characterization of the third compound, {4-[(4-amino-5-thiophen-2-yl-1,2,4-triazol-3-yl)sulfanylmethyl]-N-(4-methoxyphenyl)-1,3-thiazol-2-amine}, isolated in the same screening. Extensive computational and molecular analyses suggested that the new compound has the capability to interact with mortalin, p53, and PARP1. We provide evidence that this new compound, although required in high concentration as compared to the earlier two compounds (Mortaparib and Mortaparib^Plus) and hence called Mortaparib^Mild, also downregulates mortalin and PARP1 expression and functions in multiple ways impeding cancer cell proliferation and migration characteristics. Mortaparib^Mild is a novel candidate anticancer compound that warrants further experimental and clinical attention.

Similarities and Differences of Hsp70, hsc70, Grp78 and Mortalin as Cancer Biomarkers and Drug Targets

Background: Upregulation of Heath Shock Protein 70 (HSP70) chaperones supports cancer cell survival. Their high homology causes a challenge to differentiate them in experimental or prevention and treatment strategies. The objective of this investigation was to determine similarities and differences of Hsp70, hsc70, Grp78 and Mortalin members of the HSP70 family encoded by HSPA1, HSPA8, HSPA5 and HSPA9 genes, respectively. Methods: Literature reviews were conducted using HSPA1, HSPA5, HSPA8 and HSPA9 gene or protein names or synonyms combined with biological or cancer-relevant terms. Ingenuity Pathway Analysis was used to identify and compare profiles of proteins that directly bind individual chaperones and their associated pathways. TCGA data was probed to identify associations of hsc70 with cancer patient survival. ClinicalTrials.gov was used to identify HSP70 family studies. Results: The chaperones have similar protein folding functions. Their different cellular effects are determined by co-chaperones and client proteins combined with their intra- and extra-cellular localizations. Their upregulation is associated with worse patient prognosis in multiple cancers and can stimulate tumor immune responses or drug resistance. Their inhibition selectively kills cancer over healthy cells. Conclusions: Differences in Hsp70, hsc70, Grp78 and mortalin provide opportunities to calibrate HSP70 inhibitors for individual cancers and combination therapies.

Intracellular partners of fibroblast growth factors 1 and 2 - implications for functions

Fibroblast growth factors 1 and 2 (FGF1 and FGF2) are mainly considered as ligands of surface receptors through which they regulate a broad spectrum of biological processes. They are secreted in non-canonical way and, unlike other growth factors, they are able to translocate from the endosome to the cell interior. These unique features, as well as the role of the intracellular pool of FGF1 and FGF2, are far from being fully understood. An increasing number of reports address this problem, focusing on the intracellular interactions of FGF1 and 2. Here, we summarize the current state of knowledge of the FGF1 and FGF2 binding partners inside the cell and the possible role of these interactions. The partner proteins are grouped according to their function, including proteins involved in secretion, cell signaling, nucleocytoplasmic transport, binding and processing of nucleic acids, ATP binding, and cytoskeleton assembly. An in-depth analysis of the network of these binding partners could indicate novel, non-classical functions of FGF1 and FGF2 and uncover an additional level of a fine control of the well-known FGF-regulated cellular processes.

PCDHGB7 Increases Chemosensitivity to Carboplatin by Inhibiting HSPA9 via Inducing Apoptosis in Breast Cancer

Breast cancer is one of the most serious cancers worldwide, and chemotherapy resistance frequently drives cancer progression. Triple-negative breast cancer (TNBC) has a high recurrence rate and poor prognosis given its resistance to chemotherapy. In our previous study, we found a remarkable abnormal methylation modification of the PCDHGB7 gene in breast cancer. However, the roles of PCDHGB7 in the progression and treatment of breast cancer are unclear. In this study, we examined the effects of PCDHGB7 on the sensitivity of TNBC cells to carboplatin and investigated the underlying mechanism. By knocking down and overexpressing PCDHGB7 in HS578T and BT549 cells, we confirmed that PCDHGB7 increases TNBC cell chemosensitivity to carboplatin. Mechanistically, we found that PCDHGB7 negatively regulates the expression of HSPA9, uplifting its inhibition on P53 translocation and caspase-3 activation. Thus, we demonstrated that PCDHGB7 increases chemosensitivity of TNBC cells to carboplatin by inhibiting HSPA9 via inducing apoptosis. PCDHGB7 and HSPA9 represent potential therapeutic targets for chemosensitivity in breast cancer.

