title: Protein kinase C inhibition and x-linked inhibitor of apoptosis protein degradation contribute to the sensitization effect of luteolin on tumor necrosis factor-related apoptosis-inducing ligand-induced apoptosis in cancer cells.
abstract: Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is an important member of the TNF superfamily with great potential in cancer therapy. Luteolin is a dietary flavonoid commonly found in some medicinal plants. Here we found that pretreatment with a noncytotoxic concentration of luteolin significantly sensitized TRAIL-induced apoptosis in both TRAIL-sensitive (HeLa) and TRAIL-resistant cancer cells (CNE1, HT29, and HepG2). Such sensitization is achieved through enhanced caspase-8 activation and caspase-3 maturation. Further, the protein level of X-linked inhibitor of apoptosis protein (XIAP) was markedly reduced in cells treated with luteolin and TRAIL, and ectopic expression of XIAP protected against cell death induced by luteolin and TRAIL, showing that luteolin sensitizes TRAIL-induced apoptosis through down-regulation of XIAP. In search of the molecular mechanism responsible for XIAP down-regulation, we found that luteolin and TRAIL promoted XIAP ubiquitination and proteasomal degradation. Next, we showed that protein kinase C (PKC) activation prevented cell death induced by luteolin and TRAIL via suppression of XIAP down-regulation. Moreover, luteolin inhibited PKC activity, and bisindolylmaleimide I, a general PKC inhibitor, simulated luteolin in sensitizing TRAIL-induced apoptosis. Taken together, these results present a novel anticancer effect of luteolin and support its potential application in cancer therapy in combination with TRAIL. In addition, our data reveal a new function of PKC in cell death: PKC activation stabilizes XIAP and thus suppresses TRAIL-induced apoptosis.
authors.affiliation: Department of Community, Occupational, and Family Medicine, Yong Loo Lin School of Medicine, National University of Singapore, Republic of Singapore.
authors.forename: Ran-Xin
authors.initials: RX
authors.lastname: Shi
authors.forename: Choon-Nam
authors.initials: CN
authors.lastname: Ong
authors.forename: Han-Ming
authors.initials: HM
authors.lastname: Shen
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology; D000971:Antineoplastic Combined Chemotherapy Protocols / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects*; D051017:Apoptosis Regulatory Proteins; D053148:Caspase 3; D020169:Caspases / Q000378:metabolism; D045744:Cell Line, Tumor; D015536:Down-Regulation; D004357:Drug Synergism; D004789:Enzyme Activation / Q000187:drug effects; D019073:HT29 Cells; D006367:HeLa Cells; D006801:Humans; D047311:Luteolin / Q000008:administration & dosage / Q000494:pharmacology*; D008562:Membrane Glycoproteins / Q000008:administration & dosage / Q000494:pharmacology*; D009369:Neoplasms / Q000188:drug therapy / Q000473:pathology; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D011493:Protein Kinase C / Q000037:antagonists & inhibitors* / Q000378:metabolism; D011506:Proteins / Q000378:metabolism*; D053221:TNF-Related Apoptosis-Inducing Ligand; D014409:Tumor Necrosis Factor-alpha / Q000008:administration & dosage / Q000494:pharmacology*; D025801:Ubiquitin / Q000378:metabolism; D051636:X-Linked Inhibitor of Apoptosis Protein
chemical_list: D000970:Antineoplastic Agents; D051017:Apoptosis Regulatory Proteins; D008562:Membrane Glycoproteins; D011506:Proteins; D053221:TNF-Related Apoptosis-Inducing Ligand; C505653:TNFSF10 protein, human; D014409:Tumor Necrosis Factor-alpha; D025801:Ubiquitin; D051636:X-Linked Inhibitor of Apoptosis Protein; C496866:XIAP protein, human; D011493:Protein Kinase C; C505407:CASP3 protein, human; D053148:Caspase 3; D020169:Caspases; D046988:Proteasome Endopeptidase Complex; D047311:Luteolin