The involvement of novel protein kinase C isozymes in influencing sensitivity of breast cancer MCF-7 cells to tumor necrosis factor-α

Protein kinase C (PKC) has been implicated in tumor necrosis factor-alpha (TNF) signaling. Structurally and functionally distinct PKC activators and selective inhibitors of PKC were used to investigate the involvement of PKC isozymes in influencing TNF sensitivity in MCF-7 cells. Activators of PKC, such as phorbol-12,13-dibutyrate (PDBu) (1.0 mu M), indolactam V (10 mu M), and bryostatin 1 (1.0 mu M) decreased the sensitivity of MCF-7 cells to TNF by 5-, 10-, and 1.7-fold, respectively. The PKC-specific inhibitor bisinodlylmaleimide II (BIM) (greater than or equal to 1 mu M) antagonized the effect of PDBu in protecting MCF-7 cells against TNF cytotoxicity. High concentrations of BIM (greater than or equal to 10 mu M) also significantly enhanced the sensitivity of MCF-7 cells to TNF. In contrast, Gd 6976, a specific inhibitor of cPKCs, did not potentiate TNF sensitivity and failed to reverse the effect of PDBu. In addition, BIM but not G6 6976 blocked PDBu-mediated down-regulation of TNF receptors. There was no correlation between downregulation of PKC alpha, -delta, and -epsilon, and protection against TNF cytotoxicity by PKC activators. A 6-hr exposure to 1.0 mu M PDBu, 10 mu M indolactam V, and 1.0 mu M bryostatin 1 caused a 1.8-, 3.5- and 1.2-fold induction, respectively, of nPKC eta in MCF-7 cells. Similar exposure to BIM but not Go 6976 led to a significant down-regulation of nPKC eta. This novel regulation of PKC eta implicates this isozyme in PDBu-mediated protection of MCF-7 cells against TNF cytotoxicity.