Tulay Irez, Ivan Rolland Karkada, Sulagna Dutta, Pallav Sengupta?

1Department of Histology and Embryology, Biruni University, Turkey

2Department of Physiology, Faculty of Medicine and Biomedical Sciences, MAHSA University, Malaysia

3Department of Oral Biology and Biomedical Sciences, Faculty of Dentistry, MAHSA University, Malaysia

ABSTRACT Obestatin is a 23-amino acid peptide hormone secreted by the stomach and is found in several tissues all over the body, such as the gastric mucosa, spleen, mammary gland, plasma as well as in the testicular Leydig cells. Obestatin seems to operate as part of the integrated gut-brain network acting as an anorectic hormone, reducing food intake and reversing body weight gain. Besides the expressions of obestatin in male reproductive tissues, it is also shown to increase testosterone secretions, thus ameliorating testicular functions. In the present scenario where the increasing prevalence in obesity is considered as one of the major causatives of worldwide declining trend of semen quality, molecules like obestatin playing roles in both metabolic and reproductive functions find importance in management of obesity-induced male infertility or subfertility. The present review article aims to provide updated concepts on obestatin and its mode of actions, and its role in modulation of male reproductive functions.

Keywords:Male infertility Obestatin Testosterone Leydig cell

1. Introduction

The worldwide prevalence of obesity is following a steadily increasing trend that parallels the declining trend in male reproductive functions[1-5]. The World Health Organization reports that 39% of the worldwide adult population is either overweight or obese[6]. Obesity is a complex multifactorial metabolic disorder[7-9]that poses a risk of acquiring several chronic diseases[10]. It affects male reproductive functions leading to impaired spermatogenesis,low sperm count, reduced number of normal motile sperms,increased sperm DNA fragmentation and poor semen quality[11-14].Management of male infertility in obese men has gained high importance in research. In this regard, obestatin, a 23-amino acid amidated peptide, caught attention for its role in food intake behavior, stomach emptying and reversal of high body weight[15,16].Its biological activities are thereby opposite to that of ghrelin, even if both are derived from same precursor protein[17]. Obestatin was detected in several tissues throughout the body that includes gastric mucosa, plasma, mammary glands, as well as in the testicular cells[18,19]. Thus, it may also act as local autocrine or paracrine factor besides its endocrine actions. Its expression in the Leydig cells of testes[19], stimulatory effects upon cellular proliferation at the peripubertal phase in male rats and upregulation of testosterone secretion[20-22]indicate its relevance in testicular functions.However, the exact roles of obestatin in regulation of male reproduction are not completely understood. Considering the global declining trend in male fertility parameters[23,24]and the concurrent increase in the prevalence of metabolic syndrome, obestatin finds potential applicability in understanding the association of energy balance and reproductive functions. This review article thus aims to present: (a) an overview on obestatin and its mode of actions, and (b)the role of obestatin in modulation of male reproductive functions in normal and obese men. This concise review may provide further insights in the management of obesity induced male infertility or subfertility.

2. Obestatin: Hormone, receptors and physiological functions

Obestatin, as reported by Zhang et al[25]is an anorexic hormone that positively influences glucose homeostasis by enhancing the pancreatic β-cell mass, decreasing insulin resistance and associated adipose tissue inflammation[26,27]. Its name is derived from two Latin words ‘obedere' meaning to devour and ‘statin' meaning to suppress[25]. Studies on human showed predominant obestatin production in the gastrointestinal tract, particularly in the stomach,duodenum, jejunum, and ileum[28]. Apart from being produced in the gastrointestinal tract, it may be also synthesized in the brain,mammary gland as well as in the testicular cells[29]. A single gene code for the common precursor protein for ghrelin and obestatin is called the ghrelin-obestatin preproprotein[25]. Ghrelin is well known for its potent signals for appetite, playing a vital role in energy homeostasis[30,31]. In contrast to ghrelin's appetite and gastrointestinal motility stimulants, obestatin has been documented to decrease food intake, reverse body weight gain, and also to enhance gastric migration[32-34].

Reports suggest that there are numerous metabolic strategies in regulation of functions of adipocytes and glucose metabolism in conditions of obesity[27]. The gene encoding both ghrelin and obestatin may get altered according to the metabolic need of the body, such that alternative insertion may result in its multiple transcript variants, while, antisense transcripts may also occur[35].

The ghrelin gene that also encodes obestatin is located in the short arm of chromosome 3 (3p) in human[33,36]. It comprises of 4 exons leading to a 117-amino acid preprohormone, preproghrelin[37].According to these studies, preproghrelin hosts the exon 1 encoded preproghrelin signal peptide which is a 28-amino acid ghrelin peptide and exons 2, 3 and 4 encoded C-ghrelin which is a 66-amino acid C-terminal peptide. This C-terminal peptide has been found in the human circulation, but its physiological functions are yet not known. It may reflect alterations in the ghrelin gene encoding a 3-deleted cut C-ghrelin exon only for encoding ghrelin but not for obestatin. Obestatin-23 sequence is the same as residues 76-98 of C-ghrelin, but it is not known whether the obestatin is post-translationally C-gly-related or an alternative gene addition product[37]. It is suggested that obestatin requires additional C-terminal amidation to be biologically active. This 23-amino acid C-terminally mixed peptide is derived from preproghrelin and is attributed to a wide variety of metabolic effects[29].

Most of the studies on obestatin focused on its role in obesity and in regulating energy homeostasis. Obestatin also appears to exert a pleiotropic effect on the cardiovascular system, possibly triggering blood pressure, cardiovascular mechanisms that modulate endothelial function and may be important in determining cardiovascular outcomes of type 2 diabetes[29,38]. The levels of normal physiological obestatin reported in the literature are very variable. Recently it has been shown that obestatin levels in rodents ranged from 1.34 to 2 560.00 pg/mL, whereas the levels reported in humans showed equal variances between 8.400 to 22.077 pg/mL[39].

