Insulin-like peptide 3 (INSL3) as an indicator of leydig cell insufficiency (LCI) in Middle-aged and older men with hypogonadism: reference range and threshold

Insulin-like peptide 3 (INSL3) is a circulating biomarker for Leydig cell functional capacity in men, also indicating Leydig Cell Insufficiency (LCI) and potential primary hypogonadism. Using results from large cohort studies we explore sources of biological and technical variance, and establish a reference range for adult men. It is constitutively secreted with little within-individual variation and reflects testicular capacity to produce testosterone. The main INSL3 assays available indicate good concordance with low technical variance; there is no effect of ethnicity. INSL3 declines with age from 35 years at about 15% per decade. Like low calculated free testosterone, and to a lesser extent low total testosterone, reduced INSL3 is significantly associated with increasing age-related morbidity, including lower overall sexual function, reflecting LCI. Consequently, low INSL3 (≤0.4 ng/ml; ca. <2 SD from the population mean) might serve as an additional biochemical marker in the assessment of functional hypogonadism (late-onset hypogonadism, LOH) where testosterone is in the borderline low range. Excluding individuals with low LCI (INSL3 ≤ 0.4 ng/ml) leads to an age-independent (> 35 years) reference range (serum) for INSL3 in the eugonadal population of 0.4 - 2.3 ng/ml, with low INSL3 prospectively identifying individuals at risk of increased future morbidity.

Serum INSL3 measured by LC-MS/MS in pubertal girls and in girls with precocious puberty during GnRH agonist treatment

Introduction

The peptide hormone Insulin-like Factor 3 (INSL3) is a biomarker of testicular Leydig cells in the male but is also expressed by the theca cells of the ovaries. With the advent of sensitive assays INSL3 can be quantified in female circulation, and we suggest that circulating INSL3 is a novel biomarker for pubertal development in girls. The aim of the study is to quantify INSL3 by LC-MS/MS in sera from normal girls during pubertal transition, and during gonadal suppression by GnRH agonist therapy in girls with central precocious puberty (CPP).

Method

The sensitivity of an established LC-MS/MS-based method for serum INSL3 was improved by switching to a state-of-the-art triple quadruple mass spectrometer (Altis Plus, Thermo).

Results

The limit of detection of the improved LC-MS/MS method for serum INSL3 was 0.01 ug/L (1.5 pM) and the inter-assay CV was < 12%. Serum INSL3 increased during the pubertal transition in healthy girls and changes correlated with the concomitant rise in other measured hormones. In some girls, but not all, INSL3, FSH, inhibin B and estradiol serum concentrations increased prior to first clinical signs of puberty. Serum INSL3 concentrations were increased at baseline in girls with CPP compared to prepubertal controls and decreased during treatment with GnRH agonist followed by a steep rise and normalization after cessation of treatment.

Conclusion

The improved method allowed for quantification of INSL3 in longitudinally collected serum samples during pubertal transition in healthy girls as well as in girls with CPP before, during and after treatment with GnRH agonist. Future studies are needed to clarify if INSL3 in combination with other biomarkers enhances the predictive value of differentiating between premature thelarche and CPP.

Inhibition of WNT/β-catenin signalling during sex-specific gonadal differentiation is essential for normal human fetal testis development

Sex-specific gonadal differentiation is directed by complex signalling promoting development in either male or female direction, while simultaneously inhibiting the opposite pathway. In mice, the WNT/β-catenin pathway promotes ovarian development and the importance of actively inhibiting this pathway to ensure normal testis development has been recognised. However, the implications of alterations in the tightly regulated WNT/β-catenin signalling during human fetal gonad development has not yet been examined in detail. Thus, the aim of this study was to examine the consequences of dysregulating the WNT/β-catenin signalling pathway in the supporting cell lineage during sex-specific human fetal gonad development using an established and extensively validated ex vivo culture model. Inhibition of WNT/β-catenin signalling in human fetal ovary cultures resulted in only minor effects, including reduced secretion of RSPO1 and reduced cell proliferation although this was not consistently found in all treatment groups. In contrast, promotion of WNT/β-catenin signalling in testes severely affected development and function. This included disrupted seminiferous cord structures, reduced cell proliferation, reduced expression of SOX9/AMH, reduced secretion of Inhibin B and AMH as well as loss of the germ cell population. Additionally, Leydig cell function was markedly impaired with reduced secretion of testosterone, androstenedione and INSL3. Together, this study suggests that dysregulated WNT/β-catenin signalling during human fetal gonad development severely impairs testicular development and function. Importantly, our study highlights the notion that sufficient inhibition of the opposite pathway during sex-specific gonadal differentiation is essential to ensure normal development and function also applies to human fetal gonads.

Insulin-like Factor 3, Basal and Human Chorionic Gonadotropin-Stimulated Testosterone as Biomarkers to Predict the Effect of Testosterone Replacement in Testicular Cancer Survivors With Mild Leydig Cell Insufficiency

INTRODUCTION

Mild Leydig cell insufficiency affects a substantial proportion of testicular cancer survivors. Previous studies have not shown a beneficial effect of testosterone replacement therapy, however, with a pronounced interindividual effect. Thus, biomarkers identifying the subgroups that might benefit are wanted. We aimed to determine if insulin-like factor 3 (INSL3), basal and human chorionic gonadotropin (hCG)-stimulated testosterone can predict the effect of testosterone replacement therapy in testicular cancer survivors with mild Leydig cell insufficiency.

