Caspase-8 and apoptosis-inducing factor mediate a cytochrome c-independent pathway of apoptosis in human colon cancer cells induced by the dietary phytochemical chlorophyllin.
Chlorophyllin (CHL), an antimutagenic and anticarcinogenic water-soluble derivative of chlorophyll, was recently found to be highly effective as a chemopreventive agent in a high-risk population exposed unavoidably to aflatoxin B(1) in the diet (P. A. Egner et al., Proc. Natl. Acad. Sci. USA, 98: 14601-14606, 2001). The current study examined the response of HCT116 human colon cancer cells to CHL treatment. Cells exposed to concentrations in the range 0.0625-0.5 mM CHL underwent growth arrest and apoptosis after 24 h, with the formation of a sub-G(1) peak in the attached cell population and nuclear condensation in the floating cell population. There was a concentration-dependent attenuation of mitochondrial membrane potential (deltapsi(m)) without the release of cytochrome c or activation of the caspase-9/caspase-3/poly(ADP-ribose) polymerase pathway. However, apoptosis-inducing factor was released from mitochondria into the cytosol and translocated to the nucleus, leading to concentration-dependent cleavage of nuclear lamins. The upstream mediators of this CHL-induced apoptosis pathway were identified as caspase-8/caspase-6 and truncated Bid, acting in conjunction with other proapoptotic members of the Bcl-2 family, such as Bak. These findings suggest that CHL might trigger apoptosis via interaction with putative "death receptors" in the plasma membrane of cancer cells, leading to initial cleavage of procaspase-8 and activation of subsequent downstream events, resulting in the destruction of nuclear lamins. Importantly, E-cadherin and alkaline phosphatase, which are indicators of cell differentiation, were strongly induced at all concentrations of CHL. Thus, in addition to being an effective blocking agent during the initiation phase, these findings support a role for CHL as a suppressing agent and as a possible novel therapeutic strategy directed toward aberrant cell proliferation in the colon.
Díaz
G Darío
GD
Linus Pauling Institute, Oregon State University, Corvallis, Oregon 97331, USA.
Li
Qingjie
Q
Dashwood
Roderick H
RH
D000469:Alkaline Phosphatase / Q000096:biosynthesis; D016588:Anticarcinogenic Agents / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects* / Q000502:physiology; D053477:Apoptotic Protease-Activating Factor 1; D015820:Cadherins / Q000096:biosynthesis; D053178:Caspase 6; D053181:Caspase 8; D053453:Caspase 9; D020169:Caspases / Q000378:metabolism*; D002454:Cell Differentiation / Q000187:drug effects / Q000502:physiology; D002467:Cell Nucleus / Q000378:metabolism; D002735:Chlorophyllides / Q000494:pharmacology*; D003110:Colonic Neoplasms / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology; D003574:Cytochrome c Group / Q000378:metabolism / Q000502:physiology*; D004789:Enzyme Activation; D006801:Humans; D007425:Intracellular Membranes / Q000187:drug effects / Q000502:physiology; D034882:Lamins / Q000378:metabolism; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D011506:Proteins / Q000378:metabolism; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000096:biosynthesis; D014407:Tumor Cells, Cultured
C506592:APAF1 protein, human; D016588:Anticarcinogenic Agents; D053477:Apoptotic Protease-Activating Factor 1; D015820:Cadherins; D002735:Chlorophyllides; D003574:Cytochrome c Group; D034882:Lamins; D011506:Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D000469:Alkaline Phosphatase; C505514:CASP6 protein, human; C505530:CASP8 protein, human; C506433:CASP9 protein, human; D053178:Caspase 6; D053181:Caspase 8; D053453:Caspase 9; D020169:Caspases; C007020:chlorophyllin