Apoptosis is induced in both drug-sensitive and multidrug-resistant hepatoma cells by somatostatin analogue TT-232

Mechanism of 5-fluorouracil induced resistance and role of piperine and curcumin as chemo-sensitizers in colon cancer

Among cancer-related deaths worldwide, colorectal cancer ranks second, accounting for 1.2% of deaths in those under 50 years and 0.6% of deaths in those between 50 and 54 years. The anticancer drug 5-fluorouracil is widely used to treat colorectal cancer. Due to a better understanding of the drug's mechanism of action, its anticancer activity has been increased through a variety of therapeutic alternatives. Clinical use of 5-FU has been severely restricted due to drug resistance. The chemoresistance mechanism of 5-FU is challenging to overcome because of the existence of several drug efflux transporters, DNA repair enzymes, signaling cascades, classical cellular processes, cancer stem cells, metastasis, and angiogenesis. Curcumin, a potent phytocompound derived from Curcuma longa, functions as a nuclear factor (NF)-κB inhibitor and sensitizer to numerous chemotherapeutic drugs. Piperine, an alkaloid found in Piper longum, inhibits cancer cell growth, causing cell cycle arrest and apoptosis. This review explores the mechanism of 5-FU-induced chemoresistance in colon cancer cells and the role of curcumin and piperine in enhancing the sensitivity of 5-FU-based chemotherapy. CLINICAL TRIAL REGISTRATION: Not applicable.

The Role of Pyruvate Kinase M2 Posttranslational Modification in the Occurrence and Development of Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is one of the deadly malignant tumors that directly leads to the death of nearly one million people worldwide every year, causing a serious burden on society. In the presence of sufficient oxygen, HCC cells rapidly generate energy through aerobic glycolysis, which promotes tumor cell proliferation, immune evasion, metastasis, angiogenesis, and drug resistance. Pyruvate kinase M2 (PKM2) is a key rate-limiting enzyme in glycolysis. In recent years, studies have found that PKM2 not only exerts pyruvate kinase activity in the process of glucose metabolism, but also exerts protein kinase activity in non-metabolic pathways to affect tumor cell processes, and its activity is flexibly regulated by various posttranslational modifications such as acetylation, phosphorylation, lactylation, ubiquitination, SUMOylation, and so forth. This review summarizes the role of posttranslational modifications of PKM2-related sites in the development of HCC.

Development of an aptamer capable of multidrug resistance reversal for tumor combination chemotherapy

Multidrug resistance (MDR) is a major factor in the failure of many forms of tumor chemotherapy. Development of a specific ligand for MDR-reversal would enhance the intracellular accumulation of therapeutic agents and effectively improve the tumor treatments. Here, an aptamer was screened against a doxorubicin (DOX)-resistant human hepatocellular carcinoma cell line (HepG2/DOX) via cell-based systematic evolution of ligands by exponential enrichment. A 50 nt truncated sequence termed d3 was obtained with high affinity and specificity for HepG2/DOX cells. Multidrug resistance protein 1 (MDR1) is determined to be a possible recognition target of the selected aptamer. Aptamer d3 binding was revealed to block the MDR of the tumor cells and increase the accumulation of intracellular anticancer drugs, including DOX, vincristine, and paclitaxel, which led to a boost to the cell killing of the anticancer drugs and lowering their survival of the tumor cells. The aptamer d3-mediated MDR-reversal for effective chemotherapy was further verified in an in vivo animal model, and combination of aptamer d3 with DOX significantly improved the suppression of tumor growth by treating a xenograft HepG2/DOX tumor in vivo. This work demonstrates the feasibility of a therapeutic DNA aptamer as a tumor MDR-reversal agent, and combination of the selected aptamer with chemotherapeutic drugs shows great potential for liver cancer treatments.

