Prothymosin alpha fragmentation in apoptosis

In Vitro Immunodetection of Prothymosin Alpha in Normal and Pathological Conditions

Prothymosin alpha (ProTα) is a highly acidic polypeptide, ubiquitously expressed in almost all mammalian cells and tissues and consisting of 109 amino acids in humans. ProTα is known to act both, intracellularly, as an anti-apoptotic and proliferation mediator, and extracellularly, as a biologic response modifier mediating immune responses similar to molecules termed as "alarmins". Antibodies and immunochemical techniques for ProTα have played a leading role in the investigation of the biological role of ProTα, several aspects of which still remain unknown and contributed to unraveling the diagnostic and therapeutic potential of the polypeptide. This review deals with the so far reported antibodies along with the related immunodetection methodology for ProTα (immunoassays as well as immunohistochemical, immunocytological, immunoblotting, and immunoprecipitation techniques) and its application to biological samples of interest (tissue extracts and sections, cells, cell lysates and cell culture supernatants, body fluids), in health and disease states. In this context, literature information is critically discussed, and some concluding remarks are presented.

Development of a specific IgY-based ELISA for prothymosin alpha, a bioactive polypeptide with diagnostic and therapeutic potential

Prothymosin alpha (ProTα) is a highly conserved polypeptide (109 amino acids in humans) with diagnostic and therapeutic potential; ProTα exerts intra- and extra-cellular biological functions associated with cell proliferation, apoptosis and immune regulation, while it has been suggested to act as a damage-associated molecular pattern (DAMP) or alarmin. In this work, chicken polyclonal anti-ProTα antibodies that had been developed several years ago were immunochemically evaluated and proven to retain immunoreactivity for ProTα, with remarkable thermal and pH stability. Moreover, the antibodies showed practically no cross-reactivity with a series of ProTα-fragments, eventually intracellularly produced -such as ProTα[1-28] (also known as Tα1) and ProTα[100-109], which exert per se biological activity and might be present in biological samples along with the intact molecule, being therefore highly specific for whole-length ProTα. Based on the above antibodies (IgYs-3e), a highly specific competitive ProTα-ELISA with well-studied analytical characteristics (intra- and inter-assay CVs: ≤5% and ≤12%, respectively, limit of detection: 2.1 ng/mL, recovery: 88–104%) was developed. The new ProTα-ELISA was applied to the analysis of supernatants of HeLa cells driven to necrosis; intact ProTα was measured in cell culture supernatants, at levels that seemed to depend on % cell necrosis.

Prothymosin alpha expression in the vertebrate testis: a comparative review

Prothymosin alpha (PTMA) is a highly acidic, intrinsically disordered protein that was first extracted from rat thymus and characterized as an immunogenic factor but soon detected in a variety of mammalian tissues. The presence of a nuclear localization signal and the adoption of a peculiar random-coil conformation are among the reasons behind its interaction with several molecular partners, hence at this time PTMA is known to be a very conserved and widely expressed molecule, involved in numerous and diverse biological processes. Only few studies have tried to weigh its possible involvement in reproduction, specifically in male gametogenesis: first reports have suggested that PTMA might be associated with the proliferative and early-meiotic phases of mammal spermatogenesis. Some years later, a comparative project on vertebrate spermatogenesis reported the isolation, for the first time, of prothymosin in a non-mammalian species, the amphibian Pelophylax esculentus. PTMA transcript and protein are localized in the germinal compartment, from spermatocytes to spermatozoa. A congruent pattern has been highlighted in studies on the fish Torpedo marmorata and Danio rerio, and in the mammal Rattus norvegicus, in which the expression of PTMA has been found in meiotic and post-meiotic germ cells inside testicular cysts and tubules. Moreover, its presence has been confirmed in rat and human spermatozoa (associated with the acrosome); its retention in the apical region of the head after the acrosome reaction revealed a striking conservation of the pattern during phylogenesis and suggested a possible role for the protein in gametogenesis and in fertilization.

TAILS N-Terminomics and Proteomics Show Protein Degradation Dominates over Proteolytic Processing by Cathepsins in Pancreatic Tumors

Deregulated cathepsin proteolysis occurs across numerous cancers, but in vivo substrates mediating tumorigenesis remain ill-defined. Applying 8-plex iTRAQ terminal amine isotopic labeling of substrates (TAILS), a systems-level N-terminome degradomics approach, we identified cathepsin B, H, L, S, and Z in vivo substrates and cleavage sites with the use of six different cathepsin knockout genotypes in the Rip1-Tag2 mouse model of pancreatic neuroendocrine tumorigenesis. Among 1,935 proteins and 1,114 N termini identified by TAILS, stable proteolytic products were identified in wild-type tumors compared with one or more different cathepsin knockouts (17%-44% of 139 cleavages). This suggests a lack of compensation at the substrate level by other cathepsins. The majority of neo-N termini (56%-83%) for all cathepsins was consistent with protein degradation. We validated substrates, including the glycolytic enzyme pyruvate kinase M2 associated with the Warburg effect, the ER chaperone GRP78, and the oncoprotein prothymosin-alpha. Thus, the identification of cathepsin substrates in tumorigenesis improves the understanding of cathepsin functions in normal physiology and cancer.