PATIENTS AND METHODS

We randomized adult testicular cancer survivors with mild Leydig cell insufficiency 1:1 to 12 months of transdermal testosterone replacement therapy (Tostran gel 2%) or placebo. INSL3, basal, and hCG-stimulated testosterone were measured at baseline. Outcomes (glucose, insulin, HbA1C, lipids, blood pressure, and body composition) were measured at baseline, 6 and 12 months. We applied a linear mixed-effect model comparing patients receiving testosterone with placebo in subgroups by biomarker.

RESULTS

We included and randomized 69 patients between October 2016 and February 2018. Patients with INSL3 and hCG-stimulated testosterone concentrations below the median had a -1.7 kg (95% CI: -3.1, -0.4) and -2.0 kg (95% CI: -3.5, -0.6) change in fat mass after 12 months of testosterone replacement therapy compared with placebo. This was not the case in patients with INSL3 and hCG-stimulated testosterone above the median. We did not find any effect of these biomarkers on glucose, insulin, HbA1c, or lipids.

CONCLUSION

Patients with INSL3 and hCG-stimulated testosterone concentrations below the median had decreased fat mass after 12 months of testosterone replacement therapy compared with placebo. It should be evaluated in larger trials if these biomarkers can be used as predictive markers identifying testicular cancer patients with mild Leydig cell insufficiency who might benefit from testosterone substitution.

Serum Insulin-like Factor 3, Testosterone, and LH in Experimental and Therapeutic Testicular Suppression

BACKGROUND

Serum insulin-like factor 3 (INSL3) is a Leydig cell biomarker, but little is known about the circulating concentration of INSL3 during hypothalamus-pituitary-testicular suppression.

AIM

To study the concomitant changes in serum concentrations of INSL3, testosterone, and LH during experimental and therapeutic testicular suppression.

METHODS

We included serum samples from 3 different cohorts comprising subjects before and after testicular suppression: (1) 6 healthy young men who were treated with androgens (Sustanon, Aspen Pharma, Dublin, Ireland); 2) 10 transgender girls (male sex assigned at birth) who were treated with 3-monthly GnRH agonist injections (Leuprorelinacetat, Abacus Medicine, Copenhagen, Denmark); and (3) 55 patients with prostate cancer who were randomized to surgical castration (bilateral subcapsular orchiectomy) or treatment with GnRH agonist (Triptorelin, Ipsen Pharma, Kista, Sweden). Serum INSL3 and testosterone concentrations were quantified in stored serum samples using validated liquid chromatography-tandem mass spectrometry methodologies, and LH was measured by an ultrasensitive immunoassay.

RESULTS

The circulating concentrations of INSL3, testosterone, and LH decreased during experimental testicular suppression in healthy young men by Sustanon injections and subsequently returned to baseline levels after release of suppression. All 3 hormones decreased during therapeutic hormonal hypothalamus-pituitary-testicular suppression in transgender girls and in patients with prostate cancer.

CONCLUSION

INSL3 resembles testosterone as a sensitive marker of testicular suppression and reflects Leydig cell function, also during exposure to exogenous testosterone. Serum INSL3 measurements may complement testosterone as a Leydig cell marker in male reproductive disorders, during therapeutic testicular suppression as well as in surveillance of illicit use of androgens.

Development of an LC-MS/MS assay for quantification of intact INSL3 in rat plasma

Background: Relatively large disulfide-linked polypeptides can serve as signaling molecules for a diverse array of biological processes and may be studied in animal models to investigate their function in vivo. The aim of this work was to develop an LC-MS/MS assay to measure a model peptide, INSL3, in rat plasma. Results: A dual enrichment strategy incorporating both protein precipitation and solid phase extraction was utilized to isolate INSL3 from rat plasma, followed by targeted LC-MS/MS detection. The method was able to measure full-length INSL3 (6.1 kDa) down to 0.2 ng/ml with acceptable accuracy and precision. Conclusion: The final assay was applied to support an exploratory pharmacokinetic study to evaluate steady-state concentrations of dosed INSL3 in rat plasma.

Biomarkers of aging

Aging biomarkers are a combination of biological parameters to (i) assess age-related changes, (ii) track the physiological aging process, and (iii) predict the transition into a pathological status. Although a broad spectrum of aging biomarkers has been developed, their potential uses and limitations remain poorly characterized. An immediate goal of biomarkers is to help us answer the following three fundamental questions in aging research: How old are we? Why do we get old? And how can we age slower? This review aims to address this need. Here, we summarize our current knowledge of biomarkers developed for cellular, organ, and organismal levels of aging, comprising six pillars: physiological characteristics, medical imaging, histological features, cellular alterations, molecular changes, and secretory factors. To fulfill all these requisites, we propose that aging biomarkers should qualify for being specific, systemic, and clinically relevant.

Urinary concentration of phthalates and bisphenol A during minipuberty is associated with reproductive hormone concentrations in infant boys

BACKGROUND

The transient postnatal activation of the hypothalamic-pituitary-gonadal hormone axis is termed minipuberty and considered an important developmental period, which is highly sensitive to endocrine disruption. Here, we explore exposure-outcome associations during minipuberty between concentrations of potentially endocrine disrupting chemicals (EDCs) in urine of infant boys and their serum reproductive hormone concentrations.