Bioenergetic alteration in gastrointestinal cancers: The good, the bad and the ugly

Cancer cells exhibit metabolic reprogramming and bioenergetic alteration, utilizing glucose fermentation for energy production, known as the Warburg effect. However, there are a lack of comprehensive reviews summarizing the metabolic reprogramming, bioenergetic alteration, and their oncogenetic links in gastrointestinal (GI) cancers. Furthermore, the efficacy and treatment potential of emerging anticancer drugs targeting these alterations in GI cancers require further evaluation. This review highlights the interplay between aerobic glycolysis, the tricarboxylic acid (TCA) cycle, and oxidative phosphorylation (OXPHOS) in cancer cells, as well as hypotheses on the molecular mechanisms that trigger this alteration. The role of hypoxia-inducible transcription factors, tumor suppressors, and the oncogenetic link between hypoxia-related enzymes, bioenergetic changes, and GI cancer are also discussed. This review emphasizes the potential of targeting bioenergetic regulators for anti-cancer therapy, particularly for GI cancers. Emphasizing the potential of targeting bioenergetic regulators for GI cancer therapy, the review categorizes these regulators into aerobic glycolysis/ lactate biosynthesis/transportation and TCA cycle/coupled OXPHOS. We also detail various anti-cancer drugs and strategies that have produced pre-clinical and/or clinical evidence in treating GI cancers, as well as the challenges posed by these drugs. Here we highlight that understanding dysregulated cancer cell bioenergetics is critical for effective treatments, although the diverse metabolic patterns present challenges for targeted therapies. Further research is needed to comprehend the specific mechanisms of inhibiting bioenergetic enzymes, address side effects, and leverage high-throughput multi-omics and spatial omics to gain insights into cancer cell heterogeneity for targeted bioenergetic therapies.

Recombinant human interleukin 24 reverses Adriamycin resistance in a human breast cancer cell line

BACKGROUND

The major cause of multidrug resistance is over-expression of membrane P-glycoprotein (P-gp). We investigated the effect of recombinant human interleukin 24 (rhIL-24) on the Adriamycin (ADM)-resistant human breast cancer cell line MCF-7/ADM.

METHODS

The cytotoxicity of rhIL-24 and ADM was determined by 3-[4,5-dimethylthiazol-2-yl], 5-diphenyl tetrazolium bromide (MTT) assays. The expression of P-gp was assessed by confocal microscopy and Western blot analysis.

RESULTS

The IC50 values for rhIL-24 in MCF-7/wild-type and MCF-7/ADM cells were 0.17 and 14.6 μM, respectively. The IC50 value of Adriamycin in MCF-7/ADM cells decreased in a dose-dependent manner when rhIL-24 was used. The resistance modulating factor (RMF) was directly proportional to the dose of rhIL24. ADM accumulation increased while P-gp expression decreased at a low dose (4 μM) of rhIL24 in MCF-7/ADM cells. The expression of P-gp was decreased at 4 μM in confocal microscopy and western blot analysis.

CONCLUSIONS

rhIL-24 circumvented the drug-resistance of MCF-7/ADM cells via activation of the transcription factor Stat 3. rhIl24 has potential to act as a P-gp inhibitor to reverse Adriamycin resistance in breast cancer.

Octreotide enhances the sensitivity of the SKOV3/DDP ovarian cancer cell line to cisplatin chemotherapy in vitro

The present study aimed to investigate the effects of octreotide (OCT) on the reversal of resistance of cisplatin-resistant cancer cells and on enhancement of the cisplatin sensitivity of cancer cells. The 3-(4,5-dimethylthiazol-2-yl)-2-5-diphenyltetrazolium bromide method and flow cytometry were used to investigate the effect of cisplatin, OCT or the combination of these two compounds on the proliferation and apoptosis of SKOV3/DDP cells. Real-time, quantitative RT-PCR was used to detect the mRNA expression of SSTR2, MDR1, MRP2, GST-π and EGFR in SKOV3/DDP cells following OCT treatment. At the concentration of 2.5-20 μg/ml, OCT significantly reduced the IC(50) value (P<0.05) and promoted apoptosis (P<0.05) in the SKOV3/DDP cells in response to cisplatin. The synergistic effect of OCT and cisplatin on SKOV3/DDP cell proliferation was observed. SSTR2 was expressed on the SKOV3/DDP cell surface. OCT increased GST-π expression (P<0.05) and reduced MRP2 and EGFR expression (P<0.05) in a dose-dependent manner. However, it had no effect on the expression of MDR1 (P>0.05). It is suggested that OCT inhibits ovarian cancer proliferation and promotes apoptosis, via the cell surface expression of SSRT2, and reverses cisplatin resistance through the inhibition of MRP2 and EGFR expression.

Octreotide induces caspase activation and apoptosis in human hepatoma HepG2 cells

AIM: To investigate the role of octreotide on cellular proliferation and apoptosis of human hepatoma (HepG2) cells.

