Induction of cIAP-2 in Human Colon Cancer Cells through PKCδ/NF-κB

Activation of protein kinase C (PKC) prevents apoptosis in certain cells; however, the mechanisms are largely unknown. Inhibitors of apoptosis (IAP) family members, including NAIP, cIAP-1, cIAP-2, XIAP/hILP, survivin, and BRUCE, block apoptosis by binding and potently inhibiting caspases. Activation of NF-κB contributes to cIAP-2 induction; however, the cellular mechanisms regulating cIAP-2 expression have not been entirely defined. In this study, we examined the role of the PKC and NF-κB pathways in the regulation of cIAP-2 in human colon cancers. We found that cIAP-2 mRNA levels were markedly increased in human colon cancer cells by treatment with the phorbol ester, phorbol-12-myristate-13-acetate (PMA), or bryostatin 1. Inhibitors of the Ca²⁺-independent, novel PKC isoforms, but not inhibitors of MAPK, PI3-kinase, or PKA, blocked PMA-stimulated cIAP-2 mRNA expression, suggesting a role of PKC in PMA-mediated cIAP-2 induction. Pretreatment with the PKCδ-selective inhibitor rottlerin or transfection with an antisense PKCδ oligonucleotide inhibited PMA-induced cIAP-2 expression, whereas cotransfection with a PKCδ plasmid induced cIAP-2 promoter activity, which, taken together, identifies a role for PKCδ in cIAP-2 induction. Treatment with the proteasome inhibitor, MG132 or inhibitors of NF-κB (e.g. PDTC and gliotoxin), decreased PMA-induced up-regulation of cIAP-2. PMA-induced NF-κB activation was blocked by either GF109203x, MG132, PDTC, or gliotoxin. Moreover, overexpression of PKCδ-induced cIAP-2 promoter activity and increased NF-κB transactivation, suggesting regulation of cIAP-2 expression by a PKCδ/NF-κB pathway. In conclusion, our findings demonstrate a role for a PKC/NF-κB-dependent pathway in the regulation of cIAP-2 expression in human colon cancer cells. These data suggest a novel mechanism for the anti-apoptotic function mediated by the PKCδ/NF-κB/cIAP-2 pathway in certain cancers.