Mortaparib, a novel dual inhibitor of mortalin and PARP1, is a potential drug candidate for ovarian and cervical cancers.

BACKGROUND: Mortalin is enriched in a large variety of cancers and has been shown to contribute to proliferation and migration of cancer cells in multiple ways. It has been shown to bind to p53 protein in cell cytoplasm and nucleus causing inactivation of its tumor suppressor activity in cancer cells. Several other activities of mortalin including mitochondrial biogenesis, ATP production, chaperoning, anti-apoptosis contribute to pro-proliferative and migration characteristics of cancer cells. Mortalin-compromised cancer cells have been shown to undergo apoptosis in in vitro and in vivo implying that it could be a potential target for cancer therapy. METHODS: We implemented a screening of a chemical library for compounds with potential to abrogate cancer cell specific mortalin-p53 interactions, and identified a new compound (named it as Mortaparib) that caused nuclear enrichment of p53 and shift in mortalin from perinuclear (typical of cancer cells) to pancytoplasmic (typical of normal cells). Biochemical and molecular assays were used to demonstrate the effect of Mortaparib on mortalin, p53 and PARP1 activities. RESULTS: Molecular homology search revealed that Mortaparib is a novel compound that showed strong cytotoxicity to ovarian, cervical and breast cancer cells. Bioinformatics analysis revealed that although Mortaparib could interact with mortalin, its binding with p53 interaction site was not stable. Instead, it caused transcriptional repression of mortalin leading to activation of p53 and growth arrest/apoptosis of cancer cells. By extensive computational and experimental analyses, we demonstrate that Mortaparib is a dual inhibitor of mortalin and PARP1. It targets mortalin, PARP1 and mortalin-PARP1 interactions leading to inactivation of PARP1 that triggers growth arrest/apoptosis signaling. Consistent with the role of mortalin and PARP1 in cancer cell migration, metastasis and angiogenesis, Mortaparib-treated cells showed inhibition of these phenotypes. In vivo tumor suppression assays showed that Mortaparib is a potent tumor suppressor small molecule and awaits clinical trials. CONCLUSION: These findings report (i) the discovery of Mortaparib as a first dual inhibitor of mortalin and PARP1 (both frequently enriched in cancers), (ii) its molecular mechanism of action, and (iii) in vitro and in vivo tumor suppressor activity that emphasize its potential as an anticancer drug.

Identification and functional characterization of nuclear mortalin in human carcinogenesis.

The Hsp70 family protein mortalin is an essential chaperone that is frequently enriched in cancer cells and exists in various subcellular sites, including the mitochondrion, plasma membrane, endoplasmic reticulum, and cytosol. Although the molecular mechanisms underlying its multiple subcellular localizations are not yet clear, their functional significance has been revealed by several studies. In this study, we examined the nuclear fractions of human cells and found that the malignantly transformed cells have more mortalin than the normal cells. We then generated a mortalin mutant that lacked a mitochondrial targeting signal peptide. It was largely localized in the nucleus, and, hence, is called nuclear mortalin (mot-N). Functional characterization of mot-N revealed that it efficiently protects cancer cells against endogenous and exogenous oxidative stress. Furthermore, compared with the full-length mortalin overexpressing cancer cells, mot-N derivatives showed increased malignant properties, including higher proliferation rate, colony forming efficacy, motility, and tumor forming capacity both in in vitro and in vivo assays. We demonstrate that mot-N promotes carcinogenesis and cancer cell metastasis by inactivation of tumor suppressor protein p53 functions and by interaction and functional activation of telomerase and heterogeneous ribonucleoprotein K (hnRNP-K) proteins.

Tumor suppression by apoptotic and anti-angiogenic effects of mortalin-targeting adeno-oncolytic virus.

