Prothymosin alpha is a nuclear protein

Novel function of prothymosin alpha as a potent inhibitor of human immunodeficiency virus type 1 gene expression in primary macrophages

CD8(+) T lymphocytes control human immunodeficiency virus type 1 (HIV-1) infection by a cytotoxic major histocompatibility complex-restricted pathway as well as by secretion of noncytotoxic soluble inhibitory factors. Several components of CD8(+) cell supernatants have been identified that contribute to the latter activity. In this study we report that prothymosin alpha (ProTalpha), a protein found in the cell culture medium of the herpesvirus saimiri-transformed CD8(+) T-cell line, K#1 50K, has potent HIV-1-inhibitory activity. Depletion of native ProTalpha from an HIV-1-inhibitory fraction of CD8(+) cell supernatants removes the inhibitory activity, supporting its role in inhibition via soluble mediators. ProTalpha is an abundant, acidic peptide that has been reported to be localized in the nucleus and associated with cell proliferation and activation of transcription. In this report we demonstrate that ProTalpha suppresses HIV-1 replication, its activity is target cell specific, and inhibition occurs following viral integration. Native and recombinant ProTalpha protein potently inhibit HIV-1 long terminal repeat (LTR)-driven gene expression in macrophages. Furthermore studies using different promoters in lentiviral vectors (cytomegalovirus and phosphoglycerate kinase) revealed that suppression of viral replication by ProTalpha is not HIV LTR specific.

Antiapoptotic function of RNA-binding protein HuR effected through prothymosin alpha

We report the antiapoptotic effect of RNA-binding protein HuR, a critical regulator of the post-transcriptional fate of target transcripts. Among the most prominent mRNAs complexing with HuR is that encoding prothymosin alpha (ProTalpha), an inhibitor of the apoptosome. In HeLa cells, treatment with the apoptotic stimulus ultraviolet light (UVC) triggered the mobilization of ProTalpha mRNA to the cytoplasm and onto heavier polysomes, where its association with HuR increased dramatically. Analysis of a chimeric ProTalpha mRNA directly implicated HuR in regulating ProTalpha production: ProTalpha translation and cytoplasmic concentration increased in HuR-overexpressing cells and declined in cells in which HuR levels were lowered by RNA interference. Importantly, the antiapoptotic influence engendered by HuR was vitally dependent on ProTalpha expression, since use of oligomers that blocked ProTalpha translation abrogated the protective effect of HuR. Together, our data support a regulatory scheme whereby HuR binds the ProTalpha mRNA, elevates its cytoplasmic abundance and translation, and thereby elicits an antiapoptotic program.

Tissue-specific and differential expression of prothymosin alpha gene during rat development

We have analyzed the RNA expression of prothymosin alpha (ProT alpha) gene during rat development in several tissues and compared it to that of two proteins related to cell proliferation: proliferating cell nuclear antigen (PCNA)/cyclin and histone H3 (H3). The expression of ProT alpha gene was found to be regulated in a developmental and tissue-specific manner. The mRNA levels of ProT alpha followed a similar time-course in liver, brain, kidney, and testis, being highly increased in the early periods of postnatal development. However, in thymus ProT alpha mRNA showed only moderate changes throughout development. Our findings suggest that ProT alpha participates in developmental processes like cell proliferation and/or differentiation.

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.

Regulation of prothymosin alpha by estrogen receptor alpha: molecular mechanisms and relevance in estrogen-mediated breast cell growth

Prothymosin alpha (PTalpha) is a small highly acidic protein found in the nuclei of virtually all mammalian tissues. Its high conservation in mammals and wide tissue distribution suggest an essential biological role. While the exact mechanism of action of PTalpha remains elusive, the one constant has been its relationship with the proliferative state of the cell and its requirement for cellular growth and survival. Recently PTalpha was found to promote transcriptional activity by sequestering the anticoactivator, REA from the Estrogen Receptor (ER) complex. We now report that Estradiol (E2) upregulates PTalpha mRNA and protein expression. Further studies indicate that ERalpha regulates PTalpha gene transcriptional activity. We have also delimited the region of PTalpha gene promoter involved in ERalpha-mediated transcriptional regulation and identified a novel ERalpha-binding element. Increased intracellular PTalpha expression in the presence of estrogens is accompanied by increased nuclear/decreased cytoplasmic localization. Increased nuclear expression of PTalpha is correlated with increased proliferation as measured by expression of Ki67 nuclear antigen. Conversely, inhibition of nuclear PTalpha expression in breast cancer cells using antisense methodology resulted in the inhibition of E2-induced breast cancer cell proliferation. Overall these studies underscore the importance of PTalpha in estrogen-induced breast cell proliferation.

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.

Identification of receptors for prothymosin alpha on human lymphocytes

Prothymosin alpha (ProTalpha) is a highly conserved and widely distributed protein whose physiological functions remain elusive. In previous work we identified high and low affinity-binding sites for ProTalpha in lymphoid cells. Here we demonstrate, by affinity cross-linking and affinity chromatography, the existence of three binding partners (31, 29, and 19 kDa) for ProTalpha in the membrane of PHA-activated lymphoblasts. These surface molecules possess the expected affinity and specificity for a ProTalpha receptor. Examination of the expression of this complex of molecules by flow cytometry reveals that they bind ProTalpha in a specific and saturable way. In addition, the distribution of the receptor on the cell surface was studied by fluorescence microscopy; a cap-like structure at one of the poles of the cells was identified. These results represent a new and promising approach in the research on ProTalpha, opening the way toward the understanding of the molecular mechanism of action of this protein.

