INVOLVEMENT OF CASPASE-1 AND CASPASE-3 IN THE PRODUCTION AND PROCESSING OF MATURE HUMAN INTERLEUKIN-18 IN MONOCYTIC THP.1 CELLS

Recently, human interleukin 18 (hIL-18) cDNA was cloned, and the recom binant protein with a tentatively assigned NH2-terminal amino acid seq uence was generated. However, natural hIL-18 has not yet been isolated , and its cellular processing is therefore still unclear. To clarify t his, we purified natural hIL-18 from the cytosolic extract of monocyti c THP.1 cells. Natural hIL-18 exhibited a molecular mass of 18.2 kDa, and the NH2-terminal amino acid was Tyr(37). Biological activities of the purified protein were identical to those of recombinant hIL-18 wit h respect to the enhancement of natural killer cell cytotoxicity and i nterferon-gamma production by human peripheral blood mononuclear cells . We also found two precursor hIL-18 (prohIL-18)-processing activities in the cytosol of THP.1 cells, These activities were blocked separate ly by the caspase inhibitors Ac-YVAD-CHO and Ac-DEVD-CHO. Further anal yses of the partially purified enzymes revealed that one is caspase-1, which cleaves prohIL-18 at the Asp(36)-Tyr(37) Site to generate the m ature hIL-18, and the other is caspase-3, which cleaves both precursor and mature hIL-18 at Asp(71)-Ser(72) and Asp(76)-Asn(77) to generate biologically inactive products, These results suggest that the product ion and processing of natural hIL-18 are regulated by two processing e nzymes, caspase-1 and caspase-3, in THP.1 cells.