Role of protein kinases in antitumor drug resistance

l-lactic acidosis confers insensitivity to PKC inhibitors by competing for uptake via monocarboxylate transporters

Targeting protein kinase C (PKC) family was found to repress the migration and resistance of non-small cell lung cancer cells to epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs). However, none of the PKC inhibitors has been approved for anticancer therapy yet due to the limited efficacy in clinical trials, and the underlying mechanisms remain unclear. l-lactic acidosis, a common condition comprising high l-lactate concentration and acidic pH in the tumor microenvironment, has been known to induce tumor metastasis and drug resistance. In this study, l-lactic acid was found to reverse the inhibitory effects of pan-PKC inhibitors GO6983 on PKC activity, cell migration, and EGFR-TKI resistance, but these effects were not affected by the modulators of lactate receptor GPR81. Interestingly, blockade of lactate transporters, monocarboxylate transporter-1 and -4 (MCT1 and MCT4), attenuated the intracellular level of GO6983, and its inhibitory effect on PKC activity, suggesting that lactic acid promotes the resistance to PKC inhibitors by competing for the uptake through these transporters rather than by activating its receptor, GPR81. Our findings explain the underlying mechanisms of the limited response of PKC inhibitors in clinical trials.

Screening of Pleural Mesothelioma Cell Lines for Kinase Activity May Identify New Mechanisms of Therapy Resistance in Patients Receiving Platin-Based Chemotherapy

Background

Malignant pleural mesothelioma (MPM) is a rare, predominantly asbestos-related and biologically highly aggressive tumor associated with a dismal prognosis. Multimodal therapy consisting of platinum-based chemotherapy is the treatment of choice. The reasons underlying the rather poor efficacy of platinum compounds remain largely unknown. Kinase activity might influence cellular response to these regimens.

Materials and Methods

For this exploratory study, we screened MPM cell lines (NCI-H2452, NCI-H2052, and MSTO-211H) differing in response to cisplatin and benign control fibroblasts (MRC-5) for overall phosphorylation patterns as well as kinase activity with respect to cellular response to cisplatin-based therapeutics. We analysed the cell lines for cellular kinases in a high-throughput manner using the highly innovative technique PamGene. Cell state analysis including apoptosis, necrosis, and cell viability was performed by using enzyme activity and fluorescent-based assays.

Results

Cisplatin alters cellular phosphorylation patterns affecting cell cycle, migration, adhesion, signal transduction, immune modulation, and apoptosis. In cisplatin-responsive cell lines, phosphorylation of AKT1 and GSK3B was decreased but could not be influenced in cisplatin-resistant NCI-H2452 cells. Cisplatin-responsive cell lines showed increased phosphorylation levels of JNK1/2/3 but decreased phosphorylation in cisplatin-resistant NCI-H2452 cells.

Conclusion

Kinase phosphorylation and activity might play a crucial role in cellular response to cytostatic agents. Cisplatin influences phosphorylation patterns with distinct features in cisplatin-resistant cells. These alterations exert a significant impact on cell cycle, migration, adhesion, signal transduction, immune modulation, and apoptosis of the respective tumor cells. Based on our results, the induction of p38 or JNK1/3, or inhibition of AKT1 by, for example, BIA-6, might offer a positive synergistic effect by induction of an apoptotic response to cisplatin-based treatment, thus potentially enhancing the clinical outcome of MPM patients.

Inhibition of protein phosphatase 1 stimulates noncanonical ER stress eIF2α activation to enhance fisetin-induced chemosensitivity in HDAC inhibitor-resistant hepatocellular carcinoma cells

Hepatocellular carcinoma (HCC) is a common fatal type of malignant tumor that has highly metastatic and recurrent properties. Fisetin is a natural flavonoid found in various vegetables and fruits which exhibits anti-cancer and anti-inflammatory properties, as well as other effects. Thus, we hypothesized that fisetin can act as an adjuvant therapy in cancer or drug-resistant cancer cells, and further investigated the molecular mechanisms underlying the development of drug-resistance in HCC cells. We found that fisetin effectively inhibited the cell viability of not only parental cells but also histone deacetylase inhibitors-resistant (HDACis-R) cells and enhanced the chemosensitivity of HCC cells. Interestingly, fisetin did not induce cell apoptosis through the activation of the endoplasmic reticulum (ER) stress sensor of protein kinase R (PKR)-like endoplasmic reticulum kinase, but rather through the non-canonical pathway of the protein phosphatase 1 (PP1)-mediated suppression of eIF2α phosphorylation. Moreover, fisetin-induced cell apoptosis was reversed by treatment with PP1 activator or eIF2α siRNA in HCC cells. Based on these observations, we suggest that PP1-eIF2α pathways are significantly involved in the effect of fisetin on HCC apoptosis. Thus, fisetin may act as a novel anticancer drug and new chemotherapy adjuvant which can improve the efficacy of chemotherapeutic agents and diminish their side-effects.

