Rat thymosin beta 4 gene. Intron-containing gene and multiple retroposons

The mouse thymosin beta15 gene family displays unique complexity and encodes a functional thymosin repeat

We showed earlier that human beta-thymosin 15 (Tb15) is up-regulated in prostate cancer, confirming studies from others that propagated Tb15 as a prostate cancer biomarker. In this first report on mouse Tb15, we show that, unlike in humans, four Tb15-like isoforms are present in mouse. We used phylogenetic analysis of deuterostome beta-thymosins to show that these four new isoforms cluster within the vertebrate Tb15-clade. Intriguingly, one of these mouse beta-thymosins, Tb15r, consists of two beta-thymosin domains. The existence of such a repeat beta-thymosin is so far unique in vertebrates, though common in lower eukaryotes. Biochemical data indicate that Tb15r potently sequesters actin. In a cellular context, Tb15r behaves as a bona fide beta-thymosin, lowering central stress fibre content. We reveal that a complex genomic organization underlies Tb15r expression: Tb15r results from read-through transcription and alternative splicing of two tandem duplicated mouse Tb15 genes. Transcript profiling of all mouse beta-thymosin isoforms (Tb15s, Tb4 and Tb10) reveals that two isoform switches occur between embryonic and adult tissues, and indicates Tb15r as the major mouse Tb15 isoform in adult cells. Tb15r is present also in mouse prostate cancer cell lines. This insight into the mouse Tb15 family is fundamental for future studies on Tb15 in mouse (prostate) cancer models.

Identification and characterization of homologues of vertebrate beta-thymosin in the marine mollusk Aplysia californica

The beta-thymosins have been known as actin-sequestering proteins, but now are recognized as molecules with multiple and diverse intracellular and extracellular functions. Two closely related proteins, beta-thymosin(His) and beta-thymosin(Gln), have been de novo sequenced by top-down mass spectrometry in the common neurobiology model, Aplysia californica. As determined by nanoelectrospray quadrupole-enhanced Fourier-Transform mass spectrometry with collisionally activated and electron-capture dissociations, both of these Aplysia beta-thymosins are acetylated and differ by a single residue in the central actin-binding domain. Profiling of individual cells and tissue by matrix-assisted laser desorption/ionization mass spectrometry reveals that these proteins are widely expressed in the Aplysia central nervous system, including in individual identified neurons, neuronal clusters, nerves and connective tissues. Newly identified beta-thymosin(His) and beta-thymosin(Gln) are also detected by mass spectrometry in hemolymph, and in releasates collected from whole ganglia. When applied exogenously, beta-thymosin proteins, purified from nerve cell extract, support the anchoring of neurons, and increase neurite sprouting and total neurite outgrowth in culture. These positive effects on neurite regeneration in cell culture suggest that the beta-thymosin proteins have an extracellular function in the central nervous system of Aplysia californica.

Molecular cloning and structural characterization of the functional human thymosin beta4 gene

Thymosin beta4 (Tbeta4), the actin-sequestering protein that regulates the polymerization and depolymerization of actins, is widely distributed in animal tissues. Expression of this gene in human cells is developmentally regulated and appears to play critical roles in tumorigenesis. As a first step toward analyzing the transcriptional regulation of human Tbeta4 (hTbeta4), we isolated the gene encoding this peptide and characterized its structure features. Human Tbeta4 gene comprises three exons and two introns distributed over 2 kb with its transcription start site at 72 bp upstream of the initiation codon. Expression of the chloramphenicol acetyltransferase (CAT) reporter constructs directed by various parts of the 5'-flanking region of this gene was evaluated by transient transfection assays using human colorectal carcinoma SW480 cells as hosts. Significant promoter activity was found in the -437 to +29 region of hTbeta4 gene even though it lacks both TATA and CCAAT boxes. However, cis-element(s) responsible for PMA-induced expression of Tbeta4 gene was not identified within its 1.5-kb 5'-flanking region. Taken together, our data provide crucial information for further dissection of the molecular mechanism(s) underlying aberrant expression of the Tbeta4 gene during malignant progression of human cancers.

Identification of the positive and negative cis-elements involved in modulating the constitutive expression of mouse thymosin beta4 gene

We previously showed that the -278 to +410 region of mouse thymosin beta4 (mT,beta4) gene supports high levels of reporter gene expression in NIH3T3 cells. This region contains part of the 5'-flanking sequences (-278 to -1), the intact first exon (+1 to +133), and portion of the first intron (+134 to +410). However, the size of this exon is much longer than those of its rat and human counterparts. To resolve the question regarding this size discrepancy, transcription start site for the mTbeta4 gene was re-examined by primer extension and bioinformatics analyses. We found that the first exon of mTbeta4 gene spans 56 bp with its cap site situated in a putative initiator highly similar to the consensus mammalian sequence. In addition, a TATA box-like motif and two consecutive downstream promoter elements were also found. To delineate the cis-elements involved in modulating the constitutive expression of mTbeta4 gene, transient transfection assay was performed. Interestingly, expression level of the reporter gene driven by the -117 to +56 region of mTbeta4 gene was approximately 8-fold higher than that directed by the SV40 promoter and significant promoter activity was found to be associated with the smaller (-56 to +56) fragment. A nuclear protein-bound silencer was located in the region between the -167 and -118 and an enhancer whose effect did not seem to be dependent on protein binding was identified in the downstream (-117 to -88) region. However, neither of these cis-elements affected reporter expression driven by a SV40 promoter. Intriguingly, mTbeta4 promoter functioned well in human colorectal (SW480) and cervical (HeLa) carcinoma cells. Taken together, our findings not only provide crucial information for further elucidation of the transcriptional regulation of mTbeta4 gene but also raise the possibility of utilizing its promoter to produce large quantity of recombinant proteins in mammalian cells.

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.

Molecular cloning and structural characterization of the rat thymosin beta15 gene

Thymosin beta15 is expressed in metastatic variants of Dunning rat prostatic carcinoma but not in a nonmetastatic variant. It is also upregulated in malignant human prostate cancers and was shown to be a predictive marker for patient outcome in prostate cancer. To explore the molecular mechanism of transcriptional regulation of thymosin beta15, we isolated and characterized the rat thymosin beta15 gene. The gene appears to exist as a single copy in the rat genome and to comprise three exons distributed over 2kb. The transcription start site was defined by primer extension analysis at 30 bp upstream of the translation start site. Sequence analysis of the 5'-flanking region of the transcription start site revealed properties consistent with promoter activity. The promoter region is GC rich and contains numerous consensus transcription factor binding sites as well as a GC box, but no TATA box. The transcriptional activity of the 5'-flanking region was analyzed by transient transfection of rat prostate cancer cells with firefly luciferase-encoding gene expression vector constructs. The isolated 5' region showed significant promoter activity. The identification of the thymosin beta15 promoter could aid our understanding of the regulation of this gene and its enhanced expression in human cancer.

Beta thymosins as actin binding peptides

The beta thymosins are a highly conserved family of strongly polar 5 kDa polypeptides that are widely distributed among vertebrate classes; most are now known to bind to monomeric actin and inhibit its polymerization. One beta thymosin, beta four, (T beta 4) is the predominant form in mammalian cells, present at up to 0.5 mM. Many species are known to produce at least two beta thymosin isoforms, in some cases in the same cell. Their expression can be separately regulated. When present outside the cell, the N-terminal tetrapeptide of beta four appears to affect cell cycle regulation; beta thymosins or smaller fragments derived from them may have additional regulatory functions. We suggest that many developmental changes in beta thymosin levels within cells and tissues may be related to changes in G-actin pool size.