Prothymosin alpha functions as a cellular oncoprotein by inducing transformation of rodent fibroblasts in vitro

Prothymosin alpha (ProTalpha), a cellular molecule known to be associated with cell proliferation, is transcriptionally up-regulated on expression of c-myc and interacts with histones in vitro and associates with histone H1 in cells. Previous studies have also shown that ProTalpha is involved in chromatin remodeling. Recent studies have shown that ProTalpha interacts with the acetyl transferase p300 and an essential Epstein-Barr virus protein, EBNA3C, involved in regulation of viral and cellular transcription. These studies suggest a potential involvement in regulation of histone acetylation through the association with these cellular and viral factors. In the current studies, we show that heterologous expression of ProTalpha in the Rat-1 rodent fibroblast cell line results in increased proliferation, loss of contact inhibition, anchorage-independent growth, and decreased serum dependence. These phenotypic changes seen in transfected Rat-1 cells are similar to those observed with a known oncoprotein, Ras, expressed under the control of a heterologous promoter and are characteristic oncogenic growth properties. These results demonstrate that the ProTalpha gene may function as an oncogene when stably expressed in Rat-1 cells and may be an important downstream cellular target for inducers of cellular transformation, which may include Epstein-Barr virus and c-myc.

Mobility within the nucleus and neighboring cytosol is a key feature of prothymosin-alpha

Prothymosin alpha is a small, unfolded, negatively charged, poorly antigenic mammalian protein with a potent nuclear localization signal. Although it is apparently essential for growth, its precise function is unknown. We examined the location and behavior of the protein bearing different epitope tags using in situ immunolocalization in COS-1 and NIH3T3 cells. Tagged prothymosin alpha appeared to be punctate and widely dispersed throughout the nucleus, with the exception of the nucleolus. A tiny cytoplasmic component, which persisted in the presence of cycloheximide and actinomycin D during interphase, became pronounced immediately before, during, and after mitosis. When nuclear uptake was abrogated, small tagged prothymosin alpha molecules, but not prothymosin alpha fused to beta-galactosidase, accumulated significantly in the cytoplasm. Tagged prothymosin alpha shared domains with mobile proteins such as Ran, transportin, and karyopherin beta, which also traverse the nuclear membrane, and co-localized with active RNA polymerase II. Mild digitonin treatment resulted in nuclei devoid of prothymosin alpha. The data do not support tight binding to any nuclear component. Therefore, we propose that prothymosin alpha is a highly diffusible bolus of salt and infer that it facilitates movement of charged molecules in highly charged environments within and near the nucleus.

Fifteen years of prothymosin alpha: contradictory past and new horizons

Prothymosin alpha (ProTalpha) is a highly acidic and small protein of only 111 amino acids with an unusual primary structure. One would expected it to play an essential role in the organism, as it has a wide distribution and is high conserved among mammals, yet its exact function remains elusive. Despite the number of effects described for ProTalpha, intracellular and extracellular, none are accepted as its physiological role. Furthermore, many other aspects of its biology still remain obscure. In this review, we discuss the structural properties, location, gene family, functions and immunomodulatory activities of and cellular receptors for ProTalpha. These topics are addressed in an attempt to reconcile opposing outlooks while emphasizing those points where scant investigations do exist. We have also re-evaluated some previous results in light of the structural properties of ProTalpha and have found that molecular mimetism could be the underlying basis. This molecular mimicry hypothesis provides a clue that must not be overlooked for a realistic appraisal of future results.

Prothymosin alpha fragmentation in apoptosis

We observed fragmentation of an essential proliferation-related human nuclear protein prothymosin alpha in the course of apoptosis induced by various stimuli. Prothymosin alpha cleavage occurred at the DDVD(99) motif. In vitro, prothymosin alpha could be cleaved at D(99) by caspase-3 and -7. Caspase hydrolysis disrupted the nuclear localization signal of prothymosin alpha and abrogated the ability of the truncated protein to accumulate inside the nucleus. Prothymosin alpha fragmentation may therefore be proposed to disable intranuclear proliferation-related function of prothymosin alpha in two ways: by cleaving off a short peptide containing important determinants, and by preventing active nuclear uptake of the truncated protein.

Functional discontinuities in prothymosin alpha caused by caspase cleavage in apoptotic cells

Our study examines the effect of apoptosis on prothymosin alpha, an abundant, nuclear protein intimately involved with proliferation of all mammalian cells. When HeLa cells were treated with actinomycin D, with etoposide, or with staurosporine following synchronization with hydroxyurea, they underwent apoptosis based on several specific criteria, including fragmentation of DNA and activation of specific caspases. Similarly treated NIH3T3 cells arrested and displayed no indicators of apoptosis. In HeLa, but not in NIH3T3 cells, prothymosin alpha levels declined precipitously and a truncated version of the protein was formed. The following observations implicate caspase activity: (1) The truncated polypeptide arose only in the treated HeLa cell cultures. (2) The appearance of the truncated polypeptide coincided with the activation of caspase 3 and the cleavage of poly(ADP-ribose) polymerase, a known caspase substrate. (3) Carbobenzoxy-DEVD-fluoromethylketone, a cell-permeable caspase 3 inhibitor, blocked cleavage and degradation of prothymosin alpha. (4) The same inhibitor, when added to mixed extracts of apoptotic and normal cells, prevented cleavage of intact prothymosin alpha. (5) Recombinant caspase 3 and, to a much lesser extent, caspase 7 truncated purified prothymosin alpha. (6) In HeLa cells, cleavage occurred at three overlapping caspase 3-like sites with the consensus sequence D-X-X-D and released 10 to 14 residues from the carboxyl terminus, including the core nuclear localization signal. Two immediate consequences of the cleavage were observed: truncated prothymosin alpha was no longer confined to the nucleus and it was deficient in phosphate. These data suggest that the disabling of prothymosin alpha is a significant event in apoptosis. J. Cell. Physiol. 182:256-268, 2000. Published 2000 Wiley-Liss, Inc.