Protein kinase C as a target for new anti-cancer agents

Cancer joins the category of diseases involving abnormalities in the rate of proliferation of cells and is associated with uncontrolled cell division, where cells either generate their own growth-promoting stimuli or neighboring cells or do not respond to growth inhibitory signals. Protein kinase C (PKC) is one of the key elements in the tumor growth signal transduction pathways and is found to be overexpressed in several malignant cell types. A way to control cell proliferation and cell differentiation is by influencing signal transduction pathways by modulation of PKC. PKC encloses 12 different isoenzymes, and each isoenzyme is found to have a different functional property. Because specific PKC isoenzyme types are present in different (malignant) cell species, they may be an attractive target in the development of anti-cancer agents. Classification and identification of the available PKC isoenzymes in different tumor cells could be useful in targeting specific tumors. PKC also tends to be overexpressed in association with the multidrug resistance pheno-type. This concise review deals with the role of PKC isoenzymes in (tumor) cell biology and evaluates the antineoplastic agents interacting on PKC isoenzymes.

Phase I study of bryostatin 1, a protein kinase C modulator, preceding cisplatin in patients with refractory non-hematologic tumors

PURPOSE

Preclinical data suggested that bryostatin-1 (bryo) could potentiate the cytotoxicity of cisplatin when given prior to this drug. We designed a phase I study to achieve tolerable doses and schedules of bryo and cisplatin in combination and in this sequence.

METHODS

Patients with non-hematologic malignancies received bryo followed by cisplatin in several schedules. Bryo was given as an 1 and a 24 h continuous infusion, while cisplatin was always given over 1 h at 50 and 75 mg/m(2); the combined regimen was repeated on an every 3-week and later on an every 2-week schedule. Bryo doses were escalated until recommended phase II doses were defined for each schedule. Patients were evaluated with computerized tomography every 2 cycles.

RESULTS

Fifty-three patients were entered. In an every 2-week schedule, the 1-h infusion of bryo became limited by myalgia that was clearly cumulative. With cisplatin 50 mg/m(2) its recommended phase II dose was 30 microg/m(2). In the 3-week schedule, dose-limiting toxicities were mostly related to cisplatin effects while myalgias were tolerable. Pharmacokinetics unfortunately proved to be unreliable due to bryo's erratic extraction. Consistent inhibition of PKC isoform eta (eta) in peripheral blood mononuclear cells was observed following bryo.

CONCLUSIONS

Bryo can be safely administered with cisplatin with minimal toxicity; however, only four patients achieved an objective response. Modulation of cisplatin cytotoxicity by bryo awaits further insight into the molecular pathways involved.

Insulin regulates P-glycoprotein in rat brain microvessel endothelial cells via an insulin receptor-mediated PKC/NF-kappaB pathway but not a PI3K/Akt pathway

Our previous study showed that insulin restored impaired function and expression of P-glycoprotein in diabetic blood-brain barrier, and further study showed that insulin up-regulated P-glycoprotein expression and function in normal blood-brain barrier, so insulin might be one of the factors that regulated the function and expression of P-glycoprotein in blood-brain barrier of diabetes. In this study, the intracellular pathways that insulin regulated the P-glycoprotein were investigated using primarily cultured rat brain microvessel endothelial cells model. The rat brain microvessel endothelial cells were incubated in normal culture medium containing 50 mU/l insulin and different concentrations of inhibitors for 72 h. The P-glycoprotein function and expression in the rat brain microvessel endothelial cells were assessed using the uptake of P-glycoprotein substrate rhodamine 123 and western blot assay, respectively. It was found that treatment of 50 mU/l insulin significantly increased P-glycoprotein function and expression in rat brain microvessel endothelial cells. This induced effect was blocked by insulin receptor antibody, insulin receptor tyrosine kinase inhibitor I-OMe-AG538, PKC inhibitor chelerythrine and NF-kappaB inhibitor pyrrolidine dithiocarbamate ammonium (PDTC). But this induced effect was not inhibited by phosphatidylinositol 3-kinase (PI3K)/Akt inhibitor LY294002. These results indicated that insulin regulated P-glycoprotein function and expression through signal transduction pathways involving activation of PKC/NF-kappaB but not PI3K/Akt pathway.

