Activation of the MAPK Pathway Enhances Sensitivity of MCF-7 Breast Cancer Cells to the Mitogenic Effect of Estradiol

Abstract

Long-term estrogen deprivation causes human breast cancer cells to develop hypersensitivity to the mitogenic effect of estradiol (E(2)). Our prior studies demonstrated an association between enhanced MAPK activation and hypersensitivity in long-term estrogen-deprived (LTED) MCF-7 cells. Herein, we report that MAPK is constitutively activated in LTED cells and not dependent on serum factors. Additionally, activated MAPK levels fall upon reversion of the hypersensitivity. Importantly, we now provide direct evidence that enhanced MAPK causes hypersensitivity to E(2). We activated MAPK in wild-type MCF-7 cells using TGFalpha, and demonstrated a 2-3 log enhancement of sensitivity to E(2). PD98059 abrogated the TGFalpha-induced effect, indicating that MAPK activation is responsible for E(2) hypersensitivity. To determine the level at which MAPK activation enhanced E(2) sensitivity, we examined the dose-response effects of E(2) on several transcriptional readouts, including ERE-reporter activity and the levels of progesterone receptor and pS2. Wild-type and LTED cells exhibited nearly identical responses to E(2), suggesting that mechanisms downstream of estrogen receptor-mediated transcription are involved in inducing hypersensitivity. In support of this possibility, LTED and TGFalpha-treated wild-type cells were hypersensitive to the effects of E(2) on the key cell cycle regulator, E2F1.

Targeting the receptor tyrosine kinase RET sensitizes breast cancer cells to tamoxifen treatment and reveals a role for RET in endocrine resistance

“…phosphorylation of ERa and/or associated co-activators (Kurokawa et al, 2000;Shim et al, 2000;Yue et al, 2002;Martin et al, 2003;Shou et al, 2004) or independent of steroid receptors that are downregulated as a result of aberrant intracellular signaling (Oh et al, 2001;Creighton et al, 2006;Massarweh et al, 2008;Pancholi et al, 2008;Sabnis et al, 2008). Although a number of clinical studies support these observations (Gutierrez et al, 2005;Dowsett et al, 2006;Arpino et al, 2007) the molecular mechanisms underlying these phenotypes remain unclear and as a consequence, effective approaches for preventing and overcoming resistance are not yet available (Johnston, 2009).…”

Crosstalk between steroid receptors and the c-Src-receptor tyrosine kinase pathways: implications for cell proliferation

“…E2-dependent and ligand-independent transcriptional activation of the ERs through protein kinases activated by growth factor pathways are synergistic (Cho and Katzenellenbogen, 1993). In addition, MAPK activation renders ERa hypersensitive to lower levels of E2 (Jeng et al, 1998;Shim et al, 2000) and more sensitive to proliferative actions of E2 (Yue et al, 2002) Progesterone receptor phosphorylation and ligand-independent activation of transcription Both PR-A and PR-B are phosphorylated at equivalent sites under basal conditions, and in response to ligand binding and growth factor signaling cascades (Figure 1, reviewed in Weigel and Zhang, 1999;Lange, 2004). In response to growth factors such as EGF, Ser294 of PR-B is rapidly (3-5 min) phosphorylated via a MAPKdependent pathway .…”

“…The demonstration that IGF-1, EGF and HER family members can activate steroid receptor transcriptional activity in the absence of ligand, and that steroids and growth factors rapidly stimulate the same intracellular signaling pathways, suggests that there may be some common stimulated pathways for cell proliferation and tumor growth. Interestingly, experiments with several pharmacological inhibitors as well as dominant-negative signaling molecules suggest that the MAPK, PI3K, c-Src and STAT pathways all play critical roles in mediating both steroid and growth factor stimulation of cancer cells (Castoria et al, 1999(Castoria et al, , 2001Yue et al, 2002;Kloth et al, 2003;Stoica et al, 2003;Yamashita et al, 2003). Both steroids and growth factors stimulate expression of several genes critical for cell cycle progression, including cyclin D1, cyclin E, and cdk inhibitors p21 and p27, and ongoing expression profiling of breast cancer cells and breast tumors may provide some information on larger subsets of genes that are common for both steroid and growth factorstimulated growth (Sotiriou et al, 2003;Frasor et al, 2004).…”

Growth factor-driven mechanisms associated with resistance to estrogen deprivation in breast cancer: new opportunities for therapy

“…The pivotal role for increased activation of MAP kinase in determining the estrogen hypersensitive growth adopted by LTED has been confirmed by abrogation of their basal proliferation and hypersensitive responses to estrogen in vivo using the MAP kinase inhibitors, PD98059 or U0126 (Shim et al 2000, Yue et al 2002. As further proof, the addition of TGFa to parental MCF-7 cells in vitro in order to augment their MAP kinase activity promoted a 2-log increase in sensitivity to growth stimulation by estradiol (Yue et al 2002. Associated growth end-points of the elevated nongenomic ERa/MAP kinase signaling pathway identified to date in LTED include phosphorylation of ELK-1, a regulator of proliferative activity, and cell membrane changes (Song et al 2002a,b, Santen et al 2003.…”

“…Thus, Santen's data suggest that it is an enhanced nongenomic estrogen/ERa mechanism that drives elevated MAP kinase/Akt signaling and thereby promotes growth in the presence of very low estrogen levels , rather than a predominant genomic pathway, where estrogens would classically interact with nuclear ERa to elevate gene transcription (for example, expression of growth factor ligands such as TGFa that could subsequently prime MAP kinase/Akt (Hutcheson et al 2003)). However, additional studies from Santen's group suggest that some degree of undefined, synergistic interaction between non-genomic and genomic pathways does occur at the level of cell cycle regulators, such as E2F1, in the promotion of LTED growth (Yue et al 2002.…”