Spontaneous activation of endothelial cells: a central role for endogenous IL-1alpha

Endothelial cells assume divergent physiologic postures, from a 'quiescent' to an 'activated' state, and this ability is thought to critically regulate homeostasis of the vascular milieu. While establishing endothelial cell lines, we observed that most of the endothelial cells exhibited 'spontaneous' activation with morphological changes once the cells surpassed confluence. In the present study, we investigated whether accumulation and subsequent availability of certain factors in the cells might induce morphologic changes indicating activated phenotype in confluent endothelial cells. Cell lysate from nonactivated confluent endothelial cells activated autologous target cells, whereas culture supernatants did not. Stimulatory activity of the cell lysate was dependent on the concentration of cell lysate, i.e., nonactivated endothelial cell lysate at 25% concentration induced a substantial level of morphologic change. The 'spontaneous' as well as the cell lysate-induced change in morphology of endothelial cells was inhibited by neutralization of interleukin (IL)-1alpha with anti-IL-1alpha antibody. Correspondingly, cell lysate from confluent non-IL-1-expressing endothelial cells did not alter the morphology of autologous confluent cells even at a higher concentration. Cells that spontaneously changed their morphology correspondingly upregulated IL-1alpha and IL-8 transcripts. The results indicate that the accumulation and availability of endogenous IL-1alpha is responsible for the 'spontaneous' activation of endothelial cells in culture. Ultimately, through this property, endothelial cells may facilitate normal in vivo host responses, including the regulation of leukocyte trafficking.