Subcellular dissemination of prothymosin alpha at normal physiology: immunohistochemical vis-a-vis western blotting perspective

BACKGROUND

The cell type, cell status and specific localization of Prothymosin α (PTMA) within cells seemingly determine its function. PTMA undergoes 2 types of protease proteolytic modifications that are useful in elucidating its interactions with other molecules; a factor that typifies its roles. Preferably a nuclear protein, PTMA has been shown to function in the cytoplasm and extracellularly with much evidence leaning on pathognomonic status. As such, determination of its cellular distribution under normal physiological context while utilizing varied techniques is key to illuminating prospective validation of its distinct functions in different tissues. Differential distribution insights at normal physiology would also portent better basis for further clarification of its interactions and proteolytic modifications under pathological conditions like numerous cancer, ischemic stroke and immunomodulation. We therefore raised an antibody against the C terminal of PTMA to use in tandem with available antibody against the N terminal in a murine model to explicate the differences in its distribution in brain cell types and major peripheral organs through western blotting and immunohistochemical approaches.

RESULTS

The newly generated antibody was applied against the N-terminal antibody to distinguish truncated versions of PTMA or deduce possible masking of the protein by other interacting molecules. Western blot analysis indicated presence of a truncated form of the protein only in the thymus, while immunohistochemical analysis showed that in brain hippocampus the full-length PTMA was stained prominently in the nucleus whereas in the stomach full-length PTMA staining was not observed in the nucleus but in the cytoplasm.

CONCLUSION

Truncated PTMA could not be detected by western blotting when both antibodies were applied in all tissues examined except the thymus. However, immunohistochemistry revealed differential staining by these antibodies suggesting possible masking of epitopes by interacting molecules. The differential localization patterns observed in the context of nucleic versus cytoplasmic presence as well as punctate versus diffuse pattern in tissues and cell types, warrant further investigations as to the forms of PTMA interacting partners.

Specific in vitro binding of a new (99m)Tc-radiolabeled derivative of the C-terminal decapeptide of prothymosin alpha on human neutrophils

Prothymosin alpha (ProTα) is a conserved mammalian polypeptide with intracellular functions associated with cell proliferation and apoptosis and an extracellular role associated with immunopotentiation. The N-terminal fragment [1-28], which is identical with the immunostimulating peptide thymosin α1 (Tα1), was earlier considered as the immunoactive region of the polypeptide; however, recent data suggest that ProTα may exert a discrete immunomodulating action through its central or C-terminal region, via targeting Toll-like receptor- 4 (TLR4). In this work, a derivative of the C-terminal fragment ProTα[100-109] (ProTα-D1) that can be radiolabeled with (99m)Tc was developed. The biological activity of the non-radioactive (185/187)rhenium-complex of this derivative ([(185/187)Re]ProTα-D1, structurally similar with [(99m)Tc]ProTα-D1) was verified through suitable in vitro bioassays on human neutrophils. Subsequent cell-binding studies revealed specific, time-dependent and saturable binding of [(99m)Tc]ProTα-D1 on neutrophils, which was inhibited by intact ProTα and ProTα[100-109], as well as by a "prototype" TLR4-ligand (lipopolysaccharide from Escherichia coli). Overall, our results support the existence of ProTα-binding sites on human neutrophils, recognizing [(99m)Tc]ProTα-D1, which might involve TLR4. [(99m)Tc]ProTα-D1 may be a useful tool for conducting further in vitro and in vivo studies, aiming to elucidate the extracellular mode of action of ProTα and, eventually, develop ProTα-based immunotherapeutics.

Prothymosin alpha protects cardiomyocytes against ischemia-induced apoptosis via preservation of Akt activation

The human prothymosin alpha (PTα) gene encodes a 12.5 kDa highly acidic nuclear protein that is widely expressed in mammalian tissues including the heart and importantly, is detectable also in blood serum. During apoptosis or necrosis, PTα changes its nuclear localization and is able to exert an important cytoprotective effect. Since the role of PTα in the heart has never been evaluated, the aim of the present study was to investigate the effects of PTα on cardiomyocytes during ischemic injury. Our data show that seven after myocardial infarction (MI), PTα expression levels are significantly increased both in blood serum and in cardiac tissue, and notably we observe that PTα translocates from the nuclei to cytoplasm and plasma membrane of cardiomyocytes following MI. Furthermore, in vitro experiments in cardiomyocytes, confirm that after 6 h of simulated ischemia (SI), PTα protein levels are upregulated compared to normoxic cells. Importantly, treatment of cardiomyocytes with a recombinant PTα (rPTα), during SI results in a significant decrease in the apoptotic response and in a robust increase in cell survival. Moreover, these effects are accompanied to a significant preservation of the activated levels of the anti-apoptotic serine-threonine kinase Akt. Consistent with our in vitro observation, rPTα-treated MI mice exhibit a strong reduction in infarct size at 24 h, compared to the MI control group and at the molecular level, PTα treatment induces activation of Akt. The present study provides for the first time the demonstration that PTα offers cardioprotection against ischemic injury by an Akt-dependent mechanism.