METHODS

In total, 36 boys participating in the COPENHAGEN Minipuberty Study had data available for both urine biomarkers of target endocrine disrupting chemicals and reproductive hormones in serum from samples collected on the same day. Serum concentrations of reproductive hormones were measured by immunoassays or by LC-MS/MS. Urinary concentrations of metabolites of 39 non-persisting chemicals, including phthalates and phenolic compounds, were measured by LC-MS/MS. Nineteen chemicals had concentrations above the limit of detection in ≥50% of children and were included in data analysis. Associations of urinary phthalate metabolite and phenol concentrations (in tertiles) with hormone outcomes (age- and sex-specific SD-scores) were analysed by linear regression. Primarily, we focused on the EU regulated phthalates; butylbenzyl phthalate (BBzP), di-iso-butyl phthalate (DiBP), di-n-butyl phthalate (DnBP), and di-(2-ethylhexyl) phthalate (DEHP) as well as bisphenol A (BPA). Urinary metabolites of DiBP, DnBP and DEHP were summed and expressed as ∑DiBPm, ∑DnBPm and ∑DEHPm.

RESULTS

Compared to boys in the lowest ∑DnBPm tertile, urinary concentration of ∑DnBPm was associated with concurrent higher luteinizing hormone (LH) and anti-Müllerian hormone (AMH) SD-scores as well as lower testosterone/LH ratio in boys in the middle ∑DnBPm tertile (estimates (CI 95%) 0.79 (0.04; 1.54), 0.91 (0.13; 1.68), and -0.88 (-1.58;-0.19), respectively). Further, higher insulin-like peptide 3 (INSL3) SD-scores and lower DHEAS SD-score in boys in the highest ∑DnBPm tertile (0.91 (0.12; 1.70) and -0.85 (-1.51;-0.18), respectively) were observed. In addition, boys in the middle and highest ∑DEHPm tertile had higher LH (1.07 (0.35; 1.79) and 0.71 (-0.01; 1.43), respectively) and in the highest ∑DEHPm tertile also higher AMH (0.85 (0.10; 1.61)) concentration SD-scores, respectively. Boys in the highest BPA tertile had significantly higher AMH and lower DHEAS concentration compared to boys in the lowest BPA tertile (1.28 (0.54; 2.02) and -0.73 (-1.45; -0.01)), respectively.

DISCUSSION

Our findings indicate that exposure to chemicals with known or suspected endocrine disrupting potential, especially the EU-regulated DnBP, DEHP and BPA, may modify male reproductive hormone concentrations in infant boys suggesting that minipuberty is a critical window sensitive to endocrine disruption.

Pituitary-testis axis dysfunction following adjuvant androgen deprivation therapy

Men with high-risk, non-metastatic prostate cancer receive adjuvant androgen deprivation therapy (ADT) for at least 2 years according to Danish guidelines. It remains unclarified if patients regain the function of the pituitary-testis axis after cessation of ADT. Thus, we aimed to investigate the function of the pituitary-testis axis following adjuvant ADT. In this study, we included men who underwent external beam radiation therapy and ADT for high-risk prostate cancer. All patients underwent assessment of testosterone deficiency (TD) symptoms, full biochemical assessment of the pituitary-testis axis, and dynamic stimulatory tests of gonadotropin (gonadotropin-releasing hormone (GnRH) test) and testosterone production (human chorionic gonadotrophin (hCG) test). Patients were diagnosed with TD based on a combination of TD symptoms and testosterone below age-specific reference ranges. TD was characterized as primary, secondary, or mixed based on serum gonadotropins and stimulatory tests. We found that among the 51 patients included in the study, the median time on ADT was 3.2 years and median time since ADT cessation was 3.8 years. Twenty-eight patients were diagnosed with TD; 10 had primary TD (testicular dysfunction), 11 secondary TD (pituitary dysfunction), and 7 mixed TD (combined pituitary and testicular dysfunction). An inadequate testosterone response to hCG stimulation was shown in 42 patients, whereas only 11 patients had a subnormal gonadotropin response to GnRH. We conclude that persistent TD is a common long-term consequence of adjuvant ADT in prostate cancer survivors, equally distributed between pituitary and testicular dysfunction. The study emphasizes the necessity for systematic follow-up of full pituitary-testis axis function in patients receiving adjuvant ADT.

Reproductive Markers of Testicular Function and Size During Puberty in Boys With and Without a History of Cryptorchidism

CONTEXT

Longitudinal data on levels of hypothalamic-pituitary-gonadal axis hormones and insulin-like growth factor I (IGF-I) during puberty in boys with a history of cryptorchidism are largely missing.

OBJECTIVE

We aimed to compare pubertal hormone levels between boys with a history of congenital cryptorchidism who experienced spontaneous testicular descent or underwent orchiopexy and boys without a history of cryptorchidism.

METHODS

This was a nested case-control study within a population-based birth cohort, with a prospective, longitudinal pubertal follow-up every 6 months (2005 to 2019). Participants were 109 Finnish boys, including boys with a history of unilateral cryptorchidism who underwent orchiopexy (n = 15), unilateral cryptorchidism who had spontaneous testicular descent (n = 15), bilateral cryptorchidism who underwent orchiopexy (n = 9), bilateral cryptorchidism who had spontaneous testicular descent (n = 7), and controls (n = 63). Serum reproductive hormone levels and testicular volumes were measured.

