Decreased AQP5 expression in irradiated rats submandibular gland

Objectives: The molecular mechanisms of radiation-induced xerostomia remain unclear. The purpose of this study was to test the hypothesis that Aquaporin-5 (AQP5 )expression down-regulation in salivary gland after irradiation may be one of the main mechanisms of radiation-induced xerostomia. Methods: Saliva were collected from rat submandibular glands by intraorally cannulation and analyzed at 3 and 30 days postirradiation. The mRNA and protein expressions of AQP5 in the submandibular glands were assessed by semi-quantitative RT-PCR and Western blot. The localization and alteration of AQP5 expression were further determined by in situ hybridization and immunohistochemistry.Results: The body weight, submandibular gland weight and saliva secretion of irradiated rats significantly decreased by 12.7 %, 24.7 %, 32.2 % on day 3 and 24.0 %, 16.6 %, 38.4 % on day 30 of postirradiation respectively. There were significant increase in protein concentration and osmolality of saliva in irradiated rats on both day 3 and 30 of postirradiation. However, there were no significant differences in total saliva protein secretion between irradiated and control rats. RT-PCR analysis showed that mRNA expression of AQP5 was significantly down regulated by 37.3 % and 51.6 % in irradiated rats on day 3 and 30 of postirradiation respectively. Immunoblotting results showed that after irradiation, AQP5 expression was reduced by 40 % and 56 % at the protein level. Immunohistochemical analysis demonstrated that AQP5 expression decreased throughout the submandibular gland. Conclusion: These results provide direct evidence that irradiation treatment impairs saliva secretion by decreasing epithelial cell membrane water permeability mediated by AQP5. Down-regulation of AQP5 expresson at least partially accounts for the radiation-induced xerostomia.

Key Words: salivary gland, radiation, AQP5,