**1. Introduction**

Naturally existing vitamin K comprises vitamin K1 (phylloquinone) and vitamin K2 (mena‐ quinone). Menaquinone‐4 (MK‐4), an analog of vitamin K2, contains a 2‐methyl‐1,4‐naphtho‐

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

quinone ring and a geranylgeranyl group (four isoprene units) as a side chain (**Figure 1**). MK‐ 4 is not commonly synthesized by bacteria and is instead alkylated from menadione, which is supplemented in animal feeds to increase vitamin K levels. In most organs, MK‐4 is converted from dietary vitamin K1 and other menaquinones [1, 2] via a process catalyzed by UbiA prenyltransferase domain containing protein 1 [3]. Furthermore, MK‐4 is prescribed as a therapeutic agent for osteoporosis and to prevent fractures in Japan [4].

**2. Steroidogenesis in testicular Leydig cells**

steroidogenesis of testosterone from cholesterol.

The major function of testicular Leydig cells is to produce testosterone in response to the pituitary luteinizing hormone (LH) as shown in **Figure 3**. The LH receptor (LHR) affects the 3′,5′‐cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) signaling pathway by associating with G proteins containing the cytoplasmic Gαs subunit. Production and secretion of testosterone in Leydig cells are tightly regulated by intracellular cAMP, a common secondary messenger. The formation and degradation of intracellular cAMP are under the control of adenylate cyclase (AC) and cyclic nucleotide phosphodiesterase (PDE), respectively. A rise in intracellular cAMP levels activates PKA, which stimulates downstream steroidogenic proteins. Steroidogenic acute regulatory (StAR) protein transports cholesterol to the inner mitochon‐ drial membrane of these cells to initiate steroidogenesis. Cytochrome P450scc (also known as CYP11A), a cholesterol side‐chain cleavage enzyme, catalyzes a cascade of reactions that convert cholesterol to the steroid hormone precursor pregnenolone, which is then transformed into testosterone [16]. Both StAR and CYP11A constitute rate‐limiting steps in the overall

Menaquinone‐4 Enhances Steroidogenesis in Testis Derived Tumor Cells Via the Elevation of cAMP Level

http://dx.doi.org/10.5772/63982

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**Figure 3.** Steroidogenic pathway in testicular Leydig cells. AC, adenylate cyclase; CREB, cAMP response element‐bind‐ ing protein; CYP11A, cholesterol side‐chain cleavage enzyme; G, G protein; HSD, hydroxysteroid dehydrogenase; LH, luteinizing hormone; LHR, LH receptor; Mito, mitochondria; PDE, cyclic nucleotide phosphodiesterase; PKA, protein

kinase A; SER, smooth endoplasmic reticulum; StAR, steroidogenic acute regulatory protein.

**Figure 1.** Chemical structure of MK‐4. MK‐4 has a 2‐methyl‐1,4‐naphthoquinone ring and a geranylgeranyl group (four isoprene units) as a side chain.

Vitamin K is a well‐known nutrient required for blood coagulation and bone metabolism. In recent years, novel functions of vitamin K against inflammation [5, 6], tumors [7–9], and ligand of the nuclear receptor PXR (also known as SXR) [10, 11] have been reported. These findings suggest the beneficial role of vitamin K, including MK‐4, in several biological processes. In rodents, MK‐4 is distributed throughout the body and is observed in high quantity in the liver, bone, brain, pancreas, and reproductive organs, even when animals are fed a low MK‐4 diet (**Figure 2**) [12–15]. However, the role of MK‐4 in these organs has not been well characterized. This chapter focuses on the functional effects of MK‐4 on steroidogenesis in testicular Ley‐ dig cells.

**Figure 2.** Vitamin K contents in rat tissues. Male Wistar rats were fed an AIN‐93G diet for three weeks. Tissue levels of vitamin K and MK‐4 were determined by fluorescent HPLC (reproduced with permission from Shirakawa et al. [14]).
