Discuss The Evidence For Non-genomic Function Of Steroid Hormones
Steroid hormones are derived from cholesterol, which is the common precursor for all steroid hormones, by steroidogenic cells of the adrenal cortex and gonads. The typical structure of a steroid hormone consists of three six membered rings with one five membered ring. They are easily distinguishable by containing a cyclopentanophenanthrene ring, also known as a steroid nucleus. The steroidogenic cells of the adrenal cortex produces two different kinds of steroid hormones named corticosteroids; these are glucocorticoids and mineralocorticoids. Both of these corticosteroids have significant functions in carbohydrate metabolism and electrolyte balance. The gonads produce other steroid hormones which are termed as sex steroids. These steroid hormones are classed as either androgens, estrogens or progestins; each with key roles within the functioning and determination of the body . Both estrogens and androgens are synthesised in the adrenal cortex and gonads, and are primarily involved with female and male secondary sexual determination. Progestins originate from the ovaries and placenta and are involved in the menstrual cycle and in maintaining pregnancy. All steroid hormones initiate their function by passing through the plasma membrane and binding to their specific intracellular receptor. The action of steroid hormones is mediated by the activation of RNA and protein synthesis. More recently however, there has been increasing evidence for steroid hormones not always initiating in this way and not being dependent on gene transcription and protein synthesis to bring about their effects. Hormones have been found to work by non-conventional signalling methods, more generally known as the non-genomic function of hormones.
Most steroid hormones follow a series of sequential steps to bring about their effects, these steps are what are known as the classic (genomic) actions of steroid hormones which are mediated through the transcription of certain genes. The hormone first of all diffuses through the plasma membrane and binds to its cytoplasmic or nuclear protein receptor. This binding takes place at a high affinity and with a high degree of specificity, which in turn releases heat shock proteins. Hormone bound receptor molecules form, which increase the affinity of the receptors for DNA hormone response elements (HREs) in the regulatory regions of the gene. The hormone-receptor complex moves into the nucleus and binds DNA HREs, as well as taking up coactivators. In turn the complex regulates transcription initiation of the target gene and the rate of RNA synthesis, and so mRNA is translated into specific proteins which can then be utilised by the body . With non-genomic function, the DNA binding of receptors and RNA synthesis that is described above is not required to illicit a response. Scientific experimental results have been obtained to present evidence for non-genomic function, as results in many different cell systems seem to be unexplainable by classic (genomic) action. In certain investigations the findings that have been made can only be explained by the non-genomic function of signal generating steroid receptors. There are many reasons which make scientists believe that there has to be the existence of non-genomic mechanisms of steroid hormones. One of the main arguments that has been put forward is that certain actions of steroid hormones have been found to act too quickly to be linked with changes in mRNA and protein synthesis. Another key point that has been discovered is that the action of some hormones still have an affect even in cells that do not or cannot synthesise protein by transcription, or lack steroid nuclear receptors. Furthermore, in cases where steroid receptor inhibitors have been used to try to prevent the action of a hormone, a response has still been found to be elicited .
Evidence of alternative functioning is in the recent findings on GH3/B6 rat pituitary tumour cells which have demonstrated non-genomic estrogen action by the rapid liberation of prolactin, after the cells have been treated with 17-estradiol . Prolactin is released almost instantaneously in the presence of estrogen and it seems that there is no way that this could have been mediated by genomic control. The genomic receptor mechanism would need the processes of transcription and translation to take place, which would take a far greater amount of time for the protein (prolactin) to be released than what was found in this particular investigation. Due to this reason, it is said that the effects of estrogen in this case, must be transmitted via specific non-genomic functioning receptors. Similarly, glucocorticoids can act at the membrane through specific receptors to exert many rapid effects on different cells and tissues, which are non characteristic of the classic receptor . It has always been regarded that glucocorticoids bind to glucocorticoid receptors and modulate their effect genomically via transcription and protein synthesis to carry out functions in processes such as hormone secretion, neuronal excitability and carbohydrate metabolism. However these particular actions have been found to take place in rat pituitary tissue within seconds or minutes to when the glucocorticoid is administered, implying that certain glucocorticoid tasks must take place independently of the genome. This particular action takes place via the same biochemical effector pathways that are responsible for mediating swift responses to neurotransmitter and peptide hormones, and is further evidence for there being non-genomic functioning of steroids in certain cell reactions. This characteristic of certain responses being too rapid to be explained by genomic function is the key concept that has been put forward to argue the case for non-genomic functions of steroid hormones existing, and is one of the most widely seen throughout varying tissue types for a range of different types of hormones. This feature has additionally been identified in the effects of aldosterone on transmembrane electrolyte movements in extra renal nonepithelial cells such as smooth muscle and human lymphocytes . It was uncovered that aldosterone significantly increased the stimulation of sodium-proton exchange by around 20-30%, in the first couple of minutes of when it was added. Aldoesterone also had secondary responses which were completely different to the immediate response that took place and started to have an effect on ion transport approximately sixty minutes after aldosterone was added; this response is said to have been controlled genomically. Due to these experimental results, it has been firmly suggested that certain biological systems must contain different recpetors for genomic and non-genomic function. Unlike the localisation of genomic receptors in the nucleus, it has been proposed that non-genomic receptors are located at or very near to the cellular plasma membrane.
