Bone mineral density and testicular failure: evidence for a role of vitamin D 25-hydroxylase in human testis

Bone mass and hormone analysis in patients with spinal cord injury: evidence for a gonadal axis disruption

CONTEXT

Bone loss is a constant finding in patients with spinal cord injury (SCI).

OBJECTIVE

We sought to evaluate potential modifiable factors that could lead to bone loss in complete motor paraplegia by examining gonadal axis hormones, vitamin D status, and bone markers.

DESIGN

This is a cross sectional.

SETTING

It includes SCI Outpatient.

PATIENTS AND OTHER PARTICIPANTS

Twenty-nine chronic male patients with SCI were compared with 17 age-matched, able-bodied men.

MAIN OUTCOME MEASURE

The bone mineral density (BMD) of lower limbs and lumbar spine were measured using dual x-ray absorptiometry. Parathormone, 25-hydroxyvitamin D [25(OH)D], collagen type I C-terminal telopeptide (CTX), and sexual hormone were measured.

RESULTS

Patients with SCI had lower BMD at the inferior limbs sites. CTX showed an inverse relationship with the time since injury. Patients had lower free T levels (SCI, 12.00 ± 2.91 vs controls, 19.51 ± 5.72; P ≤ .001), and the majority (72%) had normal/low levels of gonadotropins. Low T, however, was not related to low bone mass in patients with SCI. In the controls, the 25(OH)D level was positively correlated with the T and with the lumbar spine BMD, but these correlations were not observed in the SCI.

CONCLUSIONS

Impairment of testicular function after SCI was indicated by the low levels of T and the loss of correlation between T and 25(OH)D levels; this correlation was present in the able-bodied controls. Inappropriate levels of gonadotropins were identified in most patients, featuring a hypogonadotropic hypogonadism and suggesting a disruption of the pituitary-gonadal axis. T concentrations might not be an effective target for bone loss therapy.

Uncarboxylated osteocalcin stimulates 25-hydroxy vitamin D production in Leydig cell line through a GPRC6a-dependent pathway

Recent studies disclosed a cross talk between testis and bone. By the action of LH, Leydig cells are able to modulate bone metabolism through testosterone and insulin-like factor 3. Moreover, LH modulates the Leydig expression of CYP2R1, the key enzyme involved in vitamin D (Vit D) 25-hydroxylation. However, pathways regulating CYP2R1 expression have been poorly investigated. The cross talk from the bone to the testis of the vitamin D 25-hydroxylase CYP2R1 involves osteocalcin (OC), which is produced by the osteoblasts and stimulates the production of testosterone by the Leydig cells through its putative receptor GPRC6A, a cation-sensing G-protein-coupled receptor. The aim of this study was to investigate the possible action of OC on CYP2R1 expression and 25-hydroxy Vit D (25-OH Vit D) production in a mouse Leydig cell line (MA-10). After confirmation of the expression of GPRC6A by MA-10, we found that stimulation with either human chorionic gonadotropin or uncarboxylated-OC (ucOC) increases CYP2R1 protein expression in a dose-dependent manner and, in turn, increases the release of 25-OH Vit D in culture medium. This effect was abolished by receptor blockade with, respectively, anti-LH receptor and anti-GPRC6A antibodies. Moreover, both agonists converged to phosphorylation of Erk1/2 by a likely differential action on second messengers. Human chorionic gonadotropin induced slow "tonic" increase of intercellular calcium and accumulation of cAMP, whereas ucOC mainly induced phasic increase of cell calcium. Supporting these findings, we found that serum ucOC positively correlated with 25-OH Vit D levels in 40 overweight male patients and 21 controls. Altogether, our results suggest that OC contributes with LH to 25-OH Vit D production by Leydig cells.