RESULTS

From around onset of puberty, boys with bilateral cryptorchidism who underwent orchiopexy had significantly higher follicle-stimulating hormone (FSH) and lower inhibin B levels than controls. Boys with unilateral cryptorchidism who underwent orchiopexy had significantly higher FSH than controls, whereas inhibin B levels were similar. Testosterone, luteinizing hormone, insulin-like factor 3, and IGF-I were generally similar between groups. Testicular volume of boys with unilateral or bilateral cryptorchidism who underwent orchiopexy was smaller than that of the controls from 1 year after pubertal onset (P < 0.05).

CONCLUSION

Cryptorchid boys, particularly those with bilateral cryptorchidism who underwent orchiopexy, had altered levels of serum biomarkers of Sertoli cells and germ cells and smaller testicular volumes compared with controls.

Identification of a window of androgen sensitivity for somatic cell function in human fetal testis cultured ex vivo

BACKGROUND

Reduced androgen action during early fetal development has been suggested as the origin of reproductive disorders comprised within the testicular dysgenesis syndrome (TDS). This hypothesis has been supported by studies in rats demonstrating that normal male development and adult reproductive function depend on sufficient androgen exposure during a sensitive fetal period, called the masculinization programming window (MPW). The main aim of this study was therefore to examine the effects of manipulating androgen production during different timepoints during early human fetal testis development to identify the existence and timing of a possible window of androgen sensitivity resembling the MPW in rats.

METHODS

The effects of experimentally reduced androgen exposure during different periods of human fetal testis development and function were examined using an established and validated human ex vivo tissue culture model. The androgen production was reduced by treatment with ketoconazole and validated by treatment with flutamide which blocks the androgen receptor. Testicular hormone production ex vivo was measured by liquid chromatography-tandem mass spectrometry or ELISA assays, and selected protein markers were assessed by immunohistochemistry.

RESULTS

Ketoconazole reduced androgen production in testes from gestational weeks (GW) 7-21, which were subsequently divided into four age groups: GW 7-10, 10-12, 12-16 and 16-21. Additionally, reduced secretion of testicular hormones INSL3, AMH and Inhibin B was observed, but only in the age groups GW 7-10 and 10-12, while a decrease in the total density of germ cells and OCT4⁺ gonocytes was found in the GW 7-10 age group. Flutamide treatment in specimens aged GW 7-12 did not alter androgen production, but the secretion of INSL3, AMH and Inhibin B was reduced, and a reduced number of pre-spermatogonia was observed.

CONCLUSIONS

This study showed that reduced androgen action during early development affects the function and density of several cell types in the human fetal testis, with similar effects observed after ketoconazole and flutamide treatment. The effects were only observed within the GW 7-14 period-thereby indicating the presence of a window of androgen sensitivity in the human fetal testis.

Association of age, hormonal, and lifestyle factors with the Leydig cell biomarker INSL3 in aging men from the EMAS (European Male Aging Study) cohort

BACKGROUND

Aging in men is accompanied by a broad range of symptoms, including sexual dysfunction, cognitive and musculo-skeletal decline, obesity, type 2 diabetes, cardiovascular disease and hypertension, organ degeneration/failure, and increasing neoplasia, some of which are associated with declining levels of Leydig cell-produced testosterone. High natural biological variance, together with multiple factors that can modulate circulating testosterone concentration, may influence its interpretation and clinical implications. INSL3 is a biomarker of Leydig cell function that might provide complementary information on testicular health and its downstream outcomes.

OBJECTIVES

To characterize INSL3 as a biomarker to assess gonadal status in aging men.

METHODS & MATERIALS

The large European multi-centre EMAS cohort of community-dwelling men was analysed to determine how INSL3 relates to a range of hormonal, anthropometric, and lifestyle parameters.

RESULTS & DISCUSSION

INSL3 declines cross-sectionally and longitudinally within individuals at approximately 15% per decade from age 40, unlike testosterone (1.9% per decade), which is partly compensated by increasing pituitary LH production. Importantly, lower INSL3 in younger men appears to persist with aging. Multiple regression analysis shows that, unlike testosterone, INSL3 is negatively dependent on LH and SHBG, and positively on FSH, suggesting a different mechanism of gonadotropic regulation. Circulating INSL3 is negatively associated with increased BMI or waist circumference, and with smoking, and unlike testosterone is not affected by weight loss in the obese. Geographic variation in mean INSL3 within Europe appears to be largely explained by differences in these parameters. The results allow the establishment of a European-wide reference range for INSL3 (95% confidence interval) adjusted for increasing age.

CONCLUSION

INSL3 is a constitutive biomarker of Leydig cell functional capacity, being a robust, reliably measurable peptide, not subject to the gonadotropin-dependent short-term regulation and within-individual variation of testosterone. This article is protected by copyright. All rights reserved.

Dynamic Changes of Reproductive Hormones in Male Minipuberty: Temporal Dissociation of Leydig and Sertoli Cell Activity

CONTEXT

The male hypothalamic-pituitary-gonadal (HPG) axis is transiently active during the first months of life with surging serum concentrations of reproductive hormones. This period, termed minipuberty, appears to be essential for priming testicular function. Despite the central role for male reproductive function, longitudinal data on HPG axis activation in infancy is sparse.

