Non-genomic steroid hormone effects: membrane or intracellular receptors?A controversy regarding the identity of receptors that mediate nongenomic, transcription-independent cellular responses to steroids is presently attracting considerable scientific interest. While there is strong evidence for classic receptors belonging to the nuclear receptor superfamily to mediate nongenomic non genomic effects of steroid hormones effects in some cases, it does not exist for others. Nongenomic estrogen effects seem to predominantly involve classical estrogen receptors, both residing in cytoplasm and at eggsteroid levels in angry birds space cell membrane. On the other hand, non genomic effects of steroid hormones is increasing evidence for the existence of nonrelated membrane receptors for estrogens, mediating CNS effects. Novel membrane receptors for other steroids have been recently cloned, with the demonstration of their biological relevance still largely pending. Recent findings on new and unexpected properties of classic receptors have partially deflected the interest from novel, nonclassic membrane receptors, which are being progressively identified at geno,ic.
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Glucocorticoids GC are essential steroid hormones for human life. They regulate a series of important processes by binding with three glucocorticoid receptors GR and activating genomic and non-genomic pathways.
Additionally, it can interact with other transcription factors to affect gene expression indirectly. Glucocorticoids have been widely used as co-treatment of patients with breast cancer BC due to reduction of chemotherapy-induced side effects such as nausea, lack of appetite, and inflammation.
However, GC may exert a direct effect on tumor response to chemotherapy. In vitro , GC inhibits chemotherapy, radiation and cytokine-induced apoptosis by upregulating antiapoptotic genes and detoxifying proteins.
They also upregulate the proto-oncogene c-fms, tumor suppressor gene Nm23 , several members of the epidermal growth factor EGF signaling pathway and the estrogen sulfotransferase signaling pathway, thus indirectly inhibiting estrogen receptor activation. They inhibit the proangiogenic gene vascular endothelial growth factor VEGF ; Therefore, they could play a role in reducing angiogenesis. Interestingly, the phosphorylation status of ser in the GR is dependent on the expression of the BRCA1 gene, a tumor suppressor gene that is mutated in the majority of patients with triple negative BC.
Some clinical randomized trials have also attempted to address the effect of GC on patients with BC. Thus, in this review we summarize GC mechanisms of action and their participation in several facets of BC.
The main physiological functions of glucocorticoids GC include downregulating the immunological function immunosuppression , increasing the production of glucose [ 1 ], changing carbohydrate, protein, and lipid metabolism, regulating vascular tone, regulating bone mineralization, and affecting the central nervous system CNS [ 2 , 3 ]. GC is released in response to stress conditions. Under such conditions, the hypothalamus secretes a corticotrophin-releasing hormone that stimulates the hypophysis gland.
In turn, this gland produces the corticotrophin hormone that acts on the cortex of the suprarenal gland. It is here that GC are produced and secreted into the bloodstream, where they bind with globulins and are transported throughout the body [ 4 ]. This system is known as the hypothalamic-pituitary-adrenal axis HPA axis. Three main mechanisms of action for GC have been described.
The first is the genomic mechanism, which involves a classic cytosolic GC receptor GR , while the remaining two mechanisms are non-genomic.
One uses the classic GR but bound to the plasma membrane, and the other is dependent on a non-classic membrane GR. GR belong to the superfamily of nuclear receptors of transcription factors [ 5 ]. It relates to other steroid receptors, such as those for mineralocorticoids, androgens, estrogens, progesterone, thyroid hormones, vitamin D, and retinoic acid [ 6 ]. All of these receptors are evolutionarily conserved in mammals and it has been proposed that they originated from the multiple duplication of a common ancestor gene million years ago [ 7 ].
GR are essential for life; this has been proven in various murine models containing the mutated GR. Complete deletion of the second exon of the murine GR gene gives rise to severe anomalies in lung development, which lead to death a few hours after birth [ 8 ].
In the liver, the GR has proven to be responsible for gluconeogenesis [ 9 ]. In the CNS, GR deficiency causes irregularities in the HPA axis, and the development of a great number of physiological and behavioral changes that mimic depressive disorders [ 10 ]. The GR gene in humans is located on the fifth chromosome 5p31q region and has 9 exons.
