MicroELISA method for the determination of thymosin beta 9 discriminating between thymosin beta 9 and the structurally closely related thymosin beta 4

Development of a sensitive and specific enzyme-linked immunosorbent assay for thymosin beta15, a urinary biomarker of human prostate cancer

OBJECTIVES

In tissue-based assays, thymosin beta15 (Tbeta15) has been shown to correlate with prostate cancer (CaP) malignancy and with future recurrence. To be clinically effective, it must be shown that Tbeta15 is released by the tumor into body fluids in detectable concentrations. Toward this end, we have worked to develop a quantitative high-throughput assay that can accurately measure clinically relevant concentrations of Tbeta15 in human urine.

DESIGN AND METHODS

Sixteen antibodies were raised against recombinant Tbeta15 and/or peptide conjugates. One antibody, having stable characteristics over the wide range of pH and salt concentrations found in urine and minimal cross-reactivity with other beta thymosins, was used to develop a competitive enzyme-linked immunosorbent assay (ELISA). Urinary Tbeta15 concentration was determined for control groups; normal (N = 52), prostate intraepithelial neoplasia (PIN, N = 36), and CaP patients; untreated (N = 7) with subsequent biochemical failure, radiation therapy (N = 17) at risk of biochemical recurrence.

RESULTS

The operating range of the competition ELISA fell between 2.5 and 625 ng/mL. Recoveries exceeded 75%, and the intra- and inter-assay coefficients of variability were 3.3% and 12.9%, respectively. No cross-reactivity with other urine proteins was observed. A stable Tbeta15 signal was recovered from urine specimens stored at -20 degrees C for up to 1 year. At a threshold of 40 (ng/dL)/mug protein/mg creatinine), the assay had a sensitivity of 58% and a specificity of 94%. Relative to the control groups, Tbeta15 levels were greater than this threshold in a significant fraction of the CaP patients (P < 0.001), including 5 of the 7 patients who later experienced PSA recurrence.

CONCLUSIONS

We have established an ELISA that is able to detect Tbeta15 at clinically relevant concentrations in urine from patients with CaP. The assay will provide a tool for future clinical trials to validate urinary Tbeta15 as a predictive marker for recurrent CaP.

A novel beta-thymosin from the sea urchin: extending the phylogenetic distribution of beta-thymosins from mammals to echinoderms

The study of the phylogenetic distribution of the beta-thymosin family is important to elucidate its biological function further. A new thymosin, designated as thymosin beta 14, consisting of 40 amino acid residues and with a molecular weight of 4537 Da as determined by ion spray mass spectrometry, was isolated from the sea urchin. The N-terminus of this polypeptide is blocked by an acetyl group as found by matrix-assisted laser desorption mass spectrometric and amino acid analysis. The primary structure was elucidate by Edman degradation of the HPLC-purified thymosin beta 14 fragments produced by digestion with endoproteinase Asp-N and trypsin. Sequence comparison reveals that thymosin beta 14 is 73% homologous to thymosin beta 4, obtained from calf thymus. By isolating and characterising the structure of thymosin beta 14 from the sea urchin, an invertebrate, substantial knowledge about the phylogenetic distribution and evolution of beta-thymosins is gained.

Development of specific anti-thymosin beta 10 antipeptide antibodies for application in immunochemical techniques

We present theoretical and experimental data necessary for raising specific antibodies for thymosin beta 10, a 43-amino acid residues peptide occurring in human tissues. We postulate that thymosin beta 10 contains three major antigenic determinants (residues 2-8, 17-25, and 35-41). For antibody development, we synthesized the N-terminal fragment thymosin beta 10(1-16) as well as the C-terminal fragments thymosin beta 10(31-43) and thymosin beta 10(38-43), due to their putative antigenic properties and minimal structural similarity with the homologous peptide thymosin beta 4, which also occurs in humans. The putative antigenic determinant 17-25 is present in all beta-thymosins and was therefore not synthesized. All antisera raised against the above peptide fragments or the intact molecule of thymosin beta 10 were found capable of recognizing and binding synthetic or natural thymosin beta 10 with high specificity, showing minimal cross-reactivity with thymosin beta 4 isolated from bovine tissues or synthetic thymosin alpha 1. Due to its easy preparation and the highly specific affinity of the antibody raised against it for the intact peptide, the fragment thymosin beta 10(38-43) may be considered the antigen of choice for developing anti-thymosin beta 10 antibodies, which can eventually be applied to immunochemical studies.

