Increased P-glycoprotein messenger RNA stability in rat liver tumors in vivo

Chemotherapeutic drugs stimulate the release and recycling of extracellular vesicles to assist cancer cells in developing an urgent chemoresistance

Background

Chemotherapy is a widely used treatment for cancer. However, the development of acquired multidrug resistance (MDR) is a serious issue. Emerging evidence has shown that the extracellular vesicles (EVs) mediate MDR, but the underlying mechanism remains unclear, especially the effects of chemotherapeutic agents on this process.

Methods

Extracellular vesicles isolation was performed by differential centrifugation. The recipient cells that acquired ATP-binding cassette sub-family B member 1 (ABCB1) proteins were sorted out from co-cultures according to a stringent multi-parameter gating strategy by fluorescence-activated cell sorting (FACS). The transfer rate of ABCB1 was measured by flow cytometry. The xenograft tumor models in mice were established to evaluate the transfer of ABCB1 in vivo. Gene expression was detected by real-time PCR and Western blotting.

Results

Herein, we show that a transient exposure to chemotherapeutic agents can strikingly increase Rab8B-mediated release of extracellular vesicles (EVs) containing ABCB1 from drug-resistant cells, and accelerate these EVs to circulate back onto plasma membrane of sensitive tumor cells via the down-regulation of Rab5. Therefore, intercellular ABCB1 transfer is significantly enhanced; sensitive recipient cells acquire a rapid but unsustainable resistance to evade the cytotoxicity of chemotherapeutic agents. More fascinatingly, in the xenograft tumor models, chemotherapeutical drugs also locally or distantly increase the transfer of ABCB1 molecules. Furthermore, some Non-small-cell lung carcinoma (NSCLC) patients who are undergoing primary chemotherapy have a rapid increase of ABCB1 protein in their monocytes, and this is obviously associated with poor chemotherapeutic efficacy.

Conclusions

Chemotherapeutic agents stimulate the secretion and recycling of ABCB1-enriched EVs through the dysregulation of Rab8B and Rab5, leading to a significant increase of ABCB1 intercellular transfer, thus assisting sensitive cancer cells to develop an urgent resistant phenotype. Our findings provide a new molecular mechanism of how chemotherapeutic drugs assist sensitive cancer cells in acquiring an urgent resistance.

ABC Transporters: Regulation and Association with Multidrug Resistance in Hepatocellular Carcinoma and Colorectal Carcinoma

For most cancers, the treatment of choice is still chemotherapy despite its severe adverse effects, systemic toxicity and limited efficacy due to the development of multidrug resistance (MDR). MDR leads to chemotherapy failure generally associated with a decrease in drug concentration inside cancer cells, frequently due to the overexpression of ABC transporters such as P-glycoprotein (P-gp/MDR1/ABCB1), multidrug resistance-associated proteins (MRPs/ABCCs), and breast cancer resistance protein (BCRP/ABCG2), which limits the efficacy of chemotherapeutic drugs. The aim of this review is to compile information about transcriptional and post-transcriptional regulation of ABC transporters and discuss their role in mediating MDR in cancer cells. This review also focuses on drug resistance by ABC efflux transporters in cancer cells, particularly hepatocellular carcinoma (HCC) and colorectal carcinoma (CRC) cells. Some aspects of the chemotherapy failure and future directions to overcome this problem are also discussed.

Sodium butyrate increases P-gp expression in lung cancer by upregulation of STAT3 and mRNA stabilization of ABCB1

As a new type of anticancer drug, the effect of histone deacetylase inhibitors (HDACIs) in cancer clinical therapy is disappointing owing to drug resistance. P-glycoprotein (P-gp) is clearly recognized as a multidrug resistance protein. However, the relationship between P-gp and sodium butyrate (SB), a kind of HDACIs, has not been investigated. In this study, we found that SB increased mRNA and protein expression of P-gp in lung cancer cells and the underlying mechanisms were elucidated. We found that SB treatment enhanced the mRNA and protein expression of STAT3 rather than that of β-catenin, Foxo3a, PXR, or CAR, which were reported to directly regulate the transcription of ABCB1, a P-gp-encoding gene. Interestingly, inhibition of STAT3 expression obviously attenuated SB-increased P-gp expression in lung cancer cells, indicating that STAT3 played an important role in SB-mediated P-gp upregulation. Furthermore, we found that SB increased the mRNA stability of ABCB1. In summary, this study showed that SB increased P-gp expression by facilitating transcriptional activation and improving ABCB1 mRNA stability. This study indicated that we should pay more attention to HDACIs during cancer clinical therapy.

