The role of protein kinase C in multidrug resistance

Expression of basic fibroblast growth factor correlates with resistance to paclitaxel in human patient tumors

BACKGROUND

Preclinical results indicate acidic fibroblast growth factor (aFGF) and basic FGF (bFGF) present in solid tumors as a cause of broad-spectrum chemoresistance, whereas earlier clinical studies suggest that bFGF expression is associated with opposing outcomes in patients. We investigated the relationship between FGF expression and paclitaxel activity in tumors from bladder, breast, head and neck, ovarian, and prostate cancer patients.

MATERIALS AND METHODS

Tumors (n = 96) were maintained in three-dimensional histocultures, retaining tumor-stromal interaction. Bladder tumors were treated with paclitaxel for 2 h, and the other tumors for 24 h. Antiproliferative and proapoptotic effects of paclitaxel were quantified and correlated with expression of aFGF, bFGF, P-glycoprotein (Pgp), p53, and bcl-2.

RESULTS

Fifty-one percent (49/96) and 63% (61/96) of tumors showed aFGF and bFGF staining, respectively. aFGF expression was positively correlated with tumor stage (p < 0.01), and bFGF expression with tumor grade and Pgp expression (p < 0.05). Paclitaxel inhibited antiproliferation in 86% of tumors (83/96), with an average inhibition of 46 +/- 19% (mean +/- SD) in the responding tumors. Paclitaxel also induced apoptosis in 96% of tumors (92/96), with an average apoptotic index of 12 +/- 7% in the responding tumors. aFGF expression did not correlate with tumor sensitivity to paclitaxel, whereas bFGF expression showed an inverse correlation (p < 0.01). bFGF expression was a stronger predictor of paclitaxel resistance compared to Pgp, p53, or Bcl-2.

CONCLUSION

These results support a role of bFGF in paclitaxel resistance in human patient tumors.

Preclinical studies on NSC290205 aza-steroid alkylator activity in combination with adriamycin against lymphoid leukaemia

Summary NSC290205 (A) is an hybrid synthetic antineoplastic ester that is a combination of a d-lactam derivative of androsterone and an alkylating derivative of N,N-bis(2-chloroethyl)aniline. We tested NSC290205 for synergistic antileukaemic activity with adriamycin (ADR), (i) in vitro against the human lymphoid leukaemia cell lines: CCRF-CEM, MOLT-4, and RPMI-8226, (ii) in vivo against P388 lymphocytic and L1210 lymphoid murine leukaemias (at incipient and advanced phase). Our results indicated significant cytostatic and cytotoxic synergy of NSC290205 and ADR in vitro. We further examined these results in vivo by replacing cyclophosphamide in the standard CHOP (cyclophosphamide, hydroxydaunomycin, Oncovin, prednisone) regimen with NSC290205 (AHOP) and comparing the efficiency of these two regimens in vivo. Although treatment of P388 and L1210 with cyclophosphamide or NSC290205 alone yielded equivalent results, AHOP produced a clear benefit for survival compared with CHOP against advanced leukaemias, confirming the in vitro observations [higher percentage increase in median lifespan of treated animals over the untreated (control): 188% and 239% in L1210, 308% and 353% in P388, P < 0.01, for CHOP and AHOP respectively]. AHOP also proved to be more genotoxic and cytostatic than CHOP, inducing higher sister chromatid exchange levels and cell division delays on P388 cells in vivo. NSC290205 showed superior antineoplastic potential against lymphoid leukaemia and significant synergy with ADR, producing an excellent therapeutic outcome.

