Significance of salivary adrenomedullin in the maintenance of oral health: Stimulation of oral cell proliferation and antibacterial properties

Abstract

Adrenomedullin (ADM) promotes epithelial cell proliferation and antimicrobial activity in the gastrointestinal tract. Since ADM is also present in saliva, it was the objective of our study to investigate the role of salivary ADM in the maintenance of oral health. We found mRNA for ADM and the specific receptors CRLR-RAMP2 and CRLR-RAMP3 expressed by the salivary glands and by oral keratinocytes. The hormone was detected in the glandular tissues by western blot, being slightly bigger than the synthetic peptide, indicating a posttranslational modification. ADM was localized using immunohistochemistry and immunofluorescence. Staining specific for ADM was observed near the cell nuclei of the salivary ducts and acini. There was no correlation between ADM from matched saliva and serum of healthy volunteers. The physiological role of salivary ADM in the oral cavity was investigated by incubating buccal keratinocytes with ADM and measurement of the cell proliferation using bromodeoxyuridine (BrDU) assays. There was a significant, dose dependent increase (up to 5-fold; p < 0.001) of the BrDU incorporation after stimulation with ADM (1.5 to 50 ng/mL). The antibacterial properties of salivary ADM was studied by incubation of Gram+ and Gram− bacteria and yeast, isolated from human oral flora, with ADM (0.01–1000 ng/mL) for 24 h. Bacterial growth was inhibited dose dependently ( p < 0.001), whereas the yeast was not affected. This finding was consistent when using radial growth inhibition test on agarose plates as well as photometric measurement in microtiter plates. Our findings suggest an important role of salivary ADM in the maintenance of oral health, being involved as well as in oral cell proliferation and anti-bacterial defense.