Rapid induction of thymosin beta 4 in concanavalin A-stimulated thymocytes by translational control

Expression of thymosin beta in the rat brain following transient global ischemia

Thymosin beta (Tbeta) isoforms play an important role in the organization of the cytoskeleton by sequestering G-actin during development of the mammalian brain. In this study, we examined changes in the expression of Tbeta4 and Tbeta15 after transient global ischemia. Tbeta15 mRNA increased gradually in the dentate gyrus (DG) of the hippocampal formation from 3 h after reperfusion and peaked 9 h later. Similarly, a significant increase in Tbeta4 mRNA level was observed in the DG 12 h after reperfusion. Tbeta4 and Tbeta15 proteins were found in different cell types in control brains; Tbeta15 was expressed in a subset of doublecortin (DCX)-positive cells in the DG, whereas Tbeta4-IR was observed in DG neurons and nearby microglial cells. After ischemia, Tbeta15-IR was found in DG neurons and Tbeta4-IR in the reactivated microglial cells. Interestingly, Tbeta15-IR accumulated in the nuclei of CA1 neurons, which are vulnerable to ischemic insults. These results suggest that Tbeta4 and Tbeta15 function in different cellular contexts during ischemia-induced responses.

Roles of thymosins in cancers and other organ systems

Thymosins are small peptides, originally identified from the thymus, but now known to be more widely distributed in many tissues and cells. Thymosins are divided into three main groups, alpha-, beta-, : and gamma-thymosins, based on their isoelectric points. alpha-thymosins (ProTalpha, Talphal) have nuclear localization and are involved in transcription and/or DNA replications; whereas beta-thymosins (Tbeta4, Tbeta10, Tbetal5) have cytoplasmic localization and show high affinity to G-actin for cell mobility. Furthermore, it is well known that both alpha- and beta-thymosins play important roles in modulating immune response, vascular biology, and cancer pathogenesis. More importantly, thymosins may have significant clinical applications. They may serve as molecular markers for the diagnosis and prognosis of certain diseases. In addition, they could be molecular targets of certain diseases or be used as therapeutic agents to treat certain diseases. However, the molecular mechanisms of action of thymosins are largely unknown. This review not only presents recent advances of basic science research of thymosins and their clinical applications but provides thoughtful views for future directions of investigation on thymosins.

Regulation of apoptotic pathways in bovine γ/δ T cells

T lymphocytes bearing gamma/delta TCRs are a major population of T cells in neonatal calves and discrete subsets of gamma/delta T cells display tissue-specific accumulation and responsiveness to infection. To enhance our understanding of the immunobiology of gamma/delta T cells, we characterized the gene expression profile of circulating bovine gamma/delta T cells following stimulation with recombinant human IL-2 and ConA. Statistical analysis of microarray data identified 108 genes with significantly altered expression, including four genes associated with apoptosis. Real-time reverse transcription-PCR (RT-PCR) analysis of 15 genes related to apoptotic pathways showed that both the Fas-mediated and the mitochondrial apoptotic pathways were repressed in circulating bovine gamma/delta T cells in response to mitogen activation, indicating that stimulated peripheral bovine gamma/delta T cells are resistant to activation-induced apoptosis.

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.

beta-Thymosins, small acidic peptides with multiple functions

The beta-thymosins are a family of highly conserved polar 5 kDa peptides originally thought to be thymic hormones. About 10 years ago, thymosin beta(4) as well as other members of this ubiquitous peptide family were identified as the main intracellular G-actin sequestering peptides, being present in high concentrations in almost every cell. beta-Thymosins bind monomeric actin in a 1:1 complex and act as actin buffers, preventing polymerization into actin filaments but supplying a pool of actin monomers when the cell needs filaments. Changes in the expression of beta-thymosins appear to be related to the differentiation of cells. Increased expression of beta-thymosins or even the synthesis of a beta-thymosin normally not expressed might promote metastasis possibly by increasing mobility of the cells. Thymosin beta(4) is detected outside of cells in blood plasma or in wound fluid. Several biological effects are attributed to thymosin beta(4), oxidized thymosin beta(4), or to the fragment, acSDKP, possibly generated from thymosin beta(4). Among the effects are induction of metallo-proteinases, chemotaxis, angiogenesis and inhibition of inflammation as well as the inhibition of bone marrow stem cell proliferation. However, nothing is known about the molecular mechanisms mediating the effects attributed to extracellular beta-thymosins.

