Involvement of osteopontin as a core protein in craniopharyngioma calcification formation

Matrix proteins are considered to be essential for biomineralization and to be important factors in cranioharyngioma calcification. Osteopontin (OPN) is a noncollagenous, acidic bone-matrix glycoprotein, which binds tightly to hydroxyapatite and appears to form an integral part of the mineralized matrix, probably important to the integrity of cell-matrix interactions. OPN is also a ligand for a cell membrane receptor, CD44v6, which is required for efficient OPN binding. To investigate the role of OPN in craniopharyngioma calcification formation, we studied the involvement of OPN and CD44v6 in craniopharyngiomas. Immunohistochemical staining was used in human craniopharyngiomas to detect the relationship of OPN and degree of calcification, and immunogold localization of OPN was done to identify cell secretory granules. OPN expression was elevated in calcification craniopharyngioma samples. Immunohistochemical staining demonstrated that the OPN expression level was significantly correlated with the degree of calcification (chi(2) = 29.987, P < 0.001); meanwhile, immunoelectron microscopy revealed subcellular sites of OPN presenting in epithelial cells, suggesting that the OPN protein is probably synthesized and secreted by stellate reticulum-like cells and ameloblast-like cells. At the same time, OPN expression was paralleled by cell surface reactivity for CD44v6, an OPN functional receptor. These results suggest that OPN is possibly involved as a core protein in the formation of craniopharyngioma calcification.