The prodomain of caspase-1 enhances Fas-mediated apoptosis through facilitation of caspase-8 activation

Caspase-1 (interleukin-1 beta converting enzyme) is produced in the form of a latent precursor, which is cleaved to yield a prodomain in addition to t he p20 and p10 subunits. It has been established that the (p20/p10)(2) hete rotetramer processes the latent precursor of interleukin-1 beta into an act ive form during apoptosis, but the function of the residual prodomain of ca spase-1 (Pro-C1) has not been established. To evaluate the involvement of P ro-C1 in apoptosis, a Pro-C1 expression vector was transfected into the HeL a cell line, which is susceptible to Fas-mediated apoptosis. Expression of recombinant Pro-C1 in HeLa cells enhanced apoptosis mediated by Fas, but no t etoposide-induced apoptosis. This enhancement of Fas-mediated apoptosis w as abolished by inhibitors of caspase-8 (Ile-Glu-Thr-Asp-fluoromethyl keton e) and caspase-3 (Asp-Glu-Val-Asp-aldehyde) but was only slightly diminishe d by an inhibitor of caspase-1 (acetyl- Tyr-Val-Ala-Asp-chloromethyl ketone ). During apoptosis induced by an agonistic anti-Fas antibody, the activati on of caspase-8 and caspase-3 was more pronounced and occurred more rapidly in HeLa/Pro-C1 cells than in the empty vector transfectant (HeLa/vec) cell s; in contrast, caspase-1 was not activated in either HeLa/Pro-C1 or HeLa/v ec cells. These results demonstrate an additional and novel function for ca spase-1 in which Pro-C1 acts to enhance Fas-mediated apoptosis, most probab ly through facilitation of the activation of caspase-8.