The mycotoxin penicillic acid inhibits Fas ligand-induced apoptosis by blocking self-processing of caspase-8 in death-inducing signaling complex

Upon engagement with Fas ligand (FasL), Fas rapidly induces recruitment and self-processing of Caspase-8 via the adaptor protein Fas-associated death domain (FADD), and activated Caspase-8 cleaves downstream substrates such as Caspase-3. We have found that penicillic acid (PCA) inhibits FasL-induced Apoptosis and concomitant loss of cell viability in Burkitt's lymphoma Raji cells. PCA prevented activation of Caspase-8 and Caspase-3 upon treatment with FasL. However, PCA did not affect active Caspase-3 in FasL-treated cells, suggesting that PCA primarily blocks early signaling events upstream of Caspase-8 activation. FasL-induced processing of Caspase-8 was severely impaired in the death-inducing signaling complex, although FasL-induced recruitment of FADD and Caspase-8 occurred normally in PCA-treated cells. Although PCA inhibited the enzymatic activities of active recombinant Caspase-3, Caspase-8, and caspase-9 at similar concentrations, PCA exerted weak inhibitory effects on activation of caspase-9 and Caspase-3 in staurosporine-treated cells but strongly inhibited Caspase-8 activation in FasL-treated cells. Glutathione and cysteine neutralized an inhibitory effect of PCA on Caspase-8, and PCA bound directly to the active center cysteine in the large subunit of Caspase-8. Thus, our present results demonstrate that PCA inhibits FasL-induced Apoptosis by targeting self-processing of Caspase-8.