Molecular mechanisms of leptin action in adult rat testis: potential targets for leptin-induced inhibition of steroidogenesis and pattern of leptin receptor messenger ribonucleic acid expression

Abstract

Leptin, the product of the ob gene, is a pivotal signal in the regulation of neuroendocrine function and fertility. Although much of the action of leptin in the control of the reproductive axis is exerted at the hypothalamic level, some direct effects of leptin on male and female gonads have also been reported. Indeed, recent evidence demonstrated that leptin is able to inhibit testosterone secretion at the testicular level. However, the molecular mechanisms behind this effect remain unclear. The focus of this study was twofold: (1) to identify potential targets for leptininduced inhibition of steroidogenesis, and (2) to characterize in detail the pattern of expression and cellular distribution of leptin receptor (Ob-R) mRNA in adult rat testis. In pursuit of the first goal, slices of testicular tissue from adult rats were incubated with increasing concentrations of recombinant leptin (10 9 -10 7 M) in the presence of human chorionic gonadotropin (hCG; 10 IU/ml). In this setting, testosterone secretion in vitro was monitored, and expression levels of mRNAs encoding steroidogenic factor 1 (SF-1), steroidogenic acute regulatory protein (StAR), cytochrome P450 cholesterol sidechain cleavage enzyme (P450 scc) and 17 -hydroxysteroid dehydrogenase type III (17 -HSD) were assessed by Northern hybridization. In pursuit of the second goal, the pattern of cellular expression of the Ob-R gene in adult rat testis was evaluated by in situ hybridization using a riboprobe complementary to all Ob-R isoforms. In addition, testicular expression levels of the different Ob-R isoforms, previously identified in the hypothalamus, were analyzed by means of semi-quantitative RT-PCR. In keeping with our previous data, recombinant leptin significantly inhibited hCG-stimulated testosterone secretion. In this context, leptin, in a dose-dependent manner, was able to co-ordinately decrease the hCG-stimulated expression levels of SF-1, StAR and P450 scc mRNAs, but it did not affect those of 17 -HSD type III. In situ hybridization analysis showed a scattered pattern of cellular expression of the Ob-R gene within the adult rat testis, including Leydig and Sertoli cells. In addition, assessment of the pattern of expression of Ob-R subtypes revealed that the long Ob-Rb isoform was abundantly expressed in adult rat testis. However, variable levels of expression of Ob-Ra, Ob-Re, and Ob-Rf mRNAs were also detected, whereas those of the Ob-Rc variant were nearly negligible. In conclusion, our results indicate that decreased expression of mRNAs encoding several up-stream elements in the steroidogenic pathway may contribute, at least partially, to leptin-induced inhibition of testicular steroidogenesis. In addition, our data on the pattern of testicular expression of Ob-R isoforms and cellular distribution of Ob-R mRNA may help to further elucidate the molecular mechanisms of leptin action in rat testis.

Obesity, energy balance and spermatogenesis

“…On the other hand, the excess of leptin production by adipose tissue was reported to have deleterious effects on sperm production and to induce apoptosis on germ cells (Isidori et al 1999), which illustrates that this hormone may compromise male fertility via distinct mechanisms. For instance, leptin-induced steroidogenesis inhibition has already been reported (Tena-Sempere et al 2001), and leptin was proposed as directly responsible for the signalling link between adipose tissue and altered steroidogenesis in the testis (Isidori et al 1999).…”

Inhibitory effect of leptin on the rat ovary during the ovulatory process

“…A direct inhibitory action of leptin on steroid hormone secretion has been demonstrated independently by different groups in the ovary (Spicer & Francisco 1997, Zachow & Magoffin 1997, Agarwal et al 1999, Barkan et al 1999, Zachow et al 1999, Ghizzoni et al 2001, Kikuchi et al 2001) and other tissues (Tena-Sempere et al 2001, Cameo et al 2003. The mechanism involved for such inhibitory action has not been completely clarified, but it has been suggested that this effect is mediated by a modulation on transcriptional factors, such as StAR and P450scc (Tena-Sempere et al 2001) or overexpression on c-Jun (Barkan et al 1999).…”

“…The mechanism involved for such inhibitory action has not been completely clarified, but it has been suggested that this effect is mediated by a modulation on transcriptional factors, such as StAR and P450scc (Tena-Sempere et al 2001) or overexpression on c-Jun (Barkan et al 1999). In this paper, leptin treatment also induced significant inhibition in serum level of progesterone and again, this result was confirmed by in vitro studies where the concentration of progesterone was reduced by the presence of leptin in cultures of ovarian explants and preovulatory follicles.…”

Leptin and leptin receptor in pig spermatozoa: evidence of their involvement in sperm capacitation and survival

“…OBR mRNA (long form) has been detected in a variety of porcine tissues, including adipose tissue (Lin et al 2000). Using in situ hybridization techniques, other groups have shown that leptin receptor mRNA is expressed in Sertoli cells of adult rats (Tena-Sempere et al 2001).…”