1. INTRODUCTION:

Lead (pb) is most abundant heavy metal, has been used for centuries. Human exposure to Lead through the environment is increased several folds due to its wide range of applications in industries, cosmetics, and medicine¹. The lead-induced toxicity through oxidative stress mechanisms includes the effect of lead on the membrane, DNA, and antioxidant defense systems of cells. Based on the doses of lead exposure, it affects various target sites including lung, blood vessels, testes, sperm, liver, and brain ². Several studies indicate that production of reactive oxygen species (ROS) was considered as a possible mechanism for the adverse effect of lead. Accumulation of lead in the cells severely affects the mitochondria and alters its normal function by inducing oxidative stress. This is a predominant reason for the reduction of cellular anti-defense mechanism. However, the complete mechanism remains complex ^3–5. Lead exposure can cause the adverse effects on health include developmental neurotoxicity, reproductive dysfunction, and toxicity to the blood, kidneys, and endocrine systems ⁶.It was also reported in various studies that lead could cause a deleterious effect on the entire reproductive function.

The reproductive and growth effects of lead are still unclear⁷. To the best of our knowledge, no research work has discussed the probable mechanism of action about the lead in the male reproductive system recently. Therefore, this review of literature primarily focuses on the mechanism of action of lead on the male reproductive system and to inference the adverse effects.

MATERIALS AND METHODS:

This systemic review of the literature was limited to articles written in English language and published until August 01, 2017. A literature search was performed in MEDLINE, bibliographies of published research and review articles. The terms used for data search are lead toxicity, heavy metals, male reproductive system, testicular toxicity, and sperm parameters. These terms combined with Testis, sperm motility, viability, antioxidants, Lipid peroxidation, Superoxide Dismutase (SOD), Catalase, DNA damage, hormonal, Androgen, GnRH, Testosterone, LH, FSH, Spermatogenesis, Steroidogenesis. In the sections describing the possible disrupting mechanism of action is based on the studies carried out on human, laboratory animals (in vivo) or cell cultures (in vitro). The data with co-administration of the compounds others than lead were excluded from consideration.

Lead toxicity on Semen Parameter:

In order to get a complete overview of the possible mechanism of lead-induced toxicity on the reproductive system, all important constraints like sperm parameters, morphology, and disruption in hormones, gene expressions, spermatogenesis, and steroidogenesis were analyzed from previous research findings. The normal sperm parameters were directly affected by lead exposure. It causes alteration at the reproductive axis, sperm motility and viability, acrosome reaction, chemotaxis⁸ semen quality ⁹and structural abnormality in lead treated animals¹⁰. Lead chloride exposure significantly repressed the motility and increased the tail anomalies and immotile sperm count¹¹, the reduction of sperm motility was also found in lead chloride exposure in the human study¹². Lead exposed rats were shown the significantly lesser amount of testicular daily sperm production, epididymal sperm count, viability, motility, and increased number of tail coiled sperm^13,14. Lead exposed factory workers were affected by the higher incidence of sperm abnormalities, reduced sperm count, viability, density, and semen volume. Likewise, the concentration of lead in blood and semen was significantly higher¹⁵. The study by Eibensteiner et al. 2005 reported that there is a strong correlation between the higher lead level in blood and deterioration in several semen parameters. Semen lead concentration is another important parameter that inversely related to sperm count^16,17. The sperm chromatin structure assay performed on semen samples of 503 men confirms the higher risk for semen concentration on the blood lead concentration above 50 mg/dl¹⁸. The study carried out on paint factory workers showed the deterioration of sperm count, spermatozoa structural abnormality in correlation with high blood semen lead level without interference on FSH, LH and testosterone hormones¹⁹. Probably the pathways affect the sperm function is due to reactive oxygen species (ROS) generation²⁰. A downgrade in the semen appearance and semen quality were also observed due to lead exposure.²¹

Lead Toxicity on Body Weight:

Lead exposure can induce a significant reduction in body weight and relative testis weight²². The weights of the testes and epididymis decreased by about 13%, and ventral prostate weights and seminal vesicle, by about 29%²³. Contrarily, few studies were shown that there is no alteration in testis weight but decreased the epididymis and seminal vesicle weights²⁴.