METHODS: We studied cellular proliferation, apoptosis and the possible internal caspase-mediated apoptosis pathway involved, after treatment of HepG2 carcinoma cells with octreotide in comparison with the apoptosis caused by tumor necrosis factor-α (TNF-α). Activities of caspase-3, caspase-9, caspase-8 and caspase-2 were studied, while apoptosis was investigated through detection of DNA fragmentation and through identification of apoptotic cells with the annexin-V/propidium iodide flow cytometric method.

RESULTS: After an initial increase in HepG2 cellular proliferation, a significant inhibition was observed with 10^-8 mol/L octreotide, while TNF-α dose-dependently decreased proliferation. Early and late apoptosis was significantly increased with both substances. Octreotide significantly increased caspase-3, caspase-8 and caspase-2 activity. TNF-α significantly increased only caspase-2. Cellular proliferation was decreased after treatment with octreotide or TNF-α alone but, in contrast to TNF-α, octreotide decreased proliferation only at concentrations of 10^-8 mol/L, while lower concentrations increased proliferation.

CONCLUSION: Our findings are suggestive of caspase-mediated signaling pathways of octreotide antitumor activity in HepG2 cells, and indicate that measurements of serum octreotide levels may be important, at least in clinical trials, to verify optimal therapeutic drug concentrations.

TT232, A Novel Signal Transduction Inhibitory Compound in the Therapy of Cancer and Inflammatory Diseases

TT-232 is a structural analogue of somatostatin exhibiting strong and selective growth-inhibitory effects, inhibition of neurogenic inflammation, as well as general anti-inflammatory and analgesic potential without the wide-ranging endocrine side effects of the parent hormone and its "traditional" analogues. The anti-inflammatory action of TT-232 is mediated through the SSTR4 receptor, and its antitumor activity is mediated through the SSTR1 receptor and by the tumor-specific isoform of pyruvate kinase. Its mechanism of action is in line with a new era of molecular medicine called signal transduction therapy, where "false" intracellular or intercellular communication is inhibited or corrected without interfering with basic cell functions and machinery. TT232 has passed phase I clinical trials without toxicity and significant side effects, and phase II studies are running for oncological and anti-inflammatory indications, respectively. This compound has the perspective to become the first drug in molecularly targeted therapy of inflammation where a combined effect of anti-inflammatory, analgesic, and neurogenic inflammation-inhibiting activity can be achieved.

Nuclear translocation of the tumor marker pyruvate kinase M2 induces programmed cell death

Cancer cells often fail to respond to stimuli that normally activate their intrinsic apoptotic machinery. Moreover, they are able to adapt to hypoxia by changing their glycolytic rate. Pyruvate kinase (PK) is a rate-limiting enzyme in glycolysis that is converted to a less active dimer form of PKM2 isoenzyme during oncogenesis. Here, we show that both somatostatin and the structural analogue TT-232 interact with the PKM subtype. We further show that the PKM2 is translocated to the nucleus in response to TT-232 and different apoptotic agents. Nuclear translocation of PKM2 is sufficient to induce cell death that is caspase independent, isoform specific, and independent of its enzymatic activity. These results show that the tumor marker PKM2 plays a general role in caspase-independent cell death of tumor cells and thereby defines this glycolytic enzyme as a novel target for cancer therapy development.

Apoptotic events induced by naturally occurring retinoids ATRA and 13-cis retinoic acid on human hepatoma cell lines Hep3B and HepG2

Two hepatoma cell lines were incubated for 72 h with ATRA and its analog 13cisRA and according to MTT assay, Hep3B cells were highly susceptible whereas HepG2 cells were more resistant to the treatment. At the high concentration of 166 microM, retinoids were able to induce apoptosis in both cell lines and the highest effect was observed in HepG2 cells treated with ATRA. TUNEL-based photometric ELISA showed that at the same retinoid concentration tested by flow cytometry, both cell lines showed apoptosis whereas plasma membranes were not significantly disrupted. Inhibitors of apoptosis Bcl-xL and survivin were downregulated in Hep3B cells by treatment with both retinoids. Bax, a pro-apoptotic protein, was not significantly upregulated in Hep3B cells, but was slightly increased in HepG2 cells treated with 13cisRA. Both procaspase-3 and procaspase-8 were cleaved in Hep3B cells, suggesting apoptosis could be triggered through the extrinsic pathway. In the case of HepG2 cells, lack of caspase activation suggests a mechanism dependent on other kind of proteases.

