6E11, a highly selective inhibitor of Receptor-Interacting Protein Kinase 1, protects cells against cold hypoxia-reoxygenation injury.

Necroptosis is a programmed cell death pathway that has been shown to be of central pathophysiological relevance in multiple disorders (hepatitis, brain and cardiac ischemia, pancreatitis, viral infection and inflammatory diseases). Necroptosis is driven by two serine threonine kinases, RIPK1 (Receptor Interacting Protein Kinase 1) and RIPK3, and a pseudo-kinase MLKL (Mixed Lineage Kinase domain-Like) associated in a multi-protein complex called necrosome. In order to find new inhibitors for use in human therapy, a chemical library containing highly diverse chemical structures was screened using a cell-based assay. The compound 6E11, a natural product derivative, was characterized as a positive hit. Interestingly, this flavanone compound: inhibits necroptosis induced by death receptors ligands TNF-α (Tumor Necrosis Factor) or TRAIL (TNF-Related Apoptosis-Inducing Ligand); is an extremely selective inhibitor, among kinases, of human RIPK1 enzymatic activity with a nM Kd; has a non-ATP competitive mode of action and a novel putative binding site; is weakly cytotoxic towards human primary blood leukocytes or retinal pigment epithelial cells at effective concentrations; protects human aortic endothelial cells (HAEC) from cold hypoxia/reoxygenation injury more effectively than necrostatin-1 (Nec-1) and Nec-1s. Altogether, these data demonstrate that 6E11 is a novel potent small molecular inhibitor of RIPK1-driven necroptosis.

TRAIL induces necroptosis involving RIPK1/RIPK3-dependent PARP-1 activation.

Although TRAIL (tumor necrosis factor (TNF)-related apoptosis inducing ligand) is a well-known apoptosis inducer, we have previously demonstrated that acidic extracellular pH (pHe) switches TRAIL-induced apoptosis to regulated necrosis (or necroptosis) in human HT29 colon and HepG2 liver cancer cells. Here, we investigated the role of RIPK1 (receptor interacting protein kinase 1), RIPK3 and PARP-1 (poly (ADP-ribose) polymerase-1) in TRAIL-induced necroptosis in vitro and in concanavalin A (Con A)-induced murine hepatitis. Pretreatment of HT29 or HepG2 with pharmacological inhibitors of RIPK1 or PARP-1 (Nec-1 or PJ-34, respectively), or transient transfection with siRNAs against RIPK1 or RIPK3, inhibited both TRAIL-induced necroptosis and PARP-1-dependent intracellular ATP depletion demonstrating that RIPK1 and RIPK3 were involved upstream of PARP-1 activation and ATP depletion. In the mouse model of Con A-induced hepatitis, where death of mouse hepatocytes is dependent on TRAIL and NKT (Natural Killer T) cells, PARP-1 activity was positively correlated with liver injury and hepatitis was prevented both by Nec-1 or PJ-34. These data provide new insights into TRAIL-induced necroptosis with PARP-1 being active effector downstream of RIPK1/RIPK3 initiators and suggest that pharmacological inhibitors of RIPKs and PARP-1 could be new treatment options for immune-mediated hepatitis.

The B[a]P-increased intercellular communication via translocation of connexin-43 into gap junctions reduces apoptosis.

Gap junctions are channels in plasma membrane composed of proteins called connexins. These channels are organized in special domains between cells, and provide for direct gap junctional intercellular communication (GJIC), allowing diffusion of signalling molecules <1 kD. GJIC regulates cell homeostasis and notably the balance between proliferation, cell cycle arrest, cell survival and apoptosis. Here, we have investigated benzo[a]pyrene (B[a]P) effects on GJIC and on the subcellular localization of the major protein of gap junction: connexin-43 (Cx43). Our results showed that B[a]P increased GJIC between mouse hepatoma Hepa1c1c7 cells via translocation of Cx43 from Golgi apparatus and lipid rafts into gap junction plaques. Interestingly, inhibition of GJIC by chlordane or small interference RNA directed against Cx43 enhanced B[a]P-induced apoptosis in Hepa1c1c7 cells. The increased apoptosis caused by inhibition of GJIC appeared to be mediated by ERK/MAPK pathway. It is suggested that B[a]P could induce transfer of cell survival signal or dilute cell death signal via regulation of ERK/MAPK through GJIC.

