Steroid Receptor Regulation of Epidermal Growth Factor Signaling through Src in Breast and Prostate Cancer Cells: Steroid Antagonist Action

Abstract

Under conditions of short-term hormone deprivation, epidermal growth factor (EGF) induces DNA synthesis, cytoskeletal changes, and Src activation in MCF-7 and LNCaP cells. These effects are drastically inhibited by pure estradiol or androgen antagonists, implicating a role of the steroid receptors in these findings. Interestingly, EGF triggers rapid association of Src with androgen receptor (AR) and estradiol receptor A (ERA) in MCF-7 cells or ERB in LNCaP cells. Here, we show that, through EGF receptor (EGFR) and erb-B2, EGF induces tyrosine phosphorylation of ER preassociated with AR, thereby triggering the assembly of ER/AR with Src and EGFR. Remarkably, experiments in Cos cells show that this complex stimulates EGF-triggered EGFR tyrosine phosphorylation. In turn, estradiol and androgen antagonists, through the Src-associated receptors, prevent Src activation by EGF and heavily reduce EGFR tyrosine phosphorylation and the subsequent multiple effects, including DNA synthesis and cytoskeletal changes in MCF-7 cells. In addition, knockdown of ERa or AR gene by small interfering RNA (siRNA) almost abolishes EGFR tyrosine phosphorylation and DNA synthesis in EGF-treated MCF-7 cells. The present findings reveal that steroid receptors have a key role in EGF signaling. EGFR tyrosine phosphorylation, depending on Src, is a part of this mechanism. Understanding of EGF-triggered growth and invasiveness of mammary and prostate cancer cells expressing steroid receptors is enhanced by this report, which reveals novel aspects of steroid receptor action. (Cancer Res 2005; 65(22): 10585-93)

Inhibition of the SH3 domain-mediated binding of Src to the androgen receptor and its effect on tumor growth

“…In addition, AR or ER inhibition by steroid antagonists or siRNA prevents EGF-triggered DNA synthesis and cytoskeletal changes (Migliaccio et al, 2005). This is confirmed by the experiments in Figure 5 showing that steroid antagonists inhibit EGF-induced DNA synthesis in LNCaP and MCF-7 cells.…”

“…It has been recently reported that, in LNCaP and MCF-7 cells, EGF-induced signaling requires the association of AR and ER with Src (Migliaccio et al, 2005). In addition, AR or ER inhibition by steroid antagonists or siRNA prevents EGF-triggered DNA synthesis and cytoskeletal changes (Migliaccio et al, 2005).…”

“…An increased level of AR could sensitize cancers to low levels of residual androgens, and antiandrogens could promote androgen activation rather than inhibition (Chen et al, 2004). Also, growth factors, such as epidermal growth factor (EGF), could bypass the hormone requirement by directly activating the AR (Culig et al, 1994, Migliaccio et al, 2005. Alternatively, non-androgenic steroids could promote androgen-independent growth of prostate cancer cells through a mutated AR (Zhao et al, 2000).…”

“…AR association with Src has also been observed in androgen-stimulated immortalized fibroblasts (Castoria et al, 2003), raising the possibility that, in response to androgens, stromal cells facilitate the malignant outgrowth and the metastatic spread of prostate epithelial cells. In addition, EGF signaling, which induces S phase entry and cytoskeletal changes of prostate and breast cancer cells, requires AR/ER association with Src (Migliaccio et al, 2005).…”

“…This peptide inhibits the G1 to S transition of androgen-or estradiolstimulated prostate and mammary cancer cells in vitro as well as the growth of LNCaP prostate cell xenografts established in nude mice. Because of the role of AR in EGF-elicited signaling (Migliaccio et al, 2005), the peptide also prevents EGF-dependent DNA synthesis in normal and cancer cell lines. Use of this peptide, alone or in association with other compounds, potentially offers a novel approach to specifically inhibit human prostate and breast cancers.…”