Testosterone: An Overview; Insights into its Physiology and Clinical

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Article Information:


Group: 2003
Subgroup: Volume 1, Issue 2, Spring
Date: June 2003
Type: Review Article
Start Page: 84
End Page: 96

Authors:

  • Peter Celec
  • Faculty of Medicine, and Faculty of Natural Sciences, Comenius University, Bratislava, Slovakia
  • D Ostatnikova
  • Faculty of Medicine, Comenius University, Bratislava, Slovakia

      Correspondence:

      Affiliation: Faculty of Medicine, and Faculty of Natural Sciences, Comenius University
      City, Province: Bratislava,
      Country: Slovakia
      Tel: -
      Fax: -
      E-mail: petercelec@hotmail.com

Abstract:


Latest advances in the scientific research of testosterone reflected by a high number of recent publications make it necessary to summarize the findings in this review. Both in vitro and in vivo studies have shed light especially on the complex net of intracrinological interactions, testosterone kinetics on cellular level and its recognition by a wide spectrum of nuclear, cytoplasmatic and membranebound receptors. This recently accumulated knowledge has important pathophysiological and clinical implications. Apart from androgendependent cancers, infertility therapy, cardiovascular diseases and ageing are also among the fields of biomedical investigation where testosterone plays a crucial role. Future studies will further facilitate both, basic research and clinical medicine and uncover the secrets of cognitive effects and chronobiology of testosterone. 

Keywords: Testosterone;Androgen receptor;Contraception;Honnone replacement;Honnonedependent tumours;Intracrinology

Manuscript Body:


Introduction

Despite being among issues at the center of the research interest for more than a century, testosterone is still one ofthe most interesting endocrinological topics for scientific investiga tions. Almost all organisms on earth are either male or female. This age-old difference is evolutionary; even some protozoa distinguish between "him and her". Gender segregation has remarkable consequences for physiology and behaviour. Testosterone is crucially involved in sexual differentiation early in the development. However, the attention has shifted from defining its role as the main male sex hormone towards proving new, more or less surprising hypotheses and associations to widespread fields of physiological and pathological consequences. Medline search shows that in the first 6 months of 2002 on average 4 papers on testosterone were published daily, and the trend shows that a further growth of the interest can be expected (Fig. 1).

Fig. 1. Number of published papers on testosterone in the years 1995 - 2001 as the result of a medline
search with the keyword "testosterone"

This article aims not to cover standard physiology of the hormone, but to summarize the current findings in some of the most interesting testosterone related fields of research and to give a recent literature review on these specific topics. Gender differences in behavioural, medical and other biological characteristics became popular in general public, too. So much is known about the differentiation of the body, but so little is known about the sexual differentiation of the brain with its consequences on human behaviour and cognitive performance.
From the endocrinological point of view the most important distinction is in the complex network of sexual hormones where testosterone as an androgen plays the dominant role in the male, while progesterone and estradiol as estrogens in the female "world". The truth is, the interactions and relations are much more complicated than was thought some years before. 1 -3 Nevertheless, the high levels of testosterone produced by the Leydig cells in men seem to be of main importance in the progress of sex-specific development and physiology.4