Acid sphingomyelinase deficiency protects from cisplatin-induced gastrointestinal damage.

Cisplatin is one of the most effectively used chemotherapeutic agents for cancer treatment. However, in humans, important cytotoxic side effects are observed including dose-limiting renal damage and profound gastrointestinal symptomatology. The toxic responses to cisplatin in mice are similar to those in human patients. Here, we evaluated whether the acid sphingomyelinase (Asm) mediates at least some of the toxic in vivo effects of cisplatin. To this end, we determined the toxic effects of a single intraperitoneal dose of cisplatin (27 mg/kg) in wild type (Asm(+/+)) and Asm-deficient mice (Asm(-/-)). Tissue injury and apoptosis were determined histologically on hematoxylin-eosin and TUNEL (terminal deoxynucleotidyl transferase (TdT)-mediated nick end labeling) stainings 3, 12, 36 and 72 h after treatment. Our results revealed severe toxicity of cisplatin in Asm(+/+) mice with increased numbers of apoptotic cells in the thymus and small intestine. In marked contrast, Asm(-/-) mice were resistant to cisplatin and no apoptosis was observed in these organs after treatment. Moreover, cisplatin treatment primarily triggered apoptosis of endothelial cells in microvessels of intestine and thymus, an effect that was absent in mice lacking Asm. The data thus suggest that at least some toxic effects of cisplatin are mediated by the Asm in vivo resulting in early death of endothelial cells and consecutive organ damage.

Activation of the Fas pathway independently of Fas ligand during apoptosis induced by camptothecin in p53 mutant human colon carcinoma cells.

The present study explored the role of the cell surface receptor Fas (CD95/APO-1) in apoptosis induced by camptothecin (CPT) in the HT29 colon carcinoma cell line. CPT-induced apoptosis was associated with high molecular weight DNA fragmentation as measured by filter elution. This fragmentation was inhibited by the caspase inhibitor, z-VAD-fmk and by cycloheximide, which also prevented proteolytic activation of caspase-3 and poly(ADP-ribose)polymerase cleavage. Under such conditions, Fas, Fas ligand, Bax, and p21 expression were increased and Fas recruited the FADD adaptor. Fas expression increase was blocked by cycloheximide but not by z-VAD-fmk, consistent with caspase activation downstream from Fas. Treatment of HT29 cells with FasL or with the CH-11 agonistic anti-Fas antibody potentiated the apoptotic response of cells treated with CPT. The anti-Fas blocking antibody ZB4 and the Fas-ligand inhibitor failed to protect HT29 cells from CPT-induced apoptosis. Such a protection was obtained by transient expression of constructs encoding a dominant-negative mutant of FADD, FADD in an antisense orientation and E8 or MC159 viral proteins that inhibit Fas-induced apoptosis at the level of FADD and procaspase-8, respectively. Together, these data show that topoisomerase I-mediated DNA damage-induced apoptosis involves activation of the Fas pathway without detectable Fas-ligand requirement in CPT-treated cells.

Anticancer agents sensitize tumor cells to tumor necrosis factor-related apoptosis-inducing ligand-mediated caspase-8 activation and apoptosis.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a new cytokine that was proposed to specifically induce apoptosis of cancer cells. In tumor cells that are resistant to the cytokine, subtoxic concentrations of chemotherapeutic drugs can restore the response to TRAIL. The present study further explores the mechanisms that determine tumor cell sensitivity to TRAIL by comparing four human colon carcinoma cell lines We show that colon cancer cell sensitivity to TRAIL-induced apoptosis and cytotoxicity correlates with the expression of the death receptors TRAIL-R1 and TRAIL-R2 at the cell surface, as determined by now cytometry, whereas the two decoy receptors TRAIL-R3 and TRAIL-R4 can be detected only in permeabilized cells. Clinically relevant concentrations of cisplatin and doxorubicin sensitize the most resistant colon cancer cell lines to TRAIL-induced cell death without modifying the expression nor the localization of TRAIL receptors in these cells. TRAIL induces the activation of procaspase-8 and triggers caspase-dependent apoptosis off colon cancer cells. Cytotoxic drugs lower the signaling threshold required for TRAIL-induced procaspase-8 activation. In turn, caspase-8 cleaves Bid, a BH3 domain-containing proapoptotic molecule of the Bcl-2 family and activates effector caspases. Together, these data indicate that chemotherapeutic drugs sensitize colon tumor cells to TRAIL-mediated caspase-8 activation and apoptosis.

