Novel actions of estrogen to promote proliferation: integration of cytoplasmic and nuclear pathways

Abstract

Both steroids and growth factors stimulate proliferation of steroid-dependent tumor cells, and interaction between these signaling pathways occurs at several levels. Steroid receptors are classified as ligand-activated transcription factors, and steps by which they activate target gene transcription are well understood. Several steroid responses have now been functionally linked to other intracellular signaling pathways, including c-Src or tyrosine kinase receptors. Steroids such as 17beta-estradiol (E2), via binding to cytoplasmic or membrane-associated receptors, were also shown to rapidly activate intracellular signaling cascades such as ERK, PI3K and STATs. These E2-stimulated phosphorylations can then contribute to altered tumor cell function. ER-positive breast cancer cells, in which proliferation is stimulated by E2 and suppressed by antiestrogens, have been of particular interest in dissecting nuclear and cytoplasmic roles of estrogen receptors (ER). In some cell contexts, ER interacts directly with the intracellular tyrosine kinase c-Src and other cytoplasmic signaling and adaptor molecules, such as Shc, PI3K, MNAR, and p130 Cas. Although the hierarchy among these associations is not known, it is clear that c-Src plays a fundamental role in both growth factor and E2-stimulated cell growth, and this may also require other growth factor receptors such as those for EGF or IGF-1. STAT transcription factors represent one pathway to integrate E2 cytoplasmic and nuclear signaling. STAT5 is phosphorylated in the cytoplasm at an activating tyrosine in response to E2 or EGF, and then is translocated to the nucleus to stimulate target gene transcription. E2 stimulates recruitment of STAT5 and ER to the promoter of several proliferative genes, and STAT5 knockdown prevents recruitment of either protein to these promoters. STAT5 activation by E2 in breast cancer cells requires c-Src and EGF receptor, and inhibition of c-Src or EGFR, or knockdown of STAT5, prevents E2 stimulation of several genes and breast cancer cell proliferation. Hyperactivation of the growth factor receptor-c-Src pathway can in some contexts decrease growth responses to E2, or render cells and tumors resistant to suppressive actions of endocrine therapies. Crosstalk between growth factors and steroids in both the cytoplasm and nucleus may thus have a profound impact on complex biological processes such as cell growth, and may play a significant role in the treatment of steroid-dependent breast cancers.

Figure 1

E2 stimulates cytoplasmic and nuclear signaling. E2 ligand binds to estrogen receptors (ER), stabilizes ER dimers, and stimulates direct interaction with growth factor receptors (GFR), association with c-Src and adaptor molecules (Shc, MNAR, Cas), and stimulation of common cytoplasmic signaling pathways. ER can also initiate gene transcription in the absence of E2 via phosphorylation (circled P) and activation of receptors and coactivators by growth factor signaling cascades, or by a ligand-stimulated mechanism.

Figure 2

Functional interactions between estrogen receptors and growth factors. In the cytoplasm, both ER and growth factor receptors (GFR) can act via c-Src-stimulated pathways that include Shc, Cas and other adaptors, which impinge on the Ras/Grb/Sos pathway to MAPK and the PI3K pathway to Akt. Cytoplasmic stimulation of STAT5 phosphorylation by ER or GFR results in activated STAT5 in the nucleus, which can stimulate genes individually, or in cooperation with activated ER.

Figure 3

E2 Induces ERα, STAT5b, and Phosphorylated RNA Polymerase II Recruitment to the Cyclin D1 and c-Myc Promoters. STAT5b Knockdown Inhibits E2-induced Recruitment of ERα and Phosphorylated RNA Polymerase II. A-B, T47D cells were nucleofected with 2.5 μg of 20 μm control or STAT5b siGENOME SMARTpool siRNA (Dharmacon) per reaction, grown in E2-free conditions for three days, then treated with control media containing ethanol (V) or 10 nm E2 for 30 min. ChIP assays were performed using either no antibody (-Ab) or antibodies directed against ERα, STAT5b, and the phosphorylated RNA Pol II CTD. The immunoprecipitated DNA was quantitated by real-time PCR with the use of primer sets directed towards the E2-responsive regions of the cyclin D1 (A) and c-Myc (B) promoters. The data represent the mean ± SEM of four experiments done in triplicate. Two-way ANOVA was used to determine statistical significance for –Ab, ERα, and STAT5b. *, P < 0.05 vs. V; #, P < 0.01 vs. E2-stimulated siCon cells. Student’s t test was used to determine statistical significance for ph-Pol II. *, P < 0.05 vs. V; #, P < 0.05 vs. E2-stimulated siCon cells. Inset, Western blot verifying STAT5b knockdown prior to 30 min E2 treatment. T47D cells were nucleofected with 2.5 μg of 20 μm control or STAT5b siRNA and lysed after 72 h. Proteins were separated by 8% SDS-PAGE, transferred to nitrocellulose, and immunoblotted with antibodies specific for the STAT5 SH2 domain or β-Actin.