Mortalin promotes cell proliferation and epithelial mesenchymal transition of intrahepatic cholangiocarcinoma cells in vitro

AIMS

The prognosis of patients with intrahepatic cholangiocarcinoma (ICC) remains poor in terms of overall survival (OS) and recurrence rate. Mortalin, a stress chaperone, has been reported to be involved in carcinogenesis and metastasis. However, its role in ICC has not been defined.

METHODS

Mortalin expression in tumour samples from patients with ICC was examined by Western blot and immunohistochemistry, and correlation between its expression and clinicopathological features was assessed. In addition, invasion, migration proliferation and apoptosis, and the expression of epithelial-mesenchymal transition (EMT)-related markers in ICC cells were assessed after mortalin depletion. Finally, the prognostic significance of mortalin in patients with ICC was further evaluated by Kaplan-Meier and Cox regression analysis.

RESULTS

We provide evidence that expression of mortalin in human ICC tissues is higher than that in matched peritumoural tissues. The interference of mortalin expression inhibited the proliferation and invasion of ICC cells in vitro. Mechanistically, inhibition of mortalin expression in ICC cells upregulated E-cadherin expression and decreased vimentin and snail expression. Clinically, a high level of mortalin in ICC samples was associated with loss of E-cadherin, and increased expression of vimentin and snail. Patients with ICC and high mortalin expression had a shorter OS and a higher recurrence rate. Multivariate analysis revealed that mortalin overexpression was an independent prognostic indicator for patients with ICC.

CONCLUSIONS

Mortalin may promote cell proliferation and invasion via induction of EMT of ICC cells. A high level of mortalin may be used as a prognostic biomarker and therapeutic target for patients with ICC.

Mortalin (GRP75/HSPA9) upregulation promotes survival and proliferation of medullary thyroid carcinoma cells

Medullary thyroid carcinoma (MTC) is a neuroendocrine tumor mainly caused by mutations in the rearranged during transfection (RET) proto-oncogene. For therapy of advanced MTC, the Food and Drug Administration recently approved vandetanib and cabozantinib, the tyrosine kinase inhibitors targeting RET, vascular endothelial growth factor receptor, epidermal growth factor receptor and/or c-MET. Nevertheless, not all patients respond to these drugs, demanding additional therapeutic strategies. We found that mortalin (HSPA9/GRP75), a member of HSP70 family, is upregulated in human MTC tissues and that its depletion robustly induces cell death and growth arrest in MTC cell lines in culture and in mouse xenografts. These effects were accompanied by substantial downregulation of RET, induction of the tumor-suppressor TP53 and altered expression of cell cycle regulatory machinery and apoptosis markers, including E2F-1, p21(CIP1), p27(KIP1) and Bcl-2 family proteins. Our investigation of the molecular mechanisms underlying these effects revealed that mortalin depletion induces transient MEK/ERK (extracellular signal-regulated kinase) activation and altered mitochondrial bioenergetics in MTC cells, as indicated by depolarized mitochondrial membrane, decreased oxygen consumption and extracellular acidification and increased oxidative stress. Intriguingly, mortalin depletion induced growth arrest partly via the MEK/ERK pathway, whereas it induced cell death by causing mitochondrial dysfunction in a Bcl-2-dependent manner. However, TP53 was not necessary for these effects except for p21(CIP1) induction. Moreover, mortalin depletion downregulated RET expression independently of MEK/ERK and TP53. These data demonstrate that mortalin is a key regulator of multiple signaling and metabolic pathways pivotal to MTC cell survival and proliferation, proposing mortalin as a novel therapeutic target for MTC.

Mortalin is a prognostic factor of gastric cancer with normal p53 function

BACKGROUND

Mortalin is a heat-non-inducible member of the heat shock protein 70 family. Mortalin binds to p53 and prevents p53 from entering the nucleus. To understand the significance of mortalin in gastric cancer, we investigated the expression of mortalin and p53.

METHODS

Expression of mortalin and p53 was examined by immunohistochemical staining of 182 clinical samples of gastric cancer.