Histone deacetylase inhibitors regulate P-gp expression in colorectal cancer via transcriptional activation and mRNA stabilization

Histone deacetylase inhibitors (HDACIs) are emerging as a novel class of anti-tumor drugs. But the effect of HDACIs in tumors treatment has been disappointing, which mainly due to the acquisition of resistance to HDACIs. However, the underlying mechanisms have not been clearly understood. In this study, it was found that HDACIs SAHA and TSA increased P-gp expression in CRC cells, which has been well known to contribute to drug resistant. The mechanisms underlying these effects were investigated. We showed that HDACIs enhanced transcriptional activity of P-gp protein encoding gene ABCB1. HDACIs treatment also increased the protein and mRNA expression of STAT3, but not PXR, CAR, Foxo3a or β-catenin, which are known to be involved in ABCB transcription regulation. Interestingly, knockdown of STAT3 significantly attenuated HDACIs-induced P-gp up-regulation in colorectal cancer cells, suggesting that STAT3 plays a crucial role in HDACIs-up-regulated P-gp. Furthermore, this study revealed for the first time that HDACIs enhanced the stability of ABCB1 at post-transcriptional level. Taken together, these results proved that HDACIs induced P-gp expression by two distinct ways, transcriptional activation and mRNA stabilization. Our results suggested that more attention should be paid to the cancer treatment using HDACIs since they will induce multidrug resistance in cancer cells.

Cellular communication via microparticles: role in transfer of multidrug resistance in cancer

Multidrug resistance (MDR) continues to be a major impediment to the successful treatment of cancer. The two efflux transporters, P-glycoprotein (P-gp) and MRP1 are major contributors to cancer MDR clinically. The upregulation of P-gp leading to MDR was initially understood to occur via pre- and post-transcriptional mechanisms only. However, we demonstrated that microparticles mediate the intercellular exchange and trafficking of bioactive material, including functional P-gp and selected modulatory miRNAs. This exchange of P-gp leads to the dissemination of MDR within a cancer cell population. These findings have significant implications in understanding the cellular basis governing the intercellular acquisition of deleterious traits in cancers, serving to substantially advance our understanding of the molecular basis of the emergence of MDR in cancer clinically.

Synonymous codon changes in the oncogenes of the cottontail rabbit papillomavirus lead to increased oncogenicity and immunogenicity of the virus

Papillomaviruses use rare codons with respect to the host. The reasons for this are incompletely understood but among the hypotheses is the concept that rare codons result in low protein production and this allows the virus to escape immune surveillance. We changed rare codons in the oncogenes E6 and E7 of the cottontail rabbit papillomavirus to make them more mammalian-like and tested the mutant genomes in our in vivo animal model. While the amino acid sequences of the proteins remained unchanged, the oncogenic potential of some of the altered genomes increased dramatically. In addition, increased immunogenicity, as measured by spontaneous regression, was observed as the numbers of codon changes increased. This work suggests that codon usage may modify protein production in ways that influence disease outcome and that evaluation of synonymous codons should be included in the analysis of genetic variants of infectious agents and their association with disease.

P-glycoproteins and other multidrug resistance transporters in the pharmacology of anthelmintics: Prospects for reversing transport-dependent anthelmintic resistance

Parasitic helminths cause significant disease in animals and humans. In the absence of alternative treatments, anthelmintics remain the principal agents for their control. Resistance extends to the most important class of anthelmintics, the macrocyclic lactone endectocides (MLs), such as ivermectin, and presents serious problems for the livestock industries and threatens to severely limit current parasite control strategies in humans. Understanding drug resistance is important for optimizing and monitoring control, and reducing further selection for resistance. Multidrug resistance (MDR) ABC transporters have been implicated in ML resistance and contribute to resistance to a number of other anthelmintics. MDR transporters, such as P-glycoproteins, are essential for many cellular processes that require the transport of substrates across cell membranes. Being overexpressed in response to chemotherapy in tumour cells and to ML-based treatment in nematodes, they lead to therapy failure by decreasing drug concentration at the target. Several anthelmintics are inhibitors of these efflux pumps and appropriate combinations can result in higher treatment efficacy against parasites and reversal of resistance. However, this needs to be balanced against possible increased toxicity to the host, or the components of the combination selecting on the same genes involved in the resistance. Increased efficacy could result from modifying anthelmintic pharmacokinetics in the host or by blocking parasite transporters involved in resistance. Combination of anthelmintics can be beneficial for delaying selection for resistance. However, it should be based on knowledge of resistance mechanisms and not simply on mode of action classes, and is best started before resistance has been selected to any member of the combination. Increasing knowledge of the MDR transporters involved in anthelmintic resistance in helminths will play an important role in allowing for the identification of markers to monitor the spread of resistance and to evaluate new tools and management practices aimed at delaying its spread.

Escape from stress granule sequestration: another way to drug resistance?