Knockdown of prothymosin α leads to apoptosis and developmental defects in zebrafish embryos

Prothymosin alpha (ProTα) is an abundant nuclear protein involved in cellular processes intricately linked to development, such as cell proliferation and apoptosis. Although it is known that ProTα inhibits the formation of apoptosome and blocks caspase-3 activity, its mechanism of function in the apoptotic machinery is still under investigation. We have studied the cellular role of ProTα by knocking down its expression in HeLa cells with small hairpin RNA (shRNA) in the absence of apoptotic stimuli. Flow cytometric analysis showed that the live cell population was significantly decreased with a concomitant increase of the apoptotic populations. To understand the physiological role of ProTα within the context of embryonic development, we knocked down the Ptmab zebrafish ortholog using 2 specific morpholino oligonucleotides. Ptmab morphants exhibited growth retardation, bended trunks, and curly tails. The frequency of occurrence of the phenotypic defects was increased in a morpholino dose-dependent manner. Co-injection of ptmaa mRNA with ptmab morpholino partially rescued the morphological defects. Immunostaining with the anti-phospho-histone H3 (pH3) antibody suggested that the abnormalities of Ptmab morphants could be due to defective cell proliferation that results in growth imbalances. TUNEL fluorescent labelling and Acridine Orange staining of the morphants showed high rates of cell death in the head and tail regions. Concomitantly, the active form of caspase-3 was detected in Ptmab morphants. Our data suggest a conserved anti-apoptotic role of ProTα between zebrafish and humans, and provide the first evidence that ProTα is important for early embryogenesis.

Development of an ELISA for the quantification of the C-terminal decapeptide prothymosin α(100-109) in sera of mice infected with bacteria

Apoptosis is characterized by a series of discrete biochemical events, among which is the truncation of the nuclear polypeptide prothymosin alpha (proTα) by activated caspase-3. This early apoptotic event results in the generation of a carboxy-terminal fragment of proTα, the immunoactive decapeptide proTα(100-109). We hypothesized that the detection of increased levels of proTα(100-109) in serum can be directly correlated with the induction of massive cell apoptosis, resulting from a severe bacterial infection. Thus, using high-affinity-purified polyclonal antibodies (Abs), raised in rabbits and a prototype antibody-capture system, we developed a highly sensitive and specific competitive ELISA for proTα(100-109). The sensitivity of the ELISA (0.1ng/mL to 10μg/mL) is acceptable for the quantification of the decapeptide in serum samples. To assess our initial hypothesis, we determined the concentration of proTα(100-109) in the sera of mice infected with the bacterium Streptococcus pyogenes over the course of the infection. We show that serum concentration of proTα(100-109) was marginal to undetectable before infection, increased over time and peaked at 72h postinfection. In silico analysis suggests that the Abs generated are unlikely to cross-react with any other unrelated mouse or bacterial protein. Further validation of our ELISA using serum samples from humans, infected with bacteria, may provide a useful tool to differentiate the causative agent of a potentially lethal septic infection.

Phosphoproteomic analysis of mouse thymoma cells treated with tributyltin oxide: TBTO affects proliferation and energy sensing pathways

We report the results of phosphoproteomic analysis of mouse thymoma cells treated with tributyltin oxide (TBTO), an immunotoxic compound. After cell lysis, phosphoproteins were isolated using Phosphoprotein Purification Kit, separated by SDS-PAGE and subsequently digested with trypsin. Phosphopeptides were enriched employing titanium dioxide, and the obtained fractions were analyzed by nano-LC-MS/MS. A total of 160 phosphoproteins and 328 phosphorylation sites were identified in thymoma cells. Among the differentially phosphorylated proteins identified in TBTO-treated cells were key enzymes, which catalyze rate-limiting steps in pathways that are sensitive to cellular energy status. These proteins included acetyl-CoA carboxylase isoform 1, which catalyzes the rate-limiting step of fatty acid synthesis. Another enzyme was glutamine: fructose-6-phosphate amidotransferase, GFAT1, the first and rate-limiting enzyme for the hexoamine synthesis pathway. Pyruvate dehydrogenase (PDH), a multicomplex enzyme that catalyzes the rate-limiting step of aerobic oxidation of fuel carbohydrates, was identified in both TBTO-treated and control cells; however, phosphorylation at residue S293, known to inhibit PDH activity, was identified only in control cells. A lower expression level of ribosomal protein S6 kinase 1, a downstream kinase of the mammalian target of rapamycin signaling pathway implicated in protein synthesis through phosphorylation of 40 ribosomal S6, was observed in the treated cells. Giant kinases like AMP-activated protein kinase (AMPK) and cAMP-dependent protein kinase (PKAR1A), which are known to mediate the phosphorylation of these enzymes, were identified in TBTO-treated cells. Downregulation of proteins, such as MAPK, matrin-3 and ribonucleotide reductase, subunit RRM2, which are implicated in cell proliferation, was also observed in TBTO-treated cells. Together, the results show that TBTO affects proliferation and energy sensor pathways.