Association of serum inorganic phosphate with sex steroid hormones and vitamin D in a nationally representative sample of men

Defects in bone regulatory pathways have been linked to chronic diseases including cardiovascular disease and cancer. In men, a link between bone metabolism and gonadal hormones has been suggested. However, to date, there is lack of evidence on the association between serum inorganic phosphate (Pi) and sex steroid hormones. The objective of this study was to investigate the association between Pi, sex steroid hormones and a known Pi metabolic regulator, vitamin D, in men in the National Health and Nutrition Examination Survey III (NHANES III). From NHANES III, we selected 1412 men aged 20+ who participated in the morning session of Phase I (1988-1991) with serum measurements of Pi, sex hormones, and vitamin D. Multivariable linear regression was used to calculate crude and geometric mean Pi by total and estimated free testosterone and estradiol, sex hormone-binding globulin, androstanediol glucuronide (AAG), and vitamin D. Similar analyses were performed while stratifying by race/ethnicity and vitamin D levels. We found a lack of statistically significant difference in geometric means of Pi across quintiles of concentrations of sex hormones, indicating a tight regulation of Pi. However, Pi levels were inversely associated with calculated free testosterone in non-Hispanic black men, with geometric mean levels of Pi of 1.16 and 1.02 ng/mL for those in the lowest and highest quintiles of free testosterone, respectively (p-trend < 0.05). A similar but weaker pattern was seen between total testosterone and Pi. An inverse association was also seen between AAG and Pi in men with vitamin D concentration below the median (<24.2 ng/mL). No associations were observed among men with vitamin D levels at or above the median. Our findings suggest a weak link among sex hormones, vitamin D, and Pi in men. The observed effects of race/ethnicity and vitamin D indicate a complex association involving various regulators of Pi homeostasis.

Functional Importance of 1α,25(OH)2-Vitamin D3 and the Identification of Its Nongenomic and Genomic Signaling Pathways in the Testis

The 1α,25(OH)2-vitamin D3 (1,25-D3) is known by its classic effects on Ca2+ metabolism and regulation of cellular proliferation and differentiation. The hormone 1,25-D3 acts in the testis through nongenomic and genomic events being implicated in the success of spermatogenesis in rats and in human being. The aim of this review was to highlight the effect and intracellular pathways of 1,25-D3 to modulate the spermatogenesis. The pivotal role of 1,25-D3 in male reproduction is reinforced by the presence of VDR and 1α-hydroxylase in reproductive tract. Also, the marked expression of VDR and the VD metabolizing enzymes in human testis, ejaculatory tract, and mature spermatozoa implicates the 1,25-D3 in spermatogenesis and maturation of human spermatozoa. Among genomic events, 1,25-D3 influences the expression of calcium binding protein and stimulates aromatase gene expression through a nongenomic activation of the membrane-bound VDR receptor involving the PKA pathway in the testis. Also, 1,25-D3 stimulates amino acid transport and exocytosis in testis by nongenomic events coupled to ionic currents triggered at plasma membrane. All together, the demonstration that 1,25-D3 regulates both Sertoli cell and sperm function may be useful for the study and development of new therapeutic strategies for the male reproductive disorders.

Vitamin D and male reproduction

Vitamin D is a versatile signalling molecule with a well-established role in the regulation of calcium homeostasis and bone health. The spectrum of vitamin D target organs has expanded and the reproductive role of vitamin D is highlighted by expression of the vitamin D receptor (VDR) and enzymes that metabolize vitamin D in testis, male reproductive tract and human spermatozoa. The expression levels of VDR and CYP24A1 in human spermatozoa serve as positive predictive markers of semen quality, and VDR mediates a nongenomic increase in intracellular calcium concentration that induces sperm motility. Interestingly, functional animal models show that vitamin D is important for estrogen signalling and sperm motility, while cross-sectional studies support the positive association between serum 25-hydroxyvitamin D level and sperm motility in both fertile and infertile men. Expression of VDR and enzymes that metabolize vitamin D in fetal testis indicates a yet unknown role during development, which may be extrapolated from invasive testicular germ cell tumours where 1α,25-dihydroxyvitamin D induces a mesodermal differentiation of the pluripotent testicular cancer cells. Taken together, vitamin D signalling has a positive effect on semen quality, increases estrogen responsiveness and differentiates germ cell tumours. Future studies are needed to determine when 1α,25-dihydroxyvitamin D acts in a paracrine manner and whether systemic changes, which are subject to pharmacological modulation, could influence male reproductive function.

