**5. The MK‐4 structurally related compound, geranylgeraniol**

The C20 isoprenoid compound, geranylgeraniol (GGOH, **Figure 6**), is a functional side‐chain component (the geranylgeranyl group) of MK‐4 and has been shown to have a similar biological function as MK‐4. In addition, GGOH may also have anti‐tumorigenic effects against prostate cancer [29], colon cancer [30], leukemia [31], as well as anti‐inflammatory activity in rats [32]. Our latest study revealed a time‐ and dose‐dependent increase in testosterone and progesterone production in GGOH‐treated I‐10 cells. As expected, addition of GGOH stimu‐ lated also the PKA signaling pathway and augmented intracellular cAMP levels in I‐10 cells [33].

Menaquinone‐4 Enhances Steroidogenesis in Testis Derived Tumor Cells Via the Elevation of cAMP Level http://dx.doi.org/10.5772/63982 195

**Figure 6.** Chemical structures of isoprenoid groups; geranylgeraniol (GGOH), geraniol (GOH), farnesol (FOH), phytol (POH), and geranylgeranyl diphosphate (GGPP).

To further investigate the role of isoprenoids in steroidogenesis, other structurally related isoprenoids, such as geraniol (GOH) and farnesol (FOH) that have two and three isoprene units, respectively, as well as phytol (POH) and geranylgeranyl diphosphate (GGPP), were also examined (**Figure 6**). Accordingly, testosterone and progesterone levels were markedly increased upon treatment with POH and GGPP in I‐10 cells. In contrast, FOH increased the levels of progesterone but not testosterone, whereas GOH did not affect steroidogenesis in I‐ 10 cells [33]. These results indicate that most of the tested isoprenoids, and particularly POH, can stimulate the steroidogenic pathway in I‐10 cells to the same extent as GGOH.

In summary, the novel role of MK‐4 in stimulating steroidogenesis in I‐10 cells through regulation of cAMP/PKA signaling may depend on GGOH and other structurally related isoprenoids. In addition, we found that MK‐4, but not GGOH, enhanced glucose‐stimulated insulin secretion (GSIS) by altering cAMP levels in INS‐1 insulinoma cells (unpublished data). Some studies have reported that low testosterone levels could predict the development of type 2 diabetes and cardiovascular disease and have been linked to the increased risk of mortality in men [26–30]. It remains to be established if there is a direct connection between these diseases and vitamin K and what the different functions of MK‐4 and GGOH may be. Taken together, these findings provide novel mechanistic insights in the process of steroidogenesis and GSIS and may be useful for the development of therapeutic strategies for men.

#### **Author details**

presence of a PKA inhibitor [22]. The inconsistencies between these studies may be explained by differences in the uptake, metabolism, and solubility of each vitamin K analog used. In one example, MK‐4 was taken up faster than vitamin K1 by MG‐63 osteosarcoma cells and HepG2

In contrast to a report that MK‐4 activated PKA without increasing intracellular levels of cAMP in hepatocellular carcinoma cells [23], we showed that intracellular cAMP levels increased in a dose‐dependent manner by treatment with MK‐4 for 1.5 h in I‐10 cells (**Figure 5**). These results indicated that MK‐4 enhances testosterone and progesterone production via activation of PKA

hepatoma cells by using stable isotope‐labeled vitamin K1 and MK‐4 [28].

as well as modulation of cAMP levels in testis cells.

194 Vitamin K2 - Vital for Health and Wellbeing

**Figure 5.** Intracellular cAMP levels in I‐10 cells upon MK‐4 treatment.

[33].

**5. The MK‐4 structurally related compound, geranylgeraniol**

The C20 isoprenoid compound, geranylgeraniol (GGOH, **Figure 6**), is a functional side‐chain component (the geranylgeranyl group) of MK‐4 and has been shown to have a similar biological function as MK‐4. In addition, GGOH may also have anti‐tumorigenic effects against prostate cancer [29], colon cancer [30], leukemia [31], as well as anti‐inflammatory activity in rats [32]. Our latest study revealed a time‐ and dose‐dependent increase in testosterone and progesterone production in GGOH‐treated I‐10 cells. As expected, addition of GGOH stimu‐ lated also the PKA signaling pathway and augmented intracellular cAMP levels in I‐10 cells

Hsin‐Jung Ho, Hitoshi Shirakawa\* and Michio Komai

\*Address all correspondence to: shirakah@m.tohoku.ac.jp

Laboratory of Nutrition, Graduate School of Agricultural Science, Tohoku University, Sendai, Japan
