## 6 G-Protein-Coupled Estrogen Receptors and Non-Small Cell Lung Cancers
The presence of GPER in the cell surface or plasma membrane is supported by the experimental studies of [PERSON] and associates [52; 156]. Using HEK-293 or SKB3 breast cancer cells, the investigators showed that the cell-impermeable E2-protein conjugates, E2-BSA and E2-horseradish peroxidase, promote GPR30-dependent increment in the intracellular cAMP concentrations. To further confirm that the plasma membrane is the site of GPR30 action, the investigators completed the binding assay using [3H]17\(\beta\)-E2, followed by subcellular fractionation. Their results confirm that plasma membrane isa site of GPR30 actions with [3H]17\(\beta\)-E2 binding. Thus, in this study, G-protein activation was revealed as being associated with plasma membrane but not with microsomal or other fractions prepared from HEK-293 cells or SKBR3 breast cancer cells [156]. GPER is also localized in the cytoplasmic cell organelles such as the Golgi apparatus and endoplasmic reticulum [156; 157; 158; 159]. GPER plays a critical role in regulating several normal physiological functions mediated by estrogens as evidenced by its tenfold greater capacity of estrogen binding compared to ER\(\alpha\)[51]. Supporting experiments by [PERSON] and colleagues confirmed a GPER involvement in rapid non-genomic actions [52]. Using breast cancer cell lines, it was shown that estrogens have the ability to directly bind with GPR30 without ER\(\alpha\) and ER\(\beta\) involvement, alongside activating the ERK1/2 and mitogen-activated protein kinase (MAPK) pathways involved in the proliferation of various cells [159]. Of note, an uninterrupted or stringent MAPK pathway is involved in aggravating multiple pathophysiological conditions, including cancers. Later on, [PERSON] and associates also confirmed estrogen binding with plasma-membrane-bound GPER, followed by their concurrent non-genomic activation of various downstream signaling pathways [125]. The functional mechanism of GPER differs from those of typical nuclear receptors such as ER\(\alpha\) and ER\(\beta\). Some of the molecules and cell signaling pathways activated by GPER are adenylyl cyclase, cAMP, calcium mobilization and MAPK. Other cell signaling pathways activated by GPER include EGFR/MARL transactivation, the Hippo/yes-associated protein 1 (YAP)/transcriptional coactivator with PDZ-binding domain (TAZ) pathway and phosphoinositide 3-kinase (PI3K)/Akt pathway (Figure 3) [58; 70]. A comprehensive discussion based on the studies conducted on GPER knockout mice confirmed the perception that GPER is in fact an ER with specific physiological functions mediated through nongenomic actions [70]. In 2018, [PERSON] and colleagues described the major discoveries and events pertaining to the GPER functioning through a comprehensive review article [160]. GPERs are expressed in the lung tissues that include normal bronchial epithelial cells [102]. In complete physiological conditions, various human tissues including the lungs are subjected to E2-GPER non-genomic actions [161]. A high-level GPER expression was observed in the human and mice lung cancer tissue and in the lung cancer cell lines. In these cells, the receptors are predominantly located in the cytoplasm [54]. In lung cancer cells, high-level GPER expression is associated with high ER\(\beta\) expression [53]. A number of progrowth, pro-proliferative pathways are activated by the ligand-bound GPER, in turn activating tumor/cancer progression. Metastasis is higher in cells with GPER expression in large-cell carcinoma than squamous-cell carcinoma or adenocarcinoma [54], ably supported by the reduced extent of lung metastasis in GPER knockout (KO) mice. Multiple studies claimed that GPER acts as a modulator of neoplastic transformation [56]. In the lung cancer cells, the GPERs are linked to various MMP activation and therefore assumed to be involved in cancer cell migration [88].