Calcidiol [25(OH)D3]: from diagnostic marker to therapeutical agent

OBJECTIVE

Osteoporosis is a skeletal disorder characterized by diminished bone strength, which results in an increased risk of fracture. Currently, osteoporosis is a public health priority due to the large number of individuals affected and the detrimental effect on quality of life. Primary osteoporosis, the most common form, usually results from age-related reduction in bone mineral strength. Over time, the individual's capacity to build bone is impaired, as the synthesis of vitamin D, the hormone responsible for calcium absorption, tends to decline. As serum calcium levels decrease, metabolic control serves to increase the removal of calcium from the skeleton to make up for the deficit. The synthesis of the 'hormone' vitamin D and its control therefore become central to intervention in involutional osteoporosis syndromes. In humans, plain vitamin D (cholecalciferol), also called parental or native vitamin D, is photosynthesized in the skin and then hydroxylated in the liver into the vitamin D analog calcidiol [25(OH)D3], which is hydroxylated again in the kidney into the vitamin D analog calcitriol [1,25(OH)2D3]. The advantage of administering vitamin D analogs is that the pro-drug calcidiol avoids the effect of declines in hepatic function, while calcitriol avoids the effect of declines in hepatic and kidney function. A strategy to enhance [25(OH)D3] levels to the optimal threshold of vitamin D is supplementation with the calcidiol metabolite itself. The goal of this paper is to review published studies on the efficacy of the calcidiol metabolite in increasing 25(OH)D3 serum levels and improving skeletal health parameters in humans.

METHODS

A library search of published papers in the area of use of calcidiol in humans from 1967 to 2013 was performed (key words: calcidiol, 25-hydroxy-vitamin D3, vitamin D supplementation, vitamin D metabolism, osteomalacia).

RESULTS AND CONCLUSION

The results of the survey made it possible to conclude that calcidiol is characterized by a number of features that make the compound ideal in conditions that require supplementation with a 25-hydroxylated metabolite.

CYP2R1 is a major, but not exclusive, contributor to 25-hydroxyvitamin D production in vivo

Bioactivation of vitamin D consists of two sequential hydroxylation steps to produce 1α,25-dihydroxyvitamin D3. It is clear that the second or 1α-hydroxylation step is carried out by a single enzyme, 25-hydroxyvitamin D 1α-hydroxylase CYP27B1. However, it is not certain what enzyme or enzymes are responsible for the initial 25-hydroxylation. An excellent case has been made for vitamin D 25-hydroxylase CYP2R1, but this hypothesis has not yet been tested. We have now produced Cyp2r1 (-/-) mice. These mice had greater than 50% reduction in serum 25-hydroxyvitamin D3. Curiously, the 1α,25-dihydroxyvitamin D3 level in the serum remained unchanged. These mice presented no health issues. A double knockout of Cyp2r1 and Cyp27a1 maintained a similar circulating level of 25-hydroxyvitamin D3 and 1α,25-dihydroxyvitamin D3. Our results support the idea that the CYP2R1 is the major enzyme responsible for 25-hydroxylation of vitamin D, but clearly a second, as-yet unknown, enzyme is another contributor to this important step in vitamin D activation.

Testicular function and bone metabolism--beyond testosterone

Findings in the past few years have advanced understanding of the crosstalk between testis and bone and could contribute to defining an improved clinical approach to the biochemical diagnosis and therapeutic management of hypogonadism and male osteoporosis. This Review focuses on the Leydig cells of the testis. Other than being responsible for steroidogenesis and production of testosterone, the function of these cells is fundamental to bone health in at least two other ways: Leydig cells produce insulin-like 3 (INSL3), which has a role in osteoblast function, and they contribute to 25-hydroxylation of vitamin D. Impairment of testicular function leads to low levels of testosterone, INSL3 and 25-hydroxyvitamin D and consequently to an increased risk of osteopenia and osteoporosis.

Altered bone status in unilateral testicular cancer survivors: Role of CYP2R1 and its luteinizing hormone-dependency

BACKGROUND

Recent data suggest a potential role of testis in vitamin D activation, where Leydig cells could represent key players in this process since they express the highest amount of CYP2R1, a key enzyme involved in vitamin D 25 hydroxylation.

AIM

To evaluate bone status in unilateral orchiectomy and to assess in vivo and in vitro LH-dependency of Vitamin D 25 hydroxylation.