The interaction of (1-4)-fragment of thymosin beta 4 with calmodulin-sensitive cAMP phosphodiesterase from hypothalamus

Evidence was accumulated indicating that cyclic nucleotides are involved in regulation of growth, differentiation and function of lymphoid cells. It was previously shown that the N-fragment (1-4) of thymosin beta 4 (Ac-Ser-Asp-Lys-Pro-OH) inhibits in vivo the entry of cell populations into S-phase. In the course of the study of the interrelationship between the immune and neuroendocrine systems we have found that the tetrapeptide caused incomplete competitive inhibition of hypothalamic calmodulin (CaM)-dependent phosphodiesterase (PDE) stimulated by CaM. In the presence of the peptide, the 20-fold increase of the constant for PDE activation by CaM was accompanied by an insignificant rise in the maximum rate of cAMP hydrolysis. The value of the inhibition constant (Ki) amounted to 600 nM. In the absence of CaM, the peptide at saturating concentrations reduced the basal activity of PDE nearly 2- to 3-fold. The effect of the peptide on PDE was noncompetitive with respect to cAMP. The results support our suggestion that the tetrapeptide realizes its effects in the immuno-neuroendocrine system by the mechanism of cyclic nucleotide metabolism.

The sulfoxide of thymosin beta 4 almost lacks the polymerization-inhibiting capacity for actin

Thymosin beta 4 (T beta 4), a peptide of 43 amino acids, binds to actin monomers and inhibits filament formation. In preparations of T beta 4 from bovine lung tissue, the peptide is accompanied by a derivative in which the methionine residue in position 6 is replaced by its sulfoxide. T beta 4 sulfoxide inhibits actin polymerization to an extent approximately 20-times less than T beta 4. While an equimolar amount of T beta 4 prevented actin polymerization almost completely, polymerization with the corresponding amount of the sulfoxide proceeded in a manner similar to that of pure actin, except for a slight retardation. We showed that the decrease in the inhibitory activity is reflected by a 20-times lower affinity to actin. Interestingly, under non-polymerizing conditions, the affinity of T beta 4 sulfoxide for actin is as high as that of T beta 4 (approximately 1 microM). In accordance with this, no differences were found between T beta 4 and the sulfoxide in cross-linking experiments with the monomer, where both forms of the peptide yielded similar amounts of a 47-kDa band representing conjugates of actin and beta-thymosin, as proved by Western-blotting analysis. Likewise, both, T beta 4 and the sulfoxide retarded the exchange of G-actin-bound nucleotide to similar extents. Although the sulfoxide is presumably a product of autoxidation, it is attractive to speculate that oxidation of the methionine residue in T beta 4 may represent a regulatory switch for starting filament formation in non-muscle cells.

Alterations in actin-binding beta-thymosin expression accompany neuronal differentiation and migration in rat cerebellum

The beta 4- and beta 10-thymosins, recently identified as actin monomer-sequestering proteins, are developmentally regulated in brain. Using specific mRNA and protein probes, we have used in situ hybridization and immunohistochemical techniques to investigate the distribution of the beta-thymosin mRNAs and their proteins in developing rat cerebellum. Early in postnatal development, both beta-thymosin mRNAs were expressed at highest levels in the postmitotic, premigratory granule cells of the external granular layer; expression diminished as granule cells migrated to and differentiated within the developing internal granular layer. In addition, both beta-thymosin proteins were present in bundles of cerebellar afferent fibers in the white matter at this time. Throughout the maturation period, both proteins were present in elongating parallel fibers in the upper portion of the molecular layer. Later in cerebellar development, thymosin beta 4, but not thymosin beta 10, was expressed in Golgi epithelial cells and Bergmann processes. Thymosin beta 4 was expressed in a small population of cells with microglial morphology scattered throughout the gray and white matter. Thymosin beta 10 was detected in an even smaller population of glia. Expression of thymosin beta 4 and thymosin beta 10 in premigratory granule cells and in growing neuronal processes is consistent with the possibility that both beta-thymosins are involved in the dynamics of actin polymerization during migration and process extension of neurons.