Protein kinase C targeting in antineoplastic treatment strategies

Neoplastic cell survival is governed by a balance between pro-apoptotic and anti-apoptotic signals. Noteworthy among several anti-apoptotic signaling elements is the protein kinase C (PKC) isoenzyme family, which mediates a central cytoprotective effect in the regulation of cell survival. Activation of PKC, and subsequent recruitment of numerous downstream elements such as the mitogen-activated protein kinase (MAPK) cascade, opposes initiation of the apoptotic cell death program by diverse cytotoxic stimuli. The understanding that the lethal actions of numerous antineoplastic agents are, in many instances, antagonized by cytoprotective signaling systems has been an important stimulus for the development of novel antineoplastic strategies. In this regard, inhibition of PKC, which has been shown to initiate apoptosis in a variety of malignant cell types, has recently been the focus of intense interest. Furthermore, there is accumulating evidence that selective targeting of PKC may prove useful in improving the therapeutic efficacy of established antineoplastic agents. Such chemosensitizing strategies can involve either (a) direct inhibition of PKC (e.g., following acute treatment with relatively specific inhibitors such as the synthetic sphingoid base analog safingol, or the novel staurosporine derivatives UCN-01 and CGP-41251) or (b) down-regulation (e.g., following chronic treatment with the non-tumor-promoting PKC activator bryostatin 1). In preclinical model systems, suppression of the cytoprotective function(s) of PKC potentiates the activity of cytotoxic agents (e.g., cytarabine) as well as ionizing radiation, and efforts to translate these findings into the clinical arena in humans are currently underway. Although the PKC-driven cytoprotective signaling systems affected by these treatments have not been definitively characterized, interference with PKC activity has been associated with loss of the mitogen-activated protein kinase (MAPK) response. Accordingly, recent pre-clinical studies have demonstrated that pharmacological disruption of the primary MEK-ERK module can mimic the chemopotentiating and radiopotentiating actions of PKC inhibition and/or down-regulation.

Potent induction of human colon cancer cell uptake of chemotherapeutic drugs by N-myristoylated protein kinase C-alpha (PKC-alpha) pseudosubstrate peptides through a P-glycoprotein-independent mechanism

Phorbol ester protein kinase C (PKC) activators and PKC isozyme over-expression have been shown to significantly reduce intracellular accumulation of chemotherapeutic drugs, in association with the induction of multidrug resistance (MDR) in drug-sensitive cancer cells and enhancement of drug resistance in MDR cancer cells. These observations constitute solid evidence that PKC plays a significant role in the MDR phenotype of cancer cells. PKC-catalyzed phosphorylation of the drug-efflux pump P-glycoprotein was recently ruled out as a contributing factor in MDR. At present, the sole drug transport-related event that has been identified as a component of the role of PKC in MDR is PKC-induced expression of the P-glycoprotein-encoding gene mdr1. The objective of this study was to test the hypothesis that PKC can modulate the uptake of chemotherapeutic drugs in cancer cells independently of P-glycoprotein. We analyzed the effects of selective PKC activators/inhibitors on the uptake of radiolabelled cytotoxic drugs by cultured human colon cancer cells that lacked P-glycoprotein activity and did not express the drug efflux pump at the level of message (mdr1) or protein. We found that the selective PKC activator 12-O-tetradecanoylphorbol-13-acetate (TPA) significantly reduced uptake of [14C] Adriamycin and [3H] vincristine in human colon cancer cells devoid of P-glycoprotein activity, and that PKC-inhibitory N-myristoylated PKC-alpha pseudosubstrate synthetic peptides potently and selectively induced uptake of the cytotoxic drugs in the phorbol ester-treated and non-treated colon cancer cells. TPA treatment of the cells did not induce expression of either P-glycoprotein or its message mdr1. In contrast with [14C]Adriamycin and [3H] vincristine uptake, [3H] 5-fluorouracil uptake by the cells was unaffected by TPA and reduced by the PKC-inhibitory peptides. These results indicate that PKC activation can significantly reduce the uptake of multiple cytotoxic drugs by cancer cells independently of P-glycoprotein, and that N-myristoylated PKC-alpha pseudosubstrate peptides potently and selectively induce uptake of multiple cytotoxic drugs in cultured human colon cancer cells by a novel mechanism that does not involve P-glycoprotein and may involve PKC isozyme inhibition. Thus, N-myristoylated PKC-alpha pseudosubstrate peptides may offer a basis for the development of agents that reverse intrinsic drug resistance in human colon cancer.

