Expression and distribution of osteopontin and matrix metalloproteinase (MMP)-3 and -7 in mouse salivary glands

We investigated the expression and distribution of osteopontin in mouse salivary glands. Western blot analysis showed intense positive bands at the predicted molecular mass (about 60 kDa) in mouse parotid and sublingual glands. However, a cross-reacted band around 30 kDa was strongly detected in submandibular glands. Indirect immunofluorescent analysis showed that osteopontin was localized at the luminal (apical) membranes of the acinar cells in parotid and sublingual glands. However, it was not detected in acinar cells of submandibular glands. No expression was found in ductal cells of any glands. We also examined the expression of matrix metalloproteinase (MMP)-3 and -7. In parotid gland, MMP-3 was observed at 57 kDa, indicating a latent form, but MMP-7 was not detected. In contrast, MMP-7 definitely was observed at 28 kDa area in submandibular gland, whereas MMP-3 was not detected. These results suggest that osteopontin localizes at luminal sites of acinar cells and may be associated with saliva secretion in mouse salivary gland. It is also suggested that osteopontin may be cleaved by MMP-7 in mouse submandibular gland.