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

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 activity.

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

trast, we did not observe difference in the serum levels of inorganic phosphate between *Pxr* knockout mice and wild-type mice [32], suggesting the existence of different mechanisms

Vitamin K, SXR, and GGCX http://dx.doi.org/10.5772/63983 27

We also proposed SXR/PXR-dependent mechanism concerning vitamin K effect on articular cartilage [35]. We found that systemic *Pxr* knockout mice displayed aging-dependent wearing of articular cartilage of knee joints (**Figure 2**). Remarkable reduction of width and an enlarged gap between femoral and tibial articular cartilage were observed in *Pxr* knockout mice in 8 month-old and 13-month-old mice, but not in 4-month-old mice, indicating this is an agingdependent process. With microarray analyses using ATDC5 chondrocytic cells overexpressing human SXR, we identified *Fam20a* (family with sequence similarity 20a) as an SXR-dependent gene induced by SXR ligands. We showed FAM20A related to the higher expression of COL2A1, a main component of extracellular matrix of the articular cartilages, suggesting the cartilage-protective effect of FAM20A. These results are consistent with epidemiological studies showing relationship between vitamin K intake and osteoarthritis and supporting the

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

1 Genomics for Longevity and Health, Tokyo Metropolitan Institute of Gerontology, Tokyo,

2 Division of Gene Regulation and Signal Transduction, Research Center for Genomic Medi-

potential roles of vitamin K in preventing osteoarthritis caused by aging.

according to the mouse strains and/or environment.

**6. Conclusion**

**Author details**

Kotaro Azuma1

Japan

contributes to human culture and welfare.

and Satoshi Inoue1,2\*

\*Address all correspondence to: sinoue@tmig.or.jp

cine, Saitama Medical School, Saitama, Japan

trast, we did not observe difference in the serum levels of inorganic phosphate between *Pxr* knockout mice and wild-type mice [32], suggesting the existence of different mechanisms according to the mouse strains and/or environment.

We also proposed SXR/PXR-dependent mechanism concerning vitamin K effect on articular cartilage [35]. We found that systemic *Pxr* knockout mice displayed aging-dependent wearing of articular cartilage of knee joints (**Figure 2**). Remarkable reduction of width and an enlarged gap between femoral and tibial articular cartilage were observed in *Pxr* knockout mice in 8 month-old and 13-month-old mice, but not in 4-month-old mice, indicating this is an agingdependent process. With microarray analyses using ATDC5 chondrocytic cells overexpressing human SXR, we identified *Fam20a* (family with sequence similarity 20a) as an SXR-dependent gene induced by SXR ligands. We showed FAM20A related to the higher expression of COL2A1, a main component of extracellular matrix of the articular cartilages, suggesting the cartilage-protective effect of FAM20A. These results are consistent with epidemiological studies showing relationship between vitamin K intake and osteoarthritis and supporting the potential roles of vitamin K in preventing osteoarthritis caused by aging.
