Ectopic activation of lymphoid high mobility group-box transcription factor TCF-1 and overexpression in colorectal cancer cells.

Physical interaction between the lymphoid high mobility group (HMG)-box architectural transcription factors TCF/LEF and beta-catenin is associated with translocation of the heteromeric complex to the nucleus and regulation of target gene expression. Since formation of molecular complexes among beta-catenin, E-cadherin, p300apc and TCF/LEF depends on balanced expression of these constituents, we investigated the biosynthesis of TCF-1 in colorectal cancer. Here we report detailed analyses of activation and overexpression of lymphoid transcription factor TCF-1 in human colorectal cancer-derived cell lines. Northern blot analyses revealed considerable steady-state expression levels of TCF-1 mRNA of normal size. Genomic rearrangement of the 5' flanking region of the TCF-1 gene was excluded as a cause of ectopic expression. By contrast, CAT-reporter constructs depending on a 515-bp T-cell-regulated TCF-1 genomic upstream region were significantly activated in epithelial tumor cells. RT-PCR analyses revealed a heterogeneic population of mRNA isoforms due to alternative splicing in the TCF-1 gene. On Western blots of colorectal cancer cells, the TCF-1-specific monoclonal antibody 7H3 detected a similar heterogeneous spectrum of TCF-1 specific polypeptide chains. Interestingly, overexpression of TCF-1-specific splice forms correlated with the metastatic behavior of the analyzed cells and with overproduction of lymphoid tyrosine protein kinase p56(lck). We conclude that ectopic expression of the HMG-box factor TCF-1 is associated with late events in tumor progression.

Extensive alternative splicing and dual promoter usage generate Tcf-1 protein isoforms with differential transcription control properties.

Previously, we reported the isolation of cDNA clones representing four alternative splice forms of TCF-1, a T-cell-specific transcription factor. In the present study, Western blotting (immunoblotting) yielded a multitude of TCF-1 proteins ranging from 25-55 kDa, a pattern not simply explained from the known splice alternatives. Subsequent cDNA cloning, PCR amplification, and analysis by rapid amplification of 5' cDNA ends revealed (i) the presence of an alternative upstream promoter, which extended the known N terminus by 116 amino acids, (ii) the presence of four alternative exons, and (iii) the existence of a second reading frame in the last exon encoding an extended C terminus. Inclusion of the extended N terminus into the originally reported protein resulted in a striking similarity to the lymphoid factor Lef-1. Several of the TCF-1 isoforms, although less potent, mimicked Lef-1 in transactivating transcription through the T-cell receptor alpha-chain (TCR-alpha) enhancer. These data provide a molecular basis for the complexity of the expressed TCF-1 proteins and establish the existence of functional differences between these isoforms. Furthermore, the functional redundancy between Tcf-1 and Lef-1 explains the apparently normal TCR-alpha expression in single Tcf-1 or Lef-1 knockout mice despite the firm in vitro evidence for the importance of the Tcf/Lef site in the TCR-alpha enhancer.

The human TCF-1 gene encodes a nuclear DNA-binding protein uniquely expressed in normal and neoplastic T-lineage lymphocytes.

The TCF-1 gene encodes a putative transcription factor with affinity for a sequence motif occurring in a number of T-cell enhancers. TCF-1 mRNA was originally found to be expressed in a T cell-specific fashion within a set of human and mouse cell lines. In contrast, expression reportedly occurs in multiple nonlymphoid tissues during murine embryogenesis. We have now raised a monoclonal antibody to document expression and biochemistry of the human TCF-1 protein. As expected, the TCF-1 protein was detectable only in cell lines of T lineage. Its expression was always restricted to the nucleus. Immunohistochemistry on a panel of human tissues revealed that the TCF-1 protein was found exclusively in thymocytes and in CD3+ T cells in peripheral lymphoid tissues. Western blotting yielded a set of bands ranging from 25 kD to 55 kD, resulting from extensive alternative splicing. The TCF-1 protein was detectable in all samples of a set of 22 T-cell malignancies of various stages of maturation, but was absent from a large number of other hematologic neoplasms. These observations imply a T cell-specific function for TCF-1, a notion corroborated by recent observations on Tcf-1 knock-out mice. In addition, these results indicate that nuclear TCF-1 expression can serve as a pan-T-lineage marker in the diagnosis of lymphoid malignancies.

Circumvention of tolerance for the nuclear T cell protein TCF-1 by immunization of TCF-1 knock-out mice.

Molecular events that underlie the well-defined phenotypic changes of the differentiating thymocyte are poorly understood. A candidate gene to control thymocyte differentiation, T cell factor-1 (TCF-1)* encodes a DNA-binding protein. Its mRNA expression pattern is complex during embryogenesis, yet restricted to lymphocytes postnatally. Expression studies on TCF-1 protein have been hampered by the difficulty to raise antibodies due to extreme evolutionary conservation. TCF-1 knock-out mice, generated recently in our laboratory, have strongly decreased numbers of thymocytes, but are otherwise normal. We have used these mice to generate anti-TCF-1 antibodies. By immunization with a recombinant fusion protein, we show that TCF-1 knock-out mice readily yield antiserum titers against human and mouse TCF-1 protein. Wild-type littermates remain unresponsive to TCF-1 while they mount a high-titer antibody response to the fusion protein, Maltose Binding Protein (MBP). Subsequently, TCF-1-specific hybridomas could be prepared from the spleens of immunized knock-out mice. This study illustrates the almost complete tolerance of mice for human TCF-1 and demonstrates that this tolerance is readily broken by gene knock-out. Furthermore, the usefulness of knock-out mice for the generation of monoclonal antibodies against the gene product of interest is underscored.

Anti-(U1) small nuclear RNA antibodies in anti-small nuclear ribonucleoprotein sera from patients with connective tissue diseases.

Small nuclear ribonucleoprotein (snRNP) particles are a class of RNA-containing particles in the nucleus of eukaryotic cells. Sera from patients with connective tissue diseases often contain antibodies against the proteins present in these snRNPs. Antibodies against the RNA components of snRNPs, the U snRNAs, are thought to be rare. We tested 118 anti-snRNP sera for the presence of anti-snRNA antibodies and found them in 45 sera (38%). In all sera the antibodies (IgG and F(ab)2 fragments thereof) were exclusively directed against U1 snRNA. The anti-(U1) RNA antibodies were always accompanied by anti-(U1)RNP antibodies but were not found in sera which contain antibodies of the Sm serotype directed against all nucleoplasmic U snRNP particles. Like anti-RNP antibodies, anti-U1 RNA activity is confined to sera from patients with SLE or SLE overlap syndromes and is rarely found in patients with other connective tissue diseases. By analyzing binding to subfragments of U1 snRNA made in vitro, it was demonstrated that anti-(U1)RNA antibodies recognize epitopes distributed throughout the U1 RNA molecule. In most sera, however, either the second or the fourth hairpin loop is the main target of the antibody. The possible mechanisms that could lead to the production of this new type of autoantibody are discussed.