Is thymosin alpha 1 a thymic hormone?

Prothymosin alpha is a nonspecific facilitator of nuclear processes: studies of run-on transcription

The effect of prothymosin alpha on transcriptional elongation has been examined. The addition of prothymosin alpha to COS-1 and NIH3T3 cell nuclei engaged in run-on transcription stimulated RNA synthesis approximately two- to threefold in a dose-dependent manner. Polyglutamic acid or a random polypeptide composed of glutamic acid, alanine, and tyrosine, did not substitute for prothymosin alpha. Enhanced transcription occurred in the presence of high and low doses of actinomycin D and in the presence of alpha-amanitin, but not in nuclear extracts. The stimulatory effect was dependent on a limiting concentration of one nucleoside triphosphate and was nearly abrogated by saturating levels of precursors. In the presence of Sarkosyl, which itself increases transcription, prothymosin alpha was almost ineffectual. The data are consistent with a model in which prothymosin alpha does not interact directly with polymerases but, instead, nonspecifically decreases the barriers to diffusion of charged molecules in electrostatically charged environments.

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.

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.

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.

Thymosin-alpha1 stimulates maturation of CD34+ stem cells into CD3+4+ cells in an in vitro thymic epithelia organ coculture model

The effect of thymosin-alpha1 on thymopoiesis is largely unknown. Thymosin is found in the cortical and medullary thymic epithelia, as well as in nurse cells; thus, it is hypothesized that thymosin may affect both early and late stage of thymocyte maturation. In this study, the effect of thymosin-alpha1 on thymopoiesis was determined by coculturing in vitro CD34+ stem cells (SC) with allogeneic cultured thymic epithelia fragments (CTEF) for 1-4 weeks and analyzing T-cell maturation by flow cytometry. Thymosin-alpha1 significantly enhanced the cell number (e.g., proliferation) of mononuclear cells obtained at 2 and 4 weeks of the SC-CTEF cocultures (P < 0.01 and < 0.05, respectively). In particular, thymosin-alpha1 stimulated expression of CD3+ cells at 3 and 4 weeks (P < 0.05). The predominant subpopulation increased by thymosin stimulation was single positive mature CD4+ cells, which was confirmed to occur within the SC-CTEF thymic organ tissue by laser confocal immunofluorescence microscopy. Thymosin stimulation tended to enhance IL-7 synthesis, critical cytokine in the maturation of thymocytes. In summary, this is the first study to demonstrate that thymosin-alpha1 enhanced thymopoiesis of CD34+ stem cells in humans using an in vitro model of differentiation using stem cells and cultured thymic epithelial fragments cocultures. Furthermore, the thymosin significantly increased expression of CD3+4+ T cells.

The conformation of peptide thymosin alpha 1 in solution and in a membrane-like environment by circular dichroism and NMR spectroscopy. A possible model for its interaction with the lymphocyte membrane

The 28-residue peptide thymosin alpha1 was studied by circular dichroism and two-dimensional NMR. Circular dichroism indicates that thymosin alpha1 in water solution does not assume a preferred conformation, while in the presence of small unilamellar vesicles of dimiristoylphosphatidylcholine and dimiristoylphosphatidic acid (10:1) and in sodium dodecyl sulphate, it assumes a partly structured conformation. Presence of zinc ions produces similar effects. In a more hydrophobic environment like a solution of a mixed solvent water-2,2,2 trifluoroethanol, it adopts a structured conformation. NMR spectra indicated that in this mixture as solvent, thymosin alpha1 has a structure characterized by two regions. A beta-turn is present between residue 5 and residue 8, while the region between residues 17 and 24 shows an alpha helix conformation. These changes of conformation in different environments may be considered structural requirements in the steps of its interaction with the lymphocyte membrane. In fact, these conformational changes may correspond to the first event of the mechanism of lymphocyte activation in the immune response modulation by thymosin alpha1.

Production and function of cytokines in natural and acquired immunity to Candida albicans infection

Host resistance against infections caused by the yeast Candida albicans is mediated predominantly by polymorphonuclear leukocytes and macrophages. Antigens of Candida stimulate lymphocyte proliferation and cytokine synthesis, and in both humans and mice, these cytokines enhance the candidacidal functions of the phagocytic cells. In systemic candidiasis in mice, cytokine production has been found to be a function of the CD4+ T helper (Th) cells. The Th1 subset of these cells, characterized by the production of gamma interferon and interleukin-2, is associated with macrophage activation and enhanced resistance against reinfection, whereas the Th2 subset, which produces interleukins-4, -6, and -10, is linked to the development of chronic disease. However, other models have generated divergent data. Mucosal infection generally elicits Th1-type cytokine responses and protection from systemic challenge, and identification of cytokine mRNA present in infected tissues of mice that develop mild or severe lesions does not show pure Th1- or Th2-type responses. Furthermore, antigens of C. albicans, mannan in particular, can induce suppressor cells that modulate both specific and nonspecific cellular and humoral immune responses, and there is an emerging body of evidence that molecular mimicry may affect the efficiency of anti-Candida responses within defined genetic contexts.