Cell biological analysis of extracellular-signal regulated kinase (ERK) in the differentiation of ameloblasts

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We investigated the morphological changes of ameloblasts induced by PD98059, an inhibitor of MEK-1, to clarify a role of ERK in the process of ameloblast differentiation. Immunolocalization of extracellular regulated kinase (ERK) and MAP/ERK kinase (MEK) in enamel organ was also examined. Western blotting using the lysate of enamel organ revealed that cells in enamel organ expressed ERK and MEK. At 1 and 3 h after PD98059 administration, there was no morphological change in ameloblasts at any stages. At 6 h, abnormal morphology of ameloblasts was evident at the transitional stage. The layer of ameloblasts showed irregular appearance. Numerous apoptotic cells with nuclear condensation were seen in this layer. However, ameloblasts at other stages did not exhibit severe abnormality. Intense immunoreactivity for ERK was detected in the distal region of ameloblasts at the transitional stage. Immunoelectron microscopy revealed that ERK localized in cytoplasm close to cell adhesion structures. ERK is thought to play an important role in cell proliferation, differentiation and apoptosis. Our results suggest that ERK is involved in morphological changes and apoptosis of ameloblasts at the transitional stage. We also assessed other transcription regulating molecules, such as Sp family and NF-□B. The localization of these molecules leads to the consideration that they are closely related with the process of amelogenesis. These results indicate that several signal transduction may regulate the differentiation of ameloblasts.