Obestatin acts through G protein-coated motility receptor of the ghrelin receptor and growth hormone secretagogue receptor-1 activation[25,40]. The growth hormone secretagogue receptor is expressed inα, β and δ cells of pancreatic islets[41,42]. It has been proposed that obestatin works through two functional receptors,G protein-coated motility receptor-39 and Glucagon-like peptide 1 receptors[25,41]. Both these G protein-dependent receptors were found to correlate with the metabolic functions of obestatin[29,37,43-45]. However, the understanding of the exact intracellular mechanism of action of obestatin via its receptors needs further interventions.

3. Obestatin and regulation of obesity

As obesity is usually characterized by abnormal excessive fat deposits which are detrimental to health, many promising researches have been conducted on the appetite control mechanism and its regulating factors. Recent research trend is inclined towards endogenous appetite regulatory peptides, such as obestatin, which was found in mammalian stomach and also in some other vital organs and tissues[46]. In humans, the level of obestatin production depends on physiological and metabolic status of the subjects,varying greatly in the ones with obesity or anorexia from that in the control subjects[47-49].

As discussed earlier, obestatin, is encoded by the same gene as that for ghrelin and both these hormones are produced from a common precursor - preproghrelin, having counteracting effects on food intake[50,51]. Obestatin treatments on rats showed suppressed food intake behavior[25,44,52], inhibition of jejunal contractions and reduction in body weight[25,52]. Obestatin has also been shown to regulate pancreatic β-cell survival and insulin secretion[46]. It has also been noted that obestatin levels were reduced in morbidly obese subjects who underwent bariatric surgery[53]. A study conducted on women showed that lower plasma obestatin levels were found in obese subjects compared with subjects with normal weight.These findings agree partly with the study by Guo et al[54], in which preprandial obestatin levels were found lower in obese men and women. Studies have further shown that obestatin treatment influences phospholipid turnover and impacts lipid homeostasis,while giving persuading proof that obestatin might act to improve diet-induced restrictions in lipid digestion, and it might impact upon steroid, bile acid and glutathione digestion.

Although evidences showed that obestatin might be involved in the regulation of energy balance and body weight, there are conflicts in the exact roles of obestatin in obesity[55]. Even though several studies have witnessed the inhibitory effects of obestatin on food intake,obesity and gastrointestinal motility, a consensus is not reached

yet[56-59].

4. Obestatin and male reproductive functions

Hormones associated with the hypothalamus-pituitary-gonadal axis along with its crosstalks with other axes regulate the male reproductive functions[14]. Research suggests that the factors associated with growth and body weight homeostasis, also possess various roles in the regulation of reproductive function[60-63].Obestatin has also been claimed to play such dual roles mediating both energy homeostasis and testicular functions[64,65]. In adult male rats, it was observed that a single intravenous injection of obestatin could trigger testosterone secretion and its chronic infusion in prepubertal rats significantly increased both testosterone production and spermatogenesis[20,22]. It also significantly reversed diabetes mediated compromised male fertility parameters, such as epididymal sperm count, sperm motility, testicular enzyme activities, and sperm morphology in rats. Moreover, obestatin may have protective role against diabetes-induced testicular dysfunctions, owing to its antioxidant properties[66]. In this regard, it may be put forth that obestatin plays essential role in resisting as well as reversing metabolic disorder-induced male reproductive impairments. These attribute to the integration of its functions in metabolic homeostasis with maintenance of testicular functions[65,67].

5. Obestatin and semen quality

The effects of obestatin on reproductive functions are not well defined but it is presumed to improve testicular functions in obese subjects[66,68]. It is generally thought that ghrelin and obestatin,which control metabolism, regulate cellular metabolic activations in the reproductive system[19]. Both obestatin and ghrelin levels were found to be higher in semen than in serum[66], portraying a linear correlation between serum and semen levels of ghrelin and obestatin. In male rats, obestatin has been shown to be expressed in the testes, especially in the Leydig cells[19]. Moretti et al have also reported immunoreactivity for obestatin in the spermatozoa,seminal vesicles and prostate[68]. Obestatin levels in semen showed positive correlation with essential semen parameters such as the sperm motility and sperm concentration[65,66,69,70]. In obese mice,immunoreactivity of both ghrelin and obestatin were shown to decrease in testes. However, in a study conducted by El-Damarawi et al, exogenously administered obestatin and L-carnitine decreased obesity-related parameters and increased testosterone levels and weight reduction have been observed[65]. They also showed increase in primary and secondary spermatocytes, spermatids and Leydig cell populations after administration of obestatin[22].

Thus, it may be suggested that obestatin may improve semen quality by ameliorating spermatogenesis and by inducing proliferation of testicular cells like the Leydig cells, thereby increasing the possibility of higher rate of steroidogenesis. Moreover,its possible influence on the male accessory reproductive glands may suggest its contribution in the secretion of seminal plasma.These studies indicate that obestatin may be a potential modulator in the management of obesity-induced male infertility or subfertility.However, given the inadequate number of studies, the physiological roles of seminal obestatin remain obscure.

6. Conclusions

The present review article has reviewed the most updated available information and presented a concise concept on obestatin and its role in male reproductive functions. It is clear from the above discussions that obestatin plays vital roles both in reversing body weight and management of obesity, as well as in ameliorating male reproductive functions. Recent research trend shows an increasing interest in exploring the association of metabolic disorders with reproductive impairments. In this context, obestatin appears to be a potential hormone that may be explored to ameliorate obesity-induced male infertility or subfertility.

Conflict of interest statement

The authors declare that there is no conflict of interest.