Positive and negative regulation of apoptotic pathways by cytotoxic agents in hematological malignancies.

Most chemotherapeutic drugs can induce tumor cell death by apoptosis. Analysis of the molecular mechanisms that regulate apoptosis has indicated that anticancer agents simultaneously activate several pathways that either positively or negatively regulate the death process. The main pathway from specific damage induced by the drug to apoptosis involves activation of caspases in the cytosol by pro-apoptotic molecules such as cytochrome c released from the mitochondrial intermembrane space. At least in some cell types, anticancer drugs also upregulate the expression of death receptors and sensitize tumor cells to their cognate ligands. The Fas-mediated pathway could contribute to the early steps of drug-induced apoptosis while sensitization to the cytokine TRAIL could be used to amplify the response to cytotoxic drugs. The Bcl-2 family of proteins, that includes anti- and pro-apoptotic molecules, regulates cell sensitivity mainly at the mitochondrial level. Anticancer drugs modulate their expression (eg through p53-dependent gene transcription), their activity (eg by phosphorylating Bcl-2) and their subcellular localization (eg by inducing the translocation of specific BH3-only pro-apoptotic proteins). Very early after interacting with tumor cells, anticancer drugs also activate lipid-dependent signaling pathways that either increase or decrease cell ability to die by apoptosis. In addition, cytotoxic agents can activate protective pathways that involve activation of NFkappaB transcription factor, accumulation of heat shock proteins such as Hsp27 and activation of proteins involved in cell cycle regulation. This review discusses how modulation of the balance between noxious and protective signals that regulate drug-induced apoptosis could be used to improve the efficacy of current therapeutic regimens in hematological malignancies.

P27KiP1 overexpression inhibits the growth and doxorubicin sensitivity of HT29 human colon cancer cells in vivo.

We have previously shown that p27KiP1 plays a role in the tumor cell resistance of HT29 confluent monolayers to cytotoxic drugs in vitro. To determine whether p27KiP1 was a resistance factor to cytotoxic drugs in vivo we tested the effect of doxorubicin on p27KiP1-overexpressing HT29 tumors in nude mice. In this study we show that ectopic overexpression of p27KiP1 in HT29 human colon cancer cells decreases their tumorigenicity in vivo in nude mice. This decreased tumor growth was associated with increased p27KiP1 protein expression, studied by Western blotting in tumor extracts. Interestingly, the overexpressing-p27KiP1 tumors were significantly more resistant to intraveneous doxorubicin treatment than the control tumors. These results indicate that p27KiP1, which delays tumor growth could also increase tumor resistance to cytotoxic drugs in vivo.

STAT-1-independent upregulation of FADD and procaspase-3 and -8 in cancer cells treated with cytotoxic drugs.

We have previously shown that treatment by anticancer drugs sensitized tumor cells to Fas (APO-1/CD95)-mediated cell death. The present study demonstrates that the cytotoxic drugs cisplatin, doxorubicin and mitomycin C induce the accumulation of the Fas receptor, the FADD adaptor molecule, the procaspases-8, -3 and -2L and the proapoptotic molecule Bax in several human colon cancer cells. This upregulation is also observed in U3A myeloblastoma cells that do not express STAT-1, a transcription factor involved in the constitutive expression of procaspases. We conclude that anticancer drugs sensitize tumor cells to Fas-mediated cell death by a STAT-1-independent upregulation of molecules involved in this apoptotic pathway.

Fas ligand-independent, FADD-mediated activation of the Fas death pathway by anticancer drugs.