Use of bimanyl actin derivative (TMB-actin) for studying complexation of beta-thymosins. Inhibition of actin polymerization by thymosin beta 9

By reacting trimethylammoniobromobimane bromide (TMB bromide) with rabbit muscle actin, a fluorescent reporter group was linked to cysteine at position 374. Fluorescence of TMB-actin decreased significantly on addition of thymosin beta 4 (T beta 4), a peptide of 43 amino acid residues reported to bind to monomeric actin and to prevent filament formation. Based on this effect, we determined the KD value of the thymosin beta 4 complex as 0.8 microM, a value that is in agreement with previous determinations. In addition to the main compound thymosin beta 4, bovine tissue contains a related peptide, thymosin beta 9 (T beta 9), which has 41 amino acid residues and ca. 75% sequence homology. In the present study we show for the first time that T beta 9, similar to T beta 4, forms a 1:1 complex with monomeric actin, and hereby inhibits actin polymerization. With a KD value of 1.1 microM the affinity of T beta 9 is in the same range as that of T beta 4, suggesting that T beta 9, like T beta 4, contributes to maintaining the pool of monomeric actin in bovine non-muscle cells. Further proof of the interaction of T beta 9 with actin was provided by native PAGE, where the complex showed the reported higher mobility, as well as by crosslinking experiments. Using different crosslinking reagents, like water-soluble carbodiimide (EDC), m-maleimidobenzoyl-N-hydroxysuccinimidate (MBS), and disuccinimidylsuberate (DSS), we were able to produce conjugates of 47 kDa. In one of these (from MBS) both actin and T beta 9 could be identified by immunoblotting. When, in the MBS crosslinking experiments, native actin was replaced with (374-NEM)-actin, the 47 kDa band was not seen, indicating that Cys-374 takes part in the thiol-specific crosslinking reaction. This suggests that part of the binding site of T beta 9 must be located close to the carboxy-terminus.

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.

A thymosin beta 4 ELISA using an antibody against the N terminal fragment thymosin beta 4 [1-14]

A thymosin beta 4 ELISA was developed in which thymosin beta 4, absorbed on microwells, competed with thymosin beta 4 in solution for the binding sites of an anti-thymosin beta 4 antibody. The antibody molecules finally immobilized on the microwells were detected using a goat anti-rabbit immunoglobulin/horseradish peroxidase conjugate in combination with the substrate 2,2'-azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid) diammonium salt, and measuring the relevant optical density values. Anti-thymosin beta 4 antibodies were raised in rabbits against intact thymosin beta 4 as well as against selected fragments of the peptide, i.e., the N terminal fragments thymosin beta 4[1-14] and thymosin beta 4[1-11]. The antibody against thymosin beta 4[1-14] was used in the thymosin beta 4 ELISA, because it showed minimal cross-reactivity (0.1%) with the highly homologous peptide thymosin beta 9 as well as exhibiting the highest titre. The ELISA procedure developed, apart from showing a minimal cross-reaction with thymosin beta 9, was fast, easy to perform and exhibited good assay characteristics.

Immunohistochemical localization of thymosin beta 9 in bovine tissues

Using an indirect fluorescent antibody technique with frozen sections, the localization of thymosin beta 9 was investigated for the first time in bovine thymus, spleen, lung, muscle and liver. The antibodies used have been raised against the N-terminal fragment 1-14 of thymosin beta 9 in order to minimize the cross-reactivity with thymosin beta 4 which was found to be also present in bovine tissues. The specific antibodies against thymosin beta 9 raised in our laboratory allowed us to localize this peptide in presence of the highly homologous and always accompanying thymosin beta 4 in different tissues. Although thymosin beta 9 was first isolated from calf thymus, it could be also detected in other bovine organs. The highest density of positive immunoreaction was found to be in spleen sections. In the muscle tissue a pronounced fluorescence intensity was present in the region of the sarcolemma.