**6. Conclusion**

**5. SXR-mediated vitamin K functions on bone and cartilage**

activity.

26 Vitamin K2 - Vital for Health and Wellbeing

As described above, we proposed another mode of vitamin K function as a ligand of a nuclear receptor, SXR, and its murine ortholog, PXR. We showed that SXR is also expressed in osteoblastic cell lines and is activated by vitamin K2 [3]. We further identified SXR-dependent vitamin K-responsive genes by microarray analysis using human osteoblastic cell line, MG63 cells stably overexpressing SXR [26]. The identified genes included *Tsukushi* which encodes a protein that has a collagen-accumulating effect [27], *Matrilin2* which encodes a protein comprising extracellular matrix like collagen [28], and *Cd14* which regulates osteoblastogenesis [29] and osteoclastogenesis by inducing differentiation of B cells [30, 31]. These genes are induced even in the presence of warfarin, indicating their induction is independent of GGCX

The involvement of SXR/PXR signaling in bone metabolism *in vivo* was suggested by the bone phenotype of systemic *Pxr* knockout mice [32]. We showed that 4-month-old female *Pxr* knockout mice had lower bone mineral density in femoral bone. Micro-CT analyses revealed fragile structure in the femoral trabecular bones of *Pxr* knockout mice. By histomorphometrical analyses, enhanced bone resorption and suppressed bone formation were observed in *Pxr* knockout mice. The mechanical strength of bone from *Pxr* knockout mice was weaker than that of wild-type mice. Negishi et al. reported the phenotype of systemic *Pxr* knockout mice from different origins [33, 34]. They also observed lower bone mineral density in *Pxr* knockout mice. They proposed a mechanism involving SLC34A2, a transporter of inorganic phosphate, expressed in the intestine. They showed *Slc34a2* is a PXRresponsive gene in the intestine and this was supported by the observation that serum levels of inorganic phosphate were significantly decreased in *Pxr* knockout mice. In con-

**Figure 2.** Aging-dependent wearing of articular cartilage of the knee joint in *Pxr* knockout mice. Representative microscopic images of articular cartilage of 13-month-old wild-type and *Pxr* knockout mice are shown. Arrowheads indicate lateral articular cartilage of the tibia. This difference was not significant in 4-month-old mice, suggesting this is aging-

dependent process. Cited from Azuma et al. [35].

In this chapter, we described multiple mechanisms of vitamin K functions clarified so far and their involvement in aging-related skeletal diseases as examples for their biological significance. Besides blood coagulation, osteoporosis, and osteoarthritis, it became gradually aware that many physiological and pathological phenomena, such as fertility [36], atherosclerosis [37–39], brain development [40], dementia [41], and glucose metabolism [42–44], are related to the status of vitamin K sufficiency. We sincerely hope that vitamin K study leads to discoveries of new biological mechanisms and targets for disease prevention and treatment and eventually contributes to human culture and welfare.