Trimerization of the Fas receptor (CD95, APO-1), a membrane bound protein, triggers cell death by apoptosis. The main death pathway activated by Fas receptor involves the adaptor protein FADD (for Fas-associated death domain) that connects Fas receptor to the caspase cascade. Anticancer drugs have been shown to enhance both Fas receptor and Fas ligand expression on tumor cells. The contribution of Fas ligand-Fas receptor interactions to the cytotoxic activity of these drugs remains controversial. Here, we show that neither the antagonistic anti-Fas antibody ZB4 nor the Fas-IgG molecule inhibit drug-induced apoptosis in three different cell lines. The expression of Fas ligand on the plasma membrane, which is identified in untreated U937 human leukemic cells but remains undetectable in untreated HT29 and HCT116 human colon cancer cell lines, is not modified by exposure to various cytotoxic agents. These drugs induce the clustering of Fas receptor, as observed by confocal laser scanning microscopy, and its interaction with FADD, as demonstrated by co-immunoprecipitation. Overexpression of FADD by stable transfection sensitizes tumor cells to drug-induced cell death and cytotoxicity, whereas down-regulation of FADD by transient transfection of an antisense construct decreases tumor cell sensitivity to drug-induced apoptosis. These results were confirmed by transient transfection of constructs encoding either a FADD dominant negative mutant or MC159 or E8 viral proteins that inhibit the FADD/caspase-8 pathway. These results suggest that drug-induced cell death involves the Fas/FADD pathway in a Fas ligand-independent fashion.

p27Kip1 induces drug resistance by preventing apoptosis upstream of cytochrome c release and procaspase-3 activation in leukemic cells.

The cyclin-dependent kinase inhibitor p27Kip1 has been implicated as a drug resistance factor in tumor cells grown as spheroids or confluent monolayers. Here, we show that p27Kip1 overexpression also induces resistance to drug-induced apoptosis and cytotoxicity in human leukemic cells growing in suspension. The anti-apoptotic effect of p27Kip1 is not restricted to DNA-damaging agents but extends to the tubulin poison vinblastin, agonistic anti-Fas antibodies and macromolecule synthesis inhibitors. To further identify at which level this protein interferes with the cell death pathway, we investigated its influence on caspase activation and mitochondrial changes. Exposure of mock-transfected U937 cells to 50 microm etoposide activates procaspase-3 and the long isoform of procaspase-2 and induces mitochondrial potential decrease and cytochrome c release from mitochondria to the cytosol. All these events are prevented by p27Kip1 overexpression. p27Kip1 does not modulate Bcl-2, Bcl-X(L), Mcl-1 and Bax protein level in leukemic cells but suppresses Mcl-1 expression decrease observed in mock-transfected U937 cells undergoing etoposide-induced cell death. We conclude that p27Kip1 prevents cell death upstream of the final pathway common to many apoptotic stimuli that involves cytochrome c release from mitochondria and activation of downstream caspases.

[The Fas/Fas-ligand system: implications in the antitumor immune response and in the activity of cytotoxic agents]. / Le système Fas/Fas-ligand: implications dans la réponse immunitaire antitumorale et dans l'activité des agents cytotoxiques.

Interaction of Fas-ligand (Fas-L) with the extracytoplasmic domain of the Fas receptor can induce Fas trimerization and activation of the apoptotic cell death process. Several molecular pathways that lead to apoptosis and some of their regulatory mechanisms have been identified. Fas-related membrane receptors that contain a death domain in their intracytoplasmic domain have been identified. They constitute a death receptor family (DR1 to DR5) whose first member is the TNFR1 receptor for TNF alpha. The Fas/Fas-L system plays a role in the cytotoxic activity of immune cells and the regulation of immune response amplitude. This system could be involved in the immune response to tumor cells and the cytotoxic activity of drugs and radiations. The expression of Fas-L on the plasma membrane of numerous tumor cells allow them, in vitro, to kill Fas-expressing immune cells. This observations has suggested that tumor cells used Fas-L to induce a specific immune tolerance. However, in vivo, Fas-L expression rather induces tumor cell rejection. The quantity of Fas-L expressed on tumor cells could determine whether tumor cells are tolerated or rejected. Cytotoxic drugs and radiations modulate Fas and Fas-L expression on tumor cells. The role of Fas/Fas-L interactions in the cytotoxicity of these agents remains poorly defined. It has been clearly shown, however, that low doses of cytotoxic drugs increase Fas expression on tumor cells, thereby improving their elimination by immune cells. Drug-induced modulation of Fas expression could provide new therapeutic strategies combining chemotherapy with immunotherapy.

