antioxidants Review Improvement of Testicular Steroidogenesis Using Flavonoids and Isoflavonoids for Prevention of Late-Onset Male Hypogonadism Luc J. Martin 1,* and Mohamed Touaibia 2 1 Biology Department, Université de Moncton, Moncton, NB E1A 3E9, Canada 2 Chemistry and Biochemistry Department, Université de Moncton, Moncton, NB E1A 3E9, Canada; [email protected] * Correspondence: [email protected]; Tel.: +1-506-858-4938 Received: 19 February 2020; Accepted: 10 March 2020; Published: 13 March 2020 Abstract: Androgen production, being important for male fertility, is mainly accomplished by the Leydig cells from the interstitial compartment of the testis. Testosterone plays a critical role in testis development, normal masculinization, and the maintenance of spermatogenesis. Within seminiferous tubules, appropriate Sertoli cell function is highly dependent on testicular androgen levels and is essential to initiate and maintain spermatogenesis. During aging, testosterone production by the testicular Leydig cells declines from the 30s in humans at a rate of 1% per year. This review outlines the recent findings regarding the use of flavonoids and isoflavonoids to improve testosterone production, contributing to normal spermatogenesis and preventing age-related degenerative diseases associated with testosterone deficiency. With the cumulation of information on the actions of different flavonoids and isoflavonoids on steroidogenesis in Leydig cells, we can now draw conclusions regarding the structure-activity relationship on androgen production. Indeed, flavonoids having a 5,7-dihydroxychromen-4-one backbone tend to increase the expression of the steroidogenic acute regulatory protein (StAR), being critical for the entry of cholesterol into the mitochondria, leading to increased testosterone production from testis Leydig cells. Therefore, flavonoids and isoflavonoids such as chrysin, apigenin, luteolin, quercetin, and daidzein may be effective in delaying the initiation of late-onset hypogonadism associated with aging in males. Keywords: testis; testosterone; androgen; Leydig cells; polyphenols; flavonoids 1. Introduction Testicular Leydig cells are the main site of androgen synthesis in mammals. Androgens are responsible for the development and maintenance of male sex organs and secondary sex characteristics. Specifically, embryonic testosterone is involved in the development of Wolffian ducts, leading to the formation of the epididymis, vas deferens, and seminal vesicles. In addition, testosterone is indirectly involved in the development of the genital tubercle to form the penis, scrotum, and prostate by its conversion to 5 α-dihydrotestosterone (DHT), the most potent androgen of the body. At puberty, testosterone plays a major role in masculinization of the brain, development of male sexual behavior, initiation and support of spermatogenesis, bone growth, maturation of the external genitalia, hair growth, and regulation of gonadotropins (luteinizing hormone (LH) and follicle-stimulating hormone (FSH)) secretion. Within the seminiferous tubules, Sertoli cells are stimulated by androgen through the expression of the androgen-binding protein (ABP, SHBG). In turn, activated Sertoli cells support germ cell division and normal spermatogenesis to maintain fertility. With aging, testosterone production declines, beginning in the 30s in men, at a rate of 1% per year [1]. Different conditions, such as exposure Antioxidants 2020, 9, 237; doi:10.3390/antiox9030237 www.mdpi.com/journal/antioxidants Antioxidants 2020, 9, 237 2 of 17 Antioxidants 2020, 9, 237 2 of 17 to endocrineyear [1]. Different disruptors, conditions, obesity, such diabetes as exposure and cancer to endocrine treatments, disruptors, exacerbate obesity, this diabetes decrease and in cancer serum testosteronetreatments, levels, exacerbate leading this to decrease the development in serum testosterone of late-onset levels, hypogonadism leading to [the2]. Age-relateddevelopment functional of late‐ alterationsonset hypogonadism include decreased [2]. Age response‐related functional of testicular alterations Leydig include cells to decreased the pituitary response LH [3 ].of Intesticular addition, middle-agedLeydig cells men to the have pituitary an increased LH [3]. prevalence In addition, of middle obesity‐aged and men insulin have resistance an increased associated prevalence with of low obesity and insulin resistance associated with low serum total testosterone and SHBG levels [4]. These serum total testosterone and SHBG levels [4]. These men are mainly responsible for the increase in men are mainly responsible for the increase in testosterone‐replacement therapies over the last testosterone-replacement therapies over the last decade [5]. Men with androgen