The antiproliferative activity of the tetrapeptide Acetyl-N-SerAspLysPro, an inhibitor of haematopoietic stem cell proliferation, is not mediated by a thymosin beta 4-like effect on actin assembly

Acetyl-N-SerAspLysPro (AcSDKP), known as a negative regulator of haematopoiesis, has been principally reported as an inhibitor of haematopoietic pluripotent stem cell proliferation. The tetrapeptide sequence is identical to the N-terminus of thymosin beta 4 (T beta 4), from which it has been suggested that it may be derived. Recently, evidence was shown that T beta 4 plays a role as a negative regulator of actin polymerization leading to the sequestration of its monomeric form. The structural similarity between the N-terminus of T beta 4 and AcSDKP has raised the possibility that AcSDKP may also participate in intracellular events leading to actin sequestration. The effect of T beta 4 on the proliferation of haematopoietic cells was compared to that of AcSDKP. The results revealed that T beta 4, like AcSDKP, exerts an inhibitory effect on the entry of murine primitive bone marrow cells into cell cycle in vitro. Qualitative electrophoretic analysis and quantitative polymerization assays were used to investigate the role of AcSDKP in actin polymerization. AcSDKP does not affect actin assembly at concentrations up to 50 microM, and does not compete with T beta 4 for binding to G-actin. These results suggest that AcSDKP is not involved in cell cycle regulation via an effect on the process of actin polymerization.

Regulation of thymosin beta 4 mRNA levels during cell proliferation

The levels of thymosin beta 4 mRNA were studied throughout the cell cycle of NIH 3T3 cells. In serum deprived, quiescent cells, the levels of thymosin beta 4 were undetectable; after serum restoration, the cells were induced to proliferate and we found a pronounced increase in thymosin beta 4 mRNA levels at the G1/S transition. Thymosin beta 4 mRNA was induced even in the presence of cycloheximide. On the other hand, cycling cells that were synchronized at different stages of the cycle by means of mitotic shake-off after nocodazole arrest or a double thymidine block did not show any variation in the levels of thymosin beta 4 mRNA when they progressed synchronously through the cycle. In conclusion, the present data indicate that the thymosin beta 4 gene is regulated by cell proliferation but it is not a cell cycle-regulated gene. Finally, we studied thymosin beta 4 mRNA stability by inhibiting thymosin beta 4 gene transcription with actinomycin D. Our results suggest that thymosin beta 4 mRNA has a pronounced stability, a fact that might be relevant to account for the presence of thymosin beta 4 in enucleated cells like platelets.

Molecular interactions between G-actin, DNase I and the beta-thymosins in apoptosis: a hypothesis

The beta-thymosins are a family of < 5kDa (MW), mostly acidic, proteins which were originally defined in the immune system. Recently, specific members of this family of cytoplasmic polypeptides, namely beta-4 and beta-10, were shown to bind monomeric G-actin both in vitro and in vivo. Whilst many aspects of programmed cell death or 'apoptosis' remain to be defined, the Ca2+/Mg(2+)-dependent endonuclease, DNase I does feature in this process. Monomeric G-actin binds to and inhibits the DNA-degrading activity of DNase I. Given that the intracellular abundance of thymosins beta-4 and beta-10 is related to cell division and differentiation and that anticancer/morphogenic agents such as retinoic acid (RA) and cyclic AMP modulate expression of their respective genes, it is possible that these G-actin sequestering proteins play significant roles in apoptosis perhaps mediated via DNase I.

Amplification-independent overexpression of thymosin beta-10 mRNA in human renal cell carcinoma

The structurally related small (< 5 kD) polypeptides, namely thymosins beta-4 and beta-10, were originally defined in the rat immune system. Previously it was shown that both the beta-4 and beta-10 genes are constitutively expressed at higher levels in neoplastic human kidney. Also, it was shown that human embryonic kidney contained more of these proteins than the adult tissue. The present study used a human thymosin beta-10 cDNA to examine the possibility that overexpression of the beta-10 mRNA in renal cell carcinoma was due to gene amplification. Southern blot analysis of genomic DNA extracted from normal and neoplastic tissue indicated no amplification of the thymosin beta-10 gene in RCC. No amplification or rearrangements were found in the human RAR-alpha gene in normal versus RCC tissue. Decreased expression of both the thymosin beta-4 and beta-10 proteins in the normal adult human kidney was found to be derived from a corresponding decrease in levels of the cognate mRNAs. These findings suggest that the thymosin beta-10 gene is deregulated in renal cell carcinoma.

