Adrenomedullin is expressed during rodent dental tissue development and promotes cell growth and mineralization

Background information. ADM (adrenomedullin) has pleiotropic effects, including regulation of inflammation, infection, angiogenesis, mineralized-tissue formation and development. Recently, we demonstrated up-regulation of the ADM transcript in diseased pulpal tissue while the protein is sequestered within the dentine extracellular matrix during dentinogenesis. The present study aimed to characterize ADM localization during rodent dental tissue development and determine its potential effects on dental cells. Finally, we sought to profile ADM transcript levels in adult organs and tissues to compare its expression in teeth relative to other tissues. Results. Immunohistochemical analysis of developmental rat oral tissues indicated that, at E16 (embryonic day 16), ADM was present in dental epithelium and, by El 8, ADM localized to the dental papilla and inner and outer dental epithelia. By E20, ADM was detected in secretory odontoblasts and ameloblasts and exhibited a similar expression profile to that of the key dentinogenesis signalling molecule, TGF-beta 1 (transforming growth factor-beta 1). Cell growth analysis in the dental MDPC-23, OD-21 and control 3T3 cell lines exposed to ADM (range 10(-15)-10(-7) M) together with EDTA-extracted DMPs (dentine matrix proteins) (range 0.00001-1000 mg/ml) containing comparable concentrations of ADM demonstrated that ADM stimulated a biphasic response in dental cell growth, comparable with that of DMPs, with peak stimulation observed at similar to 10(-11) M. For mineralization analysis, cell lines were exposed to combinations of 50 mu g/ml ascorbic acid, 10 mM beta-G (beta-glycerophosphate), 10(-8) M DEX (dexamethasone) and ADM (range 10-15-10-7 M). The results demonstrated that ADM could substitute for DEX to stimulate mineralization. Postnatally, multiple tissue expression profiling indicated abundant ADM levels in tongue and pulpal tissues. Conclusions. During oral and dental tissue development ADM initially localizes to epithelial tissue, whereas during later stages it is present in mineralized secreting cells, including odontoblasts. ADM may regulate proliferation and mineralization processes during development, whereas, in adulthood, it may be important for maintaining dental tissue homoeostasis.