Abstract
Late-onset hypogonadism (LOH) is a situation where a middle-aged or older man has low serum testosterone in conjunction with diffuse symptoms resembling those of genuine male hypogonadism. Testosterone replacement therapy has become a popular choice for the treatment of LOH. Aging is a process that includes irreversible changes because of a large variety of endogenous and environmental factors. Paternal aging also causes genetic and epigenetic changes in spermatozoa that damage male reproductive functions through adverse effects on sperm quality and count, as well as on sexual organs and also on the hypothalamic-pituitary testicular axis. If on one hand, hormone production, spermatogenesis, and testes undergo changes as a man ages, on the other hand, the offspring of older fathers show high prevalence of genetic abnormalities, childhood cancers, and several disorders. Information on the impact of age on male fertility is of growing importance, therefore, further studies should investigate the onset of changes in the reproductive function and its effects on aging men. The aim of this chapter is to briefly discuss the effects of aging on the male reproductive system and function.
Keywords
Aging; Epigenetics; Infertility; Late-onset hypogonadism; Male fertility; Oxidative stress;
Paternal age; Replacement therapy; Semen parameters; Spermatogenesis;
Spermatozoa; Testes; Testosterone.
Introduction
Aging of males exerts effects on reproductive organs and tissues and such changes seem to evolve progressively without a well-marked threshold of age. Among those alterations are the testicular changes that are most often accompanied by variations on the levels of reproductive hormones [1, 2]. In menopausal women the cessation of ovarian function is due to the inevitable decline and final exhaustion of the oocyte pool, paralleled by increase in the levels of follicle-stimulating hormone (FSH). However, in males, spermatogenesis continues throughout life [2]. As they age, men experience a gradual and progressive decline in reproductive function when compared to women.
18 Also, in contrast to menopause, which is universal and a well characterized timed process related with absolute gonadal failure, this condition in men is characterized by insidious beginning and slow progression [3]. This variability is a trait of male reproductive system aging and of the general male aging process. As reported in several recent studies, androgens play an important role within the development and maintenance of male reproductive and sexual functions. Diminished levels of circulating androgens lead to innate abnormalities of the male reproductive tract. Indeed, testosterone levels decrease as age advances. The symptoms caused by this decline can be regarded as a normal part of aging. At more advanced age, this is reported to be associated with reduced fertility, sexual disorders, decreased muscle formation and bone mineralization, disturbances of metabolism, and psychological feature dysfunction.
Moreover, low testosterone levels are also related to many chronic diseases, and symptomatic patients could benefit from testosterone treatment [4] though this remains a matter of intense debate. Though the biochemical changes induced by aging in the male reproductive tract remain largely unknown, we will briefly discuss the mechanisms known so far.
Impact of Aging on Testicular Anatomy and Physiology
Generally, as men grow older, they retain their fertility though some changes may occur.
Indeed, they develop certain physiological changes affecting the endocrine system and testicular function (Fig 1.3). Notwithstanding individual variations, alterations in testicular morphology are one of the several effects of aging on the reproductive system of males.
Testicular function declines with advancing age, but this reduction has a magnitude similar as that of other body organs [5-7]. Still, the decreased efficiency in testicular function has been the subject of a great number of studies, which evidenced the relationship between testicular function and age [8-12]. During aging there is a thickening of the basal membrane of the seminiferous tubules, accompanied with a reduction in both the height of the seminiferous epithelium and in the vascularization of the testes [13, 14], which are associated with testicular hernia-like protrusions [15]. Sertoli cells and germ cells represent up to 90% of the testicular volume, while Leydig cells contribute to less than 1%. Aging leads to a reduction in the number of Sertoli cells and Leydig cells [2, 16] which results in a decrease of testicular volume [1]. In fact, a negative association between increasing age and reduction in testicular volume for men over 80 years was already established [17]. In general, the observed mean testicular volume between 20 and 30 years of age is 16.5 cm3 and the maximal testis volume is observed at 25 years of age, after which there is a slight but significant decline to a mean volume of 14 cm3
19 between 80 and 90 years of age [6]. Compared to the age group 18–40 years, men aged over 75 years have a 31% smaller mean testicular volume. This variation in the testes volume is related with higher average serum levels of gonadotropins and lower serum free testosterone [1]. Age-related increment in gonadotropins is mostly due to primary testicular failure. The aforementioned decrease in Leydig cells number is reflected in the observation that older men display a diminished secretory capacity compared with younger men when testes are stimulated with human chorionic gonadotropin or via pulsatile GnRH [18].
