Male reproduction is governed by the classical hypothalamo-hypophyseal testicular axis: Hypothalamic gonadotropin releasing hormone (GnRH), pituitary luteinizing hormone (LH) and follicle stimulating hormone (FSH) and the gonadal steroid, principally, testosterone. Thyroid hormones have been shown to exert a modulatory influence on this axis and consequently the sexual and spermatogenic function of man. This review will examine the modulatory influence of thyroid hormones on male reproduction.

Mammalian testis functions at a temperature and oxygen tension (pO(2)) lower than the core body. Hypoxia-inducible factor-1α (HIF-1α) mediates the adaptive responses to hypoxia such as production of angiogenic vascular endothelial growth factor (VEGF) in a variety of cells and tissues. VEGF production in Leydig cells is stimulated by luteinising hormone (LH)/cAMP. We have conducted experiments to find out whether HIF-1α is involved in LH/cAMP-induced secretion of VEGF by Leydig cell-derived MA-10 cells. Both cobalt chloride (CoCl(2)), an inducer of hypoxia, and 8-Br-cAMP enhanced HIF-1α activity followed by an increase in VEGF secretion. However, there was no change in mRNA levels of HIF-1α. Inhibition of HIF-1α activity by cyclosporine A (CsA) inhibited a rise in VEGF production in response to CoCl(2) as well as 8-Br-cAMP. Inhibitors of protein kinase A (PKA), extracellular regulated kinase 1/2 (ERK1/2) and phosphatidyl inositol-3 kinase/Akt (PI3-K/Akt) inhibited the increase in VEGF levels in response to both CoCl(2) and 8-Br-cAMP. The data suggest that HIF-1α is a mediator of hypoxia- as well as 8-Br-cAMP-stimulated production of VEGF in MA-10 cells; both the stimuli act through a common signalling cascade.

We have earlier shown that cobalt chloride (CoCl2)-induced hypoxia and second messenger 8-bromoadenosine 3', 5'-cyclic adenosine monophosphate (8-Br-cAMP) stimulates vascular endothelial growth factor (VEGF) production in Leydig tumor cell derived MA-10 cells. Both stimuli follow common signal transduction pathways including protein kinase A (PK-A), extracellular regulated kinase 1/2 (ERK1/2), and phosphatidyl inositol-3 kinase/akt (PI3-K/Akt) pathways in the stimulation of VEGF by MA-10 cells. In the present study we investigated the role of CoCl2 and 8-Br-cAMP on steroid production in MA-10 cells. The MA-10 cells were cultured in Waymouth MB 752/1 medium, supplemented with 15% heat inactivated horse serum. Progesterone was estimated by radioimmunoassay (RIA).We report that 8-Br-cAMP stimulated progesterone production by the MA-10 cells whereas CoCl2 inhibited the same. Also, 8-Br-cAMP stimulated steroidogenic acute regulatory protein (StAR) and cytochrome P450 side-chain cleavage enzyme (P450scc) mRNAs expression. However, CoCl2 had no effect on StAR mRNA. Cobalt chloride directly inhibited the expression of P450scc mRNA. The decrease in progesterone production could be attributed to three different mechanisms, (1) an increase in production of reactive oxygen species (ROS), (2) an increase in HIF-1α activity, and (3) ultimately a decrease in the level of cytochrome P450 side chain cleavage (CYT P450scc). Hypoxia has an action and mechanism of action similar to that of gonadotropins on VEGF production, whereas they have a contrasting effect on steroidogenesis. This study suggests that hypoxia could be as important as gonadotropins in regulating Leydig cell steroidogenesis.

Our aim was (i) to investigate the hypothalamo-hypophyseal-gonadal axis in hyperthyroid Indian males, (ii) to rule out the modulatory role of adrenal steroids on it and (iii) to determine if the simultaneous rise in oestradiol and luteinising hormone (LH) in hyperthyroid males is due to a positive feedback action of oestradiol on pituitary LH release. Age- and BMI-matched men were divided into two groups, I, euthyroid subjects (n = 17) and II, hyperthyroid patients (n = 12) on the basis of their thyroid hormone levels. Serum levels of thyroid-stimulating hormone, triiodothyronine, thyroxine, LH, follicle-stimulating hormone (FSH), prolactin, E(2), T, P(4), sex hormone binding globulin and dehydroepiandrosterone sulphate (DHEAS) were assayed. Mean levels of T and E(2) were approximately two times higher in group II in comparison with group I. DHEAS levels were similar in both groups ruling out any adrenal involvement. Mean serum LH level was 2.6 folds higher in group II in comparison with group I. Mean serum levels of FSH were higher in group II, it was marginally nonsignificant. On the basis of these and previous observations, we hypothesise that endocrinological dimorphism in human male and female is not rigid; a sustained rise in serum oestradiol probably induces a positive feedback action on pituitary leading to elevated gonadotrophin levels.

8-Br-cAMP: 8-bromo - 3', 5' cyclic adenosine monophosphate; CoCl: cobalt chloride; HIF-1α: hypoxia inducible factor -1α; LH: luteinizing hormone; T: 3, 5, 3'-L-triiodothyronine; VEGF: vascular endothelial growth factor.

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Subclinical hypothyroid men characterized by a rise in only thyroid stimulating hormone (TSH) levels, and normal thyroid hormone levels showed a fall in their serum progesterone and testosterone levels. This suggested a role of TSH in regulating Leydig cell steroidogenesis. Therefore, we investigated the direct role of TSH on steroid production and secretion using a mouse Leydig tumor cell line-1 (MLTC-1). MLTC-1 cells were treated with different doses of TSH isolated from porcine pituitary as well as recombinant TSH. Steroid secretion was measured by radioimmunoassay (RIA). The mRNA levels of steroidogenic enzymes were quantitated by real-time polymerase chain reaction (RT-PCR), whereas the corresponding protein levels were determined by western blot. In MLTC-1 cells, pituitary TSH as well as recombinant TSH inhibited progesterone and testosterone secretion in a dose-dependent manner. The inhibitory action of TSH on steroid secretion was unique and not mimicked by other anterior pituitary hormones including follicle stimulating hormone and adrenocorticotropic hormone. Recombinant TSH showed no effect on steroidogenic acute regulatory protein and CYP11A1, the enzymes catalyzing the nonsteroidogenic and steroidogenic rate-limiting steps of steroid synthesis, respectively. Recombinant TSH was shown to inhibit steroidogenesis in MLTC-1 cells by inhibiting the 3-β hydroxy steroid dehydrogenase mRNA and protein levels, the enzyme that catalyzes the conversion of pregnenolone to progesterone. This inhibitory effect of TSH is probably direct as both mRNA and protein of the TSH receptor were shown to be present in the MLTC-1 cells.

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