Prothymosin alpha

Prothymosin alpha (ProT alpha) is a highly acidic protein widely distributed in mammalian cells. Since its discovery in 1984, the biological role of this protein has been controversial. Initially, ProT alpha was considered a thymic factor with a hormonal-like role in the maturation of T-lymphocytes. However, molecular and cellular analyses led to conclude that ProT alpha is a nuclear protein required in proliferation events while failing to show a clear immunological effect. The involvement of ProT alpha in changes in the compaction state of chromatin has been recently elucidated with the demonstration that this protein induces the unfolding of chromatin fibres in a process that seems to be mediated by the interaction of ProT alpha with histone H1. This finding opens up new perspectives in the study of the dynamics of the genetic material in mammalian cells. Furthermore, the relationship between ProT alpha and apoptosis as well as with proliferation makes this protein an attractive target in the search for modulators of cell death and tumour growth.

Does prothymosin alpha affect the phosphorylation of elongation factor 2?

Prothymosin alpha is a small, acidic, essential nuclear protein that plays a poorly defined role in the proliferation and survival of mammalian cells. Recently, Vega et al. proposed that exogenous prothymosin alpha can specifically increase the phosphorylation of eukaryotic elongation factor 2 (eEF-2) in extracts of NIH3T3 cells (Vega, F. V., Vidal, A., Hellman, U., Wernstedt, C., and Domínguez, F. (1998) J. Biol. Chem. 273, 10147-10152). Using similar lysates prepared by four methods (detergent lysis, Dounce homogenization, digitonin permeabilization, and sonication) and three preparations of prothymosin alpha, one of which was purified by gentle means (the native protein, and a histidine-tagged recombinant prothymosin alpha expressed either in bacteria or in COS cells), we failed to find a response. A reconstituted system composed of eEF-2, recombinant eEF-2 kinase, calmodulin, and calcium was also unaffected by prothymosin alpha. However, unlike our optimized buffer, Vega's system included a phosphatase inhibitor, 50 mM fluoride, which when evaluated in our laboratories severely reduced phosphorylation of all species. Under these conditions, any procedure that decreases the effective fluoride concentration will relieve the inhibition and appear to activate. Our data do not support a direct relationship between the function of prothymosin alpha and the phosphorylation of eEF-2.

Complex regulation of prothymosin alpha in mammary tumors arising arising in transgenic mice

Expression of prothymosin alpha (PTA) has been related to cell proliferation, both normal and pathological. PTA has also been proposed to be a target of the c-myc protooncogene. To study PTA mRNA levels during pathological cell growth, and especially the effect of the activation of specific oncogenes on PTA expression, we have studied its expression in tumors that arise in transgenic mice. We found high PTA levels in mammary tumors arising in c-myc, c-neu, and v-ras transgenic mice. Levels of this protein were variable between different tumors, and there is a differential regulation of PTA respect to other putative c-myc target genes, such as Ornithine Decarboxylase (ODC). Furthermore, expression of PTA is not absolutely dependent of c-myc expression, as shown by MYC depletion experiments performed with antisense oligonucleotides. We conclude that regulation of PTA in these tumors is complex and depends on more than a single activated oncogene.

Metabolic regulation of protein-bound glutamyl phosphates: insights into the function of prothymosin alpha

Prothymosin alpha gene expression accompanies growth of all mammalian cells. The protein, which is abundant, exceedingly acidic, and localized to the nucleus, is further distinguished by the presence of clustered phosphorylated glutamic acid residues (Trumbore et al., 1997, J Biol Chem 272:26394-26404). These glutamyl phosphates are energy rich and unstable in vivo and in vitro (Wang et al., 1997, J Biol Chem 272:26405-26412). To understand the function of prothymosin alpha in greater detail, the turnover of its phosphates was examined in metabolically manipulated cells. Phosphate half-lives in growing, mock transfected, and vector-transfected COS cells were compared with the half-life in cells transfected with the prothymosin alpha gene to determine the fate of the predominantly ectopic phosphorylated protein. The values obtained--72-75 min in cells with normal levels of the protein, but 118 min in cells with surplus prothymosin alpha--led us to conclude that underutilized phosphates persist whereas functioning phosphates disperse. Cell-cycle-specific differences in the half-lives were observed in NIH3T3 cells: 72 min while cycling, 83 or 89 min during arrest in or progression through S phase, but 174 min during M-phase arrest. In the presence of actinomycin D, the value was about 145 min regardless of whether cells were quiescent or growing. In these experiments, reduced utilization of prothymosin alpha's glutamyl phosphates, signaled by an increase in their half-lives, accompanied the attenuation or abolition of transcription. Our data suggest that prothymosin alpha fuels an energy-requiring step in the production, processing, or export of RNA.

Prothymosin alpha is a chromatin-remodelling protein in mammalian cells

Prothymosin alpha (ProTalpha) is an abundant mammalian acidic nuclear protein whose expression is related to cell proliferation. Here we report that in HL-60 cells overexpressing ProTalpha, the accessibility of micrococcal nuclease to chromatin is strongly increased. In the DNA ladder generated by the nuclease activity, the sizes of the mononucleosome (146 bp, the DNA fragment that is bound to the histone octamer) and its multimers correspond to nucleosomes lacking histone H1. The percentage of histone-H1-depleted chromatin (active chromatin) is also higher in the cells overexpressing ProTalpha. On the basis of these and previous findings, we propose a biological role for ProTalpha in the remodelling of chromatin fibres through its interaction with histone H1.

