Myrtucommulone (MC) is a unique, nonprenylated acylphloroglucinol contained in the leaves of myrtle (Myrtus communis). Here, we addressed the potential of MC to induce apoptosis of cancer cells. MC potently induced cell death of different cancer cell lines (EC(50) 3-8 microM) with characteristics of apoptosis, visualized by the activation of caspase-3, -8 and -9, cleavage of poly(ADP-ribose)polymerase (PARP), release of nucleosomes into the cytosol, and DNA fragmentation. MC was much less cytotoxic for non-transformed human peripheral blood mononuclear cells (PBMC) or foreskin fibroblasts (EC(50) cell death = 20-50 microM), and MC up to 30 microM hardly caused processing of PARP, caspase-3, -8 and -9 in human PBMC. MC-induced apoptosis was mediated by the intrinsic rather than the extrinsic death pathway. Thus, MC caused loss of the mitochondrial membrane potential in MM6 cells and evoked release of cytochrome c from mitochondria. Interestingly, Jurkat cells deficient in caspase-9 were resistant to MC-induced cell death and no processing of PARP or caspase-8 was evident. In cell lines deficient in either CD95 (Fas, APO-1) signalling, FADD or caspase-8, MC was still able to potently induce cell death and PARP cleavage. Conclusively, MC induces apoptosis in cancer cell lines, with marginal cytotoxicity for non-transformed cells, via the mitochondrial cytochrome c/Apaf-1/caspase-9 pathway.

5-Lipoxygenase (5-LO) is a tightly regulated enzyme in the synthesis of bioactive lipids from arachidonic acid. Here, we demonstrate that 5-LO is regulated by caspases, which are signaling molecules that control critical biological processes by means of specific limited proteolysis. Cell splitting of the Epstein-Barr virustransformed B lymphocytic cell line BL41-E95-A caused a pronounced, but transient, reduction of functional 5-LO protein, accompanied by the appearance of a 62-kDa 5-LO cleavage product. In parallel, splitting of BL41-E95-A cells induced activation of caspase-6 (casp-6) and casp-8. Caspase activation and 5-LO degradation were blocked by the protein-synthesis inhibitor cycloheximide, and cell-permeable peptide inhibitors of casp-6 and casp-8 prevented 5-LO cleavage. Activation of casp-6 and casp-8 was connected to subsequent enhancement of cell proliferation, whereas selective caspase inhibition blocked cell growth. Last, isolated human 5-LO was cleaved by recombinant casp-6 in vitro to a 58-kDa fragment. Based on site-directed mutagenesis studies, 5-LO is cleaved by casp-6 after Asp-170, which in a homology-based 3D model of 5-LO is located on the enzyme periphery. We suggest that splitting of BL41-E95-A cells induces de novo synthesis of a protein involved in the activation of casp-6, which cleaves 5-LO.apoptosis ͉ leukotriene ͉ lymphocytes T he enzyme 5-lipoxygenase (5-LO) initiates the synthesis of the bioactive leukotrienes (LTs) from arachidonic acid (for review, see ref. 1). LTs cause constriction and mucus secretion in the lung, increase vascular permeability, and are potent vasoconstrictors of coronary arteries, but they also induce inflammatory reactions (2). In view of these properties, LTs are regarded as powerful mediators that may have crucial roles in asthma and inflammatory disorders (2), and in vascular diseases (3) and cancer (4). Expression and cellular catalysis of 5-LO is tightly regulated. Thus, 5-LO is present mainly in mature leukocytes or dentritic cells, and the capability of cells to express 5-LO is acquired during cell maturation (5). In cells that do not express 5-LO, transcription is blocked by DNA methylation (6).Compared with the intense efforts to elucidate 5-LO expression, investigation of the fate of cellular 5-LO protein has been addressed less. Pulse-chase studies in differentiated HL60 determined the t 1/2 of 5-LO to be 26 h (7). However, the components and mechanisms of 5-LO protein degradation are unknown. Among the degradative enzymes within the cell, caspases are highly regulated cysteine proteases that control critical biological processes by means of specific limited proteolysis (for review, see ref. 8). Caspases exist as latent zymogens, which, after activation, can cleave a huge number of proteins at specific consensus sites (after Asp residues). When activated, caspases transactivate other procaspases. For example, after autoactivation of procaspase-8, the downstream caspase-3 (casp-3) that activates casp-6 is recruited. Both active casp-3 and casp-6...

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