Contribution of the cyclin-dependent kinase inhibitor p27KIP1 to the confluence-dependent resistance of HT29 human colon carcinoma cells.

We have previously shown that growth of HT29 human colorectal cancer cells at confluence increased their resistance to the cytotoxic agent cisplatin. This study further explores the mechanisms of this resistance phenotype. DNA platination induced by cisplatin exposure is slightly reduced by confluence. However, at an equivalent DNA platination level, non-confluent cells accumulate in the G2/M phase of the cell cycle, demonstrate aberrant mitotic figures and die by apoptosis, while confluent cells progress slowly through the cell cycle, do not reach mitosis and are more resistant to drug-induced cell death. At a molecular level, cisplatin enhances cyclin B and p34cdc2 levels and histone H1 kinase activity in non-confluent, but not in confluent, cells. Furthermore, when HT29 cells reach confluence, expression of the cyclin-dependent kinase inhibitor p27Kip1 increases and cells accumulate in the G0/G1 phase of the cell cycle. Transfection-mediated over-expression of p27Kip1 in non-confluent HT29 cells decreases the cytotoxic activity of cisplatin as well as its ability to trigger apoptosis. Non-confluent HT29 cells over-expressing p27Kip1 are also more resistant to doxorubicin, etoposide and 5-fluorouracil. Our results suggest that p27Kip1 contributes to the confluence-dependent resistance phenotype.

Upregulation of CASP genes in human tumor cells undergoing etoposide-induced apoptosis.

Caspases are aspartate-specific cysteine proteases that play a pivotal role in drug-induced cell death. We designed RT-PCR assays to analyse the expression of CASP-3, CASP-4, CASP-6 and the long and short isoforms of CASP-2 genes in human cells. These genes heterogeneously coexpress in leukemic cell lines and bone marrow samples from patients with de novo acute myelogenous leukemia at diagnosis. Treatment of U937 and HL60 leukemic cells and HT29 colon carcinoma cells with the topoisomerase II inhibitor etoposide upregulates CASP-2 and CASP-3 genes in these cells before inducing their apoptosis. This effect of etoposide is not observed in K562 cells and bcl-2-transfected U937 cells which are less sensitive to drug-induced apoptosis. Nuclear run-on experiments demonstrate that etoposide increases CASP gene transcription in U937 cells, an effect that is prevented by Bcl-2 overexpression. Upregulation of CASP genes is associated with an enhanced synthesis of related procaspases that precedes the appearance of apoptosis markers including caspase-3 activation, poly(ADP-ribose) polymerase cleavage and internucleosomal DNA fragmentation. These results suggest that the ability of tumor cells to upregulate CASP-2 and CASP-3 genes in response to cytotoxic drugs could be predictive of their sensitivity to drug-induced apoptosis.

New insights into the kinetic resistance to anticancer agents.

Kinetic resistance plays a major role in the failure of chemotherapy towards many solid tumors. Kinetic resistance to cytotoxic drugs can be reproduced in vitro by growing the cells as multicellular spheroids (Multicellular Resistance) or as hyperconfluent cultures (Confluence-Dependent Resistance). Recent findings on the cell cycle regulation have permitted a better understanding why cancer cells which arrest in long quiescent phases are poorly sensitive to cell-cycle specific anticancer drugs. Two cyclin-dependent kinase inhibitors (CDKI) seem particularly involved in the cell cycle arrest at the G1 to S transition checkpoint: the p53-dependent p21(cip1) protein which is activated by DNA damage and the p27(kip1) which is a mediator of the contact inhibition signal. Cell quiescence could alter drug-induced apoptosis which is partly dependent on an active progression in the cell cycle and which is facilitated by overexpression of oncogenes such as c-Myc or cyclins. Investigations are yet necessary to determine the influence of the cell cycle on the balance between antagonizing (bcl-2, bcl-X(L)...) or stimulating (Bax, Bcl-X(S), Fas...) factors in chemotherapy-induced apoptosis. Quiescent cells could also be protected from toxic agents by an enhanced expression of stress proteins, such as HSP27 which is induced by confluence. New strategies are required to circumvent kinetic resistance of solid tumors: adequate choice of anticancer agents whose activity is not altered by quiescence (radiation, cisplatin), recruitment from G1 to S/G2 phases by cell pretreatment with alkylating drugs or attenuation of CDKI activity by specific inhibitors.