Biochemical Pathways and Molecular Interactions

Testosterone is a C 19 sex hormone, wh ich belongs to the family of molecules derived from cholesterol , "the mother of all steroids".
The classical pathway is through pregnenolone, 17a-hydroxypregnenolone, dehydroepiandrosterone and 4-androstene-3, 17- dione. The alternative pathway through pregnolone, progesteron and 17-ahydroxyprogesterone participates individually in tes1 6tosterone production .s The redox processes of degradation catalysed by cytochrome P 450, 3A4, 2C 19, 2C9 and 2C 1 6 produce especially androsterone, epiandrosterone and etiocholanolone. Some of the products of hepatic metabolism, e.g. testosterone- esters, still have androgenic effects.7 A special metabolite of testosterone is estradiol. The aromatase-catalysed conversion localized not only in the brain but also in the testis8 plays its crucial role in the understanding of sex hormone interactions and effects .9 Current in vitro studies demonstrate, how steroid modifying enzymes can alter the essential hormonal effect in the cells. Lo As with many other facts in natural sciences the transport and the recogn ition of testosterone by the cell are much more differentiated than only the passive diffusion through the plasmalemma and the interaction between the molecule and the old model of a cytoplasmatic receptor. I I Because of thennody-namic reasons, passive diffusion is not likely. Many other information transport systems have been discussed. Experimental evidence has been brought up for a membrane-bound testosterone channel (triggered passive diffusion). Testosterone also alters the momentary functional status of ionic channels. In prostate and the efferent duct epithelia the most important receptor type is a Na+-K +-A TPase. 12 In neurons and macrophages a Gprotein- linked androgen receptor increases cAMP or IP3 and DAG through the activation of either adenylatcyclase or phospholipase c. 13 Probably the most interesting and surprising way of steroid effects is Ca++ response, the source of calcium level increase being the mitochondrium. The pathways of nongenomic effects of steroid hormones have been reviewed and classified recently.14 Steroid molecules like estradiol are also endocytosed and internalised in the hypothalamus. In cytoplasm the molecule can activate protein kinases that change the tertian structure of a nuclear androgen receptor interacting with heat-shock proteins. 15.16 These interactions are highly specific, as a single base deletion can alter or cut off this effect. I Recent studies show that the classic androgen receptor of vertebrates is evolutionarily young and belongs to a family of steroid receptors – a part of the nuclear receptor superfamily. 18,19
The primary ancestral steroid receptor is the estrogen receptor a, all other members of the family were produced by duplication and mutation of this primary gene.20 In erythrocytes, even the cytoskeleton functions as a steroid receptor and effector. All these pathways assume a free unbound plasma steroid fraction, but the whole truth is that a membrane transporter for the complex testosterone-SHBG (sex hormone binding globulin) has also been found. It implies that particularly in some cells the steroid actions can be mediated through the total, free and SHBG - bound testosterone.13

Physiology of Testosterone

Two different but related characteristics are used to describe the function of testosterone: anabolic and androgenic. In men, the production is mainly localised in the smooth endoplasmic reticulum of Leydig cells in the testicles. In women, half of the testosterone amount is of ovarian or suprarenal origin, the other half being of extraglandular origin .21 Nowadays, it has been found that many cells and tissues in both sexes are able to produce testosterone, but until today we do not know the exact purpose and the importance of this non-conventional production, like in the brain22 or in the gastrointestinal tract where microbes like Pseudomonas testosteroni are able to produce steroids. 23 The main part of testosterone production is under strong and complex regulation – an area that is still unclear. The luteinizing hormone and the follicle stimulating hormone are known to play the crucial role?4 The interactions with the growth hormone,25 inhibin B26 and especially the intracrinological relationships with dehydroepiandrosterone27 and estrogens28 that influence the androgenic effect also through the hepatocellular induction of sex hormone binding globu lin production is questionable?9 On the other hand, the function of luteinizing hormone releasing hormone (GnRH) with the help of developed antagonists is understood in detai1.30 Studies are done to shed light on the feedback and feedforward mechanisms underlying the functional network of the hypothalamuspituitary- gonadal axis.31 Recent observations exhibited that the follicle stimulating hormone is not under the strict control of GnRH as is the luteinizing hormone32 and it is not even under the feedback regulation of androgens.33 Moreover, in vitro low molecular weight transmitter molecules like nitric oxide (NO) have also been shown to affect significantly the production of steroids by Leydig cells in a biphasic manner - stimulation at low concentrations and inhibition at high concentrations .34 Experiments that show the influence of spermatic nerves on androgen production are among the latest investigations being conducted towards understanding of the complex process of androgen production regulation .35
In summary, the level of testosterone is influenced by a number of both, endogenous and environmental factors (Fig. 2). Even a single low animal fat meal can significantly reduce the plasma concentration of androgens for hours.36 That is why the results from studies dealing with this parameter must be considered with caution . Testosterone as a hormone per definitionem is a transporter of information, but like in the information transport via telephone, where an analog-digital converter is needed, testosterone is converted into another steroid in the target tissue. Dihydrotestosterone is the main product of this reductase-catalysed transformation in the gonads, prostate, mamma, bone marrow and skin. In neural tissues, aromatase changes, only in a simple one-step reaction, the main male sex hormone into the main female hormone - estradiol.37 Only the muscle cells do not need to metabolise testosterone to get the information hidden in this hormone. Testosterone- specific receptors have been found in nearly all tissues. The effects of testosterone are widespread - morphologically and functionally. Many different bilateral interactions with the stressaxis are known.~8 Influence of ACTH on testosterone release is gender specific. In women ACTH stimulates the suprarenal cortical production, whereas in men this pathway is secondary. The Leydig cells are inhibited by stress and consequently the testosterone level in men decreases. 39 Growth is influenced by testosterone levels not only di-rectly influencing cartilage and bone tissue, but also by the regulation changes of the growth hormone axis on various levels.40