Lead Toxicity on the Antioxidant level:

Heavy metal toxicity mainly alters the antioxidant enzyme activity in respective organs. The testicular level of Glutathione peroxidase (GPx), Catalase (CAT), and Superoxide dismutase (SOD) declined significantly and the level of malondialdehyde (MDA) in plasma and tissue were elevated in lead treated animals^25,26. In testis delta-aminolevulinic acid dehydratase (delta-ALAD), the maker enzyme level also dropped after lead exposure²⁷. A significant rise in lipid peroxidation levels and a decline in CAT and SOD activity levels were observed in the testes of rats subjected to lead indicating increased oxidative stress.²⁸

Lead Toxicity on Gene expression level:

Lead exposure to laboratory animals elevated the level of interleukin-6, interleukin -10, and the nuclear factor kappa-light-chain-enhancer of activated B cells(NF-kB) expression levels²⁹ and significantly suppressed genes expression of 3 beta-hydroxysteroiddehydrogenase (3β-HSD), 17 beta-hydroxysteroiddehydrogenase (17β-HSD), proliferating -cell nuclear antigen (PCNA) genes and caspase 3 expressions^13,30. Subsequently, Peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) gene affects the intracellular ROS level and decrease the ATP in mouse testis sertoli cells³¹. Also, the Lead treated animals shown elevated level of TGFbeta, increased DNA damage³², Down regulations of Ddx3y gene expression in mice testis and inhibited spermatogenesis development.^33,34

Lead Toxicity on Reproductive Hormones:

Androgens level in serum serves as an important indicator for reproductive hormone. The male rats treated with lead causes variation in androgen level with a significant increase in serum follicle stimulating hormone (FSH) and testosterone, however no significant change in the level of luteinizing hormone (LH) was observed²¹. A study aimed at elucidating the mechanism of lead toxicity on reproductive hormone in rats showed a decreased in plasma LH and FSH concentration³⁵. Whereas, in another study by Riaz et al. 2011³⁶in lead treated rats, the level of testosterone in serum was reduced without any significant change in LH levels. This was supported by another study by R.Z. Sokol et al, 1985, which also shows no significant change in serum LH values in lead treatment animals³⁷.Another research by Thoreux-Manlay et al 1995 in lead-exposed rats, the plasma and testicular testosterone dropped by about 80%, but plasma LH dropped only by 32%. When luteinizing hormone releasing hormone (LHRH) stimulated the pituitary, the plasma LH level reached to normal range, but plasma testosterone remained significantly reduced by 37%. The sharp decrease in the testosterone: LH ratio in lead-exposed rats, combined with the significant reduction of intertubular tissue volume in the testes, indicate impaired Leydig cell function.³⁸ The pathological changes in Leydig cells brought decline androgen level ³⁹and it has been reported to alter in metabolic function of sertoli cells⁴⁰. Several attempts were made to restore the hormonal function of the reproductive system from lead exposed toxicity. Notably, Testosterone administration significantly increased sperm count, motility, viability of spermatozoa and also enhanced levels of testicular 3β- and 17β-hydroxysteroid dehydrogenases comparatively from the leadexposed rats⁴¹. Low levels exposure of lead acetate over longer periods of time would produce a similar pattern of adaptation to toxicity at the molecular and biologic levels. It was found no significant differences in serum LH and Gonadotropin-releasing hormone (GnRH) among treated animals, but a significant increase of GnRH mRNA concentrations was recorded. The signals within and between the hypothalamus and pituitary gland appear to be disturbed by long-term, low-dose Lead exposure⁴² Suppressed testosterone levels and spermatogenesis were also noticed in Lead-treated groups⁴³.