BACKGROUND: Adeno-oncolytic (Adon) viruses offer an effective cancer therapeutic tool with several advantages, including wide host cell permeability, high transduction efficiency, safety, tumor selectivity, non-invasiveness, high genetic modifiability and high level of expression of the integrated transgenes. Armed Adon viruses in which the therapeutic efficacy of virus is enhanced by their coupling with cytotoxic, anti-angiogenic or anti-vascular gene products have gained importance because they engage additional mechanisms for tumor cell killing. In the present study, we selected mortalin, a stress chaperone that is tightly involved in human carcinogenesis, constructed a mortalin-targeting Adon (mot-Adon) virus and examined its therapeutic potential both in vitro and in vivo. METHODS: Mortalin-targeting plasmid and viral vectors that harbored mortalin-specific small interfering RNA sequences were constructed. The therapeutic value of these vectors was investigated in vitro and in vivo by cell culture and nude mice tumor models. RESULTS: We demonstrate that the mot-Adon virus has selective cytotoxicity for human cancer cells in vitro. Retrovirus-mediated overexpression of mortalin protected the cells against mot-Adon virus, confirming that mortalin silencing was the real cause of cancer cell death. Although mortalin overexpression enhanced malignant properties of cancer cells in breast xenograft models, mot-Adon virus elicited an enhanced anti-tumor effect. Immunohistochemical examination of the tumors showed that the mot-Adon virus caused enhanced apoptosis (mediated by reactivation of p53) and suppression of microvessel formation. CONCLUSIONS: Mortalin is up-regulated in a large variety of tumors and hence mot-Adon virus is proposed as a candidate cancer therapeutic agent.

Cell cycle checkpoint defects contribute to genomic instability in PTEN deficient cells independent of DNA DSB repair.

Chromosomes in PTEN deficient cells display both numerical as well as structural alterations including regional amplification. We found that PTEN deficient cells displayed a normal DNA damage response (DDR) as evidenced by the ionizing radiation (IR)-induced phosphorylation of Ataxia Telangiectasia Mutated (ATM) as well as its effectors. PTEN deficient cells also had no defect in Rad51 expression or DNA damage repair kinetics post irradiation. In contrast, caffeine treatment specifically increased IR-induced chromosome aberrations and mitotic index only in cells with PTEN, and not in cells deficient for PTEN, suggesting that their checkpoints were defective. Furthermore, PTEN-deficient cells were unable to maintain active spindle checkpoint after taxol treatment. Genomic instability in PTEN deficient cells could not be attributed to lack of PTEN at centromeres, since no interaction was detected between centromeric DNA and PTEN in wild type cells. These results indicate that PTEN deficiency alters multiple cell cycle checkpoints possibly leaving less time for DNA damage repair and/or chromosome segregation as evidenced by the increased structural as well as numerical alterations seen in PTEN deficient cells.

Glycerol stimulates innate chaperoning, proteasomal and stress-resistance functions: implications for geronto-manipulation.

Aging is associated with accumulation of toxic intracellular and extracellular protein aggregates. Cells manage "aged" proteins by mobilizing their molecular chaperones or heat shock proteins that are also considered as determinants of lifespan in diverse species. In this study, we tested whether an exogenous addition of the non-toxic chemical chaperone 'glycerol' could elicit stress and geronto-protective activities. We found that glycerol enhanced chaperoning of heat-denatured proteins. In addition to stimulating proteasome activity, glycerol led to an increased expression of the stress chaperone 'mortalin' and decreased p53 function in human cells. Glycerol-fed worms exhibited thermo-tolerance and lower level of age-associated auto-fluorescence. Through the combined stimulation of the proteasome and chaperoning activities of mortalin, in particular, glycerol treatment resulted in increased survival and fitness against oxidative- and heat-stress. These results may have significant implications in the use of glycerol as a candidate geronto-modulator in development of practical interventions for "healthy aging".

CARF (collaborator of ARF) interacts with HDM2: evidence for a novel regulatory feedback regulation of CARF-p53-HDM2-p21WAF1 pathway.

We initially cloned CARF (collaborator of ARF), as a novel ARF-binding protein by a yeast interaction screen. It also interacts with p53 directly leading to ARF-independent enhancement of p53 function and in turn undergoes a negative feedback regulation. Herein we report that i) CARF interacts with HDM2 and undergoes degradation by an HDM2-dependent proteasome pathway, and ii) it acts as a transcriptional repressor of HDM2. By overexpression and silencing studies, we demonstrated that CARF exerts a vital control on the p53-HDM2-p21WAF1 pathway that is frequently altered in cancer cells.

Quantum dot-based mortalin staining as a visual assay for detection of induced senescence in cancer cells.

Quantum dots (QDs) are fluorescent nanocrystals that are emerging as fine alternatives to the conventional organic dyes. They have several advantages including greater photostability and a wider range of excitation-emission wavelengths. By using mortalin staining as a model, we initially demonstrated that the QDs are more stable and provide better resolution in protein imaging in fixed cells. With the help of an internalizing antibody, we generated internalizing QD (i-QD) and demonstrated its inertness to cell replication, structure, and viability. Based on the superior resolution, stability and inertness, we propose the use of QD staining of mortalin as a cell-based visual assay to screen for senescence-inducing drugs, proteins, and siRNAs.

