## 6 G-Protein-Coupled Estrogen Receptors and Non-Small Cell Lung Cancers
While EGFRs are transmembrane proteins, ERs are  
present in the membrane caveolae, cytoplasm including endoplasmic reticulum and mitochondria as well as in the nucleus. The carboxy terminus domain of cytoplasmic EGFR directly communicates with E2-ER dimmers. This cross-talk leads to the activation of estrogen-responsive genes in the following two ways. In the first step, the E2-ER dimer translocates to the nucleus and binds to the estrogen-response elements (EREs) of the estrogen-responsive genes and activates them for transcription. In the subsequent step (in absence of ERE), the estrogen-responsive genes, the EGFR-ER cross-talk activates various transcription factors, translocates them to the nucleus, binds to the specific transcription factor binding sites of the estrogen-responsive genes and activates them for transcription. This transcriptional activation through ERE-dependent or -independent manner leads to the expression of pro-proliferative, prosurvival and antiapoptotic proteins which complicates the tumorigenesis. Of note, E2-ER dimmers and EGF-EGFR interaction have functions independent of their cross-talk. Abbreviations: ER: Estrogen receptor. ERE: Estrogen Response Elements, E2: Estradiol, ECM: Extracellular matrix. In the in vitro cultured NSCLC cell lines, the EGFR expression was downregulated in response to estrogen and upregulated in response to anti-estrogens, indicating EGFR expression upregulated in the absence of estrogens. Conversely, the ER\(\beta\) expression is increased in response to gefitinib, an anti-EGFR molecule, and decreased in response to EGF [67]. In lung adenocarcinoma, a strong association between the ERs and EGFR mutations has been reported [179; 180]. These studies provided experimental evidence that in lung cancer tissues, the ER and EGFR pathways functionally interact with each other, thereby augmenting well for the rationale to use the combined therapy [69]. Further in vitro studies indicate that during lung carcinogenesis, the ER and EGFR interact or cross-talk with each other. Stimulation of ER has been implicated to decrease the activity of the EGFR signal, which in turn increases the activity of ER [181]. The association between ER expression and clinico-pathological parameters such as EGFR gene mutation may elucidate the prognostic role of ER expression in lung adenocarcinoma. Studies by [PERSON] and colleagues demonstrated that strong ER\(\beta\) nuclear expression was implicated in the manifested EGFR mutations, in turn resulting in its favorable prognostic significance being influenced by EGFR mutations in lung adenocarcinoma patients [182]. Additionally, similar studies by [PERSON] and colleagues revealed an association of ER\(\beta\) expression with EGFR mutations [179]. Following EGFR signaling interception by estrogens, several downstream signaling pathways such as MEK/ERK and PI3K/AKT were activated which promote lung cancer metastasis via epithelial-mesenchymal transition [183]. Potentials of Dual Targeting of Estrogen Receptors and Epidermal Growth Factor Receptors against Non-Small Cell Lung Cancers  
Overexpression of EGFR is detectable in approximately 80-85% of NSCLC patients. Selective inhibition of EGFR tyrosine kinase activities has a significant clinical impact on therapy in lung cancer patients [83]. Anti-ER and EGFR molecules are used for dual-mode-strengthened lung cancer therapy. In NSCLC, the effect of combined anti-estrogen and TKI therapy has been investigated due to the functional relationship between estrogen and EGFR pathways. Identification of specific EGFR mutation is critical in predicting response to TKI and successful lung cancer therapy. In both in vitro and in vivo studies, a combination of the ER antagonist and the EGFR tyrosine kinase inhibitor has been shown to decrease cell proliferation and tumor growth more than monotherapy either with ER antagonists or EGFR TKI [34; 60; 67]. Phase I clinical trial by [PERSON] and colleagues evaluated the efficacy of the gefitinib-fulvestrant combination.