Figure 1.3. The aging effects in the male reproductive function. As age advances the amount of gonadotropin-releasing hormone secreted from the hypothalamus also decrease and this is reflected in lower
20 luteinizing hormone pulsatile secretion. As a consequence, there is a decrease in the number of Leydig cells and the production of testosterone in hampered, leading to testosterone deficiency. Moreover the Sertoli cell mass also decreases with aging leading to decreased testicular volume and remarkable alterations in testicular morphology. Advanced paternal age has been associated with a negative impact on the quality of male germ cells and this is attributed mainly due to the aging-induced oxidative stress which impairs several functions of the spermatozoa, as is the case of motility and morphology. Apart from those changes that aging induces in spermatozoa, other molecular alterations may also occur. Epigenetic changes, mainly DNA methylation, have been associated with age-related diseases. These epigenetic modifications may be passed from the father to offspring increasing the susceptibility of the next generations for the development of several diseases. Up-arrows (grey): increased; Down-arrows (grey): decreased.
In fact, the testicular volume decrease in older men showed strong direct correlation with serum levels of inhibin B and inhibin B/FSH ratio, and indirect correlation with FSH [1]
and on LH levels [1, 6]. Serum gonadotropins levels increase due to feedback mechanisms that cause increased secretion of gonadotropins [19, 20]. These morphological and endocrine alterations also result in a reduced production of testosterone [14]. In fact, testicular androgen metabolism increases between the 11th and 40th year of age and progressively decreases between the age of 40 and 90 [8, 9].
Endocrine Dysfunction in the Aging Male
Aging is related to important alterations in the control of the hormonal axis that regulate male fertility [21]. The hypothalamic-pituitary-testicular (HPT) axis is the key regulator of the male reproductive function and controls the synthesis of sex hormones and the formation and maturation of male germ cells. As referred in previous chapters, it is constituted by three major elements, including the hypothalamus, anterior pituitary, and the testes. In this axis, gonadotropin-releasing hormone (GnRH) secreted from the hypothalamus reaches the anterior pituitary gland via the hypophyseal portal system and stimulates the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), by the gonadotropic cells, into the bloodstream. In broad terms, LH induces the production of testosterone by the Leydig cells, while FSH stimulates Sertoli cells to secrete androgen-binding protein (ABP) and inhibin and plays a vital role in spermatogenesis [22].
Independent from primary testicular dysfunction, events that affect the hypothalamic- pituitary axis have a major outcome on the reproductive tract and the reproductive potential of the aging male. Aging causes a decrease in the secretion of GnRH, which in turn leads to smaller LH pulses. In older men, there is an impairment on the response of gonadotropic cells to exogenous GnRH, a decrease of LH pulse size and/or reduction of
21 LH bioactivity [22, 23]. Moreover, it has been reported that serum FSH levels rise more pronouncedly in men after 40 years of age, reflecting a progressive tenacity of the gonadotropic support to the germinal epithelium [24-26]. This increase in FSH is concurrent with age-dependent alterations in testicular histology and semen parameters and with the slight but significant decrease in inhibin B. Nevertheless, this alteration was also described in men with apparently normal semen parameters [1, 27]. Furthermore, there is a consensus that testicular steroidogenesis decreases with aging [6]. It is well known that androgen levels suffer a decrease in men as they age [28]. As referred in previous chapters, androgens are key hormones in the male reproductive tract, having a major role in the expression of the male phenotype. They have very important roles during male sexual differentiation, development and maintenance of secondary male characteristics, and also during the initiation and maintenance of spermatogenesis [29].
The major circulating androgen in men is testicular testosterone [30], which is also the key sex hormone responsible for regulating masculinization [31]. Testosterone is also directly involved in the development and differentiation of the Wolffian duct derived structures, namely the epididymis, vas deferens and seminal vesicles [29, 32], exerting its action by binding to and activating the androgen receptor (AR), affecting intracellular signals and changing the expression of various genes [33]. Approximately half of testosterone is present in the circulation bound to sex hormone-binding globulin (SHBG) and another half to albumin, with only 0.5–3% of testosterone being unbound or free representing the biologically active fraction [7, 34, 35]. Several studies have confirmed the age-related decline of serum testosterone, which is accompanied by a marked decrease of the free and bioavailable testosterone (larger than that of total testosterone), partly due to the concomitant increase of SHBG [6, 36]. A clear decrease in the levels of testosterone precursors (progesterone, 17α-hydroxyprogesterone, 17α- hydroxypregnenolone, androstenedione, 17α-hydroxypregnenolone, pregnenolone, dehydroepiandrosterone, androstenediol and dehydroepiandrosterone sulfate) was also described in the spermatic vein and testicular tissues from older men [37, 38]. With respect to the total testosterone, the decrease becomes evident from the age of 45-50 years, however, for the fraction of free testosterone, this reduction is more pronounced and earlier when compared to total testosterone.