Nuclear translocation of apoptosis inducing factor is associated with cisplatin induced apoptosis in LNCaP prostate cancer cells

Prostate cancer (PC) is considered resistant to cisplatin chemotherapy. In order to identify novel causes of resistance to cisplatin, we explored the role of Apoptosis Inducing Factor (AIF) that mediates caspase independent apoptosis in cisplatin induced cell death in PC. Similar to treatment with pancaspase inhibitor Z-VAD-fmk, cisplatin induced apoptosis in LNCaP cells was inhibited by AIF inhibitor N-acetyl-L-cysteine (NAC), treatment of LNCaP cells with NAC prevented AIF translocation to the nucleus and over-expression of recombinant AIF gene increased apoptosis. Our results suggest that AIF is associated with cisplatin induced apoptosis in PC.

Gene expression profiling revealed novel molecular targets of docetaxel and estramustine combination treatment in prostate cancer cells

Both docetaxel and estramustine are antimicrotubule agents with antitumor activity in various cancers including prostate cancer. Clinical trials for docetaxel and estramustine combination treatment have suggested improved antitumor activity in hormone-refractory prostate cancer. However, the molecular mechanisms involved in the combination treatment with docetaxel and estramustine have not been fully elucidated. In order to establish such molecular mechanisms in both hormone insensitive (PC-3) and sensitive (LNCaP) prostate cancer cells, gene expression profiles of docetaxel- and estramustine-treated prostate cancer cells were obtained by using Affymetrix Human Genome U133A Array. Total RNA from PC-3 and LNCaP cells untreated and treated with 2 nmol/L docetaxel, 4 micromol/L estramustine, or 1 nmol/L docetaxel plus 2 micromol/L estramustine for 6, 36, and 72 hours was subjected to microarray analysis. Real-time PCR and Western blot analysis were conducted to confirm the microarray data. Clustering analysis based on biological function showed that docetaxel and estramustine combination treatment down-regulated some genes that are known to regulate cell proliferation, transcription, translation, and oncogenesis. In contrast, docetaxel and estramustine combination treatment up-regulated some genes related to induction of apoptosis, cell cycle arrest, and tumor suppression. Docetaxel and estramustine also showed differential effects on gene expression between mono- and combination treatment. Combination treatment with docetaxel and estramustine caused alternations of a large number of genes, many of which may contribute to the molecular mechanisms by which docetaxel and estramustine inhibit the growth of prostate cancer cells. These results provide novel molecular targets of docetaxel and estramustine combination treatment in prostate cancer cells. This information could be utilized for further mechanistic research and for devising optimized therapeutic strategies against prostate cancer.

Gene expression profiling revealed novel mechanism of action of Taxotere and Furtulon in prostate cancer cells

Background

Both Taxotere and Capecitabine have shown anti-cancer activity against various cancers including prostate cancer. In combination, Taxotere plus Capecitabine has demonstrated higher anti-cancer activity in advanced breast cancers. However, the molecular mechanisms of action of Taxotere and Capecitabine have not been fully elucidated in prostate cancer.

Methods

The total RNA from PC3 and LNCaP prostate cells untreated and treated with 2 nM Taxotere, 110 μM Furtulon (active metabolite of Capecitabine), or 1 nM Taxotere plus 50 μM Furtulon for 6, 36, and 72 hours, was subjected to Affymetrix Human Genome U133A Array analysis. Real-time PCR and Western Blot analysis were conducted to confirm microarray data.

Results

Taxotere and Furtulon down-regulated some genes critical for cell proliferation, cell cycle progression, transcription factor, cell signaling, and oncogenesis, and up-regulated some genes related to the induction of apoptosis, cell cycle arrest, and differentiation in both cell lines. Taxotere and Furtulon also up-regulated some genes responsible for chemotherapeutic resistance, suggesting the induction of cancer cell resistance to these agents.