Many different types of non-classic steroid receptors have been found through various investigations , with the effects or responses that have been seen, being dependent on the type of steroid, type of species and the type of tissue/cell being investigated on. The action of vitamin D metabolites on osteoblasts has given further support to show that genomic and non-genomic pathways can function by involving two different types of receptors . Vitamin D metabolites work by modulating Ca2+ influx into osteoblasts and so a specific reaction can then proceed. The use of one particular metabolite had no stimulatory activity on Ca2+ influx and was actually slightly inhibitory. However when this particular metabolite was administered along with another vitamin metabolite, there was no inhibition and an increase in stimulated influx was seen. From this it was concluded that the inhibitor that was used, did not cause any competition of hormone binding sites at the plasma membrane and so different receptors must have been present for genomic function and different receptors for non-genomic function. Furthermore, an investigation where antibody staining was used on isolated foetal rat hippocampal neurons has shown evidence for the presence of different forms of receptors. Along with the normal estrogen receptor, using antibody staining, estrogen receptor a and estrogen receptor have also been recognised. Both of these cytoplasmic localised receptors were found to have a very similar molecular structure and are related to the classic intracellular estrogen receptor. Additionally, a non classic estrogen receptor, G protein coupled receptor 30 (GPR30), has also been identified in Drosophila. GPR30 was identified as it responded to the classic estrogen receptor antagonist (ICI 182,780) in an agonist manner; it was also found to be localised to the endoplasmic reticulum and not the plasma membrane as first thought. The existence of non-genomic progesterone receptors in human spermatozoa has also been looked into . Sperm cells are seen as an ideal cell type to use in non-genomic investigations as they are 'genomically inert'. Their nucleus contains highly compacted chromatin and so barely any RNA or protein synthesis takes place within sperm cells. Three specific experimental techniques, equilibrium binding studies, western blot and ligand blot analysis were carried out on sperm lysates. In supplement to this, the effects of different concentrations of progesterone and agonists on intracellular calcium ion concentration fluxes were studied. Using the results that were obtained from all of these tests, it was indicated that there are two very distinct membrane surface receptors present for progesterone. Further investigations using immunofluorescence have also reported the additional presence of GABA receptors in spermatozoa , however there have also been conflicting reports to this theory on the existence of GABA receptors .
Experimental analysis has also shown how a steroid hormone can be modified to avert it from diffusing through the plasma membrane of a neurone, in order to see whether its response is prevented. BSA protein along with 17-estradiol can be added to a particular steroid hormone in male adult Sprague-Dawley rats and so steroid hormones covalently bind to the BSA protein . Once this hormone-BSA complex forms, the steroid can no longer cross the neuronal plasma membrane as its conformation has now been altered. However, even though the particular steroid hormone is prevented from entering the neuronal membrane and binding to its intracellular receptor, it was found that the steroid hormone still had a cellular effect and altered the excitability and behaviour of the neurone. As a result of these findings, it has been concluded that this is yet another example of how it is not solely genomic (classic) mediation that exists in cellular steroidal activities of organisms.
There are many examples of non-genomic function in a vast array of different organisms and tissue/cell types. It is widely accepted that non-genomic function does exist as it provides a practical explanation to these findings, as is discussed in this essay. However, there is also a very small proportion of the science world that do not agree with this idea of a non-genomic function of steroids existing, and believe that there are additional genomic explanations to the different experimental results that have been reported. Their case is helped by the fact that there is very strong evidence of non-genomic modulation in some cases, however it has not been found to exist in many others. Until the discovery of pharmacological agents to definitively distinguish between intracellular and membrane receptors, there will always be an aura of uncertainty. In conclusion, I believe that the various experimental findings that have been found can be used to provide a theory stating that non-genomic functions of steroid hormones actually do exist. However, I also do believe that more needs to be done, in terms of solid experimental evidence, to make this debated topic conclusive and to lie to rest any counter arguments.
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