Testicular secretion of testosterone takes place as a result of the secretion of LH by the pituitary, which is stimulated by pulsatile release of GnRH from the hypothalamus and then acts on Leydig cells to stimulate testosterone synthesis and release [31]. Total serum testosterone levels decrease with age particularly due to the decrease in the
22 number of Leydig cells, deterioration of testicular perfusion, and disturbance in diurnal rhythm of GnRH and chorionic gonadotropin secretion [39-41]. Increased LH (primary gonadal failure) or inappropriately normal LH (secondary hypogonadism) can also occur in men with low testosterone levels, and can be difficult to differentiate from the dysfunction associated with aging if there is no evidence of a pathological disruption of the HPT axis [31]. In older men, combined forms of primary and secondary hypogonadism are often observed, with a concomitant age-related decline in testosterone levels as a consequence from defects both in testicular as well as hypothalamic-pituitary function. A large part of these men have serum testosterone levels below the lower reference limits in young adults [37, 42-44] and this decrease tends to be permanent rather than transient [35], being set above 1% per year in men after the age of 40 [7, 31]. In men between 55 and 68 years of age, total testosterone decrease averages 1.4% per year, while free testosterone decreases as much as 2.7%
(concurrently with an increase of SHBG) [45]. In addition, the rate of decline in testosterone levels is also affectedby chronic disease, such as obesity, illness, serious emotional stress, and medications, and this decline can be slow down by management of health and lifestyle factors [3]. Still, while this decrease tends to be small in some men, in most it can lead to clinical hypogonadism [46], being also associated with diffuse sexual, physical and psychological symptoms in some of aging men [7, 28, 30].
This condition of general and sexual deterioration in men was firstly described by Hellers and Meyers [47], who associated it with the decreased testosterone levels and used for the first time the term male menopause to denominate it [35]. Other denominations are also used for this condition that includes the combination of low testosterone and an array of the above symptoms, such as male menopause or climacterium, andropause (which are not adequate as the physiological decline of androgen secretion does not stop abruptly, but rather occurs gradually over time), partial androgen deficiency of the aging male (PADAM) and late-onset hypogonadism (LOH), with this last one being the preferred and most widely used [7, 48, 49]. Late-onset hypogonadism, according with a recent definition “is a clinical and biochemical syndrome associated with advancing age and characterized by symptoms and a deficiency in serum testosterone levels (below the young healthy adult male reference range). This condition may result in significant detriment in the quality of life and adversely affect the function of multiple organ systems”
[50]. However, it is worth mentioning that decreased testosterone levels alone with diffuse symptoms do not justify the diagnosis of LOH neither do sexual symptoms in men with normal testosterone [46]. The diagnostic criteria for LOH has been recently defined
23 by The European Male Ageing Study (EMAS) and includes the simultaneous presence of reproducibly low serum testosterone (total testosterone <11 nmol l−1 and free testosterone <220 pmol l−1) and three other sexual symptoms (erectile dysfunction, reduced frequency of sexual thoughts and morning erections) and is said to affect 2% of 40 to 80-year-old men [7].
Controversies and Treatment of Late-Onset Hypogonadism
Several studies have associated low levels of testosterone with increased mortality, which shows that LOH has a serious impact on health [31, 46, 51]. Severe LOH is directly related with significantly increased risks to all-cause of cardiovascular and cancer- related mortality, for which not only the level of testosterone, but also the presence of sexual symptoms, contributes independently. Therefore, men affected by LOH should be subjected to treatment, since their lives are at risk [46]. There are two types of treatment for LOH: testosterone replacement therapy and lifestyle modification (including weight loss and good treatment balance of comorbidities). Due to the fact that symptoms of hypogonadism in young men are very similar to those of LOH, testosterone replacement therapy has become the most popular choice for the treatment of this aging related condition. Still, although it is tempting to speculate that just by replacing decreased testosterone levels with exogenous hormone administration all problems caused by this decrease will be corrected, this is not absolutely true and is far from being demonstrated in a convincing way [46]. One of the problems in this approach is to set the level of testosterone that causes LOH, to then proceed with the proper doses of testosterone supplementation [52]. Thus, to diagnosis LOH, in addition to testosterone serum levels, clinical symptoms should be considered before proceeding with the therapy. Moreover, possible complications such as benign prostatic hyperplasia, prostate cancer, exacerbation of sleep apnea, gynecomastia, polycythemia and liver toxicity should be also taken into consideration [53].
The main goal of therapy with testosterone is to restore testosterone levels to the physiological range in men with serum testosterone levels continuously and constantly low and that also exhibit associated symptoms of androgen deficiency. The purpose of this treatment is to improve the quality of life, well-being, sexual function, muscle strength and bone mineral density [4]. In the initial phase of treatment the preparations more suitable are those, which act at the short term, so that if any adverse effect arises, these can be immediately detected and treatment can be discontinued [54]. There are several preparations available that differ in the method of administration and pharmacokinetics, and the selection should be a joint decision by both the patient and the physician [55].