Its transcription is regulated by at least three promoters, which contain sites for the binding of diverse transcription factors. The following five isoforms of the human GR gene are known: The processing of exon 9 by alternative splicing produces the two most well-known isoforms: Its overexpression has been associated with the development of cardiovascular disease, GC-resistant asthma, ulcerative colitis, and rheumatoid arthritis.
The amino terminal side possesses a domain called the Transcriptional activation AF-1 that plays a role in gene transcription and is hormone-independent. Near this domain, a leucine zipper has been discovered that is important in GC action [ 18 ].
The DBD is composed of two zinc ions complexed with eight cysteines, a motif termed zinc fingers, which interacts with the mayor groove of the DNA double helix. Additionally, in the central domain lies the dimerization domain, which forms a helix that reacts with a similar domain in an identical receptor in order to dimerize.
On the carboxyl terminus side we find the following: These might result from changes in the transcription or changes of the half-life of the mRNA [ 20 ]. The GR bound to these proteins presents a three-dimensional 3-D structure with three exposed domains, the HSP binding domain; the DNA binding domain, and the ligand binding domain.
This serine is crucial for maximal transactivation of GC signaling. Other phosphorylatable serines have been identified at positions , , and ; however, they have been associated with the inhibition of GC signaling [ 13 ]. However GRE in some genes are one half of that size but have similar transcription activity.
The number and localization of GRE in a specific promoter is highly variable. Examples of genes upregulated by GRE are tyrosine-aminotransferase, alanine-aminotransferase, and phosphoenolpyruvate-carboxykinase, all involved in gluconeogenesis of the liver [ 14 , 24 , 25 ].
The immunosuppressive anti-inflammatory effects of GC are mediated through the transrepression function of GR, whereas undesirable side effects, such as GC resistance, are thought to occur mainly through the activation of gene transcription [ 26 ].
This binding produces gene silencing by competition and displacement in the DNA of the other transcription factor [ 27 ].
An example can be found in the promoter of the osteocalcin gene. Even though GC represses one half of the genes that they regulate, it is known, only for some of these, that they are regulated by nGRE. Other genes regulated by nGRE include proopiumelanocortin, CRH, prolactin, and the serotonin neural receptor [ 14 , 23 ].
The concentration of the GR and the specific ligand it encounters, such as Dex or Corticosterone, determines negative feedback on the HPA axis [ 28 ]. GC also regulates post-transcriptional events such as protein synthesis and secretion [ 15 ].
These sequences are considered important cis-elements and are the most relevant and conserved group of functional sequences associated with mRNA stability and translation. They can be found in different combinations, and occasionally the pentamer is not present [ 24 , 20 , 31 , 32 ].
The wide range of actions of steroids cannot be explained only by their nuclear effects. Therefore, the hypothesis has been postulated that they possess non-genomic effects. These latter effects would include those produced at the level of the plasma membrane and the ion channels. Non-genomic effects were defined by Losel and Wehling as any action that does not affect gene expression initially or directly, but that does induce rapid effects, such as activation of signal transduction pathways.
The short time that these take implies that the effects are unaffected by inhibitors of transcription and protein synthesis; they can occur in cells that do not have a nucleus, such as platelets, erythrocytes, and sperm, and they can be triggered by steroid analogs for example, bovine serum albumin [BSA] conjugated with steroid molecules that cannot access the intracellular compartment e. However, in the literature, the former effects are described as non-genomic. Its basal levels decrease rapidly in human bronchial epithelial cells treated with dexamethasone Dex.
Additionally, GC also affect actin polymerization, such as in the endometrial adenocarcinoma cells of Ishikawa and human T lymphocytes, where it even increases their migration capacities after treatment with 0.
Some therapeutic drugs specifically target the non-genomic pathway [ 36 ]. To date, the non-genomic effects of GC have been described as involving two types of receptors: It regulates three signaling pathways. First, it has been shown that it can activate p42 MAPK [ 41 ]. This inhibition occurs by different mechanisms; in mastocytes and osteoblasts, it is dependent on MAPK phosphatase-1 MKP-1 , while in human T-cells, it involves phosphorylation of Raf In endothelial cells, the same pathway is turned on by Dexamethasone Dex [ 20 ].