Fig. 2. Some of the interactions and feedback mechanisms of testosterone127 (with permission from the
authors)

However, these interactions are very complicated and are still only partially understood as reviewed in a classic paper.41 Testosterone is an anabolicum and its effects on the metabolic status, either directly or through estradiol or androstendione33 are of great importance as far as its use and abuse is concerned,42 particularly since derivatives of dehydroepiandrosterone and androstendione are available on the market and they represent oral precursors and metabolites of active androgens.43
Moreover, animal experiments have demonstrated a potential for dependence when androgens are self-administered.44 Testosterone affects various biological rhythms due to interactions with melatonin.45 During the periovulatory phase of the menstrual cycle, when testosterone levels are/ higher in comparison with other cycle phases in women, an enhanced activity of various antioxidant enzymes has been found.46 This may be related to the complex preparation of the body for possible nidation of the zygote and gravidity as similar results have been found in immune competent cells.47 Various research groups investigated the molecular basis of the action and influence of testosterone on the brain functions. 48 It seems that environmental factors like weather and photoperiod are notable modulators of the testosterone effect on the central nervous system.49 Even though it was thought that the cardiovascular system can only be pathologically affected by androgens, it seems that androgens have important physiological influences on the heart and vessels. These effects are mediated through the effects on NO synthase,)'0 Ca+ + -level changes cell-response,s I relaxin receptors in h. 57 . .. 53 t e atrIum, - renm-anglotensm system, changes of the heart cycle seen on ECG,54 prostaglandin network,55 and kidney function. 56

Testosterone in Clinical Medicine

Infertility and contraception: Androgens are one of the most important factors of male fertility. It is an old known fact that hypogonadal males are infertile. Erectile dysfunction57,58 and especially a dysregulation of spermatogenesis, 59 maybe caused by alternation of GnRH production.60 On the other hand exogenous high-dose testosterone is being proved for years as the "male pill". Invention of male oral anticonception seems to be only a question of time. Testosterone has been the first choice, but its adverse effects and the missing safety have not been solved yet. That is why today testosterone contraception or its combination with progesterone are still issues of experiments and small clinical studies,61 even though Leydig cell suppression is known for its preventing effect against ageing. 62 Ideal male contraception would be a specific inhibition of meiosis.63 The results of clinical studies conducted with the first contraceptive substances belonging to this group have not been published yet. The future of male fertility control is probably associated with research of the function of GnRH,64 its agonists and antagonists.65 It should be noted that due to the constant but not the pulse level, these drugs affect androgens and their secretion. 66 Infradian variations in testosterone levels may play a crucial role in pregnancy planning and in the therapy of infertile couples. These changes influence definitely sperm concentratI.O n 67 an d maturI. ng.6 8