Overexpression of prothymosin alpha accelerates proliferation and retards differentiation in HL-60 cells

Prothymosin alpha (ProTalpha) is an acidic nuclear protein the expression of which is related to the proliferation and differentiation processes in mammalian cells. In the present study we have stably transfected HL-60 cells, a biological system that allows the study of both proliferation and differentiation, with recombinant vectors encoding sense and antisense ProTalpha mRNA. In the HL-60 cell clones overexpressing ProTalpha we observed an acceleration in the growth rate, whereas expression of the antisense orientation showed the opposite effect. Moreover, cell-cycle analysis demonstrated that the G1-phase was shortened in the cells expressing the sense construct. Before studying how ProTalpha affects differentiation, we showed that the down-regulation of ProTalpha gene during differentiation occurs in all mammalian cell lines (HL-60, K562, U937, MEL C88, N2A and PC12) analysed. The biological effect evoked by the induction of the ProTalpha sense vector was the retardation of cell differentiation, although expression of the antisense construct showed no effect on differentiation. In conclusion, our findings provide evidence that ProTalpha is directly implicated in cellular proliferation and that the maintenance of high levels of ProTalpha inside HL-60 cells is incompatible with their ability to differentiate.

Turnover of the acyl phosphates of human and murine prothymosin alpha in vivo

Prothymosin alpha is a small, highly acidic, abundant, nuclear, mammalian protein which is essential for cell growth. Our laboratory has recently shown that primate prothymosin alpha contains stoichiometric amounts of phosphate on the glutamyl groups of the protein and that in vitro the phosphate undergoes rapid hydrolysis or transfer to a nearby serine residue. Here an assay for the presence of acyl phosphates in vivo has been developed by measuring stable phosphoserine and phosphothreonine in vitro. The assay was used to determine the half-life of the acyl phosphates on prothymosin alpha in vivo by pulse-labeling HeLa cells with [32P]orthophosphate and chasing using three different techniques: permeabilization with digitonin to allow extracellular ATP to equilibrate with the intracellular pool; electroporation in the presence of ATP to reduce the specific activity of [32P]ATP by expansion of the pool; and incubation with inorganic phosphate. Regardless of the method, the phosphate turned over with a half-life of 75-90 min. The ability of cells to phosphorylate old prothymosin alpha molecules was established by demonstrating equivalent labeling of the protein with [32P]orthophosphate in the presence and absence of cycloheximide. The half-life of the acyl phosphates was also studied in resting and growing NIH3T3 cells, with measured values of 30-35 and 70 min, respectively. Our data suggest that the "activity" of prothymosin alpha involves the turnover of its acyl phosphates and that it participates in a function common to all nucleated mammalian cells regardless of whether they are quiescent or undergoing rapid proliferation. This is the first measurement of the stability of protein-bound acyl phosphates in vivo.

A translocation t(8;14) and c-myc gene rearrangement associated with the histological transformation of B-cell acute lymphocytic leukemia (FAB-L2) into Burkitt's type (FAB-L3) leukemia

We report a case of B-cell acute lymphocytic leukemia which showed histological transformation from an FAB-L2 into a Burkitt's type (FAB-L3). Both leukemias had identical immunoglobulin heavy-chain joining gene and kappa light-chain joining gene rearrangements, indicating the clonal identity of the two leukemias. A chromosomal analysis of leukemia cells on the onset indicated normal karyotype, whereas that of the transformed FAB-L3 showed t(8;14)(q24;q32). Furthermore, the proto-oncogene c-myc was in the germline configuration in the initial leukemia but in the rearranged configuration after transformation. Presence of t(8;14)(q24;q32) and the c-myc gene rearrangement after transformation suggested that the chromosomal translocation followed by the activation of the c-myc proto-oncogene might be involved in the Burkitt's type transformation of the FAB-L2 leukemic clone, but not in the leukemogenesis of the initial FAB-L2 leukemia.

Mutational analysis of human prothymosin alpha reveals a bipartite nuclear localization signal

Mutants of human prothymosin alpha with impaired ability to inhibit yeast Saccharomyces cerevisiae. cerevisiae cell growth were characterized. Two types of prothymosin alpha-inactivating mutations were observed. Mutations that belong to the first type compromised the nuclear entry of prothymosin alpha by affecting its nuclear localization signal. Analysis of subcellular distribution of GFP-prothymosin alpha fusions revealed a bipartite nuclear localization signal that is both necessary and sufficient for nuclear import of the protein in human cells. Mutations of the second type abrogated the inhibitory action of prothymosin alpha through an unknown mechanism, without influencing the nuclear import of the protein.

Identification of estrogen-responsive genes in neuroblastoma SK-ER3 cells

To evaluate the role of estrogen receptor in the differentiation of cells of neural origin, we developed a molecular approach aimed at the identification of estrogen target genes by mRNA differential display PCR (ddPCR) in human neuroblastoma SK-ER3 cells. More than 3000 RNAs were examined, a few of which displayed a differential regulation pattern in response to 17beta-estradiol (E2). Sequence analysis of three differentially amplified ddPCR products showed homology with the growth-associated nuclear protein prothymosin-alpha (PTMA), the Bcl2-interacting protein Nip2, and one mRNA previously described by others in fetal human brain. Two ddPCR products, referred to as P4 and P10, corresponded to new DNA sequences. Northern analysis confirmed that estrogen treatment of SK-ER3 cells resulted in the upregulation and downregulation of expression of these messages. In particular, PTMA was found to accumulate at both 1 and 17 hr after E2 treatment, whereas P10 product accumulated only at 1 hr. Conversely, P4, Nip2, and the fetal brain-related mRNAs were significantly decreased by the treatment. Further time course analysis of PTMA and Nip2 mRNAs levels indicated that the hormone exerted a marked biphasic regulatory effect on expression of both messages during the course of cell differentiation. In the present study we report for the first time the identification of a panel of estrogen target genes in neural cells that provide new insights in the molecular mechanism of action of E2 in cells of neural origin.

