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The number of Leydig cells in turn is regulated by luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Like most hormones, testosterone is supplied to target tissues in the blood where much of it is transported bound to a specific plasma protein, sex hormone-binding globulin (SHBG). However, the concentrations of testosterone required for binding the receptor are far above even total circulating concentrations of testosterone in adult males (which range between 10 and 35 nM). The areas of binding are called hormone response elements (HREs), and influence transcriptional activity of certain genes, producing the androgen effects. The relationship between sex steroids and SHBG in physiological and pathological conditions is complex, as various factors may influence the levels of plasma SHBG, affecting bioavailability of testosterone. A few studies indicate that the testosterone derivative estradiol might play an important role in male aggression. In one experiment, subjects who interacted with handguns showed higher testosterone levels and aggression than those who interacted with toys.
Defects in aging Brown Norway rat Leydig cells include reductions in each of LH-stimulated cAMP production, STAR, TSPO, CYP11A1, and downstream steroidogenic enzymes 125–127. As in men, aging in Brown Norway rats is characterized by reduced serum testosterone and unchanged or increased LH levels, and by the reduced ability of the Leydig cells to produce testosterone in response to LH 123, 124. In both, these decreases result from reduced testosterone production by aging Leydig cells, not from a reduction in cell numbers. Although there is agreement that testosterone replacement in young hypogonadal men is relatively safe and has beneficial effects, exogenous testosterone typically will suppress LH, resulting in reduced Leydig cell testosterone production and therefore in the suppression of spermatogenesis. Moreover, a cell-autonomous AMPK-dependent mechanism actively represses steroidogenesis, thus preventing excessive production of steroid hormones .
MA-10 cells remain the most widely used of the cell lines, and have been instrumental in advancing our knowledge of Leydig cell steroid formation and regulation. These cells retain functional LHCGR-mediated steroidogenesis, producing progesterone, testosterone, and estradiol. In 1987, Finaz and colleagues developed the K9 Leydig cell line, cells that were able to produce testosterone in response to hormone treatment and had characteristics of normal Leydig cells .
Such an approach, or targeting TSPO with specific drug ligands, hold promise for providing new means by which to increase serum testosterone levels without administering LH-suppressive testosterone. As a result, the negative regulatory roles of γ and ε were ablated and therefore the cells produced more steroids both acutely at the initiation of steroidogenesis, or long-term, respectively. In subsequent studies, cell penetrating peptide sequences conjugated to a short sequence of VDAC1 containing S167, and of STAR containing S194, were shown to successfully compete out ε and γ interactions in MA-10 cells 72, 73.
In 1975, Cooke and colleagues showed that inhibition of protein synthesis affected LH-induced steroid production by Leydig cells . The finding by Hall et al that Leydig cells metabolize cholesterol to testosterone was of particular importance because it is this quality that defines steroidogenic cells. However, it is unlikely that these cells contribute significantly to testosterone production in the adult 9, 21. Initially, LH is not required either for the development of fetal Leydig cells or for their androgen production 8, 13. In the rat, the fetal Leydig cells begin to produce testosterone by gestational day 15.5, with peak production just prior to birth. LH is not required either for the development of fetal Leydig cells or for their initial testosterone production. Fetal Leydig cells produce the high levels of testosterone that are required for the differentiation of the male genitalia and for brain masculinization.
5α-DHT binds to the same androgen receptor even more strongly than testosterone, so that its androgenic potency is about 5 times that of T. Androgens such as testosterone have also been found to bind to and activate membrane androgen receptors. Only the free amount of testosterone can bind to an androgenic receptor, which means it has biological activity. The part of the total hormone concentration that is not bound to its respective specific carrier protein is the free part. This additional information could suggest, contrarily, that testosterone may encourage greed or selfishness. When controlling for the effects of belief in having received testosterone, women who have received testosterone make fairer offers than women who have not received testosterone. This could explain why some studies find a link between testosterone and pro-social behaviour, if pro-social behaviour is rewarded with social status.

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