Müllerian Inhibiting Substance lowers testosterone in luteinizing hormone-stimulated rodents

Müllerian Inhibiting Substance (MIS) expression is inversely proportional to the serum concentration of testosterone in males after birth and in vitro studies have shown that MIS can lower testosterone production by Leydig cells. Also, mice overexpressing MIS exhibited Leydig cell hypoplasia and lower levels of serum testosterone, but it is not clear whether this is a result of MIS affecting the development of Leydig cells or their capacity to produce testosterone. To examine the hypothesis that MIS treatment will result in decreased testosterone production by mature Leydig cells in vivo, we treated luteinizing hormone (LH)-stimulated adult male rats and mice with MIS and demonstrated that it can lead to a several-fold reduction in testosterone in serum and in testicular extracts. There was also a slight decrease in 17-OH-progesterone compared to the more significant decrease in testosterone, suggesting that MIS might be regulating the lyase activity of cytochrome P450c17 hydroxylase/lyase (Cyp17), but not its hydroxylase activity. Northern analysis showed that, in both MIS-treated rats and mice, the mRNA for Cyp17, which catalyzes the committed step in androgen synthesis, was down-regulated. In rats, the mRNA for cytochrome P450 side-chain cleavage (P450scc) was also down-regulated by MIS. This was not observed in mice, indicating that there might be species-specific regulation by MIS of the enzymes involved in the testosterone biosynthetic pathway. Our results show that MIS can be used in vivo to lower testosterone production by mature rodent Leydig cells and suggest that MIS-mediated down-regulation of the expression of Cyp17, and perhaps P450scc, contributes to that effect.

缪勒管抑制物质（Müllerian Inhibiting Substance, MIS）的表达与出生后男性体内的血清睾酮浓度呈负相关，体外研究已证实MIS可降低莱迪希细胞的睾酮合成能力。另有研究显示，过表达MIS的小鼠会出现莱迪希细胞发育不全及血清睾酮水平降低，但目前尚不清楚这一现象是源于MIS对莱迪希细胞发育的影响，还是对其睾酮合成能力的作用。为验证“体内MIS处理可降低成熟莱迪希细胞的睾酮生成”这一假说，我们对经黄体生成素（luteinizing hormone, LH）刺激的成年雄性大鼠与小鼠施以MIS处理，结果证实其可使血清及睾丸组织提取物中的睾酮水平呈数倍下降。与睾酮的显著降低相比，17-羟孕酮仅出现小幅下降，这提示MIS或可调控细胞色素P450c17羟化酶/裂解酶（cytochrome P450c17 hydroxylase/lyase, Cyp17）的裂解活性，而非其羟化酶活性。Northern印迹分析显示，经MIS处理的大鼠与小鼠体内，催化雄激素合成关键步骤的Cyp17 mRNA表达均出现下调。在大鼠体内，细胞色素P450侧链裂解酶（cytochrome P450 side-chain cleavage, P450scc）的mRNA表达同样被MIS下调，但该现象并未在小鼠体内观察到，这表明MIS对睾酮生物合成通路相关酶的调控可能存在物种特异性。本研究结果证实，MIS可在活体中降低啮齿类成熟莱迪希细胞的睾酮生成能力，并提示MIS介导的Cyp17（或许还有P450scc）表达下调是该效应的重要机制之一。