OBJECTIVE

To explore the dynamics of HPG hormone activity in healthy male infants, to assess the association of HPG axis activity and testicular volume, and to establish reference curves for serum levels of reproductive hormones.

DESIGN

Prospective, longitudinal birth cohort (the COPENHAGEN Minipuberty Study, 2016-2018, 1-year follow-up).

SETTING

Population-based.

PATIENTS OR OTHER PARTICIPANTS

Healthy, male, term, singleton newborns were followed from birth on with repeated clinical examinations including blood sampling during a 1-year follow-up. A total of 128 boys contributed to this study, while 119 participated in the postnatal follow-up.

MAIN OUTCOME MEASURES

Serum reproductive hormone concentrations and testicular volume.

RESULTS

Reproductive hormone concentrations showed marked dynamics during the first 6 months of age. Gonadotropins, total testosterone, and insulin-like factor 3 peaked at around 1 month of age. Inhibin B, anti-Müllerian hormone, and testicular volume peaked at around 4 to 5 months. Correlations largely recapitulated typical HPG axis pathways but also differed significantly from adult men.

CONCLUSIONS

We demonstrate a temporal dissociation of Leydig and Sertoli cell activity during male minipuberty and provide reference curves for reproductive hormones.

Insulin‐like peptide 3 in domestic animals with normal and abnormal reproductive functions, in comparison to rodents and humans

Background

Methods

Main findings

Conclusion

Prenatal exposure to phenols and benzophenones in relation to markers of male reproductive function in adulthood

INTRODUCTION

Environmental exposure during fetal life may disrupt testicular development. In humans, a limited number of studies have investigated whether these adverse effects persist into adulthood. Using data from a prospective, population-based birth cohort study, The Copenhagen Mother-Child cohort, the objective was to assess if there is an association between fetal exposure to selected phenols and benzophenones and markers of testicular function in adult men.

METHODS

Pregnant women were recruited in 1997-2001. Their sons were examined clinically at 18-20 years of age, with focus on adult markers of reproductive function (anogenital distance (AGD), semen quality and reproductive hormones). In total, 101 18-20-year-old men were included, whose mothers during pregnancy had a serum sample drawn and analyzed for bisphenol A (BPA) and seven other simple phenols, as well as six benzophenones. To investigate the association between chemical levels (in tertiles, T1-T3) in relation to markers of reproductive function, univariate and multiple linear regression analyses were performed.

RESULTS

In fully adjusted analyses, increased levels of luteinizing hormone (LH) were observed with higher fetal exposure to BPA (percentage difference (95%CI)) (T2: 12% (-8%,36%) and T3: 33% (10%,62%), compared to T1) and benzophenone-3 (BP-3) (T2: 21% (-2%,49%), T3: 18% (-4%,45%)), while no clear association was seen to total testosterone (TT). Higher levels of BPA and BP-3 were associated with a lower TT/LH ratio, although only significant for BPA (p-trend=0.01). No associations were seen to AGD or markers of semen quality.

CONCLUSION

In conclusion, high exposure to BPA and BP-3 was associated with a compensated reduced Leydig cell function but no other changes in markers of reproductive health. As maternal levels of BPA and BP-3 were not correlated, separate effects may be at play. Larger studies on long-term reproductive consequences of prenatal exposures are warranted to validate our findings.

The Leydig cell biomarker INSL3 as a predictor of age-related morbidity: Findings from the EMAS cohort

BACKGROUND

Insulin-like peptide 3 (INSL3) is a constitutive hormone secreted in men by the mature Leydig cells of the testes. It is an accurate biomarker for Leydig cell functional capacity, reflecting their total cell number and differentiation status.

OBJECTIVES

To determine the ability of INSL3 to predict hypogonadism and age-related morbidity using the EMAS cohort of older community-dwelling men.

MATERIALS & METHODS

Circulating INSL3 was assessed in the EMAS cohort and its cross-sectional and longitudinal relationships to hypogonadism, here defined by testosterone (T) <10.5nmol/l, and a range of age-related morbidities determined by correlation and regression analysis.

RESULTS & DISCUSSION

While INSL3 is an accurate measure of primary hypogonadism, secondary and compensated hypogonadism also indicate reduced levels of INSL3, implying that testicular hypogonadism does not improve even when LH levels are increased, and that ageing-related hypogonadism may combine both primary and secondary features. Unadjusted, serum INSL3, like calculated free testosterone (cFT), LH, or the T/LH ratio reflects hypogonadal status and is associated with reduced sexual function, bone mineral density, and physical activity, as well as increased occurrence of hypertension, cardiovascular disease, cancer, and diabetes. Using multiple regression analysis to adjust for a range of hormonal, anthropometric, and lifestyle factors, this relationship is lost for all morbidities, except for reduced bone mineral density, implying that INSL3 and/or its specific receptor, RXFP2, may be causally involved in promoting healthy bone metabolism. Elevated INSL3 also associates with hypertension and cardiovascular disease. When unadjusted, INSL3 in phase 1 of the EMAS study was assessed for its association with morbidity in phase 2 (mean 4.3 years later); INSL3 significantly predicts 7 out of 9 morbidity categories, behaving as well as cFT in this regard. In contrast, total T was predictive in only 3 of the 9 categories.

