EVIDENCE FOR APOPTOSIS SIGNAL-REGULATING KINASE-1 IN THE REGENERATINGPALATAL EPITHELIUM UPON ACUTE INJURY

Apoptosis signal-regulating kinase 1 (ASK1), a recently identified mit ogen-activated protein (MAP) kinase kinase kinase, is a key element in the mechanism of stress-and cytokine-induced apoptosis. However, path ophysiologic roles of ASK1 in vivo are poorly understood. In the prese nt study, we analyzed the ASK1 expression in injured rat palate using an immunohistochemical approach to investigate the roles of ASK1 durin g the process of wound healing. In the normal rat palatal epithelium, a weak cytoplasmic staining of ASK1 was observed in keratinocytes of t he prickle cell layer. After mucoperiosteal injury of the palate, ASK1 was clearly observed in the suprabasal keratinocytes surrounding the wound. ASK1 expression was most evident at Day 2 after injury in the e dge of the migrating epithelium. Thereafter, the intensity of ASK1 sta ining decreased gradually until the re-epithelialization was completed at Day 10 to 14. A staining with the terminal deoxynucleotidyl transf erase-mediated dUTP-biotin nick-end-labeling method identified a numbe r of apoptotic keratinocytes in the suprabasal layers at the healing e dge. Active induction of epithelial apoptosis was readily detectable f rom Day 5 after injury. In double-staining analysis, the temporal and spatial pattern of ASK1 expression correlated well with the appearance of apoptotic keratinocytes. p38 MAP kinase, a downstream component of ASK1, was found to be activated at the sites of ASK1 expression, sugg esting that increased expression of ASK1 leads to activation of downst ream MAP kinase signaling pathway in vivo. These results suggest a sig nificant contribution of ASK1 to the epithelial apoptosis in the proce ss of mucoepithelial wound repair.