A third pathway has been described that initiates by the activation of proteins with SH3 domains such as Src and Ras. The latter can activate the MAPK signaling pathway [ 24 , 34 , 45 ].
The non-classic mGR is an acidic glycoprotein of 63 kDa that was identified in neuronal plasma membranes of the amphibian Taricha granulosa as a functional receptor of GC [ 14 , 46 ].
It has completely different pharmacological characteristics from those of classic mGR. Additionally, it presents high-affinity binding for corticosterone, but not for hormones that classically bind GR, such as aldosterone and Dex [ 47 ].
It has seven alpha helixes coupled with G proteins. Other membrane-resident proteins capable of binding GC have been identified in the liver of chickens, rats, and mice. However, in vitro and in vivo experiments suggest that classic and non-classic mGR are the those that play a crucial role in mediating some of the neurophysiologic- and behavior-related non-genomic effects of GC [ 21 , 50 ].
Before, during, and after chemotherapy in patients with BC, GC, that is, cortisone, methylprednisolone, hydrocortisone, ketoprogesterone, fluorometholone, prednisone, and prednisolone [ 52 ] are administered at various doses. These drugs reduce the secondary effects of chemotherapy, such as nausea and vomiting, and protect the normal tissue of patients with cancer against the long-term effects of genotoxic drugs [ 53 ].
This effect should not be observed as trivial, because a lack of control of these symptoms can cause the patient to abandon therapy. Some tumor responses may be a consequence of anti-inflammatory activity rather than anti-tumor activity. GC is commonly administered with monoclonal antibodies used in BC therapy, such as Trastuzumab, which mediates antibody-dependent cellular cytotoxicity.
However, there is evidence that GC may inhibit antibody-dependent cellular cytotoxicity [ 52 ]. In , an increase in the frequency of metastases upon GC co-treatment was observed in patients with BC [ 54 ].
Some authors have associated the likelihood of developing metastases with GC therapy and suggest that patients who have been administered steroids may develop metastases due to the possible influence of immunosuppressive effects [ 54 ] that could inhibit immunosurveillance and allow tumor progression. In vitro experiments suggest that Dex protects cancer cells from the cytotoxic effects of several chemotherapy agents, such as ionizing radiation, carboplatin, cisplatin, and actinomycin D [ 53 ].
It is noteworthy that the concentrations of GC utilized in these experiments is on the same order of magnitude as the plasma concentration of GC in patients with cancer observed a few hours after they receive a dose of GC. The mechanism by which GC induce chemotherapy resistance depends on the cytotoxic agent in question, but it has been shown that GC increase levels of several known key mediators of gene expression, such as the following: Therefore, it should be tested whether GC induce resistance to chemotherapy in vivo and in randomized clinical trials.
If GC do convey chemotherapy resistance to patients, then the antiemetic effect is not worthwhile, and the need for non-steroidal antiemetic drugs without such secondary effects is urgent [ 53 ].
In one study of BC xenografts, Dex pre-treatment significantly reduced paclitaxel-induced apoptosis. More recently, microarray analysis of Dex-induced anti-apoptotic genes in BC cell lines revealed MKP-1 and SGK-1 proteins to be of particular importance in GC-induced chemotherapy resistance in BC, with glucocorticoid treatment upregulating both of these genes [ 56 ].
Clearly, the balance between GC-induced pro- and anti-apoptotic events in specific cell types plays an important role in cell death or survival, and the pros and cons of including GC in cancer treatments should be investigated further [ 56 ].
GC have also been shown to inactivate apoptosis pathways during normal mammary gland development [ 2 ]. In order to do this, the trials have included pre- and post-menopausal women. The trials have concluded that GC enhance chemotherapy more efficiently in post-menopausal than in premenopausal women, that there is no correlation between GC administration and estrogen receptor status, and that GC administration induces adrenocortical inhibition, which might reduce the synthesis of endogenous GC [ 52 ].
However, contact with exogenous GC is not the only source to which a cancer is exposed. This raises the question of what effect does endogenous GC have on non-hematologic malignancy. Therefore, future trials should measure the levels of endogenous GC in patients prior to treatment [ 52 ].
The presence or absence of GR in BC biopsies is controversial. GR is strongly expressed in metaplastic carcinomas and malignant tumors, but is not expressed in non-metaplastic carcinomas [ 2 ].