Conclusions

Taxotere and Furtulon caused the alternation of a large number of genes, many of which may contribute to the molecular mechanisms by which Taxotere and Furtulon inhibit the growth of prostate cancer cells. This information could be utilized for further mechanistic research and for devising optimized therapeutic strategies against prostate cancer.

Phase II trial of the combination of bryostatin-1 and cisplatin in advanced or recurrent carcinoma of the cervix: a New York Gynecologic Oncology Group study

OBJECTIVES

Bryostatin-1 is a macrocyclic lactone that has been shown to regulate protein kinase C (PKC) activity and thereby potentially inhibit tumor invasion, angiogenesis, cell adhesion, and multidrug resistance. In preclinical experiments, bryostatin-1 induces tumor growth inhibition and enhances cytotoxicity when combined with other agents including cisplatin in cervical cancer cells. It was therefore anticipated that combination bryostatin-1-cisplatin therapy would be effective in patients with cervical cancer. The current study was conducted to evaluate this therapeutic approach in patients with recurrent or advanced-stage cervical carcinoma.

METHODS

An IRB-approved New York Gynecologic Oncology Group (NYGOG) trial was activated for patients with a histological diagnosis of metastatic cervical cancer or in patients with recurrent disease not eligible for surgery or radiation. Enrolled patients received bryostatin-1 (50-65 microg/m(2)) as a 1-h infusion followed by cisplatin (50 mg/m(2)). The combined treatment was administered every 21 days.

RESULTS

Fourteen patients were enrolled. The majority of patients had squamous cell carcinoma. Ten out of fourteen patients had recurrent disease. Fifty percent of the patients received bryostatin at 50 microg/m(2) and 50% received bryostatin at 65 microg/m(2). Seventy-one percent completed two cycles of treatment. The most common grade II-III toxicities were myalgia, anemia, and nausea or vomiting. One patient developed a hypersensitivity reaction and one developed grade III nephrotoxicity. Seventy-one percent (10/14) of patients were evaluated for tumor response. Eight out of ten (80%) of patients had progressive disease and 2/10 (20%) had stable disease. There were no treatment responses.

CONCLUSIONS

Despite promising preclinical data, this clinical trial indicates that the combination of cisplatin and bryostatin-1 at the doses and schedule used is not effective in patients with advanced-stage or recurrent cervical cancer. There is even the possibility of therapeutic antagonism. The development of a serum assay for bryostatin-1 and additional mechanistic studies would be useful for future bryostatin clinical trials.

Antitumor activity of protein kinase C inhibitors and cisplatin in human head and neck squamous cell carcinoma lines

Protein kinase C (PKC) plays a pivotal role in signal transduction involved in the control of cell proliferation, differentiation and apoptosis. Interference with such signaling pathways may result in altered tumor cell response to antineoplastic drugs. We investigated the effects of two selective PKC inhibitors as single agents and in combination with cisplatin in cell lines derived from squamous cell carcinomas of the head and neck (SCCHN). Safingol (Saf) is directed against the regulatory domain, whereas chelerythrine (Che) interacts with the catalytic domain of PKC. In six SCCHN cell lines (UM-SCC 11B, 14A, 14C and 22B, 8029NA, and a 5-fold cisplatin-resistant subline 8029DDP). PKC activities ranged between 1 and 158 IU/1 x 10(7) cells, and they were inversely proportional to the amount of cellular epidermal growth factor receptor. Using the colorimetric MTT assay, PKC inhibitors Saf and Che showed comparable dose-dependent growth inhibition. The 50% inhibitory concentrations (IC50) were between 3.8-8.6 microM for Saf and 8.5-13.6 microM for Che with no relationship to PKC activity or cisplatin sensitivity of the respective cell lines. Combinations of cisplatin (IC50 = 0.4-5.8 microg/ml) and either PKC inhibitor (5 microM Saf, 10 microM Che) led to a significant decrease of cisplatin IC50 values in most cell lines. However, comparison with theoretical additive dose-response curves showed additive rather than synergistic effects for both PKC inhibitors.

