Estrogens exert a variety of effects on growth and development of different tissues. The effects of estrogens are mediated via estrogen receptors (ER), intracellular proteins that function as ligand-activated transcription factors and belong to the nuclear receptor super-family (25). Two mammalian ERs have been identified: ERα and ERβ. The structure of both receptors is similar and consists of six domains named A through F from the N-to C-terminus, encoded by 8 to 9 exons (26). Domains A and B are located at the N-terminus and contain an agonist-independent transcriptional activation domain (activation function 1, or AF-1). The C domain is a highly conserved central DNA-binding domain composed of two zinc fingers through which ER interacts with the major groove and the phosphate backbone of the DNA helix. The C-terminus of the protein contains domains E and F and functions as ligand-binding domain (LBD–domain E) and AF-2–domain F (Fig. 6.3).
Activation of transcription via the ER is a multistep process. The initial step requires activation of the ER via various mechanisms (Fig. 6.4). For example, estrogens such as 17β-estradiol can diffuse into the cell and bind to the LBD of the ER. Upon ligand binding, the ER undergoes conformational changes followed by a dissociation of various bound proteins, mainly heat shock proteins 90 and 70 (Hsp90 and Hsp70). Activation of the ER also requires phosphorylation by several protein kinases, including casein kinase II, protein kinase A, and components of the Ras/Mapk (mitogen-activated protein kinase) pathway (26). Four phosphorylation sites of the ER are clustered in the NH2 terminus with the AF-1 region.
The activated ER elicits a number of different genomic as well as nongenomic effects on intracellular signaling pathways. The classical steroid signaling pathway involves binding of the activated estrogen receptor to an estrogen responsive element (ERE) on the genome as homodimers and subsequent stimulation of transcription (27,28). The minimal consensus sequence for
Table 6.1 Proto-Oncogenes hst, int-2
Ha-ras, N-ras, N-ras fgr, lck, src, yes cot, plm-1, mos, raf/mil jun, ets-1, ets-2,fos, gil 1,rel, ski, vav lyl-1, maf, myb, myc, L-myc, N-myc, evi-1 the ERE is a 13 bp palindromic inverted repeat (IR) and is defined as 5 -GGTCAnnnTGACC-3 . Genes that are regulated by activated ERs include early gene responses such as c-myc, c-fos, and d-jun, and genes encoding for growth factors such as insulin growth factor (IGF-1 and IGF-2), epidermal growth factor (EGF), transforming growth factor-α, and colony-stimulating factor (CSF-1).
Table 6.2 Tumor Suppressor Genes
Figure 6.3 Structure of the two mammalian estrogen receptors. ERα (595 amino acids) and ERβ (530 amino acids) consist of six domains (A–F from the N-to C-terminus). Domains A and B at the N-terminus contain an agonist-independent transcriptional activation domain (activation function 1, or AF-1). The C domain is the central DNA-binding sequence (DBD). Domains E and F function as ligand binding domain (LBD) and activation function 2 (AF-2). Also shown is the structure of the ER ligand 17β-estradiol.