SUBJECTS AND METHODS

125 normotestosteronemic patients with testicular cancer (TC), featured by unilateral orchiectomy and 41 age-matched healthy male controls were studied in the Center for Human Reproduction Pathology at the University of Padova. To evaluate LH-dependency of Vitamin D 25 hydroxylation in vitro, Leydig cell cultures were stimulated with hCG and assessed for CYP2R1 expression, whereas in vivo 10 hypogonadotropic hypogonadal (HH) patients were evaluated before and after treatment with gonadotropins for bone metabolism markers. Hormonal pattern and bone metabolism markers were measured in all subjects, whereas 105 patients and 41 controls underwent bone densitometry by DEXA.

RESULTS

In TC patients 25-hydroxyvitamin D levels were significantly lower compared to controls. Furthermore, 23.8% of patients with TC displayed low bone density (Z-score <-2 SD). None of the 41 control subjects showed any significant alteration of BMD. In vitro and in vivo studies revealed that CYP2R1 expression in Leydig cells appeared to be hCG dependent.

CONCLUSION

Our data show an association between TC and alteration of the bone status, despite unvaried androgen and estrogen levels, suggesting the evaluation of bone status and possible vitamin D deficiency in TC survivors.

Vitamin D in human reproduction: a narrative review

BACKGROUND

Special attention has been given to the effect of vitamin D supplementation on fertility outcomes in both sexes.

AIMS

The purpose of this narrative review was to elucidate the role of vitamin D in male and female reproduction, providing current evidence from both animal and human studies.

MATERIALS AND METHODS

Using PubMed and Medline, we searched for publications during the last 30 years regarding the role of vitamin D in human reproduction.

RESULTS

Accumulating evidence from animal and human studies suggests that vitamin D is involved in many functions of the reproductive system in both genders. In women, vitamin D status has been associated with in vitro fertilization (IVF) outcome, features of polycystic ovarian syndrome (PCOS) and endometriosis. Although several data converge towards a beneficial effect of vitamin D supplementation in metabolic disturbances in women with PCOS, a significant knowledge gap precludes the establishment of a clear cause-effect relationship. In men, vitamin D status has been associated with semen quality and sperm count, motility and morphology. There is evidence for a favourable effect of vitamin D supplementation on semen quality, testosterone concentrations and fertility outcomes.

DISCUSSION

Studies with superior methodological characteristics are needed in order to establish a role for vitamin D on the treatment of female and male infertility.

CONCLUSIONS

Recent data on vitamin D provide new insights in the complex pathogenesis and treatment of infertility.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Vitamin D metabolism, sex hormones, and male reproductive function

The spectrum of vitamin D (VD)-mediated effects has expanded in recent years, and VD is now recognized as a versatile signaling molecule rather than being solely a regulator of bone health and calcium homeostasis. One of the recently identified target areas of VD is male reproductive function. The VD receptor (VDR) and the VD metabolizing enzyme expression studies documented the presence of this system in the testes, mature spermatozoa, and ejaculatory tract, suggesting that both systemic and local VD metabolism may influence male reproductive function. However, it is still debated which cell is the main VD target in the testis and to what extent VD is important for sex hormone production and function of spermatozoa. This review summarizes descriptive studies on testicular VD metabolism and spatial distribution of VDR and the VD metabolizing enzymes in the mammalian testes and discusses mechanistic and association studies conducted in animals and humans. The reviewed evidence suggests some effects of VD on estrogen and testosterone biosynthesis and implicates involvement of both systemic and local VD metabolism in the regulation of male fertility potential.

Vitamin D and fertility: a systematic review

BACKGROUND

Vitamin D has been well-known for its function in maintaining calcium and phosphorus homeostasis and promoting bone mineralization. There is some evidence that in addition to sex steroid hormones, the classic regulators of human reproduction, vitamin D also modulates reproductive processes in women and men.

AIM

The aim of this review was to assess the studies that evaluated the relationship between vitamin D and fertility in women and men as well as in animals.

METHODS

We performed a systematic literature search in Pubmed for relevant English language publications published until October 2011.