Expression of somatostatin receptors in normal and cirrhotic human liver and in hepatocellular carcinoma

BACKGROUND

Somatostatin analogues have been used with conflicting results to treat advanced hepatocellular carcinoma (HCC). The aim of this study was to investigate expression of somatostatin receptor (SSTR) subtypes in human liver, and to examine the effect of selective SSTR agonists on proliferation, apoptosis, and migration of hepatoma cells (HepG2, HuH7) and hepatic stellate cells (HSCs).

METHODS

Expression of SSTRs in cell lines, normal and cirrhotic liver, and HCC was examined by immunohistochemistry and reverse transcription-polymerase chain reaction. Effects of SSTR agonists on proliferation and apoptosis of tumour cells and HSCs were assessed by the 5-bromo-2' deoxyuridine and TUNEL methods, respectively. The influence of SSTR agonists on migration was investigated using Boyden chambers.

RESULTS

In normal liver, both hepatocytes and HSCs were negative for all five SSTRs. Cirrhotic liver and HCC as well as cultured hepatoma cells and HSCs expressed all five SSTRs, both at the protein and mRNA levels, except for HuH7 cells which did not immunoreact with SSTR3. None of the agonists influenced proliferation or apoptosis. However, compared with untreated cells, L-797,591, an SSTR1 agonist, reduced migration of HepG2, HuH7, and HSCs significantly to 88 (7)% (p<0.05), 83 (11)% (p<0.05), and 67 (13)% (p<0.01), respectively.

CONCLUSIONS

Cirrhotic liver and HCC express SSTRs. Although the somatostatin analogues used in this study did not affect proliferation and apoptosis, stimulation of SSTR1 may decrease invasiveness of HCC by reducing migration of hepatoma cells and/or HSCs. Clinical trials evaluating somatostatin analogues for the treatment of HCC should take these findings into account.

Antineoplastic effects of octreotide on human gallbladder cancer cells in vitro

AIM: To investigate whether octreotide can inhibit the growth of human gallbladder cancer cells in vitro and to elucidate the antineoplastic mechanism of octreotide in gallbladder cancer.

METHODS: A human gallbladder cancer cell line, GBC-SD, was cultured in vitro. The antiproliferative effects of octreotide were examined by means of an MTT assay and a colony forming ability assay. Morphological variation was investigated under scanning electron microscopy and transmission electron microscopy. Cell cycle analysis and apoptosis rate was evaluated by flow cytometry (FCM) after staining by propidium iodide. DNA fragmentation was assayed by agarose gel electrophoresis. Immunohistochemical staining was performed to evaluate the expressions of mutant-type p53 and bcl-2.

RESULTS: The growth curve and colony forming ability assay showed significant inhibition of octreotide to the proliferation of GBC-SD cells in culture in a time- and dose-dependent manner. After exposure to octreotide, GBC-SD cells showed typically apoptotic characteristics, including morphological changes of chromatin condensation, vacuolar degeneration, nucleus fragmentation and apoptotic body formation. In FCM profile apoptotic cells showed increased sub-G₁ peaks in the octreotide group, significantly higher than the control group (P = 0.013). There was also an augmentation in the cell proportion of G₀/G₁ phase (P = 0.015), while the proportion of S phase and G₂/M phase remained unchanged (P = 0.057 and P = 0.280, respectively). DNA agarose gel electrophoresis displayed a ladder after exposure to 1 000 nmol/L octreotide. After being treated with octreotide, the expressions of both mutant-type p53 and bcl-2 decreased considering the percentage of positive cells (P < 0.05).

CONCLUSION: Octreotide has a negative action to the proliferation of GBC-SD cells, and the mechanism may be related to cytostatic and cytotoxic effects. The reduction of mutant-type p53 and bcl-2 expressions may be associated with the apoptosis induced by octreotide.