Sensitization of cancer cells treated with cytotoxic drugs to fas-mediated cytotoxicity.

BACKGROUND: The transmembrane receptor Fas, together with its protein-binding partner (Fas ligand), is a key regulator of programmed cell death (i.e., apoptosis). Fas and Fas ligand also influence the ability of cytotoxic T lymphocytes and natural killer cells to eliminate tumor cells. However, by inducing apoptosis in activated T cells, the Fas/Fas ligand system may protect some tumor cells from clearance by the immune system. Anticancer drugs enhance Fas ligand expression on the surface of Fas receptor-expressing leukemia cells, thus suggesting that apoptosis caused by these drugs may be mediated via the Fas/Fas ligand system. PURPOSE: This study was conducted to further investigate the relationship between the modulation of Fas receptor gene and protein expression by treatment of cells with cytotoxic drugs and the immune clearance of tumor cells. METHODS: Fas expression on human HT29 colon carcinoma cells treated with a variety of anticancer drugs (cisplatin, doxorubicin, mitomycin C, fluorouracil, and camptothecin) was analyzed by use of quantitative flow cytometry. Human HCT8R and HCT116 colon carcinoma cells and human U937 leukemia cells were treated with cisplatin only and analyzed in the same way. Fas ligand messenger RNA and protein levels were studied by use of a reverse transcription-polymerase chain reaction assay and by flow cytometry. Fas gene expression and messenger RNA levels in cisplatin-treated HT29 cells were characterized by use of in vitro nuclear run-on and northern blot hybridization assays. The cytotoxic activities of agonistic anti-Fas antibodies, Fas ligand, and allogeneic peripheral blood leukocytes, in the absence or presence of Fas-blocking monoclonal antibodies, against tumor cells were assessed by methylene blue staining and chromium-51 release assays. RESULTS: Clinically relevant concentrations of cisplatin, doxorubicin, mitomycin C, fluorouracil, or camptothecin enhanced Fas receptor expression on the plasma membrane of HT29 cells. Cisplatin-mediated increases in Fas expression were confirmed in HCT8R, HCT116, and U937 cells. The enhancement of Fas protein expression was associated with an increased sensitivity of cisplatin-treated tumor cells to agonistic anti-Fas antibodies, to soluble Fas ligand, and to allogeneic peripheral blood leukocyte-mediated cytotoxicity. Each of these effects was blocked by co-treatment of the cells with antagonistic anti-Fas antibody. CONCLUSION AND IMPLICATIONS: In addition to their direct cytotoxic effects, chemotherapeutic drugs sensitize tumor cells to Fas-mediated cytotoxicity and Fas-dependent immune clearance. On the basis of these findings, new strategies might be developed to improve the efficacy of these drugs.

Colon-cancer cell variants producing regressive tumors in syngeneic rats, unlike variants yielding progressive tumors, attach to interstitial collagens through integrin alpha2beta1.

Nine clones of tumor cells, derived from a single rat colon carcinoma, were analyzed for their adhesive properties and in vivo growth patterns. Four clones (denoted REG) gave rise to regressively growing tumors. Cells from the 4 REG clones attached significantly better to collagen types I and III than did cells from the 5 clones (denoted PRO) which grew progressively in vivo. In contrast, REG and PRO clones did not differ in their attachment to collagen type IV, laminin or fibronectin. The attachment of REG cells to collagen was dependent on Mg2+, but not Ca2+. Monospecific rabbit IgG to rat integrin beta 1-chain inhibited REG cell attachment to collagen, demonstrating involvement of a beta 1 integrin in this process. PRO and REG cells expressed an underglycosylated beta 1 chain (Mr approximately 105,000) that was somewhat smaller than beta 1-chains described previously on rat fibroblasts and hepatocytes (Mr approximately 115,000). Monoclonal IgG to rat integrin alpha 2 beta 1, but not to alpha 1 beta 1, readily inhibited REG cell attachment to collagen, demonstrating the involvement of integrin alpha 2 beta 1. However, beta 1 and alpha 2 integrin subunits were found in purified glycoproteins from both PRO and REG cells. This suggests that alpha 2 beta 1 integrin is expressed by both cell variants, but is functional as a collagen receptor on REG cells only. In this system of tumor-cell variants, the clear-cut differences in attachment to interstitial collagens of the 9 clones suggest a possible relationship between this attachment and the capacity to induce progressive or regressive tumors.