deficiency may present decade [5]. Men with androgen deficiency may present with decreased libido, erectile dysfunction, with decreased libido, erectile dysfunction, decreased body hair, increased fatigue, decreased muscle decreased body hair, increased fatigue, decreased muscle mass and strength, increased body fat, low mass and strength, increased body fat, low bone mineral density, irritability, inability to concentrate, bone mineral density, irritability, inability to concentrate, and poor quality of life [6,7]. Therefore, andtherapies poor quality aiming of to life increase [6,7]. Therefore, serum levels therapies of testosterone aiming to may increase contribute serum to levels relieving of testosterone part of these may contributesymptoms to and relieving may involve part of nutritional these symptoms supplementation and may with involve flavonoids. nutritional In this supplementation review, a structure with‐ flavonoids.activity relationship In this review, for aplant structure-activity polyphenols with relationship flavonoid for backbone plant polyphenols on testosterone with flavonoid production backbone will onbe testosterone investigated production to draw conclusions will be investigated from cumulated to draw literature conclusions thus from far. cumulated literature thus far. 2. Steroidogenesis2. Steroidogenesis WithinWithin the the testis, testis, androgen androgen production, production, of which of which testosterone testosterone is the is mostthe most abundant, abundant, occurs occurs almost exclusivelyalmost exclusively in Leydig in cells. Leydig Androstenedione cells. Androstenedione and dehydroepiandrosterone and dehydroepiandrosterone (DHEA) (DHEA) are also are produced also byproduced the testis. by However, the testis. theirHowever, levels their of androgenicitylevels of androgenicity are lower are than lower those than ofthose testosterone. of testosterone. Adult LeydigAdult cells Leydig do express cells do the express aromatase the aromatase enzyme (CYP19A1) enzyme (CYP19A1) which is responsible which is responsible for the conversion for the of androgenconversion into of estrogen androgen [8]. into However, estrogen this [8]. rate However, of conversion this rate of testosterone of conversion into of estradiol testosterone is minimal into comparedestradiol to is the minimal secretion compared levels ofto androgen,the secretion and levels estrogen of androgen, may instead and estrogen play a bu mayffering instead action play on a the steroidogenesisbuffering action by on Leydig the steroidogenesis cells [9]. by Leydig cells [9]. TestosteroneTestosterone synthesis synthesis from from cholesterol cholesterol involves involves the sequential the sequential action action of several of several steroidogenic steroidogenic enzymes suchenzymes as the cholesterol such as the side-chain cholesterol cleavage side‐ enzymechain cleavage (CYP11A1), enzyme cytochrome (CYP11A1), P450 17cytochromeα-hydroxylase P450/20-lyase 17α‐ (CYP17A1),hydroxylase/20 3β-hydroxysteroid‐lyase (CYP17A1), dehydrogenase 3β‐hydroxysteroid (HSD3B1 dehydrogenase in rodents and (HSD3B1 HSD3B2 in in rodents humans), and and 17βHSD3B2-hydroxysteroid in humans), dehydrogenase and 17β‐hydroxysteroid type 3 (HSD17B3), dehydrogenase catalyzing type a 3 cascade (HSD17B3), of hydroxylation, catalyzing a cascadecleavage, dehydrogenation,of hydroxylation, and cleavage, isomerization dehydrogenation, reactions. and Testosterone isomerization synthesis reactions. from Testosterone cholesterol synthesis and its from cholesterol and its conversion to other estrogenic and androgenic active metabolites is presented conversion to other estrogenic and androgenic active metabolites is presented in Figure1. in Figure 1. Figure 1. Common pathways for steroid synthesis within Leydig cells. Abbreviations for enzymes: Cyp11a1,Figure 1. P450 Common side-chain pathways cleavage; for steroid Hsd3b1, synthesis 3β-hydroxysteroid within Leydig cells. dehydrogenase; Abbreviations Cyp17a1,for enzymes: P450 17αCyp11a1,-hydroxylase P450/ 20-lyase;side‐chain Hsd17b3, cleavage; 17 Hsd3b1,β-hydroxysteroid 3β‐hydroxysteroid dehydrogenase; dehydrogenase; Srd5a1, 5 αCyp17a1,-reductase; P450 Cyp19a1, 17α‐ P450 aromatase. Abbreviations for steroids: DHEA, dehydroepiandrosterone; DHT, dihydrotestosterone. Antioxidants 2020, 9, 237 3 of 17 In Leydig cells, as in other steroidogenic tissues, cholesterol is the primary substrate for steroid synthesis. Cholesterol may be synthesized from acetyl-coenzyme A or derived from plasma reserves through the receptor-mediated endocytosis of low-density lipoprotein (LDL) particles. In each case, cholesterol is stored in esterified form in cytoplasmic lipid droplets. The contribution of the two sources of cholesterol for steroidogenesis