Epidermal Growth Factor Enhances Cellular TATA Binding Protein Levels and Induces RNA Polymerase I- and III-Dependent Gene Activity

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Abstract
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.