TT-232: a somatostatin structural derivative as a potent antitumor drug candidate

TT-232 (D-Phe-Cys-Tyr-D-Trp-Lys-Cys-Thr-NH2) has been developed as an antitumor somatostatin analog. TT-232 has no growth hormone release inhibitory effect and does not inhibit the secretion of gastric acid. This analog induces apoptosis in and exerts pronounced antiproliferative effects on various human tumors (colon, pancreas, lymphoma, leukemia, melanoma, hepatoma) cell lines. The growth of human xenografts (prostate, breast carcinoma, lymphoma, melanoma) and animal tumors (colon-26, P-388, S-180, B16, MXT) was inhibited by TT-232 (dose range: 30-750 microg/kg/day) in 54-98% of cases. Continuous long-term infusion proved to be the most effective way of administration. TT-232 combined with decarbazine or etoposide treatment enhanced the antitumor activity of these drugs on human melanoma and lymphoma xenografts, respectively. Regarding the mode of action, TT-232 activates cell cycle inhibitors via SSTR receptors, inhibits tyrosine kinases through interfering with the proliferative signaling cascades, and interacts with an intracellular receptor and an enzyme involved in glycolysis causing translocation of this enzyme to the nucleus, thus inducing apoptosis. TT-232 may be a promising candidate in the therapy of human malignancies.

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Enhanced inhibitive effects of combination of rofecoxib and octreotide on the growth of human gastric cancer

Our previous studies indicated that cyclooxygenase-2 inhibitor or octreotide could suppress the proliferation of gastric adenocarcinoma in vitro or in vivo. The present study was aimed to find whether rofecoxib combined with octreotide could enhance the inhibitive effects on the growth of gastric cancer. The effect of rofecoxib or octreotide on proliferation of gastric cancer cell line was determined by 3H-thymidine ribotide incorporation. The TdT-mediated dUTP nick end-labeling assay was used to detect the apopotosis. To determine their synergic antineoplastic effects, the interaction between rofecoxib and octreotide on SGC-7901 cell was evaluated by the median effect plot. After orthotopical implantion of xenografts of human gastric cancer in stomach, nude mice were given rofecoxib plus octreotide for 8 weeks. Cyclooxygenase-2 in gastric cancer tissues was measured by immunohistochemistry. Combination of rofecoxib and octreotide presented synergistic effect (combination index < 1) in the majority of responses. The inhibitory rate for xenografts in nude mice was 89.7% in rofecoxib group. Combination of rofecoxib and octreotide enhanced inhibitory rate to 98.8%. The combination greatly increased the apoptotic index (78.20% +/- 6.45%) of the xenografts as compared with that of using rofecoxib alone (46.60% +/- 3.42%); the difference was very significant (p < 0.001). Rofecoxib could inhibit the activity of cyclooxygenase-2 in the tissue of gastric adenocarcinomas of nude mice. Our results indicate that combination of rofecoxib and octreotide significantly enhances the antiproliferative effect in gastric adenocarcinoma, which might have potential therapeutic value.

The development of larger cells that spontaneously escape senescence--a step during the immortalization of a human cancer cell line

There are few information concerning the changes associated with the transition interval when slow growing, primary explanted human cancer cells are displaced by new selected faster growing cells and became an immortal cell line. In a previous paper (J. Cell. Mol. Med., 5: 49-59, 2001) we described the TV cell line derived from a laryngeal tumor which harbors human papillomavirus (HPV) gene sequences throughout more than sixty in vitro passages. In this paper we analyze the modifications observed during the crisis interval when significant amount of cells senesce but occasional cells acquire some mutations that make them immortal. Confocal microscopy analysis revealed the heterogeneity of the cells in terms of their size and nucleus/cell ratio. Proliferation capacity was assessed by flow cytometry analyzing DNA content and expression of transferrin receptor (CD71). We discussed the possibility that HPV genome sequences alleviate a proliferation block during the crisis growth arrest of human larynx carcinoma cell line and the possibility that the cells monitor their size and growth by measuring the levels of some protein whose synthesis is coupled to cell development.

Antiproliferative and apoptosis-inducing effects of hemin in hepatoma cells

Hemin is an extremely versatile molecule that may have cytotoxic or cytoprotective effects on certain cells. We investigated the effect of hemin on the growth of hepatoma cells, including the multidrug-resistant ones. Searching for new tools that interfere with the growth of hepatomas is an important area of clinical research. Cell viability and proliferation of drug-sensitive and multidrug-resistant hepatoma cell lines was determined using the trypan-blue exclusion test XTT/PMS and colony-forming ability assays. Apoptosis was assessed by confocal microscopy and DNA ladder assay. Hemin inhibited the proliferation and induced apoptosis in both drug-sensitive and multidrug-resistant hepatoma cells overexpressing functional P-glycoprotein. zVAD-fmk inhibited the hemin-induced decrease in cell viability, pointing to a role of caspases in hemin-induced apoptosis. The antiproliferative and apoptosis-inducing effects of hemin might be considered in the design of treatment for patients with hepatoma.

