LIPOPOLYSACCHARIDE ACTIVATES CASPASE-1 (INTERLEUKIN-1-CONVERTING ENZYME) IN CULTURED MONOCYTIC AND ENDOTHELIAL-CELLS

Interleukin-1 beta (IL-1 beta) is a pleiotropic proinflammatory cytoki ne. Mechanisms leading to its secretion include not only release of ne wly synthesized protein, but also cleavage of a preformed immature pre cursor protein into an active secretory form by the intracellular prot ease caspase-1 (formerly termed IL-1-converting enzyme [ICE]). Caspase -1 belongs to a rapidly growing family of cysteine proteases with subs trate specificity for aspartate involved in cellular apoptosis. We hav e used an assay determining the caspase-1 activity based on cleavage o f a fluorogenic peptide substrate to elucidate its role in lipopolysac charide (LPS)-induced secretion of IL-1 beta. We show that LPS induces moderate caspase-1 activity in the monocytic cell line THP-1, in fres hly isolated peripheral blood monocytes, and in human umbilical vein e ndothelial cells (HUVECs) in a time- and dose-dependent fashion. Caspa se-1 activation by LPS was associated with cleavage of the IL-1 beta p recursor protein that was followed by release of the mature IL-1 beta protein in monocytic cells. In contrast, subsequent release of IL-1 be ta by HUVECs was not significant. LPS-induced caspase-1 activation app eared not to result from modulation of caspase-1 transcript accumulati on and inhibition of caspase-1 activity was accomplished by two specif ic inhibitors, YVAD-CHO and YVAD-CMK, capable of alleviating the relea se of mature IL-1 beta. Taken together, these results show that LPS mo derately activates caspase-1 and that caspase-1 activation contributes to LPS induction of IL-1 beta secretion. (C) 1998 by The American Soc iety of Hematology.