To implicate ERs in the complications of NSCLCs, it is essential to determine that the NSCLC tissues are exposed to estrogens either through localized enhanced aromatase expression or via sufficient circulatory estrogens being synthesized and secreted in the gonads. Exposure of the human body to exogenous estrogens is also possible either through administration of synthetic estrogens/phytoestrogens orxenoestrogenic agents. Since both genomic or non-genomic actions of estrogens involve interactions with ERs or other associated receptors (e.g., GPER/EGFR), detecting the expression level of these receptors in the NSCLC tissues may be crucial. Indeed, aromatase activities have been detected in NSCLC cell lines as well as ~86% of tumors [108]. A number of studies including one by the Mollerup group reported the ER prevalence in lung tumors and NSCLC cell lines [113]. Additionally, a relationship of the hormonal status in the cancer-affected tissues with the expression index of ERs has been established. Of note, it is well known that premenopausal women generate large contingents of estrogens as compared to postmenopausal women and men of all age groups. A comparative analysis of ER expression extent in cancer patients, vis-à-vis age/gender group of pre- and postmenopausal women and men, revealed the highest ER expression intensity in the premenopausal women. In this study, the men cancer patients exhibited a minimal ER expression, suggesting a critical role of either circulating or local tissue level of estrogens in the ER expression [120]. Additionally, overexpression of ERs is detected in lung adenocarcinoma. Of the two ER types, the ERβ is more abundant in lung cancer. Studies have reported an ERβ overexpression in 60–80% of tumors, irrespective of gender [121]. In a noted effort, Kawai and associates explained the differential ERα detection pattern in NSCLC samples using an ERα antibody (Ab) raised against either a full-length or an N-terminus or a C-terminus ERα protein. The Ab against the C-terminus ERα region detected a predominantly cytoplasm-localized protein compared to the one raised against the N-terminus of the protein. The investigators predicted that in NSCLC, ERα is N-terminal-deleted and lacks the nuclear localization signal [122]. In a similar study on the prevalence of ERα splice variants, ERα36 was reported in NSCLC specimens, while wild-type ERα was minimally expressed. The results of this study claim that in normal lungs, the wild-type ERα is quasi-absent [123]. In comparison, both in normal physiology as well as in cultured NSCLC cells, the ERβ is predominantly localized within the nucleus [49,122]. A number of studies including that of Baik and colleagues described the ERβ nuclear localization as a more reliable prognosis factor contrary to that of the cytoplasmic ERβ expression [27]. Studies also claimed that the cytoplasmic and the nuclear ERβ co-expression were correlated with a low survival rate compared to the one without co-expression [124]. The ERs are also localized in the plasma membrane [52,125]. Studies by Gao and teammates claimed the ER involvement in NSCLC progression by modulating the membrane receptor signaling network [126]. It is predicted that the localization of ERs in the specific cellular compartment including plasma membrane, cytoplasm and/or nucleus may have a distinct function and affect the prognosis differentially via a genomic or non-genomic pathway. The ERs expressed by lung bronchial epithelial as well as NSCLC cells respond transcriptionally to E2/17β-estradiol [66,67,114]. In a Kras-activated and p53-deletion-induced lung adenocarcinoma mice model, administration of E2/17β-estradiol promoted the tumor progression. In this experimental mice model, male and ovariectomized female mice respond likewise in response to17β-estradiol administration [44]. The poorer clinical outcomes observed in NSCLC patients may be related to the proliferative as well as survival responses to 17β-estradiol. A number of studies documented the specific cell signaling molecules and associated pathways involved in the 17β-estradiol dependent proliferation, survival and growth of in vitro cultured NSCLC cell lines and tumor xenografts [60,67,121,127]. However, there is a lot of controversy regarding beneficial versus adverse effects of HRT in the human body [14,15]. Similarly, the role of ERs in NSCLC remains controversial, and the mechanisms of action of ERs in NSCLC complications are not conclusive. There is no consensus on whether ERβ expression plays a role in survival. Some studies have suggested a protective effect of ERβ nuclear expression [128,129,130], which may only be significant in men [130]. It is possible that the presence of nuclear ERβ confers a hormonal dependence for growth rather than on the other more aggressive oncogenic pathways, leading to a comparatively better survival. A number of comprehensive review articles described various aspects of ERs in the NSCLC complication [55,131].

Aromatase inhibitors that include irreversible steroid inhibitors/non-steroidal inhibitors (e.g., anastrozole, letrozole, etc.) and selective ER modulators (SERMs, e.g., tamoxifen, a competitive inhibitor with equal ERα and ERβ affinity) or selective ER downregulators (SERDs, e.g., fulvestrant/ICI182780, a pure anti-ER molecule with equal affinity for ERα and ERβ which degrades both ERα and ERβ) are used against estrogen/ER-dependent cancers. The efficacy of these molecules against NSCLC complication was examined by in vitro cell culture and animal model experiments including xenograft and ER knockout mice models and clinical studies involving NSCLC patients. Details of the estrogen activities in lung cancer complication, particularly in women, have been recently reviewed [132,133]. In cell culture experiments using A549 NSCLC cells, it was shown that lowering the estrogen concentration inhibited the A549 cell’s growth [134]. Another significant in vitro study revealed that aromatase inhibitors such as letrozole significantly decreased cell proliferation while exemestane reduced tumor growth and increased cell apoptosis, alongside inhibiting cell migration and invasion [135]. The efficacy of the SERD molecules such as fulvestrant was also examined in the in vitro cell culture based experiments and tumor xenograft mouse model. The results of these studies show significant inhibition of 17β-estradiol-induced NSCLC (cell lines) grown in culture and as tumor xenografts. In this study, the combined activity of anti-ER and anti-EGFR molecules emerged more efficacious pertaining to the anti-NSCLC activities [67]. A number of similar experimental studies focused on the efficiency of fulvestrant in NSCLC suppression [106,136,137]. Data presented by Wang and colleagues showed that dexamethasone suppresses the growth of NSCLC cell line A549 injected xenograft tumors via inducing estrogen sulfotransferases and decreasing estradiol levels in tumor tissues, suggesting that dexamethasone may be used as an anti-estrogenic agent for the NSCLC treatment [138]. Clinical observational studies consisting of 6500 breast cancer survivors subjected to anti-estrogen therapy showed lower subsequent lung cancer mortality [139]. In another similar study consisting of 2320 patients, a strong anti-estrogen therapy and lung cancer motility correlation was observed [140]. More recently, three separate and independent clinical trials have been conducted to evaluate the anti-estrogen/anti-ER therapeutic efficacy toward lung cancer [71,72,73]. Based on the results, it is predicted that, in the future, ER detection in NSCLC could be considered reliable treatment recourse for women. Comprehensive review articles discussed the estrogens and their receptors in influencing the development as well as microenvironment of lung cancers and the significance of clinical studies exploring the anti-estrogen/anti-ER therapy usefulness against lung cancers (Table 1) [141,142].