Lysine-87 is a functionally important residue in human prothymosin alpha

Human prothymosin alpha mutants were generated with the aid of random mutagenesis and screened for their ability to inhibit yeast Saccharomyces cerevisiae cell growth. Conversion of Lys-87 to Glu resulted in an inactivated prothymosin alpha mutant, which lost the ability of the wild-type protein to block yeast cell growth. We propose that prothymosin alpha may possess a bipartite rather than monopartite nuclear localization signal, which includes Lys-87, and that the above mutation destroys one part of the nuclear localization signal, thus preventing efficient nuclear uptake of prothymosin alpha.

Prothymosin alpha binds histones in vitro and shows activity in nucleosome assembly assay

Prothymosin alpha (Pro Talpha) is a polypeptide which appears to be involved in cell proliferation, though its precise function has yet to be identified. Here, we report experiments which show that calf Pro Talpha selectively binds to core histones and histone H1 in vitro. Characterization of these interactions by various procedures (including affinity chromatography on Pro T alpha-Sepharose columns, immunoblotting assay and investigation of the behaviour of mixtures of Pro T alpha and histones in solution) indicated that Pro T alpha has higher affinity for core histones (particularly H3 and H4) than for H1. Similarities between the histone-binding patterns of Pro T alpha and of poly(glutamic acid) suggest that the observed histone-binding capacity resides largely in the acidic central region of Pro T alpha. However, all five histones were also bound by T alpha 1 (a peptide corresponding to the first 28 amino acids of Pro T alpha); histone binding by the N-terminal region of Pro T alpha thus cannot be ruled out. Phosphorylation of Pro T alpha does not appear to affect these interactions. In accordance with the observed capacity for histone binding, Pro T alpha (in conjunction with ATP and some Pro T alpha-binding factor/s in a thymocyte extract) was able to induce in vitro nucleosome assembly. We discuss the possibility that Pro T alpha plays a role in chromatin remodelling.

Thymosin alpha 1 does not promote growth or oncogenic transformation

Thymosin alpha 1 (T alpha 1) is an immune modulatory peptide which has been evaluated in a variety of clinical trials. Although no in vivo adverse effects, including enhancement of tumor growth, have been noted, in vitro studies suggesting a role for T alpha 1 in cell growth have been reported. The studies presented in this report evaluated both exogenously added T alpha 1 and endogenously expressed T alpha 1 as factors which could either promote growth of tumor cells or induce transformation. No effect of exogenous T alpha 1 on cell growth was found. NIH-3T3 cells transfected with cDNA for the precursor ProThymosin alpha (Pro T alpha) expressed elevated levels of authentic T alpha 1 but did not demonstrate either enhanced proliferation in liquid culture or transformation as defined by the loss of contact inhibition or anchorage independent growth in soft agar. Thus these studies argue against the hypothesis that T alpha 1 is either an intracellular or extracellular growth promoter.

Do products of the myc proto-oncogene play a role in transcriptional regulation of the prothymosin alpha gene?

The Myc protein has been reported to activate transcription of the rat prothymosin alpha gene by binding to an enhancer element or E box (CACGTG) located in the first intron (S. Gaubatz et al., Mol. Cell. Biol. 14:3853-3862, 1994). The human prothymosin alpha gene contains two such motifs: in the promoter region at kb -1.2 and in intron 1, approximately 2 kb downstream of the transcriptional start site in a region which otherwise bears little homology to the rat gene. Using chloramphenicol acetyltransferase (CAT) reporter constructs driven either by the 5-kb human prothymosin alpha promoter or by a series of truncated promoters, we showed that removal of the E-box sequence had no effect on transient expression of CAT activity in mouse L cells. When intron 1 of the prothymosin alpha gene was inserted into the most extensive promoter construct downstream of the CAT coding region, a diminution in transcription, which remained virtually unchanged upon disruption of the E boxes, was observed. CAT constructs driven by the native prothymosin alpha promoter or the native promoter and intron were indifferent to Myc; equivalent CAT activity was observed in the presence of ectopic normal or mutant Myc genes. Similarly, expression of a transiently transfected wild-type prothymosin alpha gene as the reporter was not affected by a repertoire of myc-derived genes, including myc itself and dominant or recessive negative myc mutants. In COS-1 cells, equivalent amounts of the protein were produced from transfected prothymosin alpha genes regardless of whether genomic E boxes were disrupted, intron 1 was removed, or a repertoire of myc-derived genes was included in the transfection cocktail. More importantly, cotransfection of a dominant negative Max gene failed to reduce transcription of the endogenous prothymosin alpha gene in COS cells or the wild-type transfected gene in COS or L cells. Taken together, the data do not support the idea that Myc activates transcription of the intact human prothymosin alpha gene or reporter constructs that mimic its structure. Rather, they suggest that the human prothymosin alpha promoter and downstream elements are buffered so as to respond poorly, if at all, to transient fluctuations in transcription factors which regulate other genes.

Binding of human prothymosin alpha to the leucine-motif/activation domains of HTLV-I Rex and HIV-1 Rev

Rex of human T-cell leukemia virus type I (HTLV-I) and Rev of human immunodeficiency virus 1 (HIV-1) are post-transcriptional regulators of viral gene expression. By means of affinity chromatography, we purified an 18-kDa cellular protein that bound to the conserved leucine-motif/activation domain of HTLV-I Rex or HIV-1 Rev. The protein that was purified through a Rev-affinity column was found to bind to Rex immunoprecipitated with anti-Rex IgG from an HTLV-I-producing cell line. We analyzed the purified approximately 18-kDa protein biochemically and identified it as prothymosin alpha. The binding activity of prothymosin alpha to Rev or Rex was completely abolished when the epsilon-amino groups of its lysine residues were chemically modified by N-succinimidyl-3-(4-hydroxy-3,5-diodo- phenyl)propionate. The functional relationship between the nuclear protein prothymosin alpha and Rex-Rev is discussed.

