While ERα is traditionally recognized as a nuclear receptor that regulates gene expression, detailed evidence accumulated over the past 20 years has demonstrated that a small percentage (approx. 1%) of ERα can also be localized to the cell membrane in certain contexts to produce physiological and clinically relevant biological responses [
121]. The membrane ERα [mERα] was first discovered in 1999 [
122] and characterised in breast cancer cells by Levin and colleagues [
123,
124]. The molecular identity was shown to be a palmitoylated variant of the full length (66kDa) nuclear ERα receptor which allowed its tethering at the cell membrane in a caveolin-1 signalsome [
125,
126]. The understanding of the role of the mERα in physiology and cancer biology was hampered by the lack of specific inhibitors and agonists which could distinguish E2-mERα ligand binding from nuclear ERα signal transduction [
127]. The non-genomic and rapid actions (secs/mins) of estrogen on protein kinases, intracellular calcium and ion channel activity in colonic crypts [
128,
129,
130] distinguish it from canonical genomic E2-ERα signal transduction which shows typical long latency of hours/days to generate transcriptional responses [
131]. It is important to note that the rapid membrane-initiated responses to estrogen can be elicited at physiological sub-nanomolar concentrations of estrogen such that dose-response is of little value in distinguishing non-genomic from nuclear E2-ERα actions [
132]. One way to overcome this difficulty is the use of estrogen analogues which penetrate poorly, if at all, the cell membrane, or the generation of nuclear excluded ERα mutants [
133]. In this regard, certain specific membrane-impeded analogues of estrogen such as E2-BSA [
134] and estrogen dendrimer conjugates (EDC) [
135] which do not enter the cytosol to bind nuclear ERα have been shown to replicate the rapid non-genomic actions of free unbound estrogen (17β-estradiol). The most significant advance, however, in understanding the physiological and pathological roles of mERα have resulted from the generation by the Levin group of selective mouse models with membrane-only mERα (MOER) or nuclear-only ERα (NOER) expression [
136]. Mutations of the palmitoylation site of ERα have also provided a useful tool to dissect membrane-initiated and nuclear actions of estrogen [
137]. These studies have shown the absolute requirement for mERα expression in transducing rapid actions of estrogen on protein kinases and tyrosine kinase signaling pathways in cancer cell proliferation but not in the development of reproductive organs and tissues [
136,
137,
138]. The presence of ERα on the cell membrane, in addition to its primarily nuclear localization, expands the range of estrogen actions through combined non-genomic and genomic regulation of cell differentiation and proliferation through an expanded signalsome [
139,
140].
Regarding the role of membrane ERα in colon cancer, research in this specific area is still evolving, and the understanding of its implications is not yet fully established. However, many of the signaling pathways regulated by membrane estrogen actions in colon (EGFR, ERK-MAPK, PI3K-Akt and Wnt) are involved in cell proliferation, survival, and migration in colon cancer cells (
Figure 3), [
141]. For example, activation of membrane ERα in colonic epithelial cells isolated from females has been associated with the rapid activation of mitogenic signaling cascades, including the MAPK pathway, PI3K/Akt pathway, and Src kinase signaling [
142,
143,
144], which can promote cell growth and survival. Additionally, membrane ERα has been implicated in modulating epithelial-mesenchymal transition in breast [
145] and colon cancer cells [
146], a process involved in CRC tumour invasion and metastasis [
147].
The current data indicate that mERα is the primary endogenous ER mediator of rapid E2 responses although a membrane ERβ (mERβ) is also found co-expressed with mERα in cancer cells to regulate cell proliferation [
148]. The role of mERβ is less well-studied in cancer biology although ERβ may be present at the cell membrane in a palmitoylated form in colon cancer cells to inhibit cell proliferation [
149]. Studies have demonstrated that mERβ can produce rapid non-genomic actions of estrogen on ERK and JNK kinase activity when expressed in Chinese hamster ovarian cells [
150] while other studies have shown E2-mERβ to rapidly activate p38 MAPK in human colon cancer cells [
151]. The membrane ER subtypes appear to mimic their respective nuclear ER responses to estrogen i.e., pro-proliferative, pro-tumorigenic effects via nuclear ERα and mERα while conversely producing anti-proliferative, anti-tumorigenic effects via both nuclear ERβ and mERβ. In this way, estrogen non-genomic interactions with membrane ERα and membrane ERβ can modulate cell proliferation, apoptotic pathways and cell death in CRC [
152].