CONCLUSION

Together with its low within-individual variance, these findings suggest that assessing INSL3 in men could offer important insight into the later development of disease in the elderly.

Serum Insulin-like Factor 3 Levels Are Reduced in Former Androgen Users, Suggesting Impaired Leydig Cell Capacity

CONTEXT

Illicit use of anabolic androgenic steroids (AAS) is frequently observed in men and is associated with subsequent testosterone deficiency although the long-term effect on gonadal function is still unclear. Serum insulin-like factor 3 (INSL3) has been suggested to be a superior biomarker of Leydig cell secretory capacity compared to testosterone.

OBJECTIVE

This study aimed to investigate serum INSL3 concentrations in AAS users.

METHODS

This community-based, cross-sectional study included men aged 18 to 50 years, involved in recreational strength training and allocated to 1 of 3 groups: never-AAS users as controls (n = 44), current (n = 46), or former AAS users (n = 42) with an average duration since AAS cessation of 32 (23 ; 45) months.

RESULTS

Serum INSL3 was lower in current AAS users and former AAS users than in controls, median (interquartile range), 0.04 µg/L (nondetectable [ND]-0.07 µg/L) and 0.39 µg/L (0.24-0.62 µg/L) vs 0.59 µg/L (0.45-0.72 µg/L), P less than .001. Former AAS users exhibited lower serum INSL3 levels than controls in a multivariable linear regression even after adjusting for serum total testosterone (TT) and other relevant confounders, (B) (95% CI), -0.16 µg/L (95% CI, -0.29 to -0.04 µg/L), P equal to .011. INSL3 and TT were not associated in the model, P equal to .821. Longer accumulated AAS duration (log2) was associated with lower serum INSL3 in former AAS users, (B) (95% CI), -0.08 (95% CI, -0.14 to -0.01), P equal to .022. Serum INSL3, but not inhibin B or testosterone, was associated with testicular size in a multivariate linear regression, (B) (95% CI); 4.7 (95% CI, 0.5 to 8.9), P equal to .030.

CONCLUSION

Serum INSL3 is reduced years following AAS cessation in men, independently of testosterone, suggesting persistently impaired Leydig cell capacity.

Male seminal parameters are not associated with Leydig cell functional capacity in men

BACKGROUND

Insulin-like peptide 3 (INSL3) is a constitutive, secreted peptide produced in the male uniquely by the Leydig cells of the testes. It is a biomarker for Leydig cell functional capacity, which is a measure of the numbers and differentiation status of these steroidogenic cells and lacks the biological and technical variance of the steroid testosterone. This retrospective study was carried out to examine the relationship between seminal parameters and the Leydig cell compartment, and secondarily to assess other factors responsible for determining Leydig cell functional capacity.

METHODS

INSL3 was assessed together with seminal, anthropometric, and hormonal parameters in a Swedish cohort of 18-year-old men, representing the average population, and in a smaller, more heterogeneous cohort of men visiting an Australian infertility clinic.

RESULTS AND DISCUSSION

Average INSL3 concentration at 18 years is greater than that reported at younger or older ages and indicated a large 10-fold variation. In neither cohort was there a relationship between INSL3 concentration and any semen parameter. For the larger, more uniform Swedish cohort of young men, there was a significant negative relationship between INSL3 and BMI, supporting the idea that adult Leydig cell functional capacity may be established during puberty. In both cohorts, there was a significant relationship between INSL3 and FSH, but not LH concentration. No relationship was found between INSL3 and androgen receptor trinucleotide repeat polymorphisms, reinforcing the notion that Leydig cell functional capacity is unlikely to be determined by androgen influence alone. Nor did INSL3 correlate with the T/LH ratio, an alternative measure of Leydig cell functional capacity, supporting the view that these are independent measures of Leydig cell function.

Serum insulin-like factor 3 quantification by LC-MS/MS in male patients with hypogonadotropic hypogonadism and Klinefelter syndrome

PURPOSE

Insulin-like factor 3 (INSL3) is an emerging testicular marker, yet larger studies elucidating the clinical role of INSL3 in patients with hypogonadism are lacking. The aim was to describe serum INSL3 concentrations analyzed by LC-MS/MS methodology in males with hypogonadotropic hypogonadism (HH) and Klinefelter syndrome (KS).

METHODS

This was a combined study from two tertiary centers in Denmark and France analyzing INSL3 concentrations by LC-MS/MS. In total, 103 patients with HH and 82 patients with KS were grouped into treated (HH: n = 96; KS: n = 71) or untreated (HH: n = 7; KS: n = 11). Treatment modalities included testosterone and hCG. Serum concentrations and standard deviation (SD) scores of INSL3, total testosterone, and LH according to age and treatment were evaluated.

RESULTS

In both HH and KS, INSL3 concentrations were low. In HH, INSL3 was low regardless of treatment, except for some hCG-treated patients with normal concentrations. In untreated HH, testosterone was low, while normal to high in most testosterone- and hCG-treated patients. In untreated KS, INSL3 and testosterone concentrations were low to normal, while in testosterone-treated KS, serum INSL3 was low in most patients. INSL3 SD scores were significantly lower in untreated HH than in untreated KS (p = 0.01).