Prothymosin alpha: in search of a function

Prothymosin alpha is an acidic nuclear protein that is expressed at high levels in a wide variety of cell types. Accumulating data correlate prothymosin expression with alterations in the proliferative state of cells. Some data indicates that prothymosin may actually be necessary, if not sufficient, for proliferation, and that prothymosin may function in a c-myc associated pathway. Prothymosin is highly conserved through evolution suggesting a key function, however, that function remains unknown.

Human prothymosin alpha inhibits division of yeast Saccharomyces cerevisiae cells, while its mutant lacking nuclear localization signal does not

Effect of human prothymosin alpha and its mutant over-produced in S. cerevisiae on yeast cell division was studied. Wild-type prothymosin alpha appeared to block division of yeast cells. Its inhibitory action could be abolished by deletion of the last nine carboxy-terminal amino acids of prothymosin alpha containing nuclear localization signal, thus pointing to the nucleus as a compartment, where prothymosin alpha performs its action.

The prothymosin alpha gene is specifically expressed in ectodermal and mesodermal regions during early postimplantation mouse embryogenesis

Prothymosin alpha (ProT alpha) is a highly acidic nuclear protein, once believed to have an extracellular immunoregulatory role but more recently implicated in cell proliferation and/or differentiation. Several recent studies have revealed that ProT alpha mRNA is present during embryogenesis. However, these studies did not investigate the spatial distribution of ProT alpha mRNA in the embryo. Here we present a detailed study of the spatial distribution of ProT alpha mRNA during the early stages of postimplantation development (6.5-12.5 dpc) of the mouse. Three findings are of particular interest. First, ProT alpha mRNA levels increase during the early postimplantation stages (6.5-8.5 dpc) of mouse embryogenesis. Second, ProT alpha mRNA is not uniformly distributed in the mouse embryo, but is present in a spatially specific manner. Third, we have observed that the mouse ProT alpha gene is expressed almost exclusively in ectodermal and mesoderm-derived structures, and not in cells which give rise to the definitive endoderm.

An E-box element localized in the first intron mediates regulation of the prothymosin alpha gene by c-myc

In RAT1A fibroblasts, expression of the prothymosin alpha gene is under the transcriptional control of the c-myc proto-oncogene. We have now cloned the rat gene encoding prothymosin alpha and show that the cloned gene is regulated by c-myc in vivo. We find that regulation by c-myc is mediated by sequences downstream of the transcriptional start site, whereas the promoter is constitutive and not regulated by c-myc. We have identified an enhancer element within the first intron that is sufficient to mediate a response to Myc and Max in transient transfection assays and to activation of estrogen receptor-Myc chimeras in vivo. We find that this element contains a consensus Myc-binding site (CACGTG). Disruption of this site abolishes the response to Myc and Max in both transient and stable assays. Mutants of either Myc or Max that are deficient for heterodimerization fail to regulate the prothymosin alpha gene, suggesting that a heterodimer between Myc and Max activates the prothymosin alpha gene. Our data define the prothymosin alpha gene as a bona fide target gene for c-myc.

An E-box element localized in the first intron mediates regulation of the prothymosin alpha gene by c-myc

In RAT1A fibroblasts, expression of the prothymosin alpha gene is under the transcriptional control of the c-myc proto-oncogene. We have now cloned the rat gene encoding prothymosin alpha and show that the cloned gene is regulated by c-myc in vivo. We find that regulation by c-myc is mediated by sequences downstream of the transcriptional start site, whereas the promoter is constitutive and not regulated by c-myc. We have identified an enhancer element within the first intron that is sufficient to mediate a response to Myc and Max in transient transfection assays and to activation of estrogen receptor-Myc chimeras in vivo. We find that this element contains a consensus Myc-binding site (CACGTG). Disruption of this site abolishes the response to Myc and Max in both transient and stable assays. Mutants of either Myc or Max that are deficient for heterodimerization fail to regulate the prothymosin alpha gene, suggesting that a heterodimer between Myc and Max activates the prothymosin alpha gene. Our data define the prothymosin alpha gene as a bona fide target gene for c-myc.

Prothymosin alpha binds to histone H1 in vitro

Previous studies have shown that prothymosin alpha (ProT alpha) is a nuclear acidic protein implicated in cell proliferation. To identify proteins that interact with ProT alpha we have used ligand-blotting assays. We report here that purified ProT alpha binds specifically to histone H1 in a dose dependent manner. Polyglutamic acid, an analog of the central acidic domain of ProT alpha, strongly inhibits the above interaction, suggesting that the binding of ProT alpha to histone H1 is mediated through its acidic domain.

Distribution of prothymosin alpha in rat and human adrenal cortex

BACKGROUND

Prothymosin alpha (ProT) is a polypeptide widely distributed in the organism and expressed by cell types with a high proliferative capacity. The aim of the current work was to investigate if ProT was localized in the progenitor compartment of the adrenal cortex which, following the cell migration theory, corresponds to the zona glomerulosa.

METHODS

We studied by immunohistochemical and in situ hybridization methods the distribution of ProT in rat and human adrenal cortex. Immunohistochemical techniques for the study of the proliferating cell nuclear antigen (PCNA) and incorporation of 5-bromo-2'-deoxyuridin during DNA synthesis were also done. Immunoelectron microscopic procedures were performed to determine the exact subcellular localization of ProT.

RESULTS

ProT was found in the zona glomerulosa cells, but not in the cells of the remaining cortical layers (zonae fasciculata and reticularis). Glomerulosa cells showed immunostaining for ProT only in the nuclei, excluding the nucleoli. Variability in immunostaining intensity was found between different glomerulosa cells. In situ hybridization of ProT mRNA confirmed that ProT synthesis in adrenal cortex occurs only in the zona glomerulosa. The results obtained for proliferating cell nuclear antigen and incorporation of 5-bromo-2'-deoxyuridin confirmed that adrenocortical proliferation occurs in the zona glomerulosa. Ultrastructural immunocytochemistry showed labelling for ProT over the euchromatin, but not on the heterochromatin aggregations nor the nucleoli.