Overexpression of P-glycoprotein, encoded by the MDR1 (multidrug resistance 1) gene, is often responsible for multidrug resistance and chemotherapy failure in cancer. We have demonstrated that, in leukaemic cells, P-glycoprotein expression is regulated at the translational level. More recently, we have shown that in cells overexpressing P-glycoprotein, MDR1 mRNA does not aggregate into translationally silent stress granules. Importantly, this is not unique for MDR1, since other transcripts encoding transmembrane proteins, and which are thus translated at the endoplasmic reticulum, follow the same pattern. By using a series of chimaeric transcripts, we have demonstrated that transcript localization at the endoplasmic reticulum bypasses the signals dictating stress granule sequestration. Polysome profile analyses and protein synthesis experiments indicate that, upon stress withdrawal, endoplasmic-reticulum-bound transcripts resume translation faster than those at the cytosol, which have been sequestered into stress granules. This may represent a novel mechanism by which drug-resistant cells respond quickly to stress, helping them to survive the cytotoxic effect of chemotherapeutic drugs.

P-glycoprotein expression in immortalised rat brain endothelial cells: comparisons following exogenously applied hydrogen peroxide and after hypoxia-reoxygenation

Levels of multidrug efflux transporter P-glycoprotein (P-gp) on endothelial cells lining brain blood vessels are important for limiting access of many compounds to the brain. In vivo studies have indicated that ischaemia-reperfusion that generates reactive oxygen species also increases P-gp levels in brain endothelial cells. To investigate possible mechanisms, in vitro studies were performed on immortalised (GPNT) and primary rat brain endothelial cells. Exposure to hydrogen peroxide (200 microM) resulted in intracellular oxidative stress as detected from higher levels of dichlorofluorescein fluorescence and raised levels of P-gp protein, mdr1a and mdr1b transcripts and, in GPNT cells, increased mdr1a and mdr1b promoter activity. The P-gp protein increases were abolished by pre-treatment with polyethylene glycol-catalase and were curtailed by co-culture with primary rat astrocytes. Exposure of GPNT cells to 6 h hypoxia followed by 24 h reoxygenation produced less intracellular oxidative stress as judged from smaller increments in dichlorofluorescein fluorescence but still resulted in raised levels of P-gp protein, an effect partially abolished by pre-treatment with polyethylene glycol-catalase. However, transcript levels and promoter activities were not significantly increased. These data suggest that hydrogen peroxide contributes to P-gp up-regulation following hypoxia-reoxygenation but the underlying mechanisms of its actions differ from those occurring after direct hydrogen peroxide application.

The role of mammalian ribonucleases (RNases) in cancer

Ribonucleases (RNases) are a group of enzymes that cleave RNAs at phosphodiester bonds resulting in remarkably diverse biological consequences. This review focuses on mammalian RNases that are capable of, or potentially capable of, cleaving messenger RNA (mRNA) as well as other RNAs in cells and play roles in the development of human cancers. The aims of this review are to provide an overview of the roles of currently known mammalian RNases, and the evidence that associate them as regulators of tumor development. The roles of these RNases as oncoproteins and/or tumor suppressors in influencing cell growth, apoptosis, angiogenesis, and other cellular hallmarks of cancer will be presented and discussed. The RNases under discussion include RNases from the conventional mRNA decay pathways, RNases that are activated under cellular stress, RNases from the miRNA pathway, and RNases with multifunctional activity.

Oncogenic Ras and transforming growth factor-beta synergistically regulate AU-rich element-containing mRNAs during epithelial to mesenchymal transition

Colon cancer progression is characterized by activating mutations in Ras and by the emergence of the tumor-promoting effects of transforming growth factor-beta (TGF-beta) signaling. Ras-inducible rat intestinal epithelial cells (RIE:iRas) undergo a well-described epithelial to mesenchymal transition and invasive phenotype in response to H-RasV12 expression and TGF-beta treatment, modeling tumor progression. We characterized global gene expression profiles accompanying Ras-induced and TGF-beta-induced epithelial to mesenchymal transition in RIE:iRas cells by microarray analysis and found that the regulation of gene expression by the combined activation of Ras and TGF-beta signaling was associated with enrichment of a class of mRNAs containing 3' AU-rich element (ARE) motifs known to regulate mRNA stability. Regulation of ARE-containing mRNA transcripts was validated at the mRNA level, including genes important for tumor progression. Ras and TGF-beta synergistically increased the expression and mRNA stability of vascular endothelial growth factor (VEGF), a key regulator of tumor angiogenesis, in both RIE:iRas cells and an independent cell culture model (young adult mouse colonocyte). Expression profiling of human colorectal cancers (CRC) further revealed that many of these genes, including VEGF and PAI-1, were differentially expressed in stage IV human colon adenocarcinomas compared with adenomas. Furthermore, genes differentially expressed in CRC are also significantly enriched with ARE-containing transcripts. These studies show that oncogenic Ras and TGF-beta synergistically regulate genes containing AREs in cultured rodent intestinal epithelial cells and suggest that posttranscriptional regulation of gene expression is an important mechanism involved in cellular transformation and CRC tumor progression.