Expression of Prothymosin alpha during the spermatogenesis of the spotted ray Torpedo marmorata

In this study, we show that Prothymosin alpha (Ptma), a small, unfolded, negatively charged protein, is present in the cartilaginous fish Torpedo marmorata. The ptma gene is functional and peculiarly controlled during the male spermatogenesis of T. marmorata, as revealed by in situ hybridization and by immunocytochemistry studies. The data show that the ptma transcript is present in stage-specific germ cells, i.e. spermatocytes II and round spermatids. The Ptma protein is detectable in spermatocytes II, in round and elongated spermatids as well as in spermatozoa before their release from cysts, while it is not evident in spermatozoa located in male genital tracts. The ptma transcript and protein are also evident in some Leydig cells, located among maturing cysts containing meiotic and differentiating male cells. No expression for ptma is observed within Sertoli cells. Furthermore, immunolocalization procedures demonstrate that the protein is preferentially localized in the cytoplasm, whereas a nuclear localization is observed in round and elongated spermatids. The possibility that Ptma is involved in testis activity is discussed.

Immunocytological and preliminary immunohistochemical studies of prothymosin alpha, a human cancer-associated polypeptide, with a well-characterized polyclonal antibody

Prothymosin alpha (ProTalpha) is a nuclear polypeptide of great biological and, possibly clinical, importance, because its expression levels have been associated with early diagnosis/prognosis of human cancer. It is therefore interesting to raise easily available and cost-effective antibodies that would be applied to develop reliable ProTalpha immunodiagnostics. In this study, New Zealand white rabbits and laying hens were parallel immunized against intact ProTalpha or the synthetic fragments ProTalpha[1-28], ProTalpha[87-109], and ProTalpha[101-109], all conjugated to keyhole limpet hemocyanin (KLH). The corresponding antibodies G and Y were immunochemically evaluated in parallel with ELISA and Western blot systems and applied to fluorescence immunocytology experiments using various cancer cell lines and normal cells. The antibody G raised against ProTalpha[101-109]/KLH had excellent functional characteristics in the Western blot and immunocytology experiments, where the fluorescent signal was almost exclusively shown in the cell nucleus independently of the cells assayed. The above antibody has been applied to preliminary IHC staining of human cancer prostate tissues, leading to a high percentage of clearly and intensively stained nuclei in the adenocarcinoma tissue; this antibody can be further used in cancer tissue immunostaining and in research concerning the role of ProTalpha in tumorigenesis.

Prothymosin alpha plays a key role in cell death mode-switch, a new concept for neuroprotective mechanisms in stroke

After stroke or traumatic damages, both necrotic and apoptotic neuronal death cause a loss of functions including memory, sensory perception, and motor skills. From the fact that necrosis has a nature to expand, while apoptosis to cease the cell death cascade in the brain, it is considered that the promising target for the rapid treatment for stroke is the necrosis. In this study, I introduce the discovery of prothymosin alpha (ProTalpha), which inhibits neuronal necrosis, and propose its potentiality of clinical use for stroke. First of all, it should be noted that ProTalpha inhibits the neuronal necrosis induced by serum-free starvation or ischemia-reperfusion stress, which causes a rapid internalization of GLUT1/4, leading a decrease in glucose uptake and cellular ATP levels. Underlying mechanisms are determined to be through an activation of Gi/o, phospholipase C and PKCbetaII. ProTalpha also causes apoptosis later through a similar mechanism. However, we found that ProTalpha-induced apoptosis is completely inhibited by the concomitant treatment with neurotrophins, which are up-regulated by ischemic stress in the brain. Of most importance is the finding that the systemic injection of ProTalpha completely inhibits the brain damages, motor dysfunction and learning memory defect induced by cerebral ischemia-reperfusion stress. As ProTalpha almost entirely prevents the focal ischemia-induced motor dysfunction 4 h after the start of ischemia, this protein seems to have a promising potentiality for clinical use.

Proteomic Analysis Identifies Oxidative Stress Induction by Adaphostin

PURPOSE

Activities distinct from inhibition of Bcr/abl have led to adaphostin (NSC 680410) being described as "a drug in search of a mechanism." In this study, proteomic analysis of adaphostin-treated myeloid leukemia cell lines was used to further elucidate a mechanism of action.

EXPERIMENTAL DESIGN

HL60 and K562 cells treated with adaphostin for 6, 12, or 24 h were analyzed using two-dimensional PAGE. Differentially expressed spots were excised, digested with trypsin, and analyzed by liquid chromatography-tandem mass spectrometry. The contribution of the redox-active hydroquinone group in adaphostin was also examined by carrying out proteomic analysis of HL60 cells treated with a simple hydroquinone (1,4-dihydroxybenzene) or H(2)O(2).

RESULTS

Analysis of adaphostin-treated cells identified 49 differentially expressed proteins, the majority being implicated in the response to oxidative stress (e.g., CALM, ERP29, GSTP1, PDIA1) or induction of apoptosis (e.g., LAMA, FLNA, TPR, GDIS). Interestingly, modulation of these proteins was almost fully prevented by inclusion of an antioxidant, N-acetylcysteine. Validation of the proteomic data confirmed GSTP1 as an adaphostin resistance gene. Subsequent analysis of HL60 cells treated with 1,4-dihydroxybenzene or H(2)O(2) showed similar increases in intracellular peroxides and an almost identical proteomic profiles to that of adaphostin treatment. Western blotting of a panel of cell lines identified Cu/Zn superoxide dismutase (SOD) as correlating with adaphostin resistance. The role of SOD as a second adaphostin resistance gene was confirmed by demonstrating that inhibition of SOD using diethyldithiocarbamate increased adaphostin sensitivity, whereas transfection of SOD I attenuated toxicity. Importantly, treatment with 1,4-dihydroxybenzene or H(2)O(2) replicated adaphostin-induced Bcr/abl polypeptide degradation, suggesting that kinase inhibition is a ROS-dependent phenomenon.

