**4. Effect of arsenic on tyrosine phosphorylation**

Arsenic is mainly present in four forms namely arsenate (As(V)), arsenite (As (III)), MMA (monomethylarsonic acid), and DMA (dimethylarsenic acid) [126]. Trace quantities of arsenic were found in drinking water of rats, hamsters, goats, chickens and humans [127]. Arsenic-induced male infertility is reported to cause abnormal sperms, decreased sperm count, and decreased sperm motility in both humans and animals [128–130]. Exposure of the cells to arsenic increased total cellular tyrosine phosphorylation of 110–120, 90, 70, 56, and 40 kDa proteins [131]. Arsenic-induced tyrosine-phosphorylation in EGFR [132]. It is not known how arsenic induces the activation of EGFR either by the conformational changes or by dimerization of EGFR, which results in the activation of EGFR [133]. It was proposed that arsenic might activate EGFR through generation of ROS that, in turn, triggered the conformational changes in the receptor [134, 135]. The arsenic-induced activation of EGFR recruits Sh-c and phosphorylates its tyrosine residues, which results in enhancement of the interactions between Sh-c and Grb2. Signals are then relayed to the downstream signaling proteins [132]. Inhibition of EGFR kinase blocked arsenicinduced activation of MAPKs [136]. Arsenic may activate with the vicinal sulfhydryl groups of the Src molecule, (2) direct interactions with extracellular matrix proteins to induce integrin rearrangements, or (3) the generation of ROS [137, 138]. Biscardi and colleagues found that Src was able to phosphorylate EGFR at two unique tyrosine residues, distinct from the autophosphorylation sites, to activate EGFR in association with the activation of other cell signaling proteins [139, 140]. Arsenic induces Src and that activates downstream proteins e.g., MAPKs via EGFR-dependent and EGFRindependent pathways [138, 141]. Shim et al. [142] reported that arsenic inhibits Ca2+ influx into antigen-activated mast cells and inhibit tyrosine phosphorylation. These results indicate that the target of arsenic is upstream of the Ca2+ influx which is a major pathway of sperm capacitation as well. Thus, further detailed studies are warranted to find out the effects of arsenic on sperm capacitation mechanism.

Six months exposure to sodium arsenite (1, 5, or 25 mg/L) reduced Voltagedependent anion channel protein 3 (VDAC3), which leads to impaired capacitation and fertilization process in male rats [143, 144]. cAMP activates the serine/ threonine Kinase and cAMP-dependent protein kinase catalytic subunit alpha (PRKACA), which in turn activates tyrosine through phosphorylation. Blocking of PRKACA altered the tyrosine phosphorylation at the protein level which results in impairment of capacitation of sperm [143, 144]. Arsenic exposure on the proteome and metabolome in rat testis leads to 36 up-regulated and 34 down-regulated proteins and 13 metabolites (8 high and 5 low). Theses altered proteins were related to spermatogenesis, fertilization, fertility, and mating behavior which may be mediated by the ERK/AKT/NF-κB-dependent signaling pathway [143, 144]. However, these studies indicate the toxic effect of arsenic, but arsenic-induced male reproductive toxicity, particularly effect on capacitation and tyrosine phosphorylation mechanisms are still far from being completely understood.

#### **5. Effect of lead (Pb) on tyrosine phosphorylation**

It is well known that there has been a worldwide decrease in human male fertility in recent years. One of the main factors affecting this is environmental pollution. Lead is one of the major heavy metal contaminants that threatens the health of animals and human beings at global level. It is a naturally occurring element and widely used in acid batteries, paints, smelters, and paper printing. It accumulates into human and animal blood, bone and soft tissues with a half-life of 35 days in blood and 20–30 years in bone via contaminated food, and drinking water [145]. Pb has also been reported to accumulate in the epididymus and some glands [146, 147] and is considered a male reproductive toxicant [148]. The mechanism of toxicity of Pb is still not very clear. Pb mainly targets events of spermatogenesis and spermatozoa function via free radical generation, apoptosis, motility, and DNA fragmentation, and ultimately declines the rate of fertilization [149]. Recently Hassan et al. [150] reported that exposure of 20 mg PbAc/kg bwt, orally in rats for 45 days resulted in significant decrease in testis weight, spermatozoa count, testosterone levels, and antioxidant enzymes levels. Histological study indicated that Pb -exposed group was devoid of germ cells and maturation arrest with the formation of giant primary spermatocytes. Some studies reported that Pb has the ability to displace zinc and results in alteration in Ca2+ mediated process [151].

Capacitation is highly Ca2+ dependent process which means lead exposure could inhibit or induce the capacitation. Only few studies are reported about the effect of Pb on tyrosine phosphorylation during capacitation. Yuanqiao et al. [152] reported that 10–100 μM lead acetate dose-dependently inhibited total and progressive motility measures, capacitation and progesterone-induced acrosome reaction in humans. It also decreased the intracellular concentrations of cyclic adenosine monophosphate (cAMP) and intracellular calcium (Ca2+ )i, and reduced the tyrosine phosphorylation of sperm proteins, all of which are thought to be key factors in regulation of capacitation. These findings suggest that lead inhibits human sperm functions by reducing the levels of sperm intracellular cAMP, (Ca2+)i and tyrosine phosphorylation of sperm proteins *in-vitro*. Voltage-dependent Ca2+ channels, known as Catsper, are mainly involved in regulation of capacitation by mediating Ca2+ influx [153]. Therefore, it can be postulated that Pb exposure decreases intracellular Ca2+ by inhibiting progesterone -induced acrosome reaction via voltage-dependent channels. Further concentration and time dependent studies are warranted to explicate the effects of Pb on sperm capacitation and tyrosine signaling mechanism.

#### **6. Conclusions**

Heavy metals affect tyrosine phosphorylation during capacitation of spermatozoa and lead to male infertility. Alteration in tyrosine signaling might be a result of *Effect of Heavy Metals on Tyrosine Kinases Signaling during Sperm Capacitation DOI: http://dx.doi.org/10.5772/intechopen.99261*

various stress conditions which are produced by heavy metals in cells like oxidative stress, apoptosis, mitochondrial damage, calcium influx and change in osmolarity of cells. Particularly Hg, Pb, As and Cd inhibit or induce tyrosine phosphorylation of sperm proteins. There are several factors including animal species and strains, gender, age, stress, genetic disorders, nutritional status, smoking, alcohol consumption, use of medicines, and concomitant exposure to other chemicals or even physical factors which will influence both the metabolism and the dose–response relationships including reproduction that affects biological processes specifically signaling mechanism. Therefore, extensive research is warranted focusing on tyrosine phosphorylation signaling during sperm capacitation using large sample size or population with minimum dose which are reported in human blood after exposure of lead, mercury, arsenic and cadmium. It is now generally accepted that the mammalian testes are very sensitive to heavy metals, and these induce changes in the testicular biochemical functions via ROS and DNA damage that ultimately affect the fertilizing ability particularly capacitation in spermatozoa.