Anticancer drug-mediated induction of multidrug resistance-associated genes and protein kinase C isozymes in the T-lymphoblastoid cell line CCRF-CEM and in blasts from patients with acute lymphoblastic leukemias

The major determinants mediating drug resistance in acute lymphoblastic leukemias (ALL) unresponsive to chemotherapy, are still unclear. For example, it is still unknown whether selection or induction processes are responsible for drug resistance here or whether protein kinase C (PKC) isozymes contribute to the resistant phenotype. Therefore, inducibility of resistance factors or PKC isozymes genes was examined in CCRF-CEM cells treated with diverse anticancer drugs--adriamycin, camptothecin, etoposide or vincristine--at sublethal concentrations for 24 h. MDR1, MRP1, LRP and PKC isozyme alpha, beta(1), beta(2), epsilon, iota, eta, theta, zeta gene expression was determined by cDNA-PCR. We found significant dose-dependent, mostly combined, induction of the MDR1, MRP1 and LRP genes. Significantly enhanced gene expression of the majority of PKC isozyme genes was found after treatment with camptothecin. PKCzeta was upregulated throughout by each anticancer drug applied in this setting. A series of selected CCRF-CEM-derived multidrug resistance (MDR) sublines also showed enhanced expression of the PKC isozymes compared to the parental cell line. MDR1 and PKCeta gene expression levels were correlated highly significantly. Blasts from two patients with ALL during the first week of monotherapy with steroids revealed combined induction of the MDR1, multidrug resistance-associated protein 1 (MRP1), lung cancer resistance-related protein (LRP) and most PKC isozymes, predominantly PKCzeta. Another patient with T-ALL, who failed to respond to four months of intensive chemotherapy, showed an enhanced MRP1 gene expression combined with markedly overexpression of PKCeta and PKCtheta. Furthermore, the camptothecin and etoposide-mediated induction of resistance factors in the CCRF-CEM cell line could be suppressed by staurosporine, a rather unspecific inhibitor of protein kinases. However, selective inhibitors of PKC isozymes (bisindolylmaleimide GO 6850, indolocarbazole GO 6976) produced no significant effects here. Therefore, the PKC isozymes eta, theta and zeta are of interest as potential targets to overcome drug resistance in ALL.

Cyclic AMP and fluconazole resistance in Saccharomyces cerevisiae

Cyclic AMP (cAMP) is involved in the stress response in Saccharomyces cerevisiae. We show that cAMP is required for resistance to fluconazole in S. cerevisiae. In addition, activation of Ras2, a regulator of cAMP generation, results in some protection from fluconazole toxicity in a fashion independent of the efflux transporter Pdr5p.

Pharmacokinetics and metabolism of the staurosporine analogue CGP 41 251 in mice

Studies with CGP 41 251 (I), an N-benzoylstaurosporine derivative and PKC-alpha inhibitor, revealed that oral administration of 400 microg/day of the compound to wild type mice on four successive days reversed multi drug resistance (Killion et al. Oncology Research 7: 453-459, 1995). In our study, the same regimen of administration was followed with the primary objective to establish the pharmacokinetics and metabolism of the compound and to substantiate at which plasma concentrations of CGP 41 251 multi drug resistance (MDR) reversal can be expected. Concentrations of CGP 41 251 and metabolites in plasma were determined by a validated high performance liquid chromatography (HPLC) method with fluorescence detection. Structural characterization of the metabolites was performed with HPLC and mass spectrometric detection. In our experiment extensive metabolism of CGP 41 251 was found. The presence of five hydroxylated metabolites of CGP 41 251 (I) was confirmed and two metabolites were structurally elucidated as CGP 50 750 (III) and CGP 52 421 (V). Maximal concentrations of 73 ng/ml, 1.9 ng/ml and 126 ng/ml for CGP 41 251 (I), III and V were found, respectively. The mass spectra of the other three metabolites indicate that these are oxidized nitrogens or hydroxylated compounds. As yet, the oxidation or hydroxylation sites have not been established. This study has revealed new information about CGP 41 251 pharmacokinetics and metabolism. Target levels between 10-100 ng/ml may be important to achieve in further clinical trials with CGP 41 251 as MDR modulator.

Protein kinase C isoform expression and activity alter paclitaxel resistance in vitro

OBJECTIVE

The aim of this study was to assess the relationship of protein kinase C (PKC) isoform expression and functional activity to the development of multidrug resistance in gynecologic malignancies.

