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.