Mechanisms of Resistance to Chemotherapy in Gastric Cancer.

Although surgical resection is the standard curative therapy for gastric cancer, these tumors are often diagnosed at an advanced stage, when surgery is not recommended. Alternative treatments such as radiotherapy and chemotherapy achieve only very modest results. There is therefore an urgent need to advance in this field of oncologic gastroenterology. The poor response of gastric cancer to chemotherapy is usually due to a combination of mechanisms of chemoresistance (MOC), which may include a reduction in drug uptake (MOC-1a), enhanced drug efflux (MOC-1b), a reduced proportion of active agents in tumor cells due to a reduction in pro-drug activation or an enhancement in drug inactivation (MOC-2), changes in the expression/function of the molecular targets of anticancer drugs (MOC-3), an enhanced ability of cancer cells to repair anticancer drug-induced DNA damage (MOC-4), and decreased expression/function of pro-apoptotic factors or up-regulation of anti-apoptotic genes (MOC-5). Two major goals of modern pharmacology aimed at overcoming this situation are the prediction of a lack of response to chemotherapy and the identification of the underlying mechanisms accounting for primary or acquired refractoriness to anticancer drugs. These are important issues if we are to select the best pharmacological regime for each patient and develop novel strategies to overcome chemoresistance. The present review reports updated information regarding the mechanisms of chemoresistance (from MOC-1 to MOC-5) in gastric cancer, the advances made in the prediction of the failure of chemotherapeutic treatment, and novel strategies based on gene therapy currently being developed to treat these tumors.

Genetic variants in genes involved in mechanisms of chemoresistance to anticancer drugs.

Refractoriness to the pharmacological treatment of cancer is dependent on the expression levels of genes involved in mechanisms of chemoresistance and on the existence of genetic variants that may affect their function. Thus, changes in genes encoding solute carriers may account for considerable inter-individual variability in drug uptake and the lack of sensitivity to the substrates of these transporters. Moreover, changes in proteins involved in drug export can affect their subcellular localization and transport ability and hence may also modify the bioavailability of antitumor agents. Regarding pro-drug activation or drug inactivation, genetic variants are responsible for changes in the activity of drug-metabolizing enzymes, which affect drug clearance and may determine the lack of response to anticancer chemotherapy. The presence of genetic variants may also decrease the sensitivity to pharmacological agents acting through molecular targets or signaling pathways. Recent investigations suggest that changes in genes involved in DNA repair may affect the response to platinum-based drugs. Since most anticancer agents activate cell death pathways, the evasion of apoptosis plays an important role in chemoresistance. Several genetic variants affecting death-receptor pathways, the mitochondrial pathway, downstream caspases and their natural modulators, and the p53 pathway, whose elements are mutated in more than half of tumors, and survival pathways, have been reported. The present review summarizes the available data regarding the role of genetic variants in the different mechanisms of chemoresistance and discusses their potential impact in clinical practice and in the development of tools to predict and overcome chemoresistance.

Molecular bases of liver cancer refractoriness to pharmacological treatment.

Hepatocellular carcinoma and cholangiocarcinoma are the two most important primary malignancies of the liver. These are among the tumours with the lowest response to pharmacological treatment based on currently available drugs. This is due either to the existence of refractoriness of the initial tumour or to the ability of cancer cells to develop chemoresistance during treatment. Liver cancers share some of the mechanisms responsible for drug refractoriness with other types of tumours, such as a reduction in drug uptake; enhanced drug export; intracellular inactivation of the active agent; alteration of the molecular target; an increase in the activity of the target route to be inhibited, or the appearance or stimulation of alternative routes; enhanced repair of drug-induced modifications in the target molecules, and the activation/ inhibition of intracellular signalling pathways, all of which lead to a negative balance between the apoptosis/survival of tumour cells. The aim of the present article is to review how these mechanisms of chemoresistance affect the different families of drugs that are being or have been used to treat hepatocellular carcinoma and cholangiocarcinoma. A better understanding of the molecular bases of drug refractoriness is needed in order to develop novel drugs or pharmacological strategies aimed at overcoming resistance to anticancer agents.

Overview of the molecular bases of resistance to chemotherapy in liver and gastrointestinal tumours.

Primary malignancies of the liver and the gastrointestinal tract constitute one of the main health problems worldwide. Together, these types of tumour are the first cause of death due to cancer, followed by lung and breast cancer respectively. One important limitation in the treatment of these tumours is that, with a few exceptions, they exhibit marked resistance to currently available drugs. Moreover, most of them develop chemoresistance during treatment. The mechanisms responsible for drug refractoriness in gastrointestinal tumours include a reduction in drug uptake; enhanced drug export; intracellular inactivation of the effective agent; alteration of the molecular target; an increase in the activity of the target route to be inhibited or the appearance or stimulation of alternative routes; enhanced repair of drug-induced modifications in the target molecules, and the activation/inhibition of intracellular signalling pathways, which leads to a negative balance between the apoptosis/survival of tumour cells. A better understanding of these mechanisms is needed in order to develop accurate tests to predict the lack of response to chemotherapy and novel approaches aimed at overcoming resistance to anticancer agents. The purpose of the present review is to offer an updated overview of the molecular mechanisms of resistance to cytostatic drugs in the most frequent types of primary malignant tumour affecting the liver and gastrointestinal tract.