METHODS

Paclitaxel-resistant subclones (T30 and T30-Res) of the Mes-sa human uterine sarcoma cell line were selected through exposure to paclitaxel in vitro. Indices of relative drug resistance were determined by the MTT (3-[4,5-dimethyl-2-thiazolyl]-2, 5-diphenyl-2H-tetrazolium bromide) assay. Differences in the expression pattern of PKC isoforms were assessed by Western blot of cell lysates. Finally, the influence of PKC activity (i.e., translocation to the plasma membrane, confirmed by Western blot of plasma membrane bound protein) on resistance to paclitaxel was examined with the MTT assay in cells preincubated with PMA.

RESULTS

The indices of relative paclitaxel resistance of Mes-sa, Mes-sa-T30, and Mes-sa-T30-Res were 1-, 5-, and 11-fold, respectively. Five (alpha, gamma, iota, lambda, and mu) of the 11 known PKC isoforms were detected in all cell lysates. Only PKC-alpha and PKC-gamma expression increased with increasing indices of paclitaxel resistance. Interestingly, PMA induction of PKC activity reversed resistance to paclitaxel in all cell lines by 2- to 3-fold, and this reversal of drug resistance was associated with a time-dependent translocation of PKC-alpha and PKC-gamma to the plasma membrane compartment.

CONCLUSIONS

Increased expression of only the PKC-alpha and PKC-gamma isoforms correlates with increasing levels of paclitaxel resistance in Mes-sa cells in this in vitro experimental model. However, increased functional activity of these and other PKC isoforms leads to reversal in paclitaxel resistance. Therefore, PKC activating mechanisms normally present in primary tumor cells may be compromised in drug-resistant clones.

Phosphorylation of the DNA repair protein APE/REF-1 by CKII affects redox regulation of AP-1

The DNA repair protein apurinic endonuclease (APE/Ref-1) exerts several physiological functions such as cleavage of apurinic/apyrimidinic sites and redox regulation of the transcription factor AP-1, whose activation is part of the cellular response to DNA damaging treatments. Here we demonstrate that APE/Ref-1 is phosphorylated by casein kinase II (CKII). This was shown for both the recombinant APE/Ref-1 protein (Km=0.55 mM) and for APE/Ref-1 expressed in COS cells. Phosphorylation of APE/Ref-1 did not alter the repair activity of the enzyme, whereas it stimulated its redox capability towards AP-1, thus promoting DNA binding activity of AP-1. Inhibition of CKII mediated phosphorylation of APE/Ref-1 blocked mutagen-stimulated increase in AP-1 binding. It also abrogated the induction of c-Jun protein and rendered cells more sensitive to induced DNA damage. Thus, phosphorylation of APE/Ref-1 appears to be involved in regulating the different physiological activities of the enzyme. CKII mediated phosphorylation of APE/Ref-1 and concomitant increase in AP-1 binding activity appears to be a novel mechanism of cellular stress response, forcing transcription of AP-1 target gene(s) the product(s) of which may exert protective function.

Protein kinases and multidrug resistance

The role of protein kinases in the multidrug resistance phenotype of cancer cell lines is discussed with an emphasis on protein kinase C and protein kinase A. Evidence that P-glycoprotein is phosphorylated by these kinases is summarised and the relationship between P-glycoprotein phosphorylation and the multidrug-resistant phenotype discussed. Results showing that protein kinase C, particularly the alpha subspecies, is overexpressed in many MDR cell lines are described: this common but by no means universal finding seems to be drug- and cell line-dependent and in only in a few cases is there a direct correlation between protein kinase C activity and multidrug resistance. From co-immunoprecipitation results it is suggested that P-glycoprotein is a specific protein kinase C receptor, as well as being a substrate. Revertant experiments provide conflicting results as to a direct relationship between expression of P-glycoprotein and protein kinase C. Evidence that protein kinase A influences P-glycoprotein expression at the gene level is well documented and the mechanisms by which this occurs are becoming clarified. Results on the relationship between protein kinase C and multidrug resistance using many inhibitors and phorbol esters are difficult to interpret because such compounds bind to P-glycoprotein. In spite of huge effort, a direct involvement of protein kinase C in regulating multidrug resistance has not yet been firmly established. However, evidence that PKC regulates a Pgp-independent mechanism of drug resistance is accumulating.