Phorbol ester-induced loss of colchicine ultrasensitivity in chronic lymphocytic leukaemia lymphocytes

Prolonging microtubule dysruption enhances the immunogenicity of chronic lymphocytic leukaemia cells

Cytotoxic chemotherapies do not usually mediate the expression of an immunogenic gene programme in tumours, despite activating many of the signalling pathways employed by highly immunogenic cells. Concomitant use of agents that modulate and complement stress-signalling pathways activated by chemotherapeutic agents may then enhance the immunogenicity of cancer cells, increase their susceptibility to T cell-mediated controls and lead to higher clinical remission rates. Consistent with this hypothesis, the microtubule inhibitor, vincristine, caused chronic lymphocytic leukaemia (CLL) cells to die rapidly, without increasing their immunogenicity. Protein kinase C (PKC) agonists (such as bryostatin) delayed the death of vincristine-treated CLL cells and made them highly immunogenic, with increased stimulatory abilities in mixed lymphocyte responses, production of proinflammatory cytokines, expression of co-stimulatory molecules and activation of c-Jun N-terminal kinase (JNK), p38 and nuclear factor kappa B (NF-kappaB) signalling pathways. This phenotype was similar to the result of activating CLL cells through Toll-like receptors (TLRs), which communicate 'danger' signals from infectious pathogens. Use of PKC agonists and microtubule inhibitors to mimic TLR-signalling, and increase the immunogenicity of CLL cells, has implications for the design of chemo-immunotherapeutic strategies.

Protein kinases in the regulation of apoptosis in B-cell chronic lymphocytic leukemia

The involvement of several protein kinase pathways in the regulation of apoptosis and cell survival has been analyzed in a wide range of models. This article reviews current understanding of the protein kinases involved in the control of apoptosis in B-cell chronic lymphocytic leukemia (B-CLL) cells. Protein kinase C (PKC), phosphatidylinositol 3-kinase (P13K) and nuclear factor-kappa B (NF-kappaB) play important roles in the survival of these leukemic cells. These survival pathways affect proteins involved in the control of apoptosis by altering their expression or function. The elucidation of the signal transduction network involved in the survival of B-CLL cells could provide novel pharmacological targets for the therapy of B-CLL.

Involvement of protein kinase C and phosphatidylinositol 3-kinase pathways in the survival of B-cell chronic lymphocytic leukemia cells

B-cell chronic lymphocytic leukemia (B-CLL) is characterized by the accumulation of long-lived CD5(+) B lymphocytes. TPA (12-O-tetradecanoylphorbol 13- acetate) and interleukin-4 (IL-4) inhibit apoptosis of B-CLL lymphocytes ex vivo. We used specific inhibitors of protein kinase C (PKC), extracellular-regulated kinase (ERK), and phosphatidylinositol 3-kinase (PI3-kinase) to study their involvement in TPA- and IL-4-induced survival of B-CLL lymphocytes. BisI, a specific inhibitor of PKC, induced apoptosis and inhibited the antiapoptotic activity of TPA and IL-4. B-CLL cells have a basal PKC activity that was increased by TPA but not by IL-4. TPA, but not IL-4, induced ERK activation. However, the inhibition of ERK activation did not affect the viability of B-CLL lymphocytes, demonstrating that this pathway is not involved in their survival. Inhibition of PI3-kinase by LY294002 induced apoptosis of B-CLL cells and inhibited the survival effect of IL-4 and TPA. In addition, Akt, a downstream effector of PI3-kinase activity, was phosphorylated by TPA and IL-4 in B-CLL cells, though PI3-kinase had no effect on PKC-dependent phosphorylation of Akt. Furthermore, the inhibition of PKC or PI3-kinase increased dexamethasone- and fludarabine-induced apoptosis ex vivo in the presence of survival factors. These results demonstrate that PKC and PI3-kinase are involved in the survival of B-CLL cells and suggest that inhibitors of these pathways could be combined with the drugs used in the treatment of B-CLL.

Effects of protein kinase C modulators on multidrug resistance in human glioma cells

To identify the role of protein kinase C (PKC) in multidrug resistance, the effects of phorbol-12-myristate-13-acetate (PMA), a PKC activator, or calphostin C, a PKC inhibitor, on intracellular vincristine accumulation and expression of P-glycoprotein phosphorylation were studied in one multidrug-resistant and three multidrug-sensitive human glioma cell lines. Basal PKC activities and immunoreactivities of PKC-alpha and -zeta were higher in multidrug-resistant cells than in multidrug-sensitive cells. There was no significant difference in the immunoreactivity of PKC-delta between multidrug-resistant and -sensitive cells, and immunoreactive PKC-beta, -gamma, and -epsilon were not detected in either multidrug-resistant or -sensitive cells. The treatment of multidrug-resistant cells with 100 nM PMA for 2 hours resulted in the activation not of PKC-zeta but of PKC-alpha, with concomitant decrease in vincristine accumulation and increase in P-glycoprotein phosphorylation. The exposure of multidrug-resistant cells to 100 nM PMA for 24 hours induced down-regulation not of PKC-zeta but of PKC-alpha, with concurrent decrease in vincristine accumulation, and reduced but still increased P-glycoprotein phosphorylation. The treatment of multidrug-resistant cells with 100 nM calphostin C for 2 hours decreased immunoreactive PKC-zeta and not immunoreactive PKC-alpha, inducing increase in vincristine accumulation, with concomitant decrease in P-glycoprotein phosphorylation. There was no evidence of significant change in vincristine accumulation in multidrug-sensitive cells treated with PMA or calphostin C. This may suggest that at least two isozymes of PKC, PKC-alpha and -zeta, are involved in P-glycoprotein phosphorylation and that vincristine efflux function in multidrug-resistant human glioma cells is closely associated with P-glycoprotein phosphorylation and is decreased by PKC inhibitor.