CONCLUSIONS

The dichotomy between lower INSL3 and higher testosterone concentrations, particularly observed in hCG-treated patients with HH, confirms that INSL3 is a different marker of Leydig cell function than testosterone. However, the clinical application of INSL3 in males with hypogonadism remains unclear.

Serum Concentrations and Gonadal Expression of INSL3 in Eighteen Males With 45,X/46,XY Mosaicism

OBJECTIVE

Insulin-like factor 3 (INSL3) is produced in the testes and has been proposed as a circulating biomarker of Leydig cell capacity, but remains undescribed in 45,X/46,XY mosaicism. The aim was to examine serum concentrations and gonadal expression of INSL3 in 45,X/46,XY mosaicism.

METHODS

Retrospectively collected data from medical records, gonadal tissue samples, and prospectively analyzed serum samples from eighteen male patients with 45,X/46,XY mosaicism (one prepubertal, four testosterone-treated, 13 untreated) were included. Biochemical, clinical, and histological outcomes were evaluated according to serum INSL3 concentrations, quantified by LC-MS/MS methodology, and gonadal INSL3 immunohistochemical expression.

RESULTS

Serum INSL3 concentrations spanned from below to above the reference range. In untreated patients, the median serum INSL3 SD score was -0.80 (IQR: -1.65 to 0.55) and no significant difference was observed between INSL3 and testosterone. There was no clear association between serum INSL3 and External Genitalia Score at diagnosis, spontaneous puberty, or sperm concentration. INSL3 and CYP11A1 expression overlapped, except for less pronounced INSL3 expression in areas with severe Leydig cell hyperplasia. No other apparent links between INSL3 expression and histological outcomes were observed.

CONCLUSIONS

In this pilot study, serum INSL3 concentrations ranged and seemed independent of other reproductive hormones and clinical features in males with 45,X/46,XY mosaicism. Discordant expression of INSL3 and CYP11A1 may explain low INSL3 and normal testosterone concentrations in some patients. Further studies are needed to elucidate the divergence between serum INSL3 and testosterone and the potential clinical use of INSL3.

Physiology and evolution of the INSL3/RXFP2 hormone/receptor system in higher vertebrates

Although the insulin-like peptide hormone INSL3 and its cognate receptor RXFP2 (relaxin-family peptide receptor 2) have existed throughout chordate evolution, their physiological diversification appears to be linked closely with mammalian emergence and radiation. In contrast, they have been lost in birds and reptiles. Both hormone and receptor are expressed from autosomal genes which have maintained their synteny across vertebrate evolution. Whereas the INSL3 gene comprises only two exons closely linked to the JAK3 gene, RXFP2 is normally encoded by 18 exons. Both genes, however, are subject to alternative splicing to yield a variety of possibly inactive or antagonistic molecules. In mammals, the INSL3-RXFP2 dyad has maintained a probably primitive association with gametogenesis, seen also in fish, whereby INSL3 promotes the survival, growth and differentiation of male germ cells in the testis and follicle development in the ovary. In addition, however, the INSL3/RXFP2 system has adopted a typical 'neohormone' profile, essential for the promotion of internal fertilisation and viviparity; fetal INSL3 is essential for the first phase of testicular descent into a scrotum, and also appears to be associated with male phenotype, in particular horn and skeletal growth. Circulating INSL3 is produced exclusively by the mature testicular Leydig cells in male mammals and acts as a potent biomarker for testis development during fetal and pubertal development as well as in ageing. As such it can be used also to monitor seasonally breeding animals as well as to investigate environmental or lifestyle conditions affecting development. Nevertheless, most information about INSL3 and RXFP2 comes from a very limited selection of species; it will be especially useful to gain further information from a more diverse range of animals, especially those whose evolution has led them to express unusual reproductive phenotypes.

Longitudinal Increases in Serum Insulin-like Factor 3 and Testosterone Determined by LC-MS/MS in Pubertal Danish Boys

BACKGROUND

Serum concentrations of the peptide hormone insulin-like factor 3 (INSL3) is a candidate marker for improved distinction between constitutional delay of growth and puberty (CDGP) and permanent hypogonadotropic hypogonadism (HH) in boys.

AIM

To assess the possible diagnostic role of LC-MS/MS-based INSL3 measurements as a marker of imminent puberty by comparison with testosterone (T) and luteinizing hormone (LH) levels in serum longitudinally collected from 18 healthy boys throughout puberty.

RESULTS

The first increase in serum LH was detected on average 4 months earlier, as compared with the first observed increases in INSL3 and T. When comparing the 2 testicular hormones only, we found that in 22% (4 of 18) of the boys the first increase in serum INSL3 was observed prior to the first observed increase in T, whereas in 44% (8 of 18) the first increase in T was observed before the first observed increase in INSL3. In the remaining 6 boys, the 2 testicular hormones showed the first increase at the same examination.

CONCLUSION

In some boys with delayed puberty, the first indication of testicular maturation may be detectable by observing serum INSL3. Further studies of LC-MS/MS determination of serum INSL3 in patients with CDGP and HH are warranted.

Evaluation of Serum Insulin-like Factor 3 Quantification by LC-MS/MS as a Biomarker of Leydig Cell Function

BACKGROUND

The peptide hormone insulin-like factor 3 (INSL3) is a marker for Leydig cell function and the clinical use of serum INSL3 measurements has been suggested by several groups.

