Regulation of TNF-Mediated Cell Death in Breast Cancer Cells

Dongmei Lu, Regulation of TNF-mediated cell death in breast cancer cells. Doctor of Philosophy (Biochemistry & Molecular Biology), May 2006, 173 pp., 32 illustrations, 329 references. Protein kinase C-ε (PKCε), a novel PKC, has been shown to attenuate tumor necrosis factor-α (TNF)-induced apoptosis in breast cancer cells. The purpose of this dissertation is to delineate the mechanism(s) by which PKCε exerts its antiapoptotic effect. Comparison of PKCε level in several breast cancer cells revealed that PKCε level alone could not explain sensitivity of breast cancer cells to TNF. Protein kinase B/Akt (Akt) was constitutively active in breast cancer cells resistant to TNF. Inhibition of phosphatidyl-inositol 3-kinase (P13-K0 by Ly294003 increased TNF-mediated apoptosis in MCF-7 cells that overexpress Akt and sensitized BT-20 and SKBR-3 cells that express constitutively active (CA) Akt to TNF. PKC inhibitor bisindolylmaleimide (BIM) also sensitized BT-20 and MCF-7 cells to TNF. Overexpression of CA-Akt in MCF-7 cells attenuated TNF-induced apoptosis. Therefore, both PKCε and Akt are important for deciding TNF sensitivity. The cross-talk between PKCε and Akt was examined in MCF-7 cells. PKCε overexpression increased basal Akt phosphorylation and enhanced TNF-induced Akt activation. Knockdown of PKCε by siRNA decreased TNF-induced Akt activation. Depletion of Akt abolished the antiapoptotic effect of PKCε. Akt was constitutively associated with PKCε and DNA-dependent protein kinase (DNA-PK), and this association was increased by TNF. Knockdown of DNA-PK diminished the effect of PKCε on Akt phosphorylation and increased TNF-mediated apoptosis. These results suggest that PKCε activates Akt via DNA-PK to mediate its antiapoptotic function. We also investigated whether PKCε regulates mitochondrial cell death pathway by inhibiting the proapoptotic function of Bcl-2 family member Bax. Overexpression of wild-type but not dominant-negative PKCε inhibited TNF-mediated mitochondrial depolarization. Depletion of Bax inhibited TNF-induced apoptosis. PKCε overexpression abolished Bax dimerization and translocation to mitochondria, while PKCε depletion had the opposite effect. Bax was associated with PKCε in PKCε-overexpressing cells. These results indicate that PKCε attenuates mitochondrial cell death pathway by inhibiting Bax translocation. These findings demonstrate the PKCε activates Akt via DNA-PK and inhibits proapoptotic Bax to mediate its antiapoptotic effect in breast cancer cells. An understanding of the mechanism(s) by which PKCε inhibits apoptosis in breast cancer cells is important for developing more effective cancer therapies.