An antibody-conjugated internalizing quantum dot suitable for long-term live imaging of cells.

Quantum dots (QD) are fluorescent semiconductor nanocrystals that are emerging as superior alternatives to the conventional organic dyes used in biological applications. Although QDs offer several advantages over conventional fluorescent dyes, including greater photostability and a wider range of excitation and (or) emission wavelengths, their toxicity has been an issue in its wider use as an analytic, diagnostic and therapeutic tool. We prepared a conjugate QD with an internalizing antibody and demonstrated that the QD-antibody conjugate is efficiently internalized into cells and is visible even after multiple divisions. We demonstrate that the internalized QD is nontoxic to cells and provides a sensitive tool for long-term molecular imaging.

Stress chaperones, mortalin, and pex19p mediate 5-aza-2' deoxycytidine-induced senescence of cancer cells by DNA methylation-independent pathway.

DNA demethylating agents are used to reverse epigenetic silencing of tumor suppressors in cancer therapeutics. Understanding of the molecular and cellular factors involved in DNA demethylation-induced gene desilencing and senescence is still limited. We have tested the involvement of two stress chaperones, Pex19p and mortalin, in 5-Aza-2' deoxycytidine (5AZA-dC; DNA demethylating agent)-induced senescence. We found that the cells overexpressing these chaperones were highly sensitive to 5AZA-dC, and their partial silencing eliminated 5AZA-dC-induced senescence in human osteosarcoma cells. We demonstrate that these chaperones modulate the demethylation and chromatin remodeling-dependent (as accessed by p16(INK4A) expression) and remodeling-independent (such as activation of tumor suppressor p53 pathway) senescence response of cells. Furthermore, we found the direct interactions of 5AZA-dC with these chaperones that may alter their functions. We conclude that both mortalin and Pex19p are important mediators, prognostic indicators, and tailoring tools for 5AZA-dC-induced senescence in cancer cells.

Quantum dot-based protein imaging and functional significance of two mitochondrial chaperones in cellular senescence and carcinogenesis.

Mortalin/mtHSP70 and HSP60 are heat-shock proteins that reside in multiple subcellular compartments, mitochondria being the dominant compartment. We present here biochemical evidence for their in vivo and in vitro interactions. By the use of quantum dots (powerful tools used for simultaneous imaging of multiple proteins), we visualized minute differences in the subcellular niche of these two proteins in normal and cancer cells. Knockdown of either of these two by shRNA expression plasmids caused growth arrest of osteosarcoma cells. However, interestingly, whereas an overexpression of mortalin extended in vitro life span of normal fibroblasts (TIG-1), overexpression of HSP60 was neutral. We demonstrate the minute differences in subcellular distribution of mortalin and HSP60, their involvement in tumorigenesis, and functional distinction in pathways involved in senescence.

Activation of wild type p53 function by its mortalin-binding, cytoplasmically localizing carboxyl terminus peptides.

The Hsp70 family member mortalin (mot-2/mthsp70/GRP75) binds to a carboxyl terminus region of the tumor suppressor protein p53. By in vivo co-immunoprecipitation of mot-2 with p53 and its deletion mutants, we earlier mapped the mot-2-binding site of p53 to its carboxyl terminus 312-352 amino acid residues. In the present study we attempted to disrupt mot-2-p53 interactions by overexpression of short p53 carboxyl-terminal peptides. We report that p53 carboxyl-terminal peptides (amino acid residues 312-390, 312-352, 323-390, and 323-352) localize in the cytoplasm, whereas 312-322, 337-390, 337-352, and 352-390 locate mostly in the nucleus. Most interestingly, the cytoplasmically localizing p53 peptides harboring the residues 323-337 activated the endogenous p53 function by displacing it from p53-mortalin complexes and relocating it to the nucleus. Such activation of p53 function was sufficient to cause growth arrest of human osteosarcoma and breast carcinoma cells.

Identification and characterization of molecular interactions between mortalin/mtHsp70 and HSP60.