RESULTS

Mortalin-positive and aberrant p53-positive tumors were found in 75.2 and 49.5 % of cases, respectively. Mortalin-positive tumors were deeper in invasion and had more lymph node and liver metastases compared with mortalin-negative tumors (P < 0.01, P < 0.05, respectively). Mortalin-positive tumors had worse prognosis compared with mortalin-negative tumors (P = 0.035). Moreover, in tumors with normal p53 function, mortalin-positive tumors had worse prognosis compared with mortalin-negative tumors (P = 0.017).

CONCLUSIONS

Mortalin has a great impact on gastric cancer with normal p53. Therefore, mortalin is a target molecule for treatment of gastric cancer, as well as a promising prognostic factor, especially in tumors with normal p53.

Expression profile of telomere-associated genes in multiple myeloma

To further contribute to the understanding of multiple myeloma, we have focused our research interests on the mechanisms by which tumour plasma cells have a higher survival rate than normal plasma cells. In this article, we study the expression profile of genes involved in the regulation and protection of telomere length, telomerase activity and apoptosis in samples from patients with monoclonal gammopathy of undetermined significance, smouldering multiple myeloma, multiple myeloma (MM) and plasma cell leukaemia (PCL), as well as several human myeloma cell lines (HMCLs). Using conventional cytogenetic and fluorescence in situ hybridization studies, we identified a high number of telomeric associations (TAs). Moreover, telomere length measurements by terminal restriction fragment (TRF) assay showed a shorter mean TRF peak value, with a consistent correlation with the number of TAs. Using gene expression arrays and quantitative PCR we identified the hTERT gene together with 16 other genes directly involved in telomere length maintenance: HSPA9, KRAS, RB1, members of the Small nucleolar ribonucleoproteins family, A/B subfamily of ubiquitously expressed heterogeneous nuclear ribonucleoproteins, and 14-3-3 family. The expression levels of these genes were even higher than those in human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs), which have unlimited proliferation capacity. In conclusion, the gene signature suggests that MM tumour cells are able to maintain stable short telomere lengths without exceeding the short critical length, allowing cell divisions to continue. We propose that this could be a mechanism contributing to MM tumour cells expansion in the bone marrow (BM).

A mortalin/HSPA9-mediated switch in tumor-suppressive signaling of Raf/MEK/extracellular signal-regulated kinase

Dysregulated Raf/MEK/extracellular signal-regulated kinase (ERK) signaling, a common hallmark of tumorigenesis, can trigger innate tumor-suppressive mechanisms, which must be inactivated for carcinogenesis to occur. This innate tumor-suppressive signaling may provide a potential therapeutic target. Here we report that mortalin (HSPA9/GRP75/PBP74) is a novel negative regulator of Raf/MEK/ERK and may provide a target for the reactivation of tumor-suppressive signaling of the pathway in cancer. We found that mortalin is present in the MEK1/MEK2 proteome and is upregulated in human melanoma biopsy specimens. In different MEK/ERK-activated cancer cell lines, mortalin depletion induced cell death and growth arrest, which was accompanied by increased p21(CIP1) transcription and MEK/ERK activity. Remarkably, MEK/ERK activity was necessary for mortalin depletion to induce p21(CIP1) expression in B-Raf(V600E)-transformed cancer cells regardless of their p53 status. In contrast, in cell types exhibiting normal MEK/ERK status, mortalin overexpression suppressed B-Raf(V600E)- or ΔRaf-1:ER-induced MEK/ERK activation, p21(CIP1) expression, and cell cycle arrest. Other HSP70 family chaperones could not effectively replace mortalin for p21(CIP1) regulation, suggesting a unique role for mortalin. These findings reveal a novel mechanism underlying p21(CIP1) regulation in MEK/ERK-activated cancer and identify mortalin as a molecular switch that mediates the tumor-suppressive versus oncogenic result of dysregulated Raf/MEK/ERK signaling. Our study also demonstrates that p21(CIP1) has dual effects under mortalin-depleted conditions, i.e., mediating cell cycle arrest while limiting cell death.