24 Testosterone replacement therapy is available in the form of oral preparations, intramuscular injections and transdermal gel or patches [56] (Table 14.1), namely intramuscular injections of testosterone esters (such as TestovironTM (Schering, Germany) and SustanonTM (Organon, The Netherlands)), testosterone pellets for subcutaneous implantation and long-acting injectable testosterone esters (such as testosterone-bucyclate or testosterone undecanoate), oral preparations (such as MesterolonTM (Proviron, Schering, Germany) or testosterone undecanoate (AndriolTM, Organon, The Netherlands)). There are also transdermal delivery systems of testosterone, scrotal (TestodermTM, Alza, USA) and non-scrotal (AndrodermTM, Smithkline Beecham, UK) patches, among other systems that are being constantly developed. In all these options the replacement drug must be given daily to exert and adequate biological effect [28]. Numerous studies have shown the health benefits of testosterone replacement, which include increased strength and durability of erection and improved sexual performance of men with LOH [63]; increased sexual function, bone density, muscle mass, improved mood, quality of life and cognitive function, as well as increase in power [53]. Moreover, recent EMAS data also showed that weight gain or loss has an evident inverse correlation with circulating testosterone and vice versa.
Testosterone replacement treatment had important effects on body composition, resulting in a meaningful increase in lean body mass, and a significant decrease in fat mass without a change in body weight [39]. However, adverse effects caused by testosterone replacement treatment have also been described, such us increased acne, oily skin, gynecomastia, risk of prostate cancer, cardiovascular disease and lipid changes, and also increased hematocrit levels [53, 63, 65, 66]. There are also recent studies that drawn attention concerning cardiovascular complication in ageing men during testosterone replacement therapy [5, 14, 29]. Prostate and breast cancer are absolute contraindications to testosterone replacement therapy, severe lower urinary tract symptoms caused by benign prostatic hypertrophy (as defined by an International Prostate Symptom Score [IPSS]), untreated or poorly controlled congestive heart failure, and untreated sleep apnea are relative contraindications [66]. Nonetheless, the indications and contraindications for testosterone treatment of LOH, including short and long term benefits and risks, still await evidence‑based information. The current information about the benefits and risks of testosterone replacement therapy is not yet complete and accurate because it still relies mostly in data derived from studies that are insufficiently powered, poorly controlled and of short duration [7].
25 Table 14.1: Testosterone preparations used for replacement therapy
Preparation Administration Advantages Disadvantages Testosterone
undecanoate
Oral; 2-6 cps every 6 h
Absorbed through the lymphatic system, with consequent reduction of liver involvement.
Variable levels of
testosterone above and below the mid-range. Need for several doses per day with intake of fatty food [57].
Intramuscular;
one injection Every 10-14 weeks
Steady-state testosterone levels
without fluctuation.
Long-acting preparation that cannot allow drug withdrawal in case of onset of side effects [28].
Testosterone cypionate
Intramuscular;
one injection every 2-3 weeks
Short-acting preparation that allows drug withdrawal in case of onset of side effects.
Possible variation of testosterone levels [58, 59].
Testosterone enanthate
Intramuscular;
one injection every 2-3 weeks
Short-acting preparation that allows drug withdrawal in case of onset of side effects.
Possible variation of testosterone levels [58, 59].
Transdermal testosterone
Gel or skin patches; daily application
Steady-state testosterone level without fluctuation.
Skin irritation at the site of application and risk of interpersonal transfer [60, 61].
Sublingual testosterone
Sublingual;
daily doses
Rapid absorption and achievement of physiological serum level of testosterone.
Local irritation [62, 63].
Buccal testosterone
Buccal tablet;
two doses per day.
Rapid absorption and achievement of physiological serum level of testosterone.
Irritation and pain at the site of application [62, 63].
Subdermal depots
Subdermal implant every 5- 7 months
Long duration and constant serum testosterone level.
Risk of infection and
extrusion of the implants [57, 64, 65].
Biochemical Modifications in Spermatozoa of the Aging Male
Spermatogenesis is a continuous process that allows males to produce sperm from the beginning of puberty until late age. Notwithstanding, spermatozoa are continuously produced with advancing age, there is a growing body of evidence indicating that advanced paternal age is linked with a negative impact on the quality of male germ cells [67, 68]. It was reported that males after 40 years of age exhibited a gradual decrease in sperm motility [10]. A negative correlation between advanced age and sperm motility in healthy men has also been reported [9]. Nieschlag and collaborators found a negative correlation between sperm motility and aging after comparing sperm between old (60-88 years of age) and young (24-37 years of age) individuals [69]. Similarly, the values of sperm motility for old individuals were significantly lower in the group of males with 36 to 40 years of age when compared with the group of males with ≤25 years of age [70].
However, it must be taking into account that potential confounders, such as lifestyle