Study of apoptosis in human liver cancers

AIM: To investigate the action of apoptosis in occurrence of liver cacinomas in vivo and the biological effect of Solanum lyratum Thumb on BEL-7404 cell line inducing apoptosis in vitro.

METHODS: The apoptosis in the liver carcinoma was detected with terminal deoxynucl neotidyl transferase mediated dUTP nick end labelling (TUNEL); the cancer cells cultured in DMED medium were treated with extract of Solanum lyratum Thumb and observed under microscope, and their DNA was assayed by gel electrophoresis.

RESULTS: In vivo apoptotic cells in the cancer adjacent tissues inceased; in vitro treatment of liver cancers with extract of Solanum lyratum Thumb could induce the cells to manifest a typical apoptotic morphology. Their DNA was fractured and a characteristic ladder pattern could be found using electrophoresis.

CONCLUSION: In vivo the apoptosis of carcinomas was lower; maybe the cells divided quickly and then the cancers occurred. In the cancer adjacent tissues, the apoptosis pricked up, and in vitro Solanum lyratum Thumb could induce the apoptosis of BEL-7404 cells.

Octreotide for cancer of the liver and biliary tree

Inoperable liver tumors have an unfavorable natural course despite various therapeutic modalities. Octreotide, a somatostatin analog, has shown considerable antitumor activity on animal models of various hepatic tumors and on isolated cell culture lines. In this paper, a review of the experimental evidence is presented. Moreover clinical papers of case reports of uncontrolled studies of patients are also reviewed. The majority of clinical studies provide evidence of a clinical and biochemical response of liver endocrine tumors while regression of tumor size is a rare event. A randomized controlled trial of octreotide in the treatment of advanced hepatocellular carcinoma has shown a significant survival benefit in the treated patients. Literature reports indicate a stimulatory effect of octreotide on Kupffer cells as a possible antitumor mechanism, but other antiproliferative actions of octreotide have been suggested but not proved. Finally the question of the presence and affinity of somatostatin receptors on liver tumor tissue is discussed. In conclusion, according to our experience, octreotide administration is the best available treatment for advanced inoperable hepatocellular carcinoma and future better patient selection, based on receptor subtypes, might further improve the results.

Modulation of protein kinase C in antitumor treatment

PKC isoenzymes were found to be involved in proliferation, antitumor drug resistance and apoptosis. Therefore, it has been tried to exploit PKC as a target for antitumor treatment. PKC alpha activity was found to be elevated, for example, in breast cancers and malignant gliomas, whereas it seems to be underexpressed in many colon cancers. So it can be expected that inhibition of PKC activity will not show similar antitumor activity in all tumors. In some tumors it seems to be essential to inhibit PKC to reduce growth. However, for inhibition of tumor proliferation it may be an advantage to induce apoptosis. In this case an activation of PKC delta should be achieved. The situation is complicated by the facts that bryostatin leads to the activation of PKC and later to a downmodulation and that the PKC inhibitors available to date are not specific for one PKC isoenzyme. For these reasons, PKC modulation led to many contradicting results. Despite these problems, PKC modulators such as miltefosine, bryostatin, safingol, CGP41251 and UCN-01 are used in the clinic or are in clinical evaluation. The question is whether PKC is the major or the only target of these compounds, because they also interfere with other targets. PKC may also be involved in apoptosis. Oncogenes and growth factors can induce cell proliferation and cell survival, however, they can also induce apoptosis, depending on the cell type or conditions in which the cells or grown. PKC participates in these signalling pathways and cross-talks. Induction of apoptosis is also dependent on many additional factors, such as p53, bcl-2, mdm2, etc. Therefore, there are also many contradicting results on PKC modulation of apoptosis. Similar controversial data have been reported about MDR1-mediated multidrug resistance. At present it seems that PKC inhibition alone without direct interaction with PGP will not lead to successful reversal of PGP-mediated drug efflux. One possibility to improve chemotherapy would be to combine established antitumor drugs with modulators of PKC. However, here also very contrasting results were obtained. Many indicate that inhibition, others, that activation of PKC enhances the antiproliferative activity of anticancer drugs. The problem is that the exact functions of the different PKC isoenzymes are not clear at present. So further investigations into the role of PKC isoenzymes in the complex and interacting signalling pathways are essential. It is a major challenge in the future to reveal whether modulation of PKC can be used for the improvement of cancer therapy.