Expression in human trophoblast and choriocarcinoma cell lines, BeWo, Jeg-3 and JAr of genes involved in the hepatobiliary-like excretory function of the placenta.

Using cytokeratin-7-positive trophoblast cells (hTr) isolated from human term placentas and the choriocarcinoma cell lines (hCC) BeWo, Jeg-3 and JAr, the expression of genes involved in the hepatobiliary excretion of cholephilic compounds was investigated by RT-PCR/sequencing followed by measurement of the absolute abundance of mRNA by real-time RT-PCR. Although mRNA of BSEP was detectable and its expression confirmed by Western blotting, its very low expression (higher in hTr than in whole placenta and hCC) did not permit its detection by immunohistochemistry. In hTr, the expression was high for OATP-B/2B1, OATP-8/1B3, MRP1, MRP3, BCRP, FIC1, RARalpha, FXR and SHP, low for OSTalpha, MRP2, MRP4, MRP8, MDR1, CAR and SXR, very low for OATP-A/1A2 and MDR3, and not detectable for OATP-C/1B1, HNF1alpha and HNF4. Expression patterns in hCC mimicked those in hTr, although some important cell line-specific differences were found. The functionality of transporters expressed in hCC was confirmed by their ability to take up and export estradiol 17beta-d-glucuronide in a self-inhibitable and temperature-sensitive manner. In conclusion, several transporters, export pumps, and nuclear receptors involved in the liver excretory function may play a similar role in the placenta, whose specific aspects can be studied by selectively using BeWo, Jeg-3 or JAr cells.

Excretion of foetal bilirubin by the rat placenta-maternal liver tandem.

Using plasma membrane vesicles from human trophoblast, carrier-mediated transport of unconjugated bilirubin (UCBR) has been reported. In the present work, using the in situ perfused rat placenta-maternal liver tandem, the relevance of this pathway in vivo was investigated. After single-pass perfusion of rat placenta through the umbilical artery with 0.25 micromol [(3)H]-UCBR, approximately 15 per cent of it was taken up by the placenta, detected in maternal serum (>96 per cent was unconjugated) and subsequently secreted into maternal bile (approximately 15 per cent of administered dose; >88 per cent was glucuronidated bilirubin). Co-administration through the umbilical artery of 0.25 micromol [(3)H]-UCBR and 2.5 micromol unlabelled UCBR, bromosulfophthalein, cholic acid or biliverdin IXalpha, reduced [(3)H]-UCBR placenta uptake, and the amount of radioactivity found in the maternal serum and bile. Co-administration into maternal jugular vein of 0.1 micromol [(3)H]-UCBR-a dose 3-fold higher than that reaching the maternal compartment in placenta perfusion experiments-and 1.0 micromol bromosulfophthalein, cholic acid or biliverdin IXalpha, resulted in no marked inhibition of the amount of radioactivity bile output. When antipyrine and [(3)H]-UCBR were continuously co-infused to the mother, similar antipyrine concentrations in maternal and foetal serum were reached in approximately 15 min, while progressive increase in [(3)H]-bilirubin concentrations in maternal serum above 70 microM was accompanied by a very low transfer of this compound into foetal compartment where [(3)H]-bilirubin concentrations were always <10 microM. These results suggest that the transfer of UCBR across the rat placenta occurs, without biotransformation, via a foetal-to-maternal mainly unidirectional pathway that can be cis-inhibited by UCBR and other cholephilic organic anions.

Effect of ursodeoxycholic acid on the impairment induced by maternal cholestasis in the rat placenta-maternal liver tandem excretory pathway.