CONCLUSION

Adaphostin should be classified as a redox-active-substituted dihydroquinone.

Mitochondrial ribosomal protein S36 delays cell cycle progression in association with p53 modification and p21WAF1/CIP1 expression

Ribosomal biogenesis is correlated with cell cycle, cell proliferation, cell growth and tumorigenesis. Some oncogenes and tumor suppressors are involved in regulating the formation of mature ribosome and affecting the ribosomal biogenesis. In previous studies, the mitochondrial ribosomal protein L41 was reported to be involved in cell proliferation regulating through p21(WAF1/CIP1) and p53 pathway. In this report, we have identified a mitochondrial ribosomal protein S36 (mMRPS36), which is localized in the mitochondria, and demonstrated that overexpression of mMRPS36 in cells retards the cell proliferation and delays cell cycle progression. In addition, the mMRPS36 overexpression induces p21(WAF1/CIP1) expression, and regulates the expression and phosphorylation of p53. Our result also indicate that overexpression of mMRPS36 affects the mitochondrial function. These results suggest that mMRPS36 plays an important role in mitochondrial ribosomal biogenesis, which may cause nucleolar stress, thereby leading to cell cycle delay.

Prothymosin α Interacts with Free Core Histones in the Nucleus of Dividing Cells

The acidic protein prothymosin alpha (ProTalpha), with a broad presence in mammalian cells, has been widely considered to have a role in cell division, through an unrevealed mechanism in which histones may be involved in view of their ability to interact with ProTalpha in vitro. Results of co-immunoprecipitation experiments presented here demonstrate that ProTalpha interacts in vivo with core histones in proliferating B-lymphocytes (NC-37 cells). This interaction occurs with histones H3, H2A, H2B and H4 located free in the nucleoplasm, whereas no interaction was detected with histone H1, mono-nucleosome particles or chromatin. Moreover, the core histones form part of a nuclear multiprotein complex of about 700 kDa separated by ProTalpha-Sepharose affinity, with components including H3 and H4 acetyltranferases, H3 methyltransferases, hnRNP isotypes A3, A2/B1 and R, ATP-dependent and independent DNA helicases II, beta-actin and vimentin, all co-purifying by gel filtration. This indicates that the interaction of ProTalpha with core histones in the nucleus may be related to the structural modification of histones H3 and H4, and hence to chromatin activity, raising the possibility that the other proteins in the nuclear complex may play a role in this process.

The immunologically active site of prothymosin alpha is located at the carboxy-terminus of the polypeptide. Evaluation of its in vitro effects in cancer patients

Prothymosin alpha (proTalpha) is a 109 amino acid long polypeptide presenting distinct immunoenhancing activity in vitro and in vivo. Recent reports suggest that in apoptotic cells, proTalpha is cleaved by caspases at its carboxy(C)-terminus generating potentially bioactive fragments. In this study, we identified the peptide segment of proTalpha presenting maximum immunomodulatory activity. Calf thymus proTalpha was trypsinised, and the five fragments produced (spanning residues 1-14, 21-30, 31-87, 89-102 and 103-109) were tested for their ability to stimulate healthy donor- and cancer patient-derived peripheral blood mononuclear cell (PBMC) proliferation in autologous mixed lymphocyte reaction (AMLR), natural killer and lymphokine-activated killer cell activity, intracellular production of perforin, upregulation of adhesion molecules and CD25 expression. ProTalpha(89-102) and proTalpha(103-109) significantly fortified healthy donor-lymphocytes' immune responses to levels comparable to those induced by intact proTalpha. These effects were more pronounced in cancer patients, where peptides proTalpha(89-102) and proTalpha(103-109) partly, however significantly, restored the depressed AMLR and cytolytic ability of PBMC, by simulating the biological activity exerted by intact proTalpha. ProTalpha(1-14), proTalpha(21-30) and proTalpha(31-87) marginally upregulated lymphocyte activation. This is the first report showing that proTalpha's immunomodulating activity can be substituted by its C-terminal peptide(s). Whether generation and externalization of such immunoactive proTalpha fragments occurs in vivo, needs further investigation. However, if these peptides can trigger immune responses, they may eventually be used therapeutically to improve some PBMC functions of cancer patients.

