Effects of sequential exposure to lipopolysaccharide and heat stress on dental pulp cells

In the present study, we examined the effects of sequential exposure to bacterial lipopolysaccharide (LPS) and heat stress on dental pulp cells. LPS induced the proliferation of pulp cells through the activation of p38 MAPK. HSP27 was expressed in cells with or without LPS during the entire period of heat stress, while transiently phosphorylated by short-term heat stress. In LPS-treated cells, short-term heat stress also induced the phosphorylation of HSF1. The immediate phosphorylation of HSF1 and HSP27 in LPS-treated cells by short-term heat stress occurred dependent on the activation of p38 MAPK. However, with long-term heat stress, the activation of HSF1 and induction of HSP27 occurred independent of p38 MAPK. Further, full activation of Akt in LPS-treated cells was immediately induced by short-term heat stress and lasted during the entire period of heat stress. IkappaB alpha was induced and phosphorylated throughout sequential exposure to LPS and heat stress. These results suggest that LPS has the unique effects on the cytoprotection and the cell death of pulp cells during heat stress through the modification and the activation of heat stress responsive molecules, HSF1 and HSP27, and cell survival molecules, Akt and NF-kappaB/IkappaB alpha.