AIM

(1) To establish a reference range for liquid chromatography-tandem mass spectrometry (LC-MS/MS) of serum INSL3 in healthy boys and men; and (2) to compare the associations of serum INSL3 and testosterone (T) to pubertal stage, lifestyle factors, diurnal variation, body composition, and human chorionic gonadotropin (hCG) stimulation.

RESULTS

In a reference range based on LC-MS/MS analysis of serum from 1073 boys and men, INSL3 increased from levels close to the detection limit (0.03 µg/L) in prepubertal boys to a maximum mean level of 1.3 µg/L (95% CI, 0.9-2.7) in young men (19-40 years of age) and decreased slightly in older men (0.1 µg/L per decade). Serum T, but not INSL3, was associated with body mass index or body fat percentage and with alcohol consumption. Smoking was positively associated with serum T, but negatively associated with INSL3. There were significant diurnal variations in both INSL3 and T in men (P < 0.001), but serum INSL3 varied substantially less, compared with serum T (± 11% vs ± 26%). Mean serum INSL3 increased after hCG stimulation, but less than T (+ 17% vs + 53%). In both healthy men and in patients suspected of testicular failure, baseline serum INSL3 was more closely associated to the hCG-induced increase in serum T than baseline T itself.

CONCLUSION

Measurement of serum INSL3 by LC-MS/MS has promise as a marker of testicular disorders.

Updated MS²PIP web server delivers fast and accurate MS² peak intensity prediction for multiple fragmentation methods, instruments and labeling techniques

MS²PIP is a data-driven tool that accurately predicts peak intensities for a given peptide's fragmentation mass spectrum. Since the release of the MS²PIP web server in 2015, we have brought significant updates to both the tool and the web server. In addition to the original models for CID and HCD fragmentation, we have added specialized models for the TripleTOF 5600+ mass spectrometer, for TMT-labeled peptides, for iTRAQ-labeled peptides, and for iTRAQ-labeled phosphopeptides. Because the fragmentation pattern is heavily altered in each of these cases, these additional models greatly improve the prediction accuracy for their corresponding data types. We have also substantially reduced the computational resources required to run MS²PIP, and have completely rebuilt the web server, which now allows predictions of up to 100 000 peptide sequences in a single request. The MS²PIP web server is freely available at https://iomics.ugent.be/ms2pip/.

Mass Spectrometry Supports That the Structure of Circulating Human Insulin-Like Factor 3 Is a Heterodimer

The structure of the testicular peptide hormone insulin-like factor 3 (INSL3) has been the subject of discussion for more than a decade. Some studies support that the central C-domain of INSL3 is proteolytically removed and that INSL3 is secreted by the testicular Leydig cells into circulation as a small heterodimer consisting of an A- and a B-chain linked by two disulfide bridges. Other studies support that the INSL3 peptide remains uncleaved and that the predominant structure of circulating INSL3 is the larger pro-form. Furthermore, the structure of INSL3 could differ between species, and both structural forms of INSL3 could, in principle, be present in circulation. Recently, we have developed a mass spectrometry (MS)-based method for INSL3 in human serum that provides new information about the structure of circulating INSL3. Based on recent and newly presented data, we argue that in healthy men, the common, and probably the only, form of circulating INSL3 is the smaller AB heterodimer. For the first time, we demonstrate that the same analytical principle, with slight modifications, can also be applied to sera from other species, and we show that the INSL3 AB heterodimer is also present in serum from rodents. Improved understanding of the structure and biochemistry of circulating INSL3 could be valuable for the interpretation of INSL3 as a marker for reproductive and developmental disorders in humans and domesticated animals.

Insulin-Like Peptide 3 (INSL3) Serum Concentration During Human Male Fetal Life

Context: Insulin-like peptide 3 (INSL3), a protein hormone produced by Leydig cells, may play a crucial role in testicular descent as male INSL3 knockout mice have bilateral cryptorchidism. Previous studies have measured human fetal INSL3 levels in amniotic fluid only. Objective: To measure INSL3 serum levels and mRNA in fetal umbilical cord blood and fetal testes, respectively. Design: INSL3 concentrations were assayed on 50 μl of serum from male human fetal umbilical cord blood by a non-commercial highly sensitive and specific radioimmunoassay. For secondary confirmation, quantitative real-time PCR was used to measure INSL3 relative mRNA expression in 7 age-matched human fetal testes. Setting: UT Southwestern Medical Center, Dallas, TX and Medical University of South Carolina, Charleston, SC. Patients or other Participants: Twelve human male umbilical cord blood samples and 7 human male testes were obtained from fetuses 14-21 weeks gestation. Male sex was verified by leukocyte genomic DNA SRY PCR. Interventions: None. Main Outcome Measures: Human male fetal INSL3 cord blood serum concentrations and testicular relative mRNA expression. Results: INSL3 serum concentrations during human male gestational weeks 15-20 were 2-4 times higher than published prepubertal male levels and were 5-100 times higher than previous reports of INSL3 concentrations obtained from amniotic fluid. Testicular fetal INSL3 mRNA relative expression was low from weeks 14-16, rose significantly weeks 17 and 18, and returned to low levels at week 21. Conclusions: These findings further support the role of INSL3 in human testicular descent and could prove relevant in uncovering the pathophysiology of cryptorchidism.