We investigated the effects of ursodeoxycholic acid (UDCA; 60 microg/day/100 g b.wt.) on the impairment induced by maternal obstructive cholestasis during pregnancy (OCP) in the rat placenta-maternal liver tandem excretory pathway. A blunted catheter was implanted in the common bile duct on day 14 of pregnancy, and the tip was cut on day 21. [(14)C]Glycocholate (GC) was then administered through the umbilical artery of "in situ" perfused placenta (placental transfer test) or through the maternal jugular vein (biliary secretion test), and GC bile output was measured. OCP impaired both GC placental transfer and maternal biliary secretion. UDCA moderately improved the latter but had a more marked beneficial effect on GC placental transfer. Histological examination revealed trophoblast atrophy and structural alterations, e.g., loss of apical membrane microvilli in OCP placentas. Gene expression level was investigated by real-time quantitative reverse transcription-polymerase chain reaction and Western blot analysis. OCP reduced both placental lactogen II (a trophoblast-specific gene) mRNA and the functional amount of epithelial tissue, determined by transplacental diffusion of antipyrin. Using a rapid filtration technique, impairment in the ATP-dependent GC transport across trophoblast apical plasma membranes obtained from OCP placentas was found. UDCA partially prevented all these changes. The expression level of organic anion transporters Oatp1, Oatp2, and Oatp4, and multidrug resistance-associated proteins Mrp1, Mrp2, and Mrp3 in whole placenta were not affected or were moderately affected by OCP but greatly enhanced by UDCA. In summary, UDCA partially prevents deleterious effects of OCP on the rat placenta-maternal liver tandem excretory pathway, mainly by preserving trophoblast structure and function.

Overcoming cisplatin resistance in vitro by a free and liposome-encapsulated bile acid derivative: BAMET-R2.

Low water solubility and development of resistance are important drawbacks in the use of cisplatin as a cytostatic agent. A novel bile acid-cisplatin complex, Bamet-R2 [cis-diamminechlorocholylglycinateplatinum (II)], with liver vectoriality, has been synthesized. Our aim was to investigate the usefulness of this compound to overcome cisplatin resistance and to determine whether its encapsulation into liposomes increases its water solubility, uptake by liver tumor cells and cytostatic activity. Highly efficient incorporation of Bamet-R2 into liposomes permitted an increase in the concentration of the drug compared with that in the initial free solution by more than 6 x 10(6)-fold, which is 10(3)-fold higher than the encapsulation obtained for cisplatin. A partially cisplatin-resistant (87-fold) monoclonal cell line (Hepa 1-6/10R) was obtained by 2 subcloning steps of a population of mouse hepatoma Hepa 1-6 cells grown in step-wise increasing cisplatin concentrations up to 10 microM. Decreased sensitivity to cisplatin was accompanied by a 3.2-fold lower drug accumulation compared to wild-type cells. Uptake was markedly increased by the binding of cisplatin to glycocholic acid in both Hepa 1-6 and Hepa 1-6/10R cells. This probably accounts for the partial overcoming (-82%) of resistance when used on Hepa 1-6/10R cells. Inclusion of Bamet-R2 into liposomes further increased the amount of the drug accumulated in both cell types and, hence, enhanced its cytostatic activity. Since both plain liposomes and Bamet-R2 have little toxicity, the formulation of this compound in liposomes may offer a substantial advantage over cisplatin in the treatment of tumors resistant to this anti-neoplastic agent.

Fetal excretion of the fluorescent bile acid derivative cholylglycylamido-fluorescein (FITC-GC) by the rat placenta-maternal liver tandem.

Bile acid transfer from the fetus to maternal bile was studied using in situ perfused rat placenta on day 21 of gestation and a fluorescent derivative of glycocholate (GC): cholylglycylamido-fluorescein (FITC-GC). Single-pass perfusion of the placenta with 0.25 mumol FITC-GC via the umbilical artery over 5 min was followed by the output of 6 per cent of this amount in maternal bile collected over the ensuing 120 min. This amount was reduced (-35 per cent) by simultaneous administration of 2.5 mumol GC through the jugular vein of the mother. This inhibition was stronger (-73 per cent) when 2.5 mumol GC was co-infused with FITC-GC through the umbilical artery. These results suggested that FITC-GC was, at least in part, transported by bile acid carriers across both the liver and the placenta. Using isolated perfused rat livers obtained from female virgin or 21-day pregnant rats, a slight increase in the residence time of FITC-GC in the liver of pregnant rats was found. However, no change in the ability of the liver to take up FITC-GC was observed. By contrast, when FITC-GC was injected into the left jugular vein of anaesthetized pregnant rats, a delayed plasma disappearance of this compound was seen, which may have been due in part to the existence of a transient and reversible FITC-GC exchange with the placental-fetal compartment. The maximal rate of FITC-GC output into bile after FITC-GC administration (1 mumol/100 g body weight) to pregnant rats was approximately 0.2 mumol/min, while maximal FITC-GC bile output was approximately 1 nmol/min when this compound was given through the umbilical artery (2.5 mumol). Therefore, the rate of FITC-GC output into bile was considered to reflect the rate of transfer across the placenta. Using this approach no saturation but rather a linear regression (slope = 1.1 microliters/min, p < 0.05) was found between placental transfer and placental perfusate concentrations in the 10-1000-mumol/l FITC-GC range. In summary, the in situ perfused rat placenta is a useful model to study the fetal excretion of cholephilic compounds, and transfer across the trophoblast would be the limiting step in the excretion of fetal bile acids by the placenta-maternal liver tandem.