HSPA9 overexpression inhibits apoptin-induced apoptosis in the HepG2 cell line

Apoptin, a small protein derived from chicken anemia virus, possesses the capacity to specifically kill tumor cells while leaving normal cells intact. Previous studies have indicated that the subcellular localization of apoptin appears to be crucial for this tumor-selective activity. Apoptin resides in the cytoplasm of normal cells; however, in cancer cells it translocates into the nucleus. In the present study, purified prokaryotic native His-apoptin served as a bait for capturing apoptin-associated proteins in both a hepatoma carcinoma cell line (HepG2) and a human fetal liver cell line (L-02). The captured proteins obtained from a pull-down assay were separated by two-dimensional gel electrophoresis. Mass spectrometry was employed to detect the effect of HSPA9 overexpression (one of the interacting proteins with apoptin in vitro) and downregulation of HSPA9 on HepG2 cells. The data revealed that HSPA9 overexpression resulted in partial distribution of apoptin in the cytoplasm. Notably, HSPA9 overexpression markedly decreased the apoptosis rate of HepG2 cells from 41.2 to 31.7%, while the downregulation of HSPA9 using small interfering RNA significantly enhanced the apoptosis of HepG2 cells. Our results suggest new insights into the localization mechanism of apoptin which is tightly associated with HSPA9 overexpression and its crucial role in cellular apoptosis both in a tumor cell line (HepG2) and a normal cell line (L-02). These findings shed new light on the elucidation of the underlying mechanism of anticancer action of apoptin.

Elevated levels of mitochondrial mortalin and cytosolic HSP70 in blood as risk factors in patients with colorectal cancer

Mortalin/GRP75 is a ubiquitous mitochondrial chaperone related to the cytosolic heat shock protein 70 (HSP70). It protects cells from senescence and apoptosis and is overexpressed in cancer cells. Cell resistance to complement-dependent cytotoxicity depends on mortalin and during complement attack mortalin is released from cells. Our goal was to determine whether cancer patients have circulating mortalin in blood. The significance of mortalin in blood to survival prospects of colorectal cancer patients was evaluated. Occurrence of extracellular soluble HSP70 (sHSP70) is documented. We developed a sensitive ELISA for mortalin. The association between mortalin level and survival was subjected to the Cox proportional hazards analysis (univariate and multivariate analyses). Mortalin concentration in serum of colorectal cancer patients was 10-214 ng/ml. Survival data of the patients were known from an earlier study of sHSP70 in these samples. Cox regression analysis indicated that high mortalin (>60 ng/ml) is a risk factor for shorter survival. Serum levels of sHSP70 and mortalin in patients were independent variables. Concurrence of high sHSP70 and mortalin was associated with rapid disease progression (HR = 4, 2.04-8.45, p < 0.001). Addition of high sHSP70 and mortalin to a baseline model of age, sex and TNM stage, significantly (p < 0.001) enhanced the risk score to 8 (3.26-20.46). This is the first demonstration of circulating mortalin in cancer patients. Analysis of mortalin in blood, and even more so of mortalin and sHSP70, adds a high prognostic value to the TNM stage and will identify colorectal cancer patients at high risk of poor survival.

Mortalin, apoptosis, and neurodegeneration

Mortalin is a highly conserved heat-shock chaperone usually found in multiple subcellular locations. It has several binding partners and has been implicated in various functions ranging from stress response, control of cell proliferation, and inhibition/prevention of apoptosis. The activity of this protein involves different structural and functional mechanisms, and minor alterations in its expression level may lead to serious biological consequences, including neurodegeneration. In this article we review the most current data associated with mortalin's binding partners and how these protein-protein interactions may be implicated in apoptosis and neurodegeneration. A complete understanding of the molecular pathways in which mortalin is involved is important for the development of therapeutic strategies for cancer and neurodegenerative diseases.

Identification and validation of mouse sperm proteins correlated with epididymal maturation

Sperm need to mature in the epididymis to become capable of fertilization. To understand the molecular mechanisms of mouse sperm maturation, we conducted a proteomic analysis using saturation dye labeling to identify proteins of caput and cauda epididymal sperm that exhibited differences in amounts or positions on two-dimensional gels. Of eight caput epididymal sperm-differential proteins, three were molecular chaperones and three were structural proteins. Of nine cauda epididymal sperm-differential proteins, six were enzymes of energy metabolism. To validate these proteins as markers of epididymal maturation, immunoblotting and immunofluorescence analyses were performed. During epididymal transit, heat shock protein 2 was eliminated with the cytoplasmic droplet and smooth muscle γ-actin exhibited reduced fluorescence from the anterior acrosome while the signal intensity of aldolase A increased, especially in the principal piece. Besides these changes, we observed protein spots, such as glutathione S-transferase mu 5 and the E2 component of pyruvate dehydrogenase complex, shifting to more basic isoelectric points, suggesting post-translational changes such dephosphorylation occur during epididymal maturation. We conclude that most caput epididymal sperm-differential proteins contribute to the functional modification of sperm structures and that many cauda epididymal sperm-differential proteins are involved in ATP production that promotes sperm functions such as motility.