Prevention of MDR development in leukemia cells by micelle-forming polymeric surfactant

Doxorubicin (Dox) incorporated in nanosized polymeric micelles, SP1049C, has shown promise as monotherapy in patients with advanced esophageal carcinoma. The formulation contains amphiphilic block copolymers, Pluronics, that exhibit the unique ability to chemosensitize multidrug resistant (MDR) tumors by inhibiting P-glycoprotein (Pgp) drug efflux system and enhancing pro-apoptotic signaling in cancer cells. This work evaluates whether a representative block copolymer, Pluronic P85 (P85) can also prevent development of Dox-induced MDR in leukemia cells. For in vitro studies murine lymphocytic leukemia cells (P388) were exposed to increasing concentrations of Dox with/without P85. For in vivo studies, BDF1 mice bearing P388 ascite were treated with Dox or Dox/P85. The selected P388 cell sublines and ascitic tumor-derived cells were characterized for Pgp expression and functional activity (RT-PCR, Western Blot, rhodamine 123 accumulation) as well as Dox resistance (3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay). The global gene expression was determined by oligonucleotide gene microarrays. We demonstrated that P85 prevented development of MDR1 phenotype in leukemia cells in vitro and in vivo as determined by Pgp expression and functional assays of the selected cells. Cells selected with Dox in the presence of P85 in vitro and in vivo exhibited some increases in IC(50) values compared to parental cells, but these values were much less than IC(50) in respective cells selected with the drug alone. In addition to mdr1, P85 abolished alterations of genes implicated in apoptosis, drug metabolism, stress response, molecular transport and tumorigenesis. In conclusion, Pluronic formulation can prevent development of MDR in leukemia cells in vitro and in vivo.

Production of P-glycoprotein from the MDR1 upstream promoter is insufficient to affect the response to first-line chemotherapy in advanced breast cancer

Multidrug resistance, the phenomenon by which cells treated with a drug become resistant to the cytotoxic effect of a variety of other structurally and functionally unrelated drugs, is often associated with the expression of P-glycoprotein, an efflux membrane pump coded by the MDR1 (ABCB1) gene. Transcription from MDR1 can start at 2 promoters: a well-characterized downstream promoter and an as yet uncharacterized upstream promoter (USP). We have previously determined that the USP is activated in some drug-resistant cell lines, in primary breast tumors and in metastatic epithelial cells isolated from the lymph nodes of breast cancer patients. In this study, we report the cloning and characterization of the MDR1 USP and studied its association with chemotherapy response in breast cancer patients. Deletion analysis indicated that a nearby endogenous retroviral long terminal repeat is not responsible for promoter activation, and that the region within the first 400 nucleotides upstream from the transcription start point contained all the elements necessary for promoter activity in drug-resistant cells. We identified an element recognized by the transcription factor NF-IL6 (activated upon interleukin-6 exposure) which is necessary for promoter activity in drug-resistant cells and plays a role in the activation of the promoter in response to interleukin-6 in breast cancer MCF-7 cells. Although transcripts from this promoter are associated with translating polyribosomes, their low abundance makes the amount of synthesized P-glycoprotein insufficient to affect the response to first-line chemotherapy in patients with advanced breast cancer.

Cobalamin potentiates vinblastine cytotoxicity through downregulation of mdr-1 gene expression in HepG2 cells

BACKGROUND

P-glycoprotein (Pgp), produced by multidrug resistance-1 gene (mdr-1), is a main mechanism developed by cancer cells to guard against anti-cancer drugs. Alterations of DNA methylation of the mdr-1 gene promoter are known to be linked to mdr-1 gene expression and are probably related to intracellular S-adenosyl-methionine. We here used HepG2 cells to determine the role of the methionine cycle (through the use of the Methionine-Synthase (MS) cofactor, cobalamin) on mdr-1 gene expression.

METHODS

Semiquantitative RT-PCR of mdr-1 gene, cellular retention of rhodamine-123, and vinblastine cytotoxicity were carried out on cells cultivated with and without cobalamin. Methylation status of the mdr-1 gene promoter was determined by methylation-specific PCR.

RESULTS

Addition of cobalamin to the cells led to an increase in MS activity, to a significant decrease in mdr-1 gene expression which is correlated to an increase in retention of the Pgp substrate Rhodamine 123. Furthermore, cobalamin potentiated cell sensitivity to vinblastine to the same range as that of the Pgp blocker verapamil and prevented methotrexate-induced up-regulation of mdr-1 gene expression. However, no modification in methylation of the mdr-1 gene promoter was observed.

CONCLUSION

Cobalamin downregulates mdr-1 gene expression, as well as Pgp expression and function, and significantly increases cytotoxicity of vinblastine. The identification of this novel way of diminishing cellular resistance to the chemotherapeutic agent vinblastine holds promises of leading to better treatments for cancer patients.