Mortalin/mtHsp70 (mitochondrial Hsp70) and HSP60 (heat-shock protein 60) are heat-shock proteins that reside in multiple subcellular compartments, with mitochondria being the predominant one. In the present study, we demonstrate that the two proteins interact both in vivo and in vitro, and that the N-terminal region of mortalin is involved in these interactions. Suppression of HSP60 expression by shRNA (short hairpin RNA) plasmids caused the growth arrest of cancer cells similar to that obtained by suppression of mortalin expression by ribozymes. An overexpression of mortalin, but not of HSP60, extended the in vitro lifespan of normal fibroblasts (TIG-1). Taken together, this study for the first time delineates: (i) molecular interactions of HSP60 with mortalin; (ii) their co- and exclusive localizations in vivo; (iii) their involvement in tumorigenesis; and (iv) their functional distinction in pathways involved in senescence.

Use of a randomized hybrid ribozyme library for identification of genes involved in muscle differentiation.

We have employed the hybrid hammerhead ribozyme-based gene discovery system for identification of genes functionally involved in muscle differentiation using in vitro myoblast differentiation assay. The major muscle regulatory genes (MyoD1, Mylk, myosin, myogenin, and Myf5) were identified endorsing the validity of this method. Other gene targets included tumor suppressors and cell cycle regulators (p19ARF and p21WAF1), FGFR-4, fibronectin, Prkg2, Pdk4, fem, and six novel proteins. Functional involvement of three of the identified targets in myoblast differentiation was confirmed by their specific knockdown using ribozymes and siRNA. Besides demonstrating a simple and an effective method of isolation of gene functions involved in muscle differentiation, we report for the first time that overexpression of Fem, a member of the sex-determining family of proteins, caused accelerated myotube formation, and its targeting deferred myoblast differentiation. This functional gene screening is not only helpful in understanding the molecular pathways of muscle differentiation but also to design molecular strategies for myopathologic therapies.

Alternative reading frame protein (ARF)-independent function of CARF (collaborator of ARF) involves its interactions with p53: evidence for a novel p53-activation pathway and its negative feedback control.

CARF, a collaborator of ARF (alternative reading frame protein), was cloned as a novel ARF-binding protein from a yeast-interaction screen. It potentiated ARF-mediated p53 function, and also caused a moderate increase in p53 activity in the absence of ARF. We herein report the molecular mechanism of ARF-independent function of CARF. By employing a variety of approaches, including overexpression of CARF, its suppression by small interfering RNA and use of protease inhibitors, we demonstrate that: (i) CARF directly interacts with wild-type p53, causing its stabilization and functional activation; and (ii) CARF and p53 levels show an inverse relationship that is instigated by a negative-feedback control via a proteasome-mediated degradation pathway.

LIM kinase-2 targeting as a possible anti-metastasis therapy.

BACKGROUND: Metastatic properties of tumors involve movement of cancerous cells from one place to another and tissue invasion. Metastatic cells have altered cell adhesion and movement that can be examined by in vitro chemotaxis assays. The Rho/ROCK/LIM kinase pathway is one of the major signaling pathways involved in tumor metastasis. It is involved in the regulation of the actin cytoskeleton. Using the randomized ribozyme library, we initially found that metastatic human fibrosarcoma cells harboring ribozyme specific for ROCK lose their metastatic properties. In this study, we have determined the effect of ribozymes specific for LIM kinase-2 on metastatic and proliferative phenotypes of human fibrosarcoma cells. METHODS: We attempted to target LIM kinase-2 (LIMK-2) expression by hammerhead ribozymes (Rz) in human metastatic fibrosarcoma cells. An effective ribozyme was selected based on the expression analysis. Cells were stably transfected with Rz specifically effective for LIMK-2 and were examined for metastatic and proliferative properties. RESULTS: Analyses of cellular phenotypes such as cell proliferation, cell migration and colony-forming efficiency revealed that the suppression of LIMK-2 expression in human fibrosarcoma cells limits their migration and dense colony-forming efficiency without affecting cell proliferation rate or viability. CONCLUSIONS: Specific targeting of metastatic and malignant properties of tumor cells by LIMK-2 ribozyme may serve as an effective therapy for invasive tumors with minimum effect on the surrounding normal cells.

Overexpressed mortalin (mot-2)/mthsp70/GRP75 and hTERT cooperate to extend the in vitro lifespan of human fibroblasts.