Mortalin is a novel mediator of erythropoietin signaling

Erythropoietin (EPO) stimulates erythroid growth by enhancing the proliferation, maturation and survival of late-stage erythroid progenitor cells. However, the entire process of EPO stimulation remains undetermined. To further clarify the intracellular mechanisms by which EPO affects the growth of erythroid progenitor cells, we analyzed proteins obtained from purified human erythroid colony-forming cells (ECFCs) cultured with or without EPO, and one of the proteins apparently related with EPO stimuli was identified as mortalin (mthsp70/PBP74/Grp75/mot-2), which is a member of the heat shock protein 70 family of chaperones. The amount of mortalin mRNA in ECFCs increased in an EPO dose-dependent manner, and ECFC growth was dependent on the amount of mortalin. Furthermore, expression of mortalin in ECFCs was suppressed by a phosphatidylinositol 3-kinase inhibitor. Finally, we analyzed gene expression patterns in ECFCs cultured with or without EPO after treatment with mortalin small interfering RNA (siRNA) using a DNA microarray. When ECFCs treated with mortalin siRNA were cultured with EPO, the expression of several genes overlapped with the profile seen in control ECFCs cultured without EPO. Our data suggest that mortalin is involved in the mediation of EPO signaling and plays an important role in stimulating the growth of erythroid progenitor cells.

Proteomic and functional alterations in brain mitochondria from Tg2576 mice occur before amyloid plaque deposition

Synaptic dysfunction is an early event in Alzheimer's disease patients and has also been detected in transgenic mouse models. In the present study, we analyzed proteomic changes in synaptosomal fractions from Tg2576 mice that overexpress mutant human amyloid precursor protein (K670N, M671L) and from their nontransgenic littermates. Cortical and hippocampal tissue was microdissected at the onset of cognitive impairment, but before deposition of amyloid plaques. Crude synaptosomal fractions were prepared by differential centrifugation, proteins were separated by 2-D DIGE and identified by MS/MS. Significant alterations were detected in mitochondrial heat shock protein 70 pointing to a mitochondrial stress response. Subsequently, synaptosomal versus nonsynaptic mitochondria were purified from Tg2576 mice brains by density gradient centrifugation. Mitochondrial proteins were separated by IEF or Blue-native gel electrophoresis in the first dimension and SDS-PAGE in the second dimension. Numerous changes in the protein subunit composition of the respiratory chain complexes I and III were identified. Levels of corresponding mRNAs remain unchanged as shown by Affymetrix oligonucleotide array analysis. Functional examination revealed impaired state 3 respiration and uncoupled respiration in brain mitochondria from young Tg2576 mice. By immunoblotting, amyloid-beta oligomers were detected in synaptosomal fractions from Tg2576 mice and reduced glucose metabolism was observed in Tg2576 mice brains by [14C]-2-deoxyglucose infusion. Taken together, we demonstrate alterations in the mitochondrial proteome and function that occur in Tg2576 mice brains before amyloid plaque deposition suggesting that mitochondria are early targets of amyloid-beta aggregates.

Mortalin sensitizes human cancer cells to MKT-077-induced senescence

Mortalin is a chaperone protein that functions in many cellular processes such as mitochondrial biogenesis, intracellular trafficking, cell proliferation and signaling. Its upregulation in many human cancers makes it a candidate target for therapeutic intervention by small molecule drugs. In continuation to our earlier studies showing mortalin as a cellular target of MKT-077, a mitochondrion-seeking delocalized cationic dye that causes selective death of cancer cells, in this work, we report that MKT-077 binds to the nucleotide-binding domain of mortalin, causes tertiary structural changes in the protein, inactivates its chaperone function, and induces senescence in human tumor cell lines. Interestingly, in tumor cells with elevated level of mortalin expression, fairly low drug doses were sufficient to induce senescence. Guided by molecular screening for mortalin in tumor cells, our results led to the idea that working at low doses of the drug could be an alternative senescence-inducing cancer therapeutic strategy that could, in theory, avoid renal toxicities responsible for the abortion of MKT-077 clinical trials. Our work may likely translate to a re-appraisal of the therapeutic benefits of low doses of several classes of anti-tumor drugs, even of those that had been discontinued due to adverse effects.