The effects of ABCB1 3'-untranslated region variants on mRNA stability

Genetic variation in ABCB1, encoding P-glycoprotein (P-gp), is a potential cause of interindividual variation in drug response. Numerous studies have focused on the effects of coding region variants on P-gp expression and function, whereas few noncoding region variants have been investigated. The 3'-untranslated region (UTR) regulates mRNA levels or stability via RNA-protein interactions with mRNA degradation machinery. mRNA stability is a key regulatory step controlling ABCB1 mRNA expression that ultimately affects P-gp levels and function. We hypothesized that ABCB1 3'-UTR polymorphisms alter mRNA stability by disrupting RNA-protein interactions. An ethnically diverse panel of DNA samples was sequenced to identify 3'-UTR polymorphisms and determine allele frequencies. The three most common variants, along with reference ABCB1, were stably expressed in cells in order to measure mRNA half-life. The calculated half-life for ABCB1 reference in HEK293 cells was 9.4 +/- 1.3 h and was similar to that estimated for the 3'-UTR variants. Endogenous ABCB1 mRNA decay was similar in lymphoblastoid cell lines carrying 3'-UTR variant and reference alleles. Although the examined ABCB1 3'-UTR variants have no effect on ABCB1 mRNA stability, these data represent one of the first attempts to determine the influence of genetic variation in UTRs on ABCB1 mRNA levels.

Role of the highly structured 5'-end region of MDR1 mRNA in P-glycoprotein expression

Overexpression of P-glycoprotein, encoded by the MDR1 (multidrug resistance 1) gene, is often responsible for multidrug resistance in acute myeloid leukaemia. We have shown previously that MDR1 (P-glycoprotein) mRNA levels in K562 leukaemic cells exposed to cytotoxic drugs are up-regulated but P-glycoprotein expression is translationally blocked. In the present study we show that cytotoxic drugs down-regulate the Akt signalling pathway, leading to hypophosphorylation of the translational repressor 4E-BP [eIF (eukaryotic initiation factor) 4E-binding protein] and decreased eIF4E availability. The 5'-end of MDR1 mRNA adopts a highly-structured fold. Fusion of this structured 5'-region upstream of a reporter gene impeded its efficient translation, specifically under cytotoxic stress, by reducing its competitive ability for the translational machinery. The effect of cytotoxic stress could be mimicked in vivo by blocking the phosphorylation of 4E-BP by mTOR (mammalian target of rapamycin) using rapamycin or eIF4E siRNA (small interfering RNA), and relieved by overexpression of either eIF4E or constitutively-active Akt. Upon drug exposure MDR1 mRNA was up-regulated, apparently stochastically, in a small proportion of cells. Only in these cells could MDR1 mRNA compete successfully for the reduced amounts of eIF4E and translate P-glycoprotein. Consequent drug efflux and restoration of eIF4E availability results in a feed-forward relief from stress-induced translational repression and to the acquisition of drug resistance.

CRD-BP shields c-myc and MDR-1 RNA from endonucleolytic attack by a mammalian endoribonuclease

The c-myc mRNA coding region determinant-binding protein (CRD-BP) has high affinity for the coding region determinant (CRD) of c-myc mRNA. Such affinity is believed to protect c-myc CRD from endonucleolytic attack. We have recently purified a mammalian endoribonuclease which can cleave within the c-myc CRD in vitro. The availability of this purified endonuclease has made it possible to directly test the interaction between CRD-BP and the endonuclease in regulating c-myc CRD RNA cleavage. In this study, we have identified the coding region of MDR-1 RNA as a new target for CRD-BP. CRD-BP has the same affinity for c-myc CRD nts 1705-1886 and MDR-1 RNA nts 746-962 with K(d) of 500 nM. The concentration-dependent affinity of CRD-BP to these transcripts correlated with the concentration-dependent blocking of endonuclease-mediated cleavage by CRD-BP. In contrast, three other recombinant proteins tested which had no affinity for c-myc CRD did not block endonuclease-mediated cleavage. Finally, we have identified RNA sequences required for CRD-BP binding. These results provide the first direct evidence that CRD-BP can indeed protect c-myc CRD cleavage initiated by an endoribonuclease, and the framework for further investigation into the interactions between CRD-BP, c-myc mRNA, MDR-1 mRNA and the endoribonuclease in cells.

A temporal study on the histopathological, biochemical and molecular responses of CCl(4)-induced hepatotoxicity in Cyp2e1-null mice

Previous study using Cyp2e1-null mice showed that Cyp2e1 is required in CCl(4)-induced liver injury at 24h, what remains unclear are the temporal changes in liver damage and the spectrum of genes involved in this process. We investigated the time-dependent liver changes that occurred at morphological, histopathological, biochemical and molecular levels in both Cyp2e1(+/+) and Cyp2e1(-/-) mice after treating with either corn oil or CCl(4) (1 ml/kg) for 2, 6, 12, 24 and 48 h. A pale orange colored liver, indicative of fatty infiltration, was observed in Cyp2e1(+/+) mice treated with CCl(4) for 24 and 48 h, while the Cyp2e1(+/+) mice treated with corn oil and Cyp2e1(-/-) mice treated with either corn oil or CCl(4) showed normal reddish brown colored liver. Ballooned hepatocytes with multiple vacuoles in their cytoplasm were observed in the livers of Cyp2e1(+/+) mice 24 and 48 h after treating with CCl(4). The levels of serum alanine aminotransferase and aspartate aminotransferase, markers for liver injury, were significantly higher at 12h, peaked at 24h and gradually decreased at 48 h after CCl(4) intoxication. In contrast, this kind of damage was not apparent in the Cyp2e1(-/-) mice treated with CCl(4). Altered expressions of genes related to liver cirrhosis, apoptosis, oxidative stress, xenobiotic detoxification, lipid metabolism, chemsensory signaling or tumorigenesis, structural organization, regeneration and inflammatory response were identified, and the time-dependent changes in expression of these genes were varied. Overall, the present study provides insights into the mechanism of CCl(4)-induced hepatotoxicity in animal models.