Effects of phosphorylation of P-glycoprotein on multidrug resistance

Cells expressing elevated levels of the membrane phosphoprotein P-glycoprotein exhibit a multidrug resistance phenotype. Studies involving protein kinase activators and inhibitors have implied that covalent modification of P-glycoprotein by phosphorylation may modulate its biological activity as a multidrug transporter. Most of these reagents, however, have additional mechanisms of action and may alter drug accumulation within multidrug resistant cells independent of, or in addition to, their effects on the state of phosphorylation of P-glycoprotein. The protein kinase(s) responsible for P-glycoprotein phosphorylation has(ve) not been unambiguously identified, although several possible candidates have been suggested. Recent biochemical analyses demonstrate that the major sites of phosphorylation are clustered within the linker region that connects the two homologous halves of P-glycoprotein. Mutational analyses have been initiated to confirm this finding. Preliminary data obtained from phosphorylation- and dephosphorylation-defective mutants suggest that phosphorylation of P-glycoprotein is not essential to confer multidrug resistance.

Induction of apoptosis in chronic lymphocytic leukemia cells and its prevention by phorbol ester

Chronic lymphocytic leukemia lymphocytes were used to study mechanisms involved in apoptosis (programmed cell death). Apoptosis, which was determined by morphological changes including cell death and by internucleosomal DNA fragmentation, occurred during culture for 1 to 2 days in a portion of the cells from three of the four patients tested. Most of the cells underwent apoptosis and DNA fragmentation was greatly enhanced when cells were cultured in the presence of the microtubule inhibitor colchicine, the topoisomerase II inhibitor etoposide, or the glucocorticoid methylprednisolone. Tumor-promoting phorbol esters inhibited spontaneous DNA fragmentation and cell death including that induced by colchicine, etoposide, and methylprednisolone, indicating that they act on an event common to apoptosis caused by diverse stimuli. Phorbol esters probably act through protein phosphorylation, since they were effective at concentrations which modulated protein kinase C (PKC) and their action was prevented by H-7, which binds to and inactivates the catalytic site of PKC. In the absence of phorbol ester, H-7 itself induced some apoptosis. These findings implicate PKC in the suppression of apoptosis, but its precise role requires systematic investigation.

Transient enhancement of multidrug resistance by the bile acid deoxycholate in murine fibrosarcoma cells in vitro

Recent studies have implicated protein kinase C (PKC) activation in drug resistance in vitro. PKC can be activated directly by phorbol-ester tumor promoters as well as by the bile acid deoxycholate. In this report, we demonstrate that deoxycholate, at concentrations that are chronically present in the lumen of the colon in vivo, mimicked phorbol-ester tumor promoters by protecting Adriamycin (ADR)-sensitive and multidrug-resistant (MDR) murine fibrosarcoma UV-2237M cells from ADR cytotoxicity. Deoxycholate also enhanced the resistance of the MDR cell line UV-2237M-ADRR to the cytotoxic effects of vincristine and vinblastine. In contrast to cytotoxic drug-selected MDR phenotypes, deoxycholate-induced drug resistance was transient and required continuous exposure to the bile acid. The protein kinase inhibitor H7 completely reversed the protection against ADR cytotoxicity conferred on UV-2237M-ADRR cells by deoxycholate, providing evidence that deoxycholate exerts its protective effects by a mechanism that involves stimulation of protein phosphorylation and not merely by detergent effects on membrane permeability. PKC consists of a family of at least seven isozymes with distinct modes of activation and substrate specificities. We previously reported that MDR UV-2237M cell lines contain higher levels of PKC activity than the parental ADR-sensitive UV-2237M cell line (O'Brian et al., FEBS Lett 246: 78-82, 1989). The present report shows that PKC-III is a major PKC isozyme in ADR-sensitive and MDR UV-2237M cell lines. Thus, the resistance to ADR induced by the phorbol esters in UV-2237M cell lines provides strong evidence that PKC-III activation confers protection against ADR on ADR-sensitive and MDR UV-2237M cell lines. Furthermore, since deoxycholate is an endogenous molecule in the colonic epithelium, our finding that physiological concentrations of deoxycholate can render cells more resistant to chemotherapeutic drugs in vitro may have implications for the biology and therapy of intestinal cancers.

Phorbol esters induce multidrug resistance in human breast cancer cells

Mechanisms responsible for broad-based resistance to antitumor drugs derived from natural products (multidrug resistance) are incompletely understood. Agents known to reverse the multidrug-resistant phenotype (verapamil and trifluoperazine) can also inhibit the activity of protein kinase C. When we assayed human breast cancer cell lines for protein kinase C activity, we found that enzyme activity was 7-fold higher in the multidrug-resistant cancer cells compared with the control, sensitive parent cells. Exposure of drug-sensitive cells to the phorbol ester phorbol 12,13-dibutyrate [P(BtO)2] led to an increase in protein kinase C activity and induced a drug-resistance phenotype, whereas exposure of drug-resistant cells to P(BtO)2 further increased drug resistance. In sensitive cells, this increased resistance was accompanied by a 3.5-fold increased phosphorylation of a 20-kDa particulate protein and a 35-40% decreased intracellular accumulation of doxorubicin and vincristine. P(BtO)2 induced resistance to agents involved in the multidrug-resistant phenotype (doxorubicin and vincristine) but did not affect sensitivity to an unrelated alkylating agent (melphalan). The increased resistance was partially or fully reversible by the calcium channel blocker verapamil and by the calmodulin-antagonist trifluoperazine. These data suggest that stimulation of protein kinase C plays a role in the drug-transport changes in multidrug-resistant cells. This may occur through modulation of an efflux pump by protein phosphorylation.