A novel isoform of pro-interleukin-18 expressed in ovarian tumors is resistant to caspase-1 and -4 processing

Interleukin-18 (IL-18) is a proinflammatory cytokine synthesized as a 24 kDa inactive precursor (pro-IL-18) by several cell types, and is processed to a bioactive molecule of 18 kDa by the proteinases Caspase-1 or caspase-4. All ovarian carcinoma cell lines express pro-IL-18, only in some instances coexpress Caspase-1, and always express caspase-4; in any case, they display a defective processing of IL-18. We analysed whether pro-IL-18, present in two ovarian carcinoma cell lysates, could be processed 'in vitro' by recombinant active Caspase-1. While most of pro-IL-18 could be cleaved by Caspase-1, a residual of pro-IL-18 appeared to be resistant. Cloning and sequence analysis of the whole pro-IL-18 open reading frame demonstrated the existence of an alternatively spliced mRNA variant, which lacked exon-3 (Delta3pro-IL-18). The 12 bp exon-3 encodes for the AEDD amino-acid sequence, which is N-terminal with respect to the cleavage site of Caspase-1. Both pro-IL-18 and Delta3pro-IL-18 mRNA isoforms were detected in all ovarian Cancer cell lines analysed, while Delta3pro-IL-18 mRNA was undetectable in normal ovarian epithelial cells. The Delta3pro-IL-18 cDNA induced synthesis of an alternative Delta3pro-IL-18 protein upon transfection into a murine cell line. The Delta3pro-IL-18 protein was resistant to proteolytic activation by Caspase-1 and -4, although it was capable to bind Caspase-1. Aternative splicing of pro-IL-18 exon-3 may represent a novel mechanism of regulation of bioactive IL-18 production in human ovarian tumors.