Identification of in vivo P-glycoprotein mRNA decay intermediates in normal liver but not in liver tumors

Post-transcriptional regulation at the level of mRNA stability is one important mechanism for over-expression of P-glycoprotein (Pgp) genes observed in cultured cells and in animals. A previous study has shown that mRNA half-lives for Pgp genes in normal liver were less than 2 h, in contrast to greater than 12 h measured in a transplantable liver tumor line. This lower turnover rate of Pgp mRNA may, in large part, contribute to the abundance of Pgp mRNA in liver tumors. The current study sought to investigate the underlying mechanism for the lower turnover rate of Pgp2 mRNA previously determined in liver tumors. As a first approach, we set out to understand the Pgp2 mRNA decay in both normal liver and liver tumors by first identifying and characterizing Pgp2 mRNA degradation intermediates. In this study, we showed that the sensitive ligation-mediated polymerase chain reaction (LM-PCR) method can be used to detect a homogenous pool of in vitro transcribed RNA down to 0.4 ng. By employing gene-specific primers in the LM-PCR method, we successfully identified four Pgp2 mRNA decay intermediates in normal liver. All four decay intermediates detected correspond to the 5' coding region of Pgp2 mRNA, and surprisingly no decay intermediates which correspond to 3' untranslated region, 3' coding region or middle coding region were found using LM-PCR. The identified decay intermediates are unique to the normal liver as they were absent or present at very low level in all three liver tumor samples analyzed. This observation supports our previous findings that the Pgp mRNA turnover rate is lower in liver tumors than in normal liver. These findings have implications for our understanding of the regulation of Pgp mRNA turnover in normal and malignant tissues.

Detection of mRNA degradation intermediates in tissues using the 3'-end poly(A)-tailing polymerase chain reaction method

It has become increasingly clear that mRNA stability is an important determinant of mRNA abundance in virtually all organisms. Although our understanding of prokaryotic lower eukaryotic mRNA stability mechanisms has progressed considerably, little is known about mammalian mRNA stability mechanisms, particularly at the tissue and animal levels. This is due largely to the lack of suitable methods to approach the problem. In this study, we have developed and refined the 3'-end poly(A)-tailing polymerase chain reaction (PCR) method to detect degradation intermediates in vivo. Using an in vitro transcribed RNA as a template, we found that the method could be used to detect a homogeneous pool of RNA down to 0.1 ng. The addition of 10 microg of total RNA from tissues decreased the sensitivity limit to 4 ng. Detection limits of the technique were determined precisely by varying the concentrations of in vitro transcribed RNA in a constant amount of total RNA and varying the concentration of total RNA while maintaining a constant amount of in vitro transcribed RNA. Our overall results showed that the poly(A)-tailing PCR method could be used to detect specific RNA species of approximately 1000 nt in a pool of heterogeneous RNA in the range of 1 in 2500 to 1 in 10,000. To our knowledge, this is the most sensitive method to date for identifying mRNA degradation intermediates. Employing sense strand gene-specific primers in this method, we have discovered the class II and class III P-glycoprotein (Pgp) mRNA degradation intermediates in normal rat tissues. This method should serve as an additional tool to help us understand mRNA decay mechanisms in tissues and at animal levels.

A conditionally replicating adenovirus targeted to tumor cells through activated RAS/P-MAPK-selective mRNA stabilization

The expression of various proteins associated with rapid responses to inflammation and/or proliferation can be controlled at the level of mRNA stability. Because tumor cells continually recapitulate intracellular programs of proliferation, we have used tumor cell-selective stabilization of mRNA as a means to control therapeutic gene expression. We describe an adenoviral vector that is conditionally replication competent in which expression of the essential adenoviral early region 1A (E1A) gene is regulated by ligation to the 3' untranslated region (UTR) of PTGS2 (also known as COX2), the gene encoding prostaglandin-endoperoxide synthase 2, allowing activated RAS/P-MAPK-specific stabilization of its mRNA. Induction of activated RAS supports replication, whereas matched cells in which activated RAS/P-MAPK is not expressed are very poor substrates for viral replication both in vitro and in vivo. Further tumor-targeting strategies will also be required to prevent viral replication at extratumoral sites where PTGS2 is normally induced. Many different genes contain 3' UTRs that control selective mRNA stability under different physiological, pathological and tumor-associated conditions. Therefore, generating tumor selectivity at the level of mRNA stability is a strategy with broad potential applicability in vector design.

