Early growth response-1 is a regulator of DR5-induced apoptosis in colon cancer cells.
BACKGROUND
Tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) induces tumour cell apoptosis by binding to death receptor 4 (DR4) and DR5. DR4 and DR5 activation however can also induce inflammatory and pro-survival signalling. It is not known how these different cellular responses are regulated and what the individual role of DR4 vs DR5 is in these processes.


METHODS
DNA microarray study was carried out to identify genes differentially expressed after DR4 and DR5 activation. RT-PCR and western blotting was used to examine the expression of early growth response gene-1 (Egr-1) and the proteins of the TRAIL signalling pathway. The function of Egr-1 was studied by siRNA-mediated knockdown and overexpression of a dominant-negative version of Egr-1.


RESULTS
We show that the immediate early gene, Egr-1, regulates TRAIL sensitivity. Egr-1 is constitutively expressed in colon cancer cells and further induced upon activation of DR4 or DR5. Our results also show that DR4 mediates a type II, mitochondrion-dependent apoptotic pathway, whereas DR5 induces a mitochondrion-independent, type I apoptosis in HCT15 colon carcinoma cells. Egr-1 drives c-FLIP expression and the short splice variant of c-FLIP (c-FLIP(S)) specifically inhibits DR5 activation.


CONCLUSIONS
Selective knockdown of c-FLIP(S) sensitises cells to DR5-induced but not DR4-induced apoptosis and Egr-1 exerts an effect as an inhibitor of the DR5-induced apoptotic pathway, possibly by regulating the expression of c-FLIP(S).
Mahalingam
D
D
Department of Biochemistry and National Centre of Biomedical Engineering Science, National University of Ireland, Galway, Ireland.
Natoni
A
A
Keane
M
M
Samali
A
A
Szegezdi
E
E
D017209:Apoptosis / Q000235:genetics*; D053446:CASP8 and FADD-Like Apoptosis Regulating Protein / Q000235:genetics / Q000502:physiology; D002277:Carcinoma / Q000235:genetics* / Q000473:pathology; D003110:Colonic Neoplasms / Q000235:genetics* / Q000473:pathology; D051766:Early Growth Response Protein 1 / Q000037:antagonists & inhibitors / Q000235:genetics / Q000502:physiology*; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D006801:Humans; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism; D050505:Mutant Proteins / Q000235:genetics / Q000502:physiology; D034741:RNA, Small Interfering / Q000494:pharmacology; D053220:Receptors, TNF-Related Apoptosis-Inducing Ligand / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D018124:Receptors, Tumor Necrosis Factor / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D011994:Recombinant Proteins / Q000494:pharmacology; D015398:Signal Transduction / Q000187:drug effects / Q000235:genetics; D053221:TNF-Related Apoptosis-Inducing Ligand / Q000494:pharmacology; D014162:Transfection; D014407:Tumor Cells, Cultured
D053446:CASP8 and FADD-Like Apoptosis Regulating Protein; C506386:CFLAR protein, human; C497325:EGR1 protein, human; D051766:Early Growth Response Protein 1; D050505:Mutant Proteins; D034741:RNA, Small Interfering; D053220:Receptors, TNF-Related Apoptosis-Inducing Ligand; D018124:Receptors, Tumor Necrosis Factor; D011994:Recombinant Proteins; D053221:TNF-Related Apoptosis-Inducing Ligand; C105190:TNFRSF10A protein, human