Periostin is a 90 kDa secreted protein, originally identified in murine 
osteoblast-like cells, with a distribution restricted to collagen-rich tissues 
and certain tumors. In this paper, we first analyzed the expression of periostin 
mRNA and protein in human fetal osteoblasts (hFOB) and human osteosarcoma (hOS) 
cell lines by RT real-time PCR and Western blot, respectively. The hFOB 1.19 and 
three hOS (MHM, KPDXM and Eggen) showed highly variable periostin mRNA levels 
and protein. Second, we showed that the expression of periostin mRNA was 
inversely related to the cells' abilities to differentiate and mineralize. Then, 
we investigated the regulation of periostin mRNA in hFOB after siRNA treatment 
and in mouse primary osteoblasts (mOB) treated with PTH. Knock-down of periostin 
mRNA, down-regulated PTHrP, but did not affect the expression of other important 
markers of differentiation such as RUNX2. In addition, periostin mRNA was 
transiently up-regulated in osteoblasts by PTH. Finally, the localization of 
periostin and its partially co-localization with collagen 1a1 mRNA and protein 
was studied in mouse embryos and postnatal pups using in situ hybridization and 
immunohistochemistry, respectively. In conclusion, the present study provides 
novel observations related to the expression, distribution and regulation of 
periostin in bone cells and extracellular matrix.