CLARIFICATION OF THE BIOLOGICAL MEANING OF MENAQUINONE-4 SYNTHESIS IN ANIMAL ORGANS

众所周知，维生素K(K)在一系列新生蛋白翻译后活化为GLA-蛋白，如凝血因子和骨GLA-蛋白。自然产生的K有两种形式：KI，植物合成的叶醌；K2，一种由包括肠道微生物在内的微生物合成的同系物家族，被命名为Menuone-n(MK-n)。然而，有报道表明，K类似物在胃肠道外的各种器官中转化为MK-4，而不涉及细菌酶。尽管研究表明MK-4在细胞分化、诱导细胞凋亡、刺激核受体配体转录等方面具有独特的功能，但这并不能完全解释MK-4在不同组织中合成的生理意义。为了帮助阐明MK-4产生的生物学作用，我们使用无菌大鼠消除由肠道菌群合成的MK-n，并确定基因表达模式与K状态的函数关系。Wistar无菌大鼠分别饲喂缺K、对照(正常水平1(1))、添加KI或添加MK-4的饲料。我们使用DNA微阵列技术获得了睾丸基因表达谱。与补充KI的动物相比，K缺乏组中胆固醇合成和类异戊二烯代谢相关基因的表达下调。定量RT-PCR分析显示，睾丸MK-4浓度与参与睾丸睾酮合成的限速酶CyP11a的表达呈正相关。此外，与对照组和补充MK-4的动物相比，缺钾组的血浆睾酮浓度降低。综上所述，这些结果提示睾丸MK-4可能是通过调节类固醇合成相关基因表达而参与类固醇激素合成的关键因子。

Vitamin K (K) is well known for its role in post-translational activation of a family of nascent proteins to Gla-proteins, such as blood clotting factors and bone Gla-proteins. There are two forms of naturally occurring K: Ki, plant-synthesized phylloquinone; and K2, a family of homologues designated as menaquinone-n (MK-n), which are synthesized by microorganisms including intestinal microflora. However, reports indicate that K analogues are converted into MK-4 in various organs outside the gastrointestinal tract without involvement of bacterial enzymes. Although studies indicate that MK-4 has unique functions related to cell differentiation, induction of apoptosis, and stimulation of transcription of a ligand for the nuclear receptor, these cannot totally explain the physiological significance of MK-4 synthesis in diverse tissues. To help elucidate the biological role of MK-4 production, we used germfree rats to eliminate MK-n synthesized by intestinal flora, and identified patterns of gene expression as a function of K status.Wistar germfree rats were fed either K-deficient, Control (normal level of 1(1), Ki-supplemented, or MK-4-supplemented diets. We obtained testicular gene expression profiles using DNA microarray techniques. Expression of genes involved in cholesterol synthesis and isoprenoid metabolism was down-regulated in the K-deficient group compared with Ki-supplemented animals. Quantitative RT-PCR analysis revealed that testicular MK-4 concentrations were positively correlated with expression of CYP11a, a rate-limiting enzyme involved in testicular testosterone synthesis. Moreover, plasma testosterone concentrations were reduced in K-deficient relative to Control and MK-4-supplemented animals. Together, these results suggest that testicular MK-4 could be a key factor involved in steroid hormone synthesis via regulation of gene expression related to steroidogenesis.