Transcript levels of thymosin beta 4, an actin-sequestering peptide, in cell proliferation

Thymosin beta 4 (beta 4) is an ubiquitous 5-kDa peptide that has been identified as an actin-sequestering peptide. In this work, Northern blot analysis was used to study the beta 4 mRNA levels during the cell cycle of rat thymocytes and hepatocytes as well as in human lymphocytes from patients with leukemia. beta 4 mRNA was found in all the stages of thymocyte and hepatocyte cell cycle, showing an increase in the S-phase which was maintained during the G2 and M phases. Incubation of splenic T-cells with concanavalin A, phorbol myristate acetate or the ionophore A23187 lead to a similar increase of beta 4 transcript during the S-phase. The increase in beta 4 mRNA observed in the G2/M boundary of the cell cycle, together with its ability to inhibit actin polymerization, suggests a possible role of beta 4 in the the morphological changes and actin redistribution occurring during the cytokinesis.

Cell cycle-related expression of poly(ADP-ribosyl)transferase in proliferating rat thymocytes

The activity profile of poly(ADP-ribosyl)transferase was assayed during a complete cell cycle of rat thymocytes stimulated in the presence of interleukin-2 by concanavalin A or monoclonal antibodies against the T-cell antigen receptor (TCRmAB). Poly ADP-ribosylation was measured in permeabilized cells by the incorporation of [adenine-3H]NAD+ into protein bound poly ADP-ribose. The polymers of ADP-ribose were separated from the monomers using dihydroxyboronyl-Bio-Rex 70 columns. The rate of poly(ADP-ribosyl)ation increases during the G1 phase with a maximum 12 h after stimulation. This increase in activity is due to enhanced de novo synthesis of poly(ADP-ribosyl)transferase which can be abolished by the addition of cycloheximide. The half-life of this enzyme during the induction period was estimated to be 4 h. A second activity peak appears during the S-phase of the cell cycle 48 h after stimulation. The maxima of the poly(ADP-ribosyl)ation rate coincide with elevated immunoreactive enzyme levels at 12 and 48 h of culture assayed by Western blotting. The mRNA levels of pADPRT do not correlate with the first maximum of activity, whereas the second maximum was accompanied by a 5-fold increase of the specific mRNA. These results suggest a translational regulation of pADPRT in the G1 phase of the cell cycle, whereas the second activity peak in the S-phase is due to an increased transcription and translation. The induction of pADPRT activity in the G1 phase of TCRmAB-stimulated cells points to a function of poly(ADP-ribosyl)ation in the proliferation of thymocytes.

Small actin-binding proteins: the beta-thymosin family

Thymosin beta 4 is a major actin monomer binding protein present at high concentration in many vertebrate cells and cell lines. The interactions of actin with thymosin beta 4, actobindin and profilin are compared. Nine beta-thymosins have been identified; six have been shown to bind to actin. Regulation of the synthesis of thymosin beta 10 and thymosin beta 4 has been found in brain development and after stimulation of several cell types, respectively. The extracellular effects of thymosin beta 4 still need clarification.

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.

Primary structure of thymosin beta 12, a new member of the beta-thymosin family isolated from perch liver

A new polypeptide termed thymosin beta 12 has been isolated from perch liver and its primary structure elucidated. This polypeptide contains 43 amino acid residues with a molecular weight of 4822 Da. The content of thymosin beta 12 from perch liver has been determined as 43 micrograms/g of tissue. The amino-terminal end of this polypeptide is blocked by an acetyl group as deciphered by fast-atom bombardment mass spectrometric analysis. Sequence analysis reveals that thymosin beta 12 is 79% homologous to thymosin beta 4, an immunomodulator which was originally isolated from calf thymus. Thymosin beta 12 also shows 84% sequence homology to thymosin beta 11, a beta 4 analog which replaces beta 4 in two species of bony fish, oscar and rainbow trout. The evolutionary implication of such results will be discussed. The isolation of a new beta 4-related peptide from perch liver which differs from beta 11 indicates that beta-thymosin peptides are widely distributed in lower vertebrate classes.