Ageing and replacement therapy: In the 4th decade of human life the function of gonads and the production of sex hormones decrease in both sexes.69 Male menopause, the socalled "andropause" 70 is, from the endocrinological perspective, no more questionable. 71 Only the term "andropause" has been replaced by the abbreviation PAD AM – partial androgen deficiency in ageing males. In older age the low testosterone level is due to the decreased steroidogenesis in the Leydig cells in men, and of the ovarium in women. In women the androgens of suprarenal origin can partly compensate for this decrease. In older men this source although being more important is not able to keep the level within the "young range" .72 The decrease of the free fraction of testosterone is about 2% per year. 73 Epidemiological studies have shown that altered nutrition in older age increases the levels of sex honnone binding globulin and so reduces the bioavailability of testosterone.74 Even if corrected for the influence of chronic illnesses, medication and other age related factors, hypogonadism occurs in about 50% of older men.72 Twenty percent of men over 70 years have levels in the normal "young" range. The majority of the rest suffer from PADAM.75 Changes of sex hormone levels are known for influencing cardiovascular risk, 76 insulin-like growth factor 1 response to growth hormone77 and other factors. Interesting are the chronobiological alternations associated with ageing. Seasonal78 and circadian79 variations seems to be deregulated. Estrogen replacement therapy during the menopause, nowadays a standard, but positive effect of low testosterone doses, has been shown in older women as we11. 80,81 Substitution of the female androgen insufficiency in older age influences both physical and psychological symptoms.82 The complete physiology of ageing is not fully understood, but the compensation of decreasing hormone levels seems to be the proper way to maintain a a longer and enhanced high quality of life. Studies concerning testosterone replacement therapy in older men have shown a lower risk of osteoporosis,83 bettered libido and improved avera II well being.84 As the decrease is of both, central (pituitary) and peripheral (testicles) origin, the clinical indication for androgen substitution is all but uncomplicated as reviewed recently.85 New delivery systems and pharmacological formulas will hopefully make the application easier and the adverse effects rare, so the compliance of the pati. ents can b e I. mprove d .8 6

Cancer: Prostate cancer contains androgen sensitiveness, wh ile dihydrotestosterone is the strongest androgen in this tissue. On the other hand, many different types of testosterone receptors have been found in the prostate. That is why the use of testosterone reductase inhibitors is at least debatable, al- though dihydrotestosterone induces the expression of this enzyme87 and the breaking of this feed-forward mechanism is theoretically possible. It is known that in 30-50% of prostate cancer cells, amplifications of the androgen nuclear receptor were found. Antibodies against the receptor have been used only in animal experiments.88 Moreover, about 80% of androgen independent prostate tumours contain cells with a mutated androgen receptor that can be activated even by glucocorticoids.89 This explains and underlines the requirement of a mandatory complete cytological examination of the tumour before starting any therapy. Some indices show the possibility that aromatase is active in prostate and that a similar mechanism like in the brain functions in the prostate tissue,9o Although the clinical relevance is not clear, it should be noted that in vitro testosterone has an apoptotic effect on different types of prostate cancer cells.91 For example, the use of finasteride can under specific conditions even stimulate the growth of the tumour.92 Regarding breast tumor it it has been confirmed that about 50% of breast tumors are estrogen-dependent and 10 % may even be androgen-resistant.93 In this 10% the use of anti estrogens stimulates the growth and the metastatic activity. It seems that a combination of high estradiol and high testosterone levels means the highest hormonal risk ratio to provoke breast cancer,94 thus hormone replacement therapy in menopause seems to be related to this high risk.95 Although clinically not accepted yet, observations in recent years clearly define the combination of estradiol and testosterone as preferable in hormone replacement therapy for older women.96


Cardiovascular diseases: The risk of cardiovascular diseases and its relation to estradiol and testosterone levels has been identified. 97 Women before the onset of menopause have a much lower risk of heart attacks than do men. This difference disappears when estrogen production decreases. There is a discrepancy in regard to the negative effect of testosterone on the HDLlLDL ratio, increasing the risk of atherosclerosis98 and coronary heart disease.99 Androgen receptors have also been found on platelets, what could partly explain the gender differences in platelet function and the risk of thrombotic disease.loo On the other hand, clinical surveys studying the outcome of interventiomil therapy after acute heart attacks did not find a clear significant gender difference lol and even the use of intravenous testosterone during ischaemic events has brought no clear benefit. 102 A scientificdiscussion about testosterone being a cardiovascular risk factor or benefit factor cont.l l1ue1s0.3'1 04 M any 1.1 1 d·I ces d emonstrate the relation between sex hormone status and blood pressure, while both testosterone and estradiol seem to have lowering effects, which are probably mediated through atrial natriuretic peptide,105 vascular endothelial growth factor l06 or through the induction of nitric oxide synthase. 107 In women for example the preeclampsia gravidarum is, however, related to high testosterone levels l08 and endothelial function seem to be positively influenced by estradiol, not by androgens. 109 Cigarette smoking, an accepted risk factor for hypertension, is related to higher SHBG levels but not to decreased level of bioavailable testosterone. IIO Taking into account the results from in vitro studies, this could be a clinical indication for specific action of total or SHBG bound hormone. 13