Lead Toxicity on Spermatogenesis and Steroidogenesis:

The activities of testicular key steroidogenic enzymes were declined along with significant depletion in cholesterol, ascorbic acid and reduced glutathione contents in lead-treated animals⁴⁴. It was observed that lead could cause degenerative changes in the spermatogenic series in many tubules, with a loss of germ cells and vacuoles inside the cytoplasm and between the germ cells⁴⁵. Research has shown that the direct exposure of high level is protected by a blood-testis barrier. the reproductive hormonal axis alteration could be the reason for direct toxic effect on the seminiferous tubules of the testes⁴⁶. A significant decrease in the serum testosterone indicates the decreased steroidogenesis⁴⁷. Lead treatment inhibited the spermatogenesis by reducing spermiation (VII and VIII) and beginning of mitosis (IX-XI) process length.¹⁴

In human studies, it was found that the spermatogenesis impairment may due to an excessive amount of lead exposure on sertoli cells which might be a result of inhibin B overproduction⁴⁸. Prolonged exposure of lead to male rats revealed the deterioration of spermatogenesis in addition with Leydig cell degeneration. The germinal function during the growing stages of testis at maturity is altered because of the disturbed steroidogenesis⁴⁹. A comparative study was carried to estimate the concentration of testosterone secreted by Leydig cells under a lead treated condition in the animal model and by stimulation of human chorionic gonadotrophin (hCG) in vitro model. There is down regulation of the enzymes cytochromes P450scc (CYP11A1) and P450c17 (CYP17) and 3 beta-hydroxysteroid dehydrogenase (3β-HSD) which are essential for steroid biosynthesis. Significantly reduced amount of testosterone indicates the impairment directly affects the steroidogenesis⁵⁰. A Higher dose of Lead induction was shown complete arrest of spermatogenesis⁵¹. A study proposed that Human Protamine (HP2) binds to Lead casing a conformational change in the protein could be a novel mechanism of toxicity to sperm. It was also proven the higher affinity HP2 to Pb(2+) which replaces the Zn(2+). These reduce the HP2-DNA interaction results in retardation in sperm chromatin condensation causes reduced fertility⁵². Furthermore, the study aimed to trace the cellular mechanism of Lead’s adverse effect on Leydig cells steroidogenesis. Lead reduced the stimulation of hCG and dibutylcAMP (dbcAMP) tends to decrease the Steroid production in MA-10 cells. It was found that Lead inhibits steroidogenesis by reducing StAR protein expression, P450scc, and 3beta-HSD enzymes by acting on calcium channel to regulate MA-10 cells⁵³. The debility in sperm function is mainly for the reason that the hormonal deterioration of spermatogenesis rather than by direct adverse effect on spermatozoa⁵⁴

Lead Toxicity on Testis Histology:

There are numerous research data support the incidences of damage caused by administration of lead. Such as, the lumens devoid of sperm in testicular architecture¹³, conspicuous degenerative changes in the testis³⁵, seminiferous epithelium degeneration, reduced number of epithelium spermatozoa²¹, sharp depressions, membrane folding and granularity at sperm head surfaces were perceived¹⁵, wide range of malformation in testis histopathology is indicates the lead inducing oxidative stress followed by diminishing steroidogenesis and spermatogenesis²⁸. Administration of lead acetate orally for 150 days were found to change the structure of seminiferous tubules and spermatozoa³⁹.The caput and corpus regions were disrupted in histoarchitecture of the epididymis. Further, the damage of basement membrane, vacuolization of cells, epithelium disorganization, and the tubules were found almost empty indicates the complete arrest of spermatogenesis⁵¹. A research work showed the ultra structural changes in form of sertoli cell cytoplasm vacuolization and increase lysosomes size and number. Chronic exposure of lead may change the sertoli cells function and to form changes in spermatogenesis⁵⁵.

In conclusion, the treatment for plumbism can be enhanced by understanding of the mechanism of action. So far the research carried out on Lead toxicity proclaimed certain points that could reveal about its mechanism. such as the reduced testosterone level without any significant change in the Luteinizing hormone proves the lead has a direct adverse effect on Leydig cells³⁷. And the down regulation of spermatozoa is mainly due to impede in hormones of spermatogenesis⁵⁵. Further research work in these area will provide a evidence to confirms the molecular mechanism.

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Received on 16.10.2017 Modified on 17.11.2017

Research J. Pharm. and Tech. 2018; 11(3): 1228-1232.

DOI: 10.5958/0974-360X.2018.00228.7