The lifespan of human foreskin fibroblasts (HFF5), cultured under standard in vitro conditions (including ambient atmospheric oxygen tension), was extended slightly by expression of exogenous mortalin (mot-2)/mthsp70/Grp75, but not by the catalytic subunit of telomerase, hTERT. Together, mot-2 and hTERT permitted bypass of senescence, a substantial extension of lifespan, and possibly immortalization. This is the first demonstration that mot-2 and telomerase can cooperate in the immortalization process.

Mortalin-MPD (mevalonate pyrophosphate decarboxylase) interactions and their role in control of cellular proliferation.

Mortalin (mot-2/GRP75/PBP74/mthsp70) is a member of the hsp70 family of proteins and is differentially distributed in normal and immortal cells. It was shown to be involved in pathways to cell senescence and immortalization. To elucidate its functional aspects, a yeast interactive screen for mortalin (mot-2) binding proteins was performed. Mevalonate pyrophosphate decarboxylase (MPD) was identified as one of the mortalin binding partners. The interactions were confirmed in mammalian cells by two-hybrid assay and in vivo coimmunoprecipitation. MPD is known to furnish prenyl groups required for prenylation, protein modification that is essential for the activity of many proteins including p21(Ras) (Ras). We have examined the effect of MPD-mot-2 interactions on the level and activity of p21(Ras) and its downstream effectors, p44 and p42 MAP kinases (ERK1/ERK2), in Ras-Raf pathway. An overexpression of mot-2 resulted in reduced level of Ras and phosphorylated ERK2. These were rescued by co-expression of MPD from an exogenous promoter demonstrating a functional link between mot-2, MPD, and Ras. Ras and its oncogenic forms act as key players in controlling proliferation of normal and cancerous cells. Assigning mot-2 upstream of p21(Ras) offers an important mechanism for influence over cell proliferation.

Mortalin imaging in normal and cancer cells with quantum dot immuno-conjugates.

Quantum dots are the nanoparticles that are recently emerging as an alternative to organic fluorescence probes in cell biology and biomedicine, and have several predictive advantages. These include their i) broad absorption spectra allowing visualization with single light source, ii) exceptional photo-stability allowing long term studies and iii) narrow and symmetrical emission spectrum that is controlled by their size and material composition. These unique properties allow simultaneous excitation of different size of quantum dots with a single excitation light source, their simultaneous resolution and visualization as different colors. At present there are only a few studies that have tested quantum dots in cellular imaging. We describe here the use of quantum dots in mortalin imaging of normal and cancer cells. Mortalin staining pattern with quantum dots in both normal and cancer cells mimicked those obtained with organic florescence probes and were considerably stable.

Rhodacyanine dye MKT-077 inhibits in vitro telomerase assay but has no detectable effects on telomerase activity in vivo.

MKT-077, a cationic rhodacyanine dye analogue, causes selective toxicity to cancer cells. Its cellular targets elucidated thus far include oncogenic Ras, F-actin, mortalin (hmot-2)/mthsp70, and telomerase. Here we report that MKT-077 causes growth arrest of cancer cells in culture independent of their Ras, p53, or telomerase status. Telomerase activity is inhibited in vitro by MKT-077 in the telomerase assay used. However, the in vivo toxicity seen in telomerase-positive cancer cells was not accompanied by inhibition of telomerase activity or telomere shortening. Furthermore, cells with an alternative mechanism for lengthening of telomeres were also sensitive to MKT-077 and showed enhanced formation of alternative mechanism for lengthening of telomeres-associated PML bodies in their nuclei. The data suggested that inhibition of telomerase activity is unlikely to be a prime cause of MKT-077-induced toxicity in cancer cells.

CARF is a novel protein that cooperates with mouse p19ARF (human p14ARF) in activating p53.

The INK4a locus on chromosome 9p21 encodes two structurally distinct tumor suppressor proteins, p16(INK4a) and the alternative reading frame protein, ARF (p19(ARF) in mouse and p14(ARF) in human). Each of these proteins has a role in senescence of primary cells and activates pathways for cell cycle control and tumor suppression. The current prevailing model proposes that p19(ARF) activates p53 function by antagonizing its degradation by MDM2. It was, however, recently shown that stabilization of p53 by p14(ARF) occurs independent of the relocalization of MDM2 to the nucleolus. We have identified a novel collaborator of ARF, CARF. It co-localizes and interacts with ARF in the nucleolus. We demonstrate that CARF is co-regulated with ARF, cooperates with it in activating p53, and thus acts as a novel component of the ARF-p53-p21 pathway.