The nuclear receptor steroid and xenobiotic receptor (SXR) is a transcriptional 
regulator activated by various biological and xenobiotic substances. We have 
recently shown that SXR is expressed in bone and that this receptor is critical 
for bone metabolism, particularly in osteoblastic cells. Vitamin K2, one of the 
critical nutrients in bone metabolism, has been demonstrated that it is a potent 
SXR agonist and modulates the expression of various bone-related genes in 
osteoblastic cells. Using microarray analysis, we identified novel SXR target 
genes that were activated by vitamin K2 in osteoblastic cells. Among them, a 
small leucine-rich repeat proteoglycan, tsukushi, has been shown to contribute 
to collagen accumulation, and the protein may interact with another vitamin 
K2-inducible SXR target, matrilin-2, a member of the matrilin family that 
functions as collagen adaptors. Besides functioning as a xenobiotic biosensor, 
our findings show that SXR is also a vitamin K2 target and an important 
transcriptional factor that regulates bone homeostasis in bone cells.