Other diseases: Testosterone levels probably influence Alzheimers disease and this diagnosis will be probably soon an indication for androgen treatment. I I I Animal and human studies confirm the positive effects during in V.I tro experI. ments. 11 2 Anl· ma I expen.m ents have even shown testosterone dependent reduction of amyloid secretion. I 13 Granule cells from the cerebellum are protected by testosterone from oxidative stress in vitro. 114 On the contrary, neurotoxicity of experimental reperfusion injury is increased with higher doses of exogenous testosterone. I 15 These divergent results may be related to the immunomodulative effect of steroids. For example after burn injury, men are at a higher risk of developing septic complications. Experiments have demonstrated that this gender difference is due to testosterone mediated suppression of the immune system. I 16 Large-scale studies have published evidence for the relation between SHBG levels and diabetes mellitus type 2 in both sexes. I 17,1 178 Although the clear mechanism of its action is unidentified, high SHBG levels seem to affect positively insulin sensitivity in diabetic patients.118 It is not known yet if this correlation and androgens play a role in the physiology of type 2 diabetes as well. The surprising result of an epidemiological study was a sex dependent correlation between total testosterone levels and impaired glucose tolerance. Higher glycaemia in oral glucose tolerance tests seems to be associated with higher androgen levels in women and lower androgen levels in men. 119
The explanation for the association between steroid hormones and glucose tolerance status, however, needs to be further investigated. Liver function , also, is affected by testosterone level. Long-term androgen treatment often results in liver steatosis. l2o On the other hand, patients with chronic liver diseases commonly have lower plasma testosterone levels. High levels of SHBG cause gynecomasty related to liver cirrhosis. The affinity of testosterone to SHBG is higher than that of estradiol. That is why the free fraction of estrogen increases and that of testosterone decreases. A feed-forward mechanism where estradiol stimulates the SHBG production and secretion and reduces free testosterone level makes this small vicious circle complete. 121 Intersexual differences in detoxifying ethanol are of interest from the social and epidemiological points of view. Testosterone is known to induce cytochromes in hepatocytes that are then partially able to alter the damage of brain tissue resulting from long lasting alcohol abuse. 122 These findings explain the higher sensitiveness to alcohol neurotoxicity among women. 123 The reason for elevated testosterone levels after acute alcohol intake is the slowed metabolism of steroid molecules in the liver. 124 Still unan-swered, on the contrary, is the question "why are high SHBG levels found in cirrhotic patients?" Maybe the chronic catabolic status influences the Leydig cells, that are no more able to produce enough androgens. Low testosterone/ estradiol ratio stimulates hepatocytes to produce SHBG. On the other hand, the liver's function is altered . Maybe the cytoplasmatic androgen receptor dipeptide gets into plasma due to damage of the hepatocytes125 where it can function as an SHBG molecule, since both molecules are morphologically very similar.

Conclusion

It can be concluded that the actual level of androgens affects a wide range of pathophysiological mechanisms. All of them have, of course, not been noted, but it should be clear that the variations of sex hormone status, whether cyclic or not, are of clinical relevance in many different diagnostic and thera- peutic problems. The use of androgens in clinical practice remains a matter of discussion .126 Evidence based medicine has proved that studies on effects of testosterone in va rious clinical situations are rare and should be a challenge for future research. Testosterone is a molecule of a w idespread importance in all organisms throughout nature. Improved detection and measurement possibilities will further improve knowledge especially on its chronobiological characteristics and its role in the ontogeny of cognitive abilities, while these constitute, at present, the latest unanswered questions about the mysteries of testosterone.

  • Acknowledgment
    The authors are grateful to Prof. Luboslav Starka, Mr. Julius Hodosy and to the anonymous revIewer for reading and revising the manuscript.

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