Overexpression of ErbB2 impairs ligand-dependent downregulation of epidermal growth factor receptors via a post-transcriptional mechanism

Abstract

The mechanism by which ErbB2 exerts its oncogenic effect is poorly defined. In this article we show that ErbB2 co-expression slows ligand-dependent growth factor receptor downregulation in NIH 3T3 transfectants. Ligand dependence of cell growth and MAP kinase signaling are retained in epidermal growth factor receptor (EGFR) transfectants but are abolished in ErbB2-expressing cells, which grow and signal constitutively. In association with this phenomenon, we have noticed that ErbB2-expressing cells contain increased amounts of EGFR, which is hyperphosphorylated. EGFR overexpressors do not contain increased levels of ErbB2, however, tending to exclude a transfection artifact caused by saturation of receptor processing. EGF treatment of EGFR transfectants results in more rapid EGFR downregulation than occurs in ErbB2 transfectants, but Northern blot analysis demonstrates reduced basal EGFR gene expression in ErbB2 transfectants. We conclude that ErbB2 expression impairs EGFR downregulation via a posttranscriptional mechanism and propose that ErbB2 overexpression may sensitize tumor cells to the mitogenic effects of heterologous growth factors by retarding degradation of liganded heterodimers.

Dominant negative knockout of p53 abolishes ErbB2-dependent apoptosis and permits growth acceleration in human breast cancer cells

“…To this end, MCF7 human breast cancer cells known to express both copies of the wild-type p53 gene (Casey et al, 1991;Balcer-Kubiczek et al, 1995;Furuwatari et al, 1998) were transiently transfected with ErbB2. As shown in Figure 2A, ErbB2 expression in these cells induces increased immunoreactivity of both activated ErbB2 and EGFR, consistent with previous studies (Huang et al, , 1999, while also inducing increased expression of p53, p21 WAF and Myc. Of note, ErbB2 expression is associated with reduced Bcl2 expression -an effect reported previously following primary overexpression of p53 (Haldar et al, 1994).…”

Dominant negative knockout of p53 abolishes ErbB2-dependent apoptosis and permits growth acceleration in human breast cancer cells

“…To this end, MCF7 human breast cancer cells known to express both copies of the wild-type p53 gene (Casey et al, 1991;Balcer-Kubiczek et al, 1995;Furuwatari et al, 1998) were transiently transfected with ErbB2. As shown in Figure 2A, ErbB2 expression in these cells induces increased immunoreactivity of both activated ErbB2 and EGFR, consistent with previous studies (Huang et al, , 1999, while also inducing increased expression of p53, p21…”

“…However, since our previous work documented a marked prolongation of EGFR signalling by ErbB2 expression (Huang et al, 1999), we elected to test the latter hypothesis by creating ErbB2 transfectants in cell lines differing solely in terms of cell cycle control functionality. To this end, MCF7 human breast cancer cells known to express both copies of the wild-type p53 gene (Casey et al, 1991;Balcer-Kubiczek et al, 1995;Furuwatari et al, 1998) were transiently transfected with ErbB2.…”

“…We previously reported that growth arrest of 3T3 cells is associated with catalytic activation of ErbB2 (Epstein et al, 1990), and more recently demonstrated that ErbB2 lengthens and intensifies mitogenic signalling by impairing ligand-dependent EGFR downregulation (Huang et al, 1999). In addition, we have shown that the functionally distinct EGFR ligands, EGF and transforming growth factor-alpha (TGFa), exert differing effects on EGFR downregulation and hence on the duration of ErbB2 co-activation: high concentrations of EGF initially cause prolonged EGFR activation associated with ErbB2 heterodimerisation, followed by eventual EGFR downregulation and signal cessation; whereas TGFa fails to downregulate EGFR, leading to sustained signalling (Gulliford et al, 1997;Ouyang et al, 1999a).…”

The epidermal growth factor receptor family: Biology driving targeted therapeutics

“…Notably, EGFR-ErbB2 heterodimers evade lysosomal degradation in favor of endocytic recycling of EGFR-ErbB2 to the cell surface, leading to increased signal duration and potency [94,98]. Therefore, ErbB2 overexpression leads to increased EGFR density on the cell surface and prolongs activation of downstream MAPK and c-Jun [96,99,100].…”