Circumvention of confluence-dependent resistance in a human multi-drug-resistant colon-cancer cell line.

Colorectal adenocarcinomas are inherently resistant to anthracyclines and other topoisomerase-II inhibitors. Resistance to doxorubicin of colon cancer cells (Caco2) depends on 2 main mechanisms. The first is typical multi-drug resistance, characterized by the mdr1 gene and its product the P170 membrane glycoprotein. P170 effluxes anthracyclines out of cancer cells and is antagonized in vitro by verapamil. The second mechanism, which develops when cell-culture density increases, we have designated confluence-dependent resistance. Confluence-dependent resistance depends on the reduced topoisomerase II content of the G0/G1-phase cells which accumulate in the confluent population. We show here that short treatments of confluent Caco2 cells with slightly toxic concentrations of DNA-damaging agents (cisplatin, melphalan or mitomycin C) produced a transient accumulation of cells in S- and G2/M-phases of the cell cycle. Concomitantly with the increase in the S-phase population, the topoisomerase II cellular level and the sensitivity of cells to doxorubicin were greatly enhanced. Overcoming confluence-dependent resistance through S-phase accumulation and inhibition of multi-drug resistance by verapamil were fully additive, and a nearly complete reversal of confluent Caco2 cells' resistance to doxorubicin was obtained when both strategies were combined.

Deleterious effect of serum proteins on the amphotericin B-induced potentiation of cisplatin in human colon cancer cells.

Inherent resistance of colon cancer cells to cis-diamminedichloroplatinum(II) (CDDP) is partly attributed to reduced drug penetration through plasma membrane. Amphotericin B (AmB), a polyene antifungal antibiotic, has been shown to increase CDDP penetration and cytotoxicity on several non-digestive cancer cell lines. We demonstrated here that AmB dramatically increases the penetration of CDDP, and to a lesser extent that of carboplatin (Carbo-P) and oxaloplatin (L-OHP), in the primary resistant HT 29 human colon cancer cells when drug incubation is performed in serum-free medium. The cytotoxicity of CDDP but not that of Carbo-P and L-OHP was increased by AmB. However, AmB-induced potentiation of CDDP penetration and toxicity was almost completely abolished when cell incubation was performed in presence of human serum. We investigated whether the dilution of human serum by a high osmotic power gelatine solution (Lomol) could restore the positive effect of AmB on CDDP accumulation in HT 29 cells. Incubation of cells with CDDP and AmB in pure Lomol resulted in a 6-fold increase in platinum cellular content. However, addition of serum (25%) in Lomol solution reduced to only 2-fold the increase in platinum cellular content provoked by AmB. These disappointing results show that AmB is probably uninteresting as a modulator of CDDP resistance in clinical practice. The use of haemodilution to restore the positive AmB effect on platinum cellular accumulation cannot be warranted.

Confluence dependent resistance (CDR) to doxorubicin and E-cadherin expression in murine mammary cells.

Confluence dependent resistance (CDR) is one of the principal mechanisms by which solid tumor cells resist anthracyclines. CDR is thought to be mediated by cell-cell contact which increases the fraction of non-proliferating resistant cells in a post confluence monolayer culture. As E-cadherin is a major Ca2+ dependent adhesion molecule, involved in cell-cell adhesion, differentiation and polarity of normal and cancerous epithelial cells, we decided to investigate its involvement in the CDR mechanism. In order to do this, we measured the intracellular accumulation and the cytotoxicity of doxorubicin (DXR) in four subclones, derived from the same parental murine mammary cell line (NMuMG), differing in their expression of E-cadherin. A significant reduction in DXR accumulation and cytotoxicity was observed in NM-f-ras-TD-CAMx, which expresses E-cadherin, suggesting that E-cadherin could play a role in the increase of drug resistance observed in confluent cancer cells.