Apoptotic and proliferating hepatocytes differ in prothymosin α expression and cell localization

Prothymosin alpha is an acidic protein, reported to be involved in cell proliferation and apoptosis, although its precise function in both processes are still unknown. Due to the importance of these processes in the pathogenesis of hepatic diseases and the need to understand the molecular mechanisms underlying these diseases we aimed to investigate the behavior of this protein in liver growth and apoptosis, in two models of hepatocytes in culture. Prothymosin alpha expression varied throughout the hepatocyte cell cycle, according to its progression. Proliferating hepatocytes showed increased expression of the protein, while apoptotic ones showed decreased levels. The subcellular location of prothymosin alpha differed according to the different phases of the cell cycle. Thus, it appeared with a stippled and widely dispersed pattern throughout the nucleus in quiescent and proliferating hepatocytes, while it became cytoplasmic in mitotic and late apoptotic cells. These results are in agreement with the idea that high levels of prothymosin alpha need to be present in the nucleus for proliferation, and programmed cell death requires low levels of prothymosin alpha outside of the nucleus. The differences in prothymosin alpha expression and localization during hepatocyte proliferation and apoptosis suggest that this protein may have a pleiotropic function that depends not only on its availability but also on its various localizations in different subcellular compartments.

Nuclear oncoprotein prothymosin alpha is a partner of Keap1: implications for expression of oxidative stress-protecting genes

Animal cells counteract oxidative stress and electrophilic attack through coordinated expression of a set of detoxifying and antioxidant enzyme genes mediated by transcription factor Nrf2. In unstressed cells, Nrf2 appears to be sequestered in the cytoplasm via association with an inhibitor protein, Keap1. Here, by using the yeast two-hybrid screen, human Keap1 has been identified as a partner of the nuclear protein prothymosin alpha. The in vivo and in vitro data indicated that the prothymosin alpha-Keap1 interaction is direct, highly specific, and functionally relevant. Furthermore, we showed that Keap1 is a nuclear-cytoplasmic shuttling protein equipped with a nuclear export signal that is important for its inhibitory action. Prothymosin alpha was able to liberate Nrf2 from the Nrf2-Keap1 inhibitory complex in vitro through competition with Nrf2 for binding to the same domain of Keap1. In vivo, the level of Nrf2-dependent transcription was correlated with the intracellular level of prothymosin alpha by using prothymosin alpha overproduction and mRNA interference approaches. Our data attribute to prothymosin alpha the role of intranuclear dissociator of the Nrf2-Keap1 complex, thus revealing a novel function for prothymosin alpha and adding a new dimension to the molecular mechanisms underlying expression of oxidative stress-protecting genes.

Prothymosin alpha associates with the oncoprotein SET and is involved in chromatin decondensation

Prothymosin alpha (ProTalpha) is a histone H1-binding protein that interacts with the transcription coactivator CREB-binding protein and potentiates transcription. Based on coimmunoprecipitation and mammalian two-hybrid assays, we show here that ProTalpha forms a complex with the oncoprotein SET. ProTalpha efficiently decondenses human sperm chromatin, while overexpression of GFP-ProTalpha in mammalian cells results in global chromatin decondensation. These results indicate that decondensation of compacted chromatin fibers is an important step in the mechanism of ProTalpha function.

The thymosins. Prothymosin alpha, parathymosin, and beta-thymosins: structure and function

The studies on thymosins were initiated in 1965, when the group of A. White searched for thymic factors responsible for the physiological functions of thymus. To restore thymic functions in thymic-deprived or immunodeprived animals, as well as in humans with primary immuno-deficiency diseases and in immunosuppressed patients, a standardized extract from bovine thymus gland called thymosin fraction 5 was prepared. Thymosin fraction 5 indeed improved immune response. It turned out that thymosin fraction 5 consists of a mixture of small polypeptides. Later on, several of these peptides (polypeptide beta 1, thymosin alpha 1, prothymosin alpha, parathymosin, and thymosin beta 4) were isolated and tested for their biological activity. The research of many groups has indicated that none of the isolated peptides is really a thymic hormone; nevertheless, they are biologically important peptides with diverse intracellular and extracellular functions. Studies on these functions are still in progress. The current status of knowledge of structure and functions of the thymosins is discussed in this review.

Prothymosin alpha is processed to thymosin alpha 1 and thymosin alpha 11 by a lysosomal asparaginyl endopeptidase

Thymosin alpha(1) (T alpha(1)) and thymosin T alpha(11) (T alpha(11)) are polypeptides with immunoregulatory properties first isolated from thymic extracts, corresponding to the first 28 and 35 amino acid residues, respectively, of prothymosin alpha (ProT alpha), a protein involved in chromatin remodeling. It has been widely supposed that these polypeptides are not natural products of the in vivo processing of ProT alpha, since neither was found in extracts in which proteolysis was prevented. Here we show that a lysosomal asparaginyl endopeptidase is able to process ProT alpha to generate T alpha(1) and T alpha(11). In view of its catalytic properties and structural and immunological analyses, this protease was identified as mammalian legumain. It selectively cleaves some of the asparaginyl-glycine residues in the ProT alpha sequence; specifically, Asn(28)-Gly(29) and Asn(35)-Gly(36) residues are cleaved with similar efficiency in vitro to generate T alpha(1) and T alpha(11), respectively. By contrast T alpha(1) is the main product detected in vivo, free in the cytosol, at concentrations similar to that of ProT alpha. The data here reported demonstrate that T alpha(1) is not an artifact but rather is naturally present in diverse mammalian tissues and raise the possibility that it has a functional role.

