Activation of PKC is sufficient to induce an apoptotic program in salivary gland acinar cells

Abstract

Accumulating evidence suggests that specific isoforms of PKC may function to promote apoptosis. We show here that activation of the conventional and novel isoforms of PKC with 12-O-tetradecanoyl phorbol-13-ester (TPA) induces apoptosis in salivary acinar cells as indicated by DNA fragmentation and activation of caspase-3. TPA-induced DNA fragmentation, caspase-3 activation, and morphologic indicators of apoptosis, can be enhanced by pretreatment of cells with the calpain inhibitor, calpeptin, prior to the addition of TPA. Analysis of PKC isoform expression by immunoblot shows that TPAinduced downregulation of PKCa and PKCd is delayed in cells pre-treated with calpeptin, and that this correlates with an increase of these isoforms in the membrane fraction of cells. TPA-induced apoptosis is accompanied by biphasic activation of the c-jun-N-terminal kinase (JNK) pathway and inactivation of the extracellular regulated kinase (ERK) pathway. Expression of constitutively activated PKCa or PKCd, but not kinase negative mutants of these isoforms, or constitutively activated PKCe, induces apoptosis in salivary acinar cells, suggesting a role for these isoforms in TPAinduced apoptosis. These studies demonstrate that activation of PKC is sufficient for initiation of an apoptotic program in salivary acinar cells.

The Localization of Protein Kinase Cδ in Different Subcellular Sites Affects Its Proapoptotic and Antiapoptotic Functions and the Activation of Distinct Downstream Signaling Pathways

“…The JNK pathway played a major role in the apoptotic effect of PKCyNuc because inhibition of JNK significantly decreased the apoptotic effect of the PKCy-Nuc. JNK has been implicated in the regulation of cell apoptosis in response to various stimuli (51,52) and has been shown to act downstream of PKCy (16,51,53,54). A role for the nuclear PKCy in the activation of JNK is further supported by recent studies that showed that the nuclear translocation of PKCy by etoposide preceded the activation of JNK in salivary gland acinar cells (30,53).…”

“…JNK has been implicated in the regulation of cell apoptosis in response to various stimuli (51,52) and has been shown to act downstream of PKCy (16,51,53,54). A role for the nuclear PKCy in the activation of JNK is further supported by recent studies that showed that the nuclear translocation of PKCy by etoposide preceded the activation of JNK in salivary gland acinar cells (30,53). Thus, JNK may represent another nuclear PKCy substrate in addition to the already well-characterized substrates, p73 (19) and DNA-PK (20).…”

Inhibition of PKCα induces a PKCδ-dependent apoptotic program in salivary epithelial cells

“…We have previously shown that activation of PKC by phorbol ester is sufficient to induce an apoptotic program in parotid salivary epithelial cells 15 and that PKCd is essential for genotoxin-induced cell death in these cells. 13,16 Here, we demonstrate that inhibition of endogenous PKCa induces an apoptotic program in salivary epithelial cells.…”

“…[5][6][7] Serine/threonine protein kinases are also known to regulate apoptosis, including the phosphoinositide 3-kinase/AKT pathway, 8,9 members of the mitogen-activated protein kinase family (MAPKs), [10][11][12] and the protein kinase C (PKC) pathway. [13][14][15][16][17] The PKC family consists of 11 isoforms, with individual isoforms exhibiting varying substrate specificity, as well as differences in their subcellular localization and response to specific stimuli. 18,19 This argues that specific isoforms of PKC play unique roles in transducing cell signals.…”

PKCα activation down-regulates ATM and radio-sensitizes androgen-sensitive human prostate cancer cells in vitro and in vivo

“…4,5 PKC activated by TPA inhibits apoptotic responses to TNFα, 6,7 chemotherapy, 8 growth factor withdrawal 9 and radiation. 10 However, in some cell types TPA-activated PKC induces, rather than attenuates apoptosis, such as in EBV-infected Burkitt Lymphoma cells, 11 immature thymocytes, 12 salivary gland acinar cells, 13 human MCF-7 breast cancer cells 14 and LNCaP cells. 1,15 Using genetic and pharmacologic inhibition of specific PKC isozymes Garcia-Bermejo et al, 2 showed that both PKCα and PKCδ mediate TPA-induced apoptosis in LNCaP cells, although the mechanism of action remains undefined.…”