Three faces of mortalin: a housekeeper, guardian and killer

Mortalin was first cloned as a mortality factor that existed in the cytoplasmic fractions of normal, but not in immortal, mouse fibroblasts. A decade of efforts have expanded its persona from a house keeper protein involved in mitochondrial import, energy generation and chaperoning of misfolded proteins, to a guardian of stress that has multiple binding partners and to a killer protein that contributes to carcinogenesis on one hand and to old age disorders on the other. Being proved to be an attractive target for cancer therapy, it also warrants attention from the perspectives of management of old age diseases and healthy aging.

Upregulation of mortalin/mthsp70/Grp75 contributes to human carcinogenesis

Mortalin, also known as mthsp70/GRP75/PBP74, interacts with the tumor suppressor protein p53 and inactivates its transcriptional activation and apoptotic functions. Here, we examined the level of mortalin expression in a large variety of tumor tissues, tumor-derived and in vitro immortalized human cells. It was elevated in many human tumors, and in all of the tumor-derived and in vitro immortalized cells. In human embryonic fibroblasts immortalized with an expression plasmid for hTERT, the telomerase catalytic subunit, with or without human papillomavirus E6 and E7 genes, we found that subclones with spontaneously increased mortalin expression levels became anchorage-independent and acquired the ability to form tumors in nude mice. Furthermore, overexpression of mortalin was sufficient to increase the malignancy of breast carcinoma cells. The study demonstrates that upregulation of mortalin contributes significantly to tumorigenesis, and thus is a good candidate target for cancer therapy.

Mortalin/GRP75 promotes release of membrane vesicles from immune attacked cells and protection from complement-mediated lysis

The membrane attack complex (MAC) of the complement system is causing membrane damage and cell death. For protection, cells have adopted several resistance mechanisms, including removal of the membrane-inserted MAC by vesiculation. To identify proteins involved in MAC vesiculation, extracellular proteins released from K562 cells in response to treatment with sub-lytic complement were separated by acrylamide gel electrophoresis and protein bands were extracted, digested into peptides and the peptides were analyzed by mass spectrometry. A 75-kDa protein that was abundant in the supernatant of complement-treated cells was identified as mortalin/GRP75. Analysis by western blotting demonstrated that as early as 5 min after exposure to sub-lytic doses of complement, mortalin was released from K562 cells. Mortalin was released after complete activation of the complement system and formation of C5b-8, and even more so when C5b-9 was formed. Other pore formers, such as streptolysin O and melittin, did not induce release of mortalin. As shown, mortalin can bind to complement C8 and C9 and is shed in vesicles containing C9 and complement MACs. Anti-mortalin antibodies reduced mortalin release from complement-treated cells and elevated the extent of cell death by complement. Inhibitors of protein kinase C and extracellular signal-regulated protein kinase also prevented mortalin release from complement-activated cells. These results suggest that mortalin/GRP75 promotes the shedding of membrane vesicles loaded with complement MAC and protects cells from complement-mediated lysis.

Mortalin is over-expressed by colorectal adenocarcinomas and correlates with poor survival

Using comparative proteomic analysis we have identified over-expression of mortalin in colorectal adenocarcinomas. Mortalin, also known as mitochondrial heat-shock protein 70 (mhsp 70), is involved in cell cycle regulation with important roles in cellular senescence and immortalization pathways. It is known to bind to and inactivate wild-type tumour suppressor protein p53 and influences the Ras-Raf-MAPK pathway. By immunostaining a colorectal cancer tissue microarray linked to a patient database, we further found that mortalin over-expression correlates with poor patient survival and, in multivariate analysis, is independent of standard prognostic variables (p = 0.04). Our findings demonstrate that mortalin over-expression may predict outcome in colorectal cancer and suggest that this protein is involved in colorectal neoplasia. Our experimental approach emphasises the analytical power of combining proteomics with tissue microarray analysis in the context of a well-defined tumour database.

An Hsp70 family chaperone, mortalin/mthsp70/PBP74/Grp75: what, when, and where?