The early induction of the actin-sequestering peptide thymosin beta 4 in thymocytes depends on the proliferative stimulus

The expression of the actin-sequestering peptide, thymosin beta 4, was analyzed in proliferating rat thymocytes, activated by diverse stimuli, during the early G1 phase and the S phase. In the presence of concanavalin A a 6.3-fold increase of thymosin beta 4 occurred already after 1 h of stimulation without elevation of the corresponding mRNA level. In contrast, during the S phase the increase of thymosin beta 4 (2.5-fold) was accompanied by a higher mRNA level, but did not exceed the growth related increase of total protein. Stimulation with a crosslinked antibody against rat T cell antigen receptor or stimulation with phorbol 12-myristate 13-acetate (PMA) and Ca(2+)-ionophore A23187, separately or in combination, did not lead to the marked increase of the thymosin beta 4 concentration in the early G1 phase but resulted in elevated thymosin beta 4 peptide and mRNA levels during the S phase. It therefore appears that protein kinase C activation and a rise in cytoplasmic Ca(2+)-concentration are not exclusively responsible for the stimulation of thymosin beta 4 specific translation in thymocytes. This assumption was reinforced by the observation that inhibition of the protein kinase C activity by 1-(5-isoquinolinylsulfony)-2-methylpiperazine (H-7) did not affect the cellular thymosin beta 4 content 1 h and 48 h after concanavalin A (Con A) stimulation.

Nucleotide and nucleic acid metabolism in rat thymocytes during cell cycle progression

A complete cell cycle of mature, concanavalin A (Con A) stimulated rat thymocytes was documented by analyzing the cell number as well as the content and synthesis of DNA and RNA. Cell cycle progression is accompanied by an elevation of class I, II and III RNA polymerase activities (about 10-fold) in the S phase maximum, 48 h after stimulation. Moreover, maximal cellular contents of DNA, ATP, ADP and AMP were observed at this culture period, whereas the RNA level peaked at 60 h. The synthesis of purine and pyrimidine nucleotides de novo was detected by use of [14C]HCO3-. Maximal incorporation rates of [14C]HCO3- into nucleotides (de novo synthesis) and of [3H]adenine into adenylates ('salvage pathway') occur during the S phase. However, the de novo synthesis rates were markedly lower than those of the 'salvage pathway'. The highest cellular level of the nucleotide precursor 5-phosphoribosyl-1-pyrophosphate (8.4-fold increase) also coincided with the S phase.

Cell-cycle-regulated expression of thymosin beta 4 in thymocytes

Thymosin beta 4 belongs to a family of ubiquitous peptides present at a high cellular content but still with an unknown intracellular function. The expression of this peptide was studied in concanavalin-A-stimulated, proliferating rat thymocytes during cell cycle progression. An early, transient 10-fold increase of the peptide occurred 1 h after stimulation without elevation of the corresponding mRNA level. This increase coincided with that of thymosin beta 4 biosynthesis. The sharp decline of the thymosin beta 4 content was not due to a secretion of the peptide into the medium. During S phase and mitosis, the biosynthetic rates as well as mRNA content, but not the cellular thymosin beta 4 concentration, increased again. After 96 h of culture the values returned to those of quiescent cells.

Differential expression of thymosin genes in human tumors and in the developing human kidney

Thymosins beta 4 and beta 10 are 2 structurally related polypeptides originally defined in the rat immune system. To date, no truly unambiguous functions have been formally ascribed to these small (less than 4.9 kDa) acidic proteins. Previous research has demonstrated relationships between expression of these genes and cell growth/differentiation. These observations prompted the present study which has used cDNA and synthetic oligonucleotide probes in combination with high-performance liquid chromatography (HPLC) to examine the differential expression of these 2 genes in normal and neoplastic human tissues and in the developing human kidney. Low levels of beta 4 and beta 10 mRNA species prevailed in normal tissues; in contrast, these gene transcripts were notably more abundant in malignant renal tumors and in the normal human embryonic kidney. These findings show that the thymosin beta 4 and beta 10 genes are constitutively expressed at higher levels in embryonic/neoplastic as compared to normal/benign tissues and that thymosin in beta 10 in particular may be a new molecular marker for renal-cell carcinoma as well as other malignancies.

Formation of acetyl-Ser-Asp-Lys-Pro, a new regulator of the hematopoietic system, through enzymatic processing of thymosin beta 4

The demonstration that AcSDKP, a new regulator of the hematopoietic system, is formed in the bone marrow by a one-step enzymatic maturation processing of thymosin beta 4 (T beta 4) is presented. AcSDKP and T beta 4 were both detected in bone marrow cells (BMC). Incubation of [3H]T beta 4 with either intact or lysed BMC led to the formation of [3H]AcSDKP, whereas the labeled tetrapeptide was not degraded under these conditions. Model enzymatic degradation of T beta 4 carried out with bacterial enzymes suggests that a mammalian endoproteinase Asp-N might be involved in the formation of AcSDKP through the specific cleavage of the Pro4-Asp5 peptidic bond of T beta 4. In contrast, alpha-prolyl-endopeptidase was ineffective in carrying out a similar processing.