EhPgp5 mRNA stability is a regulatory event in the Entamoeba histolytica multidrug resistance phenotype

The multidrug resistance (MDR) phenotype in Entamoeba histolytica is characterized by the overexpression of the EhPgp5 gene in trophozoites grown in high drug concentrations. Here we evaluated the role of EhPgp5 mRNA stability on MDR using actinomycin D. EhPgp5 mRNA from trophozoites growing without emetine had a half-life of 2.1 h, which augmented to 3.1 h in cells cultured with 90 microM and to 7.8 h with 225 microM emetine. Polyadenylation sites were detected at 118-, 156-, and 189-nucleotide (nt) positions of the EhPgp5 mRNA 3'-untranslated region. Interestingly, trophozoites grown with 225 microM emetine exhibited an extra polyadenylation site at 19 nt. The 3'-untranslated region sequence is AU-rich and has putative consensus sequences for RNA-binding proteins. We detected a RNA-protein complex in a region that contains a polypyrimidine tract (142-159 nt) and a cytoplasmic polyadenylation element (146-154 nt). A longer poly(A) tail in the EhPgp5 mRNA was seen in trophozoites grown with 225 microM emetine. Emetine stress may affect factors involved in mRNA turnover, including polyadenylation/deadenylation proteins, which could induce changes in the EhPgp5 mRNA half-life and poly(A) tail length. Novel evidence on mechanisms participating in E. histolytica MDR phenotype is provided.

P-glycoprotein (MDR1) expression in leukemic cells is regulated at two distinct steps, mRNA stabilization and translational initiation

Multidrug resistance in acute myeloid leukemia is often conferred by overexpression of P-glycoprotein, encoded by the MDR1 gene. We have characterized the key regulatory steps in the development of multidrug resistance in K562 myelogenous leukemic cells. Unexpectedly, up-regulation of MDR1 levels was not due to transcriptional activation but was achieved at two distinct post-transcriptional steps, mRNA turnover and translational regulation. The short-lived (half-life 1 h) MDR1 mRNA of naive cells (not exposed to drugs) was stabilized (half-life greater than 10 h) following short-term drug exposure. However, this stabilized mRNA was not associated with translating polyribosomes and did not direct P-glycoprotein synthesis. Selection for drug resistance, by long-term exposure to drug, led to resistant lines in which the translational block was overcome such that the stabilized mRNA was translated and P-glycoprotein expressed. The absence of a correlation between steady-state MDR1 mRNA and P-glycoprotein levels was not restricted to K562 cells but was found in other lymphoid cell lines. These findings have implications for the avoidance or reversal of multidrug resistance in the clinic.

Differential expression of c-fos and c-myc protooncogenes by estrogens, xenobiotics and other growth-stimulatory agents in primary rat hepatocytes

Mechanism(s) of tumour promotion in liver by estrogens and other xenobiotics such as alpha-hexachlorocyclohexane (HCH), 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane (DDT) and phenobarbital (PB), are not well understood although it is clear that growth stimulation is one important element in their action. To help in characterizing mechanisms of growth control by these compounds, their effects on the expression of immediate-early protooncogenes c- fos and c- myc have been examined and compared with other compounds that stimulate DNA synthesis in primary cultures of normal rat hepatocytes. Expression of c- fos was undetectable in cultures not exposed to growth factors. Although neither epidermal growth factor (EGF) nor 17beta-estradiol (E(2)) alone had marked effects on c- fos mRNA, the two acted synergistically to cause clear c- fos expression, maximal 1-2 h after growth factor addition and when test agents were added on the first day in culture. Neither insulin nor dexamethasone alone induced c- fos mRNA but stimulation of c- fos expression by EGF plus estradiol occurred earlier in the presence of insulin, and was augmented by preincubation of cells with dexamethasone. EGF + E(2)-induced c- fos mRNA was completely abolished by actinomycin D, suggesting that transcription is the major mechanism for c- fos induction by E(2) + EGF. Compounds that strongly stimulate hepatocyte DNA synthesis such as norepinephrine, pyruvate, prolactin, glutethimide, monensin, ammonium chloride, and normal rat serum when in combination with EGF, all failed (when added with EGF) to affect c- fos mRNA expression. Thus, induction of c- fos expression may be a component of estradiol's growth stimulatory effect in cultured hepatocytes but this is not the case for other compounds that strongly stimulate DNA synthesis. Unlike c- fos mRNA, c- myc mRNA was detectable in hepatocyte cultures without added growth factor, was augmented within 2 h of exposure to EGF, and was further increased by adding E(2), other estrogens or a variety of other stimulators of DNA synthesis in hepatocytes. This suggests that increased c- myc expression may be a common effect of many of these agents in combination with EGF.

