Increased expression of TATA-binding protein, the central transcription factor, can contribute to oncogenesis

Correlated break at PARK2/FRA6E and loss of AF-6/Afadin protein expression are associated with poor outcome in breast cancer

Common fragile sites (CFSs) are regions of chromosomal break that may play a role in oncogenesis. The most frequent alteration occurs at FRA3B, within the FHIT gene, at chromosomal region 3p14. We studied a series of breast carcinomas for break of a CFS at 6q26, FRA6E, and its associated gene PARK2, using fluorescence in situ hybridization on tissue microarrays (TMA). We found break of PARK2 in 6% of cases. We studied the PARK2-encoded protein Parkin by using immunohistochemistry on the same TMA. Loss of Parkin was found in 13% of samples but was not correlated with PARK2 break. PARK2 break but not Parkin expression was correlated with prognosis. Alteration of PARK2/FRA6E may cause haplo-insufficiency of one or several telomeric potential tumor suppressor genes (TSG). The AF-6/MLLT4 gene, telomeric of PARK2, encodes the Afadin scaffold protein, which is essential for epithelial integrity. Loss of Afadin was found in 14.5% of cases, and 36% of these cases showed PARK2 break. Loss of Afadin had prognostic impact, suggesting that AF-6 may be a TSG. Loss of Afadin was correlated with loss of FHIT expression, suggesting fragility of FRA6E and FRA3B in a certain proportion of breast tumors.

Underexpression of transcriptional regulators is common in metastatic breast cancer cells overexpressing Bcl-xL

Bcl-xL gene induces metastasis in the lung, lymph nodes and bone when breast cancer cells are inoculated in Nude Balb/c mice. In an attempt to identify the molecules required for diverse metastatic foci, we compared gene expression levels in tumor cells and metastatic variants with a cDNA GeneFilter containing 4000 known genes. The transcriptional regulators of alpha1-fetoprotein transcription factor, TBP-associated factor 172 (TAF-172) and the human zinc finger protein 5 (ZFP5) were downregulated. The expression of TAF-172 was inversely proportional to Bcl-xL expression (ANOVA P < 0.0001) and metastatic activity (ANOVA P < 0.0001). A protein interaction program allowed us to functionally associate Bcl-xL and TAF through TATA-binding protein (TBP), suggesting that Bcl-xL connects metabolic pathways with transcriptional machinery. The prediction included proteins involved in apoptosis, electron transfer, kinases and transcription factors. These results indicate that the selection of diverse metastatic cells from the broad spectrum of tumor cell leads to the underexpression of certain transcriptional regulators that might act as adaptor molecules to different microenvironments, and indicate that the synergistic activity of several genes is needed for the selection process in several metastatic foci.

[TBP, a universal transcription factor?]

The TATA binding protein (TBP) is a subunit of several macromolecular complexes required for transcription by the three nuclear RNA polymerases. This observation led to the idea that TBP is a "universal" transcription factor. The discovery of three TBP-related factors and a macromolecular complex which lacks TBP but can support RNA polymerase II transcription in vitro has led to a reappraisal of the universal character of TBP. Several in vivo studies have rather shown that TBP plays a specific role in the activation of a subset of cellular genes controlling the cell cycle. In mammals, the aminoterminal region of TBP plays a highly selective role in the maternal immunotolerance of pregnancy.

Epidermal growth factor enhances cellular TATA binding protein levels and induces RNA polymerase I- and III-dependent gene activity

TATA binding protein (TBP) is a central transcription factor used by all three cellular RNA polymerases. Changes in the levels of TBP have been shown to have selective effects on gene activity. Overexpression of TBP has been recently shown to contribute to cellular transformation, and elevated levels of TBP occur in a clinically significant proportion of human colon tumors relative to matched normal tissue. To understand the mechanisms by which TBP is regulated, we have analyzed whether activation of the epidermal growth factor receptor (EGFR), a membrane-bound tyrosine receptor kinase that is activated in a large number of human cancers, can serve to regulate cellular TBP. We show that treatment of mouse epidermal cells with EGF produces an increase in TBP levels, which can be blocked with an EGFR-specific inhibitor. In contrast, TBP levels remain unchanged after EGF treatment of EGFR null cells. EGF-mediated increases in TBP are regulated at the transcriptional level, as transient expression of the human TBP promoter is induced with EGF. This regulatory event is dependent upon the downstream activation of Ras and requires the activation of p38, JNK, and ERK mitogen-activated protein kinases. The consequence of elevated TBP on gene expression was further determined. Transcription by RNA polymerase (Pol) I and III was induced by EGF. Directly overexpressing TBP also stimulated transcription from these promoters. Thus, we have identified a new and important target of EGFR signaling, TBP, that contributes to EGF-mediated stimulation of RNA Pol I- and III-dependent gene activity. Since the cellular levels of the products of these genes, tRNAs and rRNAs, determine the translational capacity of cells, this event may be an important contributor to the transforming function of EGF.

RNA polymerase III transcription and cancer

RNA polymerase (pol) III synthesizes a range of essential products, including tRNA, 5S rRNA and 7SL RNA, which are required for protein synthesis and trafficking. High rates of pol III transcription are necessary for cells to sustain growth. A wide range of transformed and tumour cell types have been shown to express elevated levels of pol III products. This review will summarize what is known about the mechanisms responsible for this deregulation. Some transforming agents have been shown to stimulate expression of the pol III-specific transcription factors TFIIIB or TFIIIC2. In addition, TFIIIB is bound and activated by several oncogenic proteins, including c-Myc. Conversely, TFIIIB interacts in healthy cells with the tumour suppressors RB and p53. Indeed, the ability to limit pol III transcription through TFIIIB may contribute to their growth-suppression capacities. The function of p53 and/or RB is compromised in most if not all transformed cells; the resultant derepression of TFIIIB may provide an almost universal route to deregulate pol III transcription in cancers. In addition to effects on protein synthesis and growth, there is a precedent for a pol III product having oncogenic activity.