Differential activity of the RVF enhancer element in the decreased expression of the neu oncogene in NR-6 cells versus parental Swiss Webster 3T3 cells

Co-regulation of B-Myb expression by E2F1 and EGF receptor

Epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase that is frequently over-expressed in human cancers and is associated with tumorigenesis, and increased tumor proliferation and progression. Also found in breast tumors with high levels is B-Myb, a transcription factor whose expression is activated by E2F1/3 at the late G1 phase and the level is sustained through the S phase. Recent reports suggest a casual correlation between EGFR and B-Myb expression in primary breast carcinomas. However, the mechanism for such co-expression remains un-investigated. Here, we report that EGFR is important for B-Myb expression and the underlying mechanism involves cooperated effects from EGFR and E2F1. EGF stimulation and forced expression of EGFR significantly increase B-Myb gene activity and such increase occurs in the G1 phase. EGF-induced B-Myb expression was not significantly suppressed following inhibition of PI-3K and ERK, two major EGFR downstream pathways. In contrast, we observed EGF-induced in vivo association of nuclear EGFR to the B-Myb promoter and the association is only detected at the G1/S phase and is abolished by EGFR kinase inhibitor. As EGFR lacks DNA-binding domain but contains transactivational activity and E2F1 activates B-Myb expression in the G1/S phase, we further reasoned that nuclear EGFR might cooperate with E2F1 leading to activation of B-Myb. Indeed, we found that EGFR co-immunoprecipitated with E2F1 in an EGF-dependent manner and that EGF activated in vivo binding of E2F1 to the B-Myb promoter. Consistently, forced expression of both EGFR and E2F1 in EGFR-null CHO cells greatly enhanced B-Myb promoter activity, compared to the vector control and expression of EGFR or E2F1 alone. Promoter mutagenesis studies showed that EGF-induced activation of B-Myb promoter required both E2F and EGFR target sites. In summary, our data suggest that deregulated EGFR signaling pathway facilitate tumor cell proliferation partly via EGFR interaction with E2F1 and subsequent activation of B-Myb gene expression.

Transcriptional regulation of type I receptor tyrosine kinases in the mammary gland

The transcriptional regulation of the human EGFR3 and ERBB2 genes has been extensively studied, particularly in the context of their overexpression in breast cancer. Here we summarize published work detailing the transcription factors which interact with the promoters of these and the rat ERBB2 homologue, neu, genes and discuss their possible relevance to gene activation in cancer. In addition we review the biologically significant molecules which modulate expression of these genes and discuss the nuclear factors involved in mediating these responses. We also describe novel therapies which may result from these studies and highlight directions for future research into the control of expression of the EGFR and ERBB2 genes in the normal mammary gland and in breast cancer.

Involvement of co-activator p300 in the transcriptional regulation of the HER-2/neu gene

Overexpression of HER-2/neu is frequently found in human cancer and has been shown to enhance the metastatic potential of tumors and to induce the chemoresistance of cancer cells. The molecular mechanism(s) by which HER-2/neu expression is deregulated in cancer is not clear. We reported previously that adenovirus 5 E1A is capable of transcriptionally repressing the HER-2/neu promoter. We report here that the E1A-associated p300 protein can derepress the E1A-mediated repression of HER-2/neu in a dose-dependent manner. A p300 mutant, which lost its ability to bind to E1A, also effectively rescued the repressed HER-2/neu promoter in the presence of excess E1A inside the cells. A protein complex can bind to the p300 consensus sequences in HER-2/neu promoter. The intensity of the retarded band of the protein complex decreased significantly after preincubation of the nuclear extracts with beads that has been conjugated with anti-p300 antibody. The binding of E1A to p300 and the p300 consensus sequence in HER-2/neu promoter were crucial for the ability of E1A to repress HER-2/neu promoter, demonstrating that p300 is involved in the transcriptional regulation of HER-2/neu and serves as a target for E1A repression.