Mortalin/mthsp70/PBP74/Grp75 (called mortalin hereafter), a member of the Hsp70 family of chaperones, was shown to have different subcellular localizations in normal and immortal cells. It has been assigned to multiple subcellular sites and implicated in multiple functions ranging from stress response, intracellular trafficking, antigen processing, control of cell proliferation, differentiation, and tumorigenesis. The present article compiles and reviews information on the multiple sites and functions of mortalin in different organisms. The relevance of its differential distributions and functions in normal and immortal cell phenotypes is discussed.

Identification and characterization of molecular interactions between glucose-regulated proteins (GRPs) mortalin/GRP75/peptide-binding protein 74 (PBP74) and GRP94

A heat-shock protein (hsp) 70 family member mortalin/glucose-regulated protein (GRP) 75/peptide-binding protein 74 (PBP74) has been localized to various cellular compartments including mitochondria, endoplasmic reticulum and cytoplasmic vesicles. Here we describe its interactions with an endoplasmic reticulum protein GRP94, a member of the hsp90 family of GRPs. Interactions were identified, confirmed and characterized by far-Western screening, in vivo reporter and co-immunoprecipitation assays. Interacting domains of the two proteins were also characterized by mutational analysis. Such interactions of these two GRPs may be important for function of either or both and therefore provide important information for further studies.

An N-terminal region of mot-2 binds to p53 in vitro

The mouse mot-2 protein was earlier shown to bind to the tumor suppressor protein, p53. The mot-2 binding site of p53 was mapped to C-terminal amino acid residues 312-352, which includes the cytoplasmic sequestration domain. In the present study, we have found that both mot-1 and mot-2 bind to p53 in vitro. By using His-tagged deletion mutant proteins, the p53-binding domain of mot-2 was mapped to its N-terminal amino acid residues 253-282, which are identical in mot-1 and mot-2 proteins. Some peptides containing the p53-binding region of mot-2 were able to compete with the full-length protein for p53 binding. The data provided rationale for in vitro binding of mot-1 and mot-2 proteins to p53 and supported the conclusion that inability of mot-1 protein to bind p53 in vivo depends on secondary structure or its binding to other cellular factors. Most interestingly, the p53-binding region of mot-2 was common to its MKT-077, a cationic dye that exhibits antitumor activity, binding region. Therefore it is most likely that MKT-077-induced nuclear translocation and restoration of wild-type p53 function in transformed cells takes place by a competitional mechanism.

Regulation of System A amino acid transport in L6 rat skeletal muscle cells by insulin, chemical and hyperthermic stress

In this study we have investigated the effects of insulin, chemical and hyperthermic stresses upon the activity of the System A amino acid transporter in L6 rat muscle cells. Uptake of alpha-methyl-aminoisobutyric acid (Me-AIB), a non-metabolisable System A substrate, was increased by between 50% and 80% when muscle cells were exposed to a maximally effective concentration of insulin (100 nM), sodium arsenite (ARS, 0.5 mM) or a 42 degrees C heat shock (HS). The observed activation in System A in response to all three stimuli was maximal within 20 min and in the case of insulin and ARS primarily involved an increase in the Vmax of System A transport. In contrast, HS induced significant increases in both Vmax and Km of System A transport suggesting that hyperthermic stress may activate System A by a mechanism distinct from that mediating the effects of insulin and ARS. The hormonal stimulation of System A was blocked by the phosphoinositide 3-kinase (PI3k) inhibitor, wortmannin, but not by rapamycin or PD 98059 which respectively inhibit the mTOR and classical MAP kinase pathways. Exposure of L6 cells to ARS, but not HS, caused a 4.7-fold stimulation in MAPKAP-K2 activity that was blocked by SB 203580, a specific inhibitor of the stress activated protein kinase SAPK2/p38. However, neither SB 203580, rapamycin nor wortmannin were able to suppress the ARS- or HS-induced stimulation in System A transport. In summary, our results demonstrate that activity of the System A transporter can be rapidly upregulated in response to hormonal and stress stimuli through changes in the transport kinetics of the System A carrier. Our data show that whilst the hormonal response is PI3k dependent, the signalling mechanisms which instigate changes in System A activity in response to chemical or hyperthermic stress do not appear to involve PI3k or components of the mTOR, p42/p44 MAP kinase or SAPK2/p38 signalling pathways.