Selenocompounds induce a redox modulation of protein kinase C in the cell, compartmentally independent from cytosolic glutathione: its role in inhibition of tumor promotion

Since selenite and other redox-active selenocompounds can modify protein kinase C (PKC) in the test tube, we have determined whether or not this redox regulation occurs inside the cell despite having high concentrations of GSH and the role of this regulation in the inhibition of tumor promotion. By using phorbol ester-promoted JB6 epidermal cell transformation assay, the concentrations of selenite, selenocystine, and selenodiglutathione which are optimal for chemopreventive activity were determined. At such concentrations (0.5 to 2 microM) in the cells treated with these agents, only a slight but transient decrease in PKC activity was observed when measured with a low (5 microM), but not with a high (100 microM) concentration of ATP. However, when the cells were serum starved or pretreated with 2-deoxyglucose, there was a pronounced but transient inactivation of PKC when assayed with both low and high concentrations of ATP. The inactivation was reversed in the cell by an endogenous mechanism or by treatment with thiol agents in the test tube. In spite of a substantial (90%) depletion of GSH in the cells by pretreatment with buthionine sulfoximine, there was no further increase in the redox modification of PKC by selenite as well as no change in the inhibitory effect of selenite on the phorbol ester-stimulated induction of ornithine decarboxylase, which is an intermediate marker related to cell transformation. While GSH is known to influence certain actions of selenium, it may not be required to mediate the effects of selenite tested in this study. The water-soluble cytosolic GSH did not interfere with the redox modification of PKC probably due to the shielding of the cysteine-rich region of the enzyme by a weak hydrophobic association with the membrane. Due to the presence of cofactors in the crude cell extracts, PKC was more sensitive to selenite than in the purified form and was inactivated by low concentrations of selenite (IC50 = 0.05 microM). This modification was reversed by thiol agents as well as by NADPH. A protein disulfide reductase, which can regenerate PKC, was present in the homogenate. Conceivably, selenite and other selenocompounds induce a redox modification of cellular PKC, compartmentally independent from the cytosolic GSH, but intimately connected to a NADPH-dependent reductase system, to mediate, at least in part, some of the cancer-preventive actions.

Expression, subcellular distribution and response to phorbol esters of protein kinase C (PKC) isozymes in drug-sensitive and multidrug-resistant KB cells evidence for altered regulation of PKC-alpha

Protein kinase C (PKC) comprises a family of related phospholipid-dependent serine/threonine protein kinases. PKC has been implicated in the induction and maintenance of the multidrug-resistance (MDR) phenotype but the role of different isozymes is not well understood. We compared the expression and subcellular distribution, and membrane association and down-regulation induced by phorbol esters, of individual PKC isozymes in drug-sensitive KB-3 and multidrug-resistant KB-V1 human carcinoma cell lines. Immunoblotting with isozyme-specific antibodies indicated the presence of PKC alpha (cytosol only). PKC beta (membrane only). PKC epsilon (mainly membrane associated) and PKC zeta (both fractions). PKC delta and PKC gamma were not detected. The expression levels of PKC beta. PKC epsilon and PKC zeta were unchanged in KB-V1 cells; PKC alpha was modestly increased ( approximately 65%) in the resistant cells as further determined by enzyme assay. The cytosolic nature and increased expression of PKC alpha were confirmed by immunofluorescent localization studies. Revertant cells, obtained by culturing KB-V1 cells in a drug-free medium, regained drug sensitivity with a loss of P-glycoprotein and a concomitant decrease in expression of PKC alpha, KB-V1 cells were found to differ markedly from KB-3 cells with respect to the translocation and down-regulation specifically of PKC alpha upon exposure to 12-O-tetradecanoyl-1-phorbol-13-acetate (TPA). Treatment with 30 nM TPA for 24 h completely depleted KB-3 cells of PKC alpha whereas 1 microM TPA was required to deplete KB-V1 cells of PKC alpha. Similar results were obtained when phorbol-12, 13-dibutyrate was used instead of TPA. Defective TPA-mediated down-regulation of PKC alpha was also observed in another PKC alpha-overexpressing MDR cell line. KB-A1. Importantly, cellular uptake of radiolabeled phorbol ester was similar for both drug-sensitive and MDR cells. Sensitive and resistant cells exhibited similar expression levels of RACK1, a PKC-binding protein important in activation-induced translocation. These findings further highlight the importance of PKC alpha in the MDR phenotype, and suggest that this isozyme may be expressed in a modified form or be subject to an altered regulation in MDR cells.