CONCLUSIONS

The results presented here: 1) support the migration theory for the adrenocortical cell renewal, 2) demonstrate that ProT is present in the nuclei of proliferating cells (being associated with euchromatin), and 3) suggest that the study of ProT expression would be useful in distinguishing cycling from resting cells.

Evolution of prothymosin alpha and proliferating cell nuclear antigen (PCNA) immunoreactivity through the development of rat ovarian follicles

The cellular distribution of prothymosin alpha (ProT) was studied in ovarian follicles of adult cycling rats. We found positive granulosa and theca cells throughout follicular maturation. When both ProT and proliferating cell nuclear antigen (PCNA) immunoreactivity was studied, we observed that both proteins were expressed in the same granulosa and theca cells, although sometimes ProT immunoreactivity was weak or absent in the mitotic (M) phase. Moreover, both peptides share the nuclear distribution, but ProT immunoreactivity was never seen in nucleoli. Therefore, we conclude that in mitotic cells ProT is expressed only in actively proliferating cells, since all ProT-positive cells were also positive for PCNA. ProT and PCNA immunoreactivities during the meiotic division were studied in oocytes. The presence of PCNA was, unlike ProT, constant throughout follicle development (except atretic oocytes). Oocytes expressed ProT from primordial follicles to the eighth generation, but more developed oocytes and atretic oocytes were not immunoreactive. In hypophysectomized rats, all oocytes were immunoreactive. Interestingly, in hypophysectomized rats treated with follicle stimulating hormone (FSH) that promoted follicle development, the more developed oocytes did not show ProT immunoreactivity. Since hypophysectomized rats were not treated with luteinizing hormone we conclude that ProT expression is not required to complete meiotic division I.

Phosphorylation of human and bovine prothymosin alpha in vivo

Prothymosin alpha is post-translationally modified. When human myeloma cells were metabolically labeled with [32P]orthophosphoric acid, they synthesized [32P]prothymosin alpha. The incorporated radioactivity was resistant to DNase and RNases A, T1, and T2, but could be completely removed by alkaline phosphatase. No evidence was found for an RNA adduct as postulated by Vartapetian et al. [Vartapetian, A., Makarova, T., Koonin, E. V., Agol, V. I., & Bogdanov, A. (1988) FEBS Lett. 232, 35-38]. Thin-layer electrophoresis of partially hydrolyzed [32P]prothymosin alpha indicated that serine residues were phosphorylated. Analysis of peptides derived from bovine prothymosin alpha and human [32P]prothymosin alpha by treatment with endoproteinase Lys-C revealed that the amino-terminal 14-mer, with serine residues at positions 1, 8, and 9, was phosphorylated at a single position. Approximately 2% of the peptide in each case contained phosphate. Further digestion of the phosphopeptide with Asp-N followed by C18 reversed-phase column chromatography produced two peptides: a phosphate-free 9-mer containing amino acids 6-14 and a labeled peptide migrating slightly faster than the N-terminal 5-mer derived from the unmodified 14-mer. Positive identification of the phosphorylated amino acid was obtained by colliding the 14-residue phosphopeptide with helium in the mass spectrometer and finding phosphate only in a nested set of phosphorylated fragments composed of the first three, four, and five amino acids. The results prove that prothymosin alpha contains N-terminal acetylserine phosphate. In a synchronized population of human myeloma cells, phosphorylation occurred throughout the cell cycle. Furthermore, prothymosin alpha appeared to be stable, with a half-life slightly shorter than the generation time. Although prothymosin alpha is known to be essential for cell division, the constancy of both the amount of the protein and the degree of its phosphorylation suggests that prothymosin alpha does not directly govern mitosis.

Identification of immunoreactive forms of thymosin alpha 1 in serum and supernatants by combining HPLC and RIA

Thymosin alpha 1 (T alpha 1) is a biologically active peptide, originally isolated from the thymus and currently undergoing clinical trials as an immunomodulator in cancer patients, in individuals with chronic active hepatitis, and as an immunoenhancer of vaccines in immunocompromised individuals. Absorption of rabbit antibody to thymosin alpha 1 with a synthetic C-14 fragment of T alpha 1 results in an antiserum with increased affinity for the amino terminal region of T alpha 1 and the precursor protein prothymosin alpha (ProT alpha). Using HPLC methodologies, the predominant form of immunoreactivity in serum and thymus was T alpha 1 not the precursor. Using this assay we detected a decline in mouse serum T alpha 1 following irradiation but not thymectomy, an observation consistent with the existence of an important radiation sensitive lymphoid source of serum T alpha 1. The secretion of authentic T alpha 1 but not the precursor into culture medium by thymic epithelial cells as well as in mitogen-stimulated peripheral blood lymphocytes was also demonstrated by HPLC/RIA. HPLC analysis by molecular weight sizing columns demonstrated that unlike thymic epithelial cells or peripheral blood lymphocytes, the immunoreactive T alpha 1 (IRT alpha 1) form in the supernatants from tumor cells such as MCF-7 breast carcinoma was of a lower molecular weight than authentic T alpha 1. These studies suggest that the authentic form of T alpha 1 is the major immunoreactive form in normal serum and that it is secreted by the medullary thymic epithelial cells as well as by peripheral blood lymphocytes. An additional immunoreactive form, secreted by tumor cells has also been identified and is the subject of future studies.