Superinduction of P-glycoprotein messenger RNA in vivo in the presence of transcriptional inhibitors

P-Glycoprotein (P-gp) is comprised of a small family of plasma membrane proteins, and its presence in high amounts often correlates with multidrug resistance in cultured cells. Dramatically increased levels of a single member of P-gp mRNA (pgp2) have been observed in experimental liver carcinogenesis models, during liver regeneration, upon culturing of hepatocytes and in the uterus of pregnant animals. In all cases, the increase in mRNA level appears to be the result of an increase in mRNA half-life (stability). Previously, we have used transcriptional inhibitors alpha-amanitin and actinomycin D to measure P-gp mRNA half-life in normal liver and in liver tumors. We showed that the level of all three P-gp mRNAs decreased with time in the presence of transcriptional inhibitors, yielding measured half-lives of less than 2 h in liver but greater than 12 h in liver tumors. This observation raised the possibility that regulation of P-gp mRNA stability plays a role in liver carcinogenesis. In the present study, we measured P-gp mRNA half-life in other normal tissues to determine if a short P-gp mRNA half-life is unique to the liver. Our study reveals that in contrast to liver, measured P-gp mRNA half-lives in most tissues examined are greater than 12 h. Moreover, we observed an unexpected, marked increase in the level of pgp2 mRNA with time after injection of transcriptional inhibitors. This can only be explained if the transcriptional inhibitors directly or indirectly inhibit the normally high degradation rate of pgp2 mRNA, resulting in the superinduction of this mRNA. These findings have implications for our understanding of the regulation of P-gp gene expression and drug resistance in vivo.

Multidrug resistance in the protozoan parasite Entamoeba histolytica

In this review we discuss the mechanisms and molecules involved in the multidrug resistance (MDR) of the protozoan parasite Entamoeba histolytica. Drug resistant mutants exhibited the main characteristics presented by the MDR mammalian cells. They showed cross-resistance to several unrelated drugs that is reverted by calcium channel blockers. MDR phenotype in E. histolytica is regulated at a transcriptional level by the EhPgp1 gene, which is constitutively expressed and by the EhPgp5 gene, whose expression is induced in the presence of the drug. Transcription factors participate in the expression regulation of these genes. After over transcription, the EhPgp genes are amplified, cooperating to produce the MDR phenotype. Post-transcriptional mechanisms such as mRNA stability seem to be involved in this phenomenon. As for other mdr gene products, the EhPGP5 protein functions as a chloride current inductor or as a regulator of cellular regulatory volume decrease.

Molecular profiling of genes up-regulated during promotion by phenobarbital treatment in a medium-term rat liver bioassay

In search of genes that are steadily up-regulated during the promotion stage in carcinogenesis, suppression PCR subtractive hybridization and following northern blot screening were performed using a phenobarbital (PB)-promotion model based on a medium-term liver bioassay. Two weeks after a single injection of diethylnitrosamine (DEN; 200 mg/kg body wt, i.p.), rats were given 600 p.p.m. PB in the drinking water for up to 64 weeks. For comparison, animals fed 1 p.p.m. ethinylestradiol (EE) or 3000 p.p.m. butylated hydroxytoluene (BHT) in the diet at promotion stage were also included. Rats were subjected to partial hepatectomy (PH) at week 3. In addition, dose-dependence of PB at week 8 of promotion and responsiveness to representative non-genotoxic carcinogens without DEN initiation were examined. Fragments of a total of 67 different genes were isolated from the up-regulated gene population in the liver at day 10 of PB treatment by subtracting from basal expression of DEN + PH alone. Using northern blot screening for signal-detectable 48 genes, 16 genes showed up-regulation in the livers at week 8 of promotion, common to the PB and EE treatments with the levels being three times or more than the basal expression of unpromoted liver. The majority of these genes were also up-regulated at week 8 by BHT treatment, and were also constitutively expressed in the DEN(-), PH(-) untreated rat livers. Among the up-regulated genes common to the PB and EE promotion, and not responding to the non-genotoxic carcinogens in uninitiated liver, the following six genes showed overexpression in PB-promoted hepatocellular carcinomas at week 64, with the levels three times or more than untreated rat liver: ubiquitously expressed mammalian ABC half transporter, apolipoprotein A4, nuclear receptor binding factor-2, CD81, hypothetical protein (HSPC014) and one unidentified gene. These genes might be candidates for biomarkers in screening of non-genotoxic hepatocarcinogens by analysis in two-stage carcinogenesis models.

Induction of P-glycoprotein mRNA transcripts by cycloheximide in animal tissues: evidence that class I Pgp is transcriptionally regulated whereas class II Pgp is post-transcriptionally regulated

P-glycoprotein (Pgp) are a small family of plasma membrane proteins capable of transporting substrates across cell membranes. Class I and class II Pgp are able to transport drugs and have been shown to mediate multidrug resistance (MDR). Class III Pgp is a long chain phospholipid transporter and does not mediate MDR. The expression and regulation of Pgp genes in animal tissues are not well