Apoptosis-related fragmentation, translocation, and properties of human prothymosin alpha

Human prothymosin alpha is a proliferation-related nuclear protein undergoing caspase-mediated fragmentation in apoptotic cells. We show here that caspase-3 is the principal executor of prothymosin alpha fragmentation in vivo. In apoptotic HeLa cells as well as in vitro, caspase-3 cleaves prothymosin alpha at one major carboxy terminal (DDVD(99)) and several suboptimal sites. Prothymosin alpha cleavage at two amino-terminal sites (AAVD(6) and NGRD(31)) contributes significantly to the final pattern of prothymosin alpha fragmentation in vitro and could be detected to occur in apoptotic cells. The major caspase cleavage at D(99) disrupts the nuclear localization signal of prothymosin alpha, which leads to a profound alteration in subcellular localization of the truncated protein. By using a set of anti-prothymosin alpha monoclonal antibodies, we were able to observe nuclear escape and cell surface exposure of endogenous prothymosin alpha in apoptotic, but not in normal, cells. We demonstrate also that ectopic production of human prothymosin alpha and its mutants with nuclear or nuclear-cytoplasmic localization confers increased resistance of HeLa cells toward the tumor necrosis factor-induced apoptosis.

Cytochrome c is transformed from anti- to pro-oxidant when interacting with truncated oncoprotein prothymosin alpha

Many apoptotic signals are known to induce release to cytosol of cytochrome c, a small mitochondrial protein with positively charged amino acid residues dominating over negatively charged ones. On the other hand, in this group, it was shown that prothymosin alpha (PT), a small nuclear protein where 53 of 109 amino acid residues are negatively charged, is truncated to form a protein of 99 amino acid residues which accumulates in cytosol during apoptosis [FEBS Lett. 467 (2000) 150]. It was suggested that positively charged cytochrome c and negatively charged truncated prothymosin alpha (tPT), when meeting in cytosol, can interact with each other. In this paper, such an interaction is shown. (1) Formation of cytochrome cz.ccirf;tPT complex is demonstrated by a blot-overlay assay. (2) Analytical centrifugation of solution containing cytochrome c and tPT reveals formation of complexes of molecular masses higher than those of these proteins. The masses increase when the cytochrome c/tPT ratio increases. High concentration of KCl prevents the complex formation. (3) In the complexes formed, cytochrome c becomes autoxidizable; its reduction by superoxide or ascorbate as well as its operation as electron carrier between the outer and inner mitochondrial membranes appear to be inhibited. (4) tPT inhibits cytochrome c oxidation by H(2)O(2), catalyzed by peroxidase. Thus, tPT abolishes all antioxidant functions of cytochrome c which, in the presence of tPT, becomes in fact a pro-oxidant. A possible role of tPT in the development of reactive oxygen species- and cytochrome c-mediated apoptosis is discussed.

Many cuts to ruin: a comprehensive update of caspase substrates

Apoptotic cell death is executed by the caspase-mediated cleavage of various vital proteins. Elucidating the consequences of this endoproteolytic cleavage is crucial for our understanding of cell death and other biological processes. Many caspase substrates are just cleaved as bystanders, because they happen to contain a caspase cleavage site in their sequence. Several targets, however, have a discrete function in propagation of the cell death process. Many structural and regulatory proteins are inactivated by caspases, while other substrates can be activated. In most cases, the consequences of this gain-of-function are poorly understood. Caspase substrates can regulate the key morphological changes in apoptosis. Several caspase substrates also act as transducers and amplifiers that determine the apoptotic threshold and cell fate. This review summarizes the known caspase substrates comprising a bewildering list of more than 280 different proteins. We highlight some recent aspects inferred by the cleavage of certain proteins in apoptosis. We also discuss emerging themes of caspase cleavage in other forms of cell death and, in particular, in apparently unrelated processes, such as cell cycle regulation and cellular differentiation.

Sensing prothymosin alpha origin, mutations and conformation with monoclonal antibodies

To overcome poor immunogenicity of prothymosin alpha, a small and highly acidic nuclear protein involved in cell proliferation, production of anti-prothymosin alpha antibodies in mice immunized with free human prothymosin alpha, with prothymosin alpha coupled to different carriers and with prothymosin alpha fused to green fluorescent protein was assessed. Fusing prothymosin alpha to green fluorescent protein turned out to be the superior approach resulting in production of high titer anti-prothymosin alpha antibodies. From these studies, two highly specific anti-prothymosin alpha monoclonal antibodies recognizing epitopes within the amino terminal (2F11) and middle (4F4) portions of the human prothymosin alpha molecule were obtained and characterized. As expected, the 2F11 antibody displayed broad species specificity, whereas the 4F4 antibody appeared to be species-specific permitting discrimination of human versus rat protein. Furthermore, a combination of point mutations in prothymosin alpha that alter the properties of the protein precluded recognition by the 4F4 antibody. Intramolecular masking of the 4F4 epitope in prothymosin alpha fused to the Tat transduction peptide of human immunodeficiency virus type 1 was observed. The anti-prothymosin alpha antibodies obtained were suitable for precipitation of human prothymosin alpha from HeLa cell lysates and for immunolocalization of the endogenous prothymosin alpha within the cells. Fusion with green fluorescent protein may thus be helpful in raising antibodies against 'problematic' proteins.