Prothymosin alpha expression occurs during G1 in proliferating B or T lymphocytes

To gain insight into possible functions for prothymosin alpha in the proliferative cycle of lymphocytes, we examined the kinetics of prothymosin alpha mRNA expression in mitogen stimulated murine lymphocytes. This mRNA increases after mitogen stimulation, peaking in mid G1. This kinetics is compatible with induction of the prothymosin alpha gene by the c-myc protein (Eilers, M., Schirm, S. and Bishop, J.M. (1991) EMBO J., 10, 133-141). Thus, although prothymosin alpha mRNA is found throughout the cell cycle, the elevated expression in G1 may be associated with an increased requirement for prothymosin alpha during the G1/S transition or the S phase of the cell cycle.

Evidence for the extranuclear localization of thymosins in thymus

A new radioimmunoassay has been developed for thymosin beta 4 by generating rabbit polyclonal antibodies against the synthetic N-terminal peptide fragment 1-15 coupled to KLH. The synthetic analogue [Tyr12]-thymosin beta 4 (1-15) was used as tracer. This radioimmunoassay, with a useful range of 10-1000 pmoles, showed cross-reactivity with the second homologous beta-thymosin of man and rat (thymosin beta 10) but not of calf (thymosin beta 9). This radioimmunoassay, together with an improved radioimmunoassay for the N-terminus of parathymosin alpha, was employed for the measurement of the levels of thymosin beta 4 and parathymosin alpha in nuclear and extranuclear extracts of calf thymus. The bulk of these polypeptides was found in the extranuclear material whereas only traces were observed in the nuclear environment, which indicates the extranuclear localisation of alpha- and beta-thymosins.

Evidence for nuclear targeting of prothymosin and parathymosin synthesized in situ

To test the hypothesis that prothymosin and parathymosin contain amino acid sequences that cause them to be targeted to the cell nucleus, expression vectors were constructed containing a simian virus 40 promoter and cDNAs that would code for chimeric proteins composed of truncated human growth hormone (hGH) linked to the NH2 terminus of prothymosin or parathymosin. The truncated hGH lacked the signal peptide sequence required for its secretion. After transfection of these constructs into HeLa S3 cells, which do not normally synthesize hGH, the use of indirect immunofluorescence staining to follow the localization of the hGH chimeras demonstrated that both prothymosin and parathymosin caused targeting to the cell nucleus. Controls with a construct coding for native hGH only, and one coding for the truncated hGH lacking the signal peptide, revealed secretion into culture medium and staining in the endoplasmic reticulum and Golgi apparatus in the first case, and diffuse staining throughout the cytoplasm in the second. The results provide direct evidence, with proteins synthesized in situ, for the presence of nuclear localization signals in both prothymosin and parathymosin.

Identification of a low-Mr acidic nuclear protein as prothymosin alpha

We have purified to homogeneity a 15-kDa perchloric acid (PCA)-soluble protein from rat thymus nuclei. This highly acidic protein showed a Mr of ca. 30 kDa in acetic acid/urea gels, probably due to oligomer formation. Sequence analysis of internal tryptic and thermolytic peptides revealed that the purified protein is, in fact, prothymosin alpha, a very hydrophilic polypeptide, which has been previously classified as a thymic or immunomodulating hormone. We found that prothymosin alpha is a rather abundant nuclear protein in rat thymus; its concentration is comparable to that of a well-characterized nonhistone protein HMG-14. The subcellular localization and physicochemical properties of prothymosin alpha suggest that its function is related to those of other long polyacidic regions containing nuclear proteins.

Thymosins: both nuclear and cytoplasmic proteins

A simple procedure based on perchloric acid extraction has been developed for the preparation and purification of bovine prothymosin alpha and thymosins beta 4 and beta 9 in high yields. Spectroscopic observations show these proteins to be non-folding at neural pH. The cellular locations of human prothymosin alpha, rat parathymosin and calf thymosin beta 4, all so-called 'thymic hormones', have been studied by injection into the cytoplasm of Xenopus oocytes, followed by separate monitoring of nuclear and cytoplasmic concentrations. It is shown that human prothymosin alpha and rat parathymosin both migrate to the nucleus whilst thymosin beta 4 remains in the cytoplasm. The peptide (1-88) of calf prothymosin alpha is shown not to accumulate in the Xenopus nucleus, demonstrating that the C-terminal 21 residues, which include a KKQK sequence, are required for nuclear migration. The present data, in association with existing evidence of wide tissue distribution and the lack of signal peptides, indicate that these proteins do not behave as hormones in the usual sense of the word. It is suggested that thymosin beta 4 should be grouped separately from the pro- and parathymosins.

Cellular levels of thymosin immunoreactive peptides are linked to proliferative events: evidence for a nuclear site of action

Thymosin alpha 1 (T alpha 1), the N-terminal 28-amino acid fragment of prothymosin alpha (ProT alpha), and ProT alpha, although originally isolated from whole thymus extracts, are also present in nonthymic cells and tissues. We used an ELISA with an antibody raised against T alpha 1 to investigate the relationship between intracellular levels of thymosin immunoreactive peptide(s) (TIP) and cell proliferation in a rat small intestinal IEC-6 cell line. Increasing TIP levels were observed during cell proliferation, which decreased when proliferation was halted by cellular contact inhibition. Serum feeding of cells previously rendered quiescent by serum starvation resulted in a significant increase in TIP within 1 hr. Conversely, serum starvation decreased TIP levels within 1 hr. Peak TIP levels appeared after 3 hr of serum incubation, while maximum [3H]thymidine incorporation was noted after 9 hr, suggesting maximum TIP concentrations in the G1 phase of the proliferative cycle. Immunoelectron microscopy demonstrated an association of TIP with condensed nuclear chromatin. These results support a relation of intracellular TIP levels to IEC-6 cell proliferation and also a nuclear site of action. HPLC analysis of cellular homogenates from proliferating IEC-6 cells revealed a peak of immune reactivity that elutes in the position of T alpha 1.