RESULTS AND DISCUSSION

The vitamin D receptor (VDR) and vitamin D metabolizing enzymes are found in reproductive tissues of women and men. Vdr knockout mice have significant gonadal insufficiency, decreased sperm count and motility, and histological abnormalities of testis, ovary and uterus. Moreover, we present evidence that vitamin D is involved in female reproduction including IVF outcome (clinical pregnancy rates) and polycystic ovary syndrome (PCOS). In PCOS women, low 25-hydroxyvitamin D (25(OH)D) levels are associated with obesity, metabolic, and endocrine disturbances and vitamin D supplementation might improve menstrual frequency and metabolic disturbances in those women. Moreover, vitamin D might influence steroidogenesis of sex hormones (estradiol and progesterone) in healthy women and high 25(OH)D levels might be associated with endometriosis. In men, vitamin D is positively associated with semen quality and androgen status. Moreover, vitamin D treatment might increase testosterone levels. Testiculopathic men show low CYP21R expression, low 25(OH)D levels, and osteoporosis despite normal testosterone levels.

Effects of hypogonadism on bone metabolism in female adolescents and young adults

Gonadal steroids, including androgens and oestrogens, play a critical part in bone metabolism, and conditions associated with a deficiency of gonadal steroids can reduce BMD in adults and impair bone accrual in adolescents. In addition, other associated hormone alterations, for example, insulin-like growth factor 1 deficiency or high cortisol levels, can further exacerbate the effect of hypogonadism on bone metabolism, as can factors such as calcium and vitamin D deficiency, low body weight and exercise status. This Review discusses the effects of different hypogonadal states on bone metabolism in female adolescents and young adults, with particular emphasis on conditions associated with low energy availability, such as anorexia nervosa and athletic amenorrhoea, in which many factors other than hypogonadism affect bone. In contrast to most hypogonadal conditions, in which replacement of gonadal steroids is sufficient to normalize bone accrual rates and BMD, gonadal steroid replacement may not be sufficient to normalize bone metabolism in these states of energy deficit.

Cardiometabolic complications after androgen deprivation therapy in a man with prostate cancer: effects of 3 years intermittent testosterone supplementation

Androgen deprivation therapy (ADT) for prostate carcinoma (PCa) may cause cardiometabolic complications unless intermittent androgen blockade (IAB) is instituted. An 80-year-old caucasian man was diagnosed intermediate grade (Gleason 4 + 3) PCa and was treated with continuous ADT with triptorelin plus bicalutamide. After 6 months of treatment, he experienced an acute myocardial infarction and 1 month after hospitalization he came to our outpatient clinic for fatigue, weight gain, and hyperglycemia. Due to iatrogenic hypogonadism, we decided to proceed with IAB, but after 3 months ADT withdrawal his serum testosterone (T) was still 0.5 ng/mL. Due to very low concomitant PSA levels (0.1 ng/mL) he was then proposed intermittent T-gel supplementation (Tostrex(®)) which was initiated according to the following scheme: 6 months on and 3 months off. T-gel dose was titrated tri-weekly in order to achieve T plasma levels below 3.49 ng/mL. After 6 months on, his serum T raised to a mean value of about 2.0 ng/mL without increments in PSA. After overall 12 months on, his serum T peaked to a mean value of 3.0 ng/mL while a delay in PSA rise was seen after 24 months (0.6 ng/mL) but remained stable until the last observation carried forward (LOCF), at 45 months. No clinical and biochemical PCa progression were observed at LOCF. Reversion of iatrogenic metabolic syndrome started after 6 months of T supplementation without using any add-on treatment. This case provides support that once regression of PCa growth is attained, T supplementation may be administered in well differentiated PCa, especially if IAB is not successful in reverting iatrogenic hypogonadism and its associated cardiac and metabolic complications.

Regulation of vitamin D metabolism

Fundamental to understanding the way in which perturbations in the vitamin D endocrine system can affect human health is an appreciation of the steps involved in the production of the well-recognized active hormonal form, 1,25-dihydroxyvitamin D(3). Thus this paper focuses first on the nature and regulation of the two enzymes responsible for the production of 1,25-dihydroxyvitamin D(3), the 25-hydroxylase in the liver and the 1α-hydroxylase in the kidney. The most important regulators of the 1α-hydroxylase in the kidney are 1,25-dihydroxyvitamin D(3) itself, parathyroid hormone and FGF23. The extent and importance of extra-renal, 1,25-dihydroxyvitamin D(3) synthesis is then considered. Finally the features of the 24R-hydroxylase, which produces 24R,25-dihydroxyvitamin D(3) in the kidney and is induced by and inactivated, 1,25-dihydroxyvitamin D(3)in target cells are described.