First evidence of prothymosin alpha in a non-mammalian vertebrate and its involvement in the spermatogenesis of the frog Rana esculenta

A cDNA clone encoding for a Prothymosin alpha (Prot-alpha) has been isolated and characterized from the testis of the frog Rana esculenta. Frog Prothymosin alpha (fProt-alpha) predicted a 109 amino acid protein with a high homology to the mammalian Prot-alpha. fProt-alpha contains 28 aspartic and 25 glutamic acid residues and presents the typical basic KKQK amino acid sequence in the close carboxyl terminal region. Northern blot analysis revealed that fProt-alpha is highly expressed in the testis. A different expression of fProt-alpha transcript was found during the frog reproductive cycle with a peak in September/October in concomitance with germ cell maturation, strongly suggesting a role for this protein in the testicular activity. In situ hybridization evidenced that the only germ cells expressing fProt-alpha are the primary and secondary spermatocytes; in addition, the hybridization signal was stronger in the October testis. Taken together, our findings indicate that fProt-alpha might contribute to the efficiency of frog spermatogenesis with a role during the meiosis. This study is the first report on the isolation and characterization of a Prot-alpha in a non-mammalian vertebrate. In addition, our results indicate that the testis of the frog R. esculenta may be a useful model to increase the knowledge concerning the physiological role of Prot-alpha in vertebrates.

Retrovirus-mediated transfer of prothymosin gene inhibits tumor growth and prolongs survival in murine bladder cancer

To explore the potential use of prothymosin alpha(ProT), a putative thymic hormone, in gene therapy for bladder cancer, we generated a replication-defective recombinant retroviral vector encoding ProT and tested its antitumor effect on the MBT-2 murine bladder cancer. C3H/HeN mice injected with MBT-2 cells in conjunction with retroviruses encoding ProT exhibited smaller tumor mass, lower tumor incidence and higher survival rate, as well as higher antitumor cytotoxic activities compared with those injected with control viruses. However, such effects were not observed in severe combined immunodeficiency mice, suggesting that ProT exerts antitumor effects through its immunomodulatory activities. Cell growth in monolayer culture and colony formation in soft agar were enhanced in ProT gene-modified MBT-2 clones, and such growth-promoting activities of ProT could be reversed if its nuclear localization signal (NLS) was deleted. To circumvent the proliferation-promoting effect of ProT on tumor cells, a retroviral vector encoding ProT lacking NLS was constructed. Our results showed that retroviruses encoding NLS-deleted ProT was more efficacious than those encoding wild-type ProT in prolonging survival of tumor-bearing mice. This is the first report indicating that ProT, in particular NLS-deleted ProT, delivered by retroviral vectors may be further explored for the treatment of bladder cancer.

Interaction of yeast importin alpha with the NLS of prothymosin alpha is insufficient to trigger nuclear uptake of cargos

A proliferation-related human protein prothymosin alpha displays exclusively nuclear localization when produced in human and Saccharomyces cerevisiae cells, whereas its isolated bipartite NLS confers nuclear targeting of the GFP reporter in human but not in yeast cells. To test whether this observation is indicative of the existence of specific requirements for nuclear targeting of proteins in yeast, a set of prothymosin alpha deletion mutants was constructed. Subcellular localization of these mutants fused to GFP was determined in yeast and compared with their ability to bind yeast importin alpha (Srp1p) in vitro. The NLS of prothymosin alpha turned out to be both necessary and sufficient to provide protein recognition by importin alpha. However, the NLS-importin alpha interaction did not ensure nuclear targeting of prothymosin alpha derivatives. This defect could be complemented by adding distinct prothymosin alpha sequences to the NLS-containing import substrate, possibly by providing binding site(s) for additional components of the yeast nuclear import machinery.

Divalent metal cation binding properties of human prothymosin alpha

The divalent cation binding properties of human prothymosin alpha, an abundant nuclear protein involved in cell proliferation, were evaluated. By using prothymosin alpha retardation on a weak cation chelating resin charged with various divalent cations, specific binding of Zn2+ ions by prothymosin alpha was observed. This finding was further confirmed by the equilibrium dialysis analysis which demonstrated that, within the micromolar range of Zn2+ concentrations, prothymosin alpha could bind up to three zinc ions in the presence of 100 mM NaCl and up to 13 zinc ions in the absence of NaCl. Equilibrium dialysis analysis also revealed that prothymosin alpha could bind Ca2+, although the parameters of Ca2+ binding by prothymosin alpha were less pronounced than those of Zn2+ binding in terms of the number of metal ions bound, the KD values, and the resistance of the bound metal ions to 100 mM NaCl. The effects of Zn2+ and Ca2+ on the interaction of prothymosin alpha with its putative partners, Rev of HIV type 1 and histone H1, were examined. We demonstrated that Rev binds prothymosin alpha, and that prothymosin alpha binding to Rev but not to histone H1 was significantly enhanced in the presence of zinc and calcium ions. Our data suggest that the modes of prothymosin alpha interaction with Rev and histone H1 are distinct and that the observed zinc and calcium-binding properties of prothymosin alpha might be functionally relevant.