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

Abstract

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.

The epidermal growth factor receptor family: Biology driving targeted therapeutics

“…EGFR has been proposed as a transcription factor as localization to the nucleus is associated with increased CyclinD1 expression [64]. Nuclear EGFR has also been shown to participate with STAT3 and E2F1 transcription factors in iNOS/NO pathway activation and B-myb expression, respectively [71,72]. Targeting to the nucleus seems to require three clusters of basic amino acids in the juxtamembrane domain of all ErbB family members that share homology with known nuclear localization sequences [73].…”

Nuclear EGFR signalling network in cancers: linking EGFR pathway to cell cycle progression, nitric oxide pathway and patient survival

“…Using unbiased approaches, nuclear EGFR and HER-2 were further shown to associate with specific DNA sequences designated ATrich sequence (ATRS) and HER-2-associated sequence (HAS), respectively (Lin et al, 2001;Wang et al, 2004). Promoters that are targeted by nuclear EGFR are those of cyclin D1, iNOS and B-Myb (Lin et al, 2001;Hanada et al, 2005;Lo et al, 2005a). Nuclear HER-2 binds to promoters of cyclooxygenase-2 (COX-2), PRPK, MMP-16 and DDX-10, whereas nuclear HER-4 associates with that of b-casein (Wang et al, 2004;Williams et al, 2004).…”

“…In this regard, nuclear EGFR interacts with STAT3 and coregulates iNOS expression (Lo et al, 2005a). Nuclear EGFR/E2F1 complex activates expression of B-Myb, a positive regulator of G1/S cell cycle progression (Hanada et al, 2005). Nuclear HER-4 forms a complex with STAT5a and coactivates b-casein gene promoter (Williams et al, 2004).…”

“…Increased expression of nuclear EGFR is linked to poor clinical outcome in patients with breast carcinomas (Lo et al, 2005c) and oropharyngeal squamous cell carcinomas (Psyrri et al, 2005). Consistently, nuclear accumulation of EGFR correlates with increased expression of cyclin D1, inducible nitric oxide synthase (iNOS) and B-Myb, all of which are frequently overexpressed in human cancers and associated with increased cell proliferation (Lin et al, 2001;Hanada et al, 2005;Lo et al, 2005a). Furthermore, most recent reports indicate a plausible mechanism underlying nuclear EGFR-mediated gene regulation, which involves a physical interaction with other transcription factors, signal transducer and activator of transcription 3 (STAT3) and E2F1 (Hanada et al, 2005;Lo et al, 2005a).…”

“…Consistently, nuclear accumulation of EGFR correlates with increased expression of cyclin D1, inducible nitric oxide synthase (iNOS) and B-Myb, all of which are frequently overexpressed in human cancers and associated with increased cell proliferation (Lin et al, 2001;Hanada et al, 2005;Lo et al, 2005a). Furthermore, most recent reports indicate a plausible mechanism underlying nuclear EGFR-mediated gene regulation, which involves a physical interaction with other transcription factors, signal transducer and activator of transcription 3 (STAT3) and E2F1 (Hanada et al, 2005;Lo et al, 2005a). A cellular mechanism that can potentially account for nuclear import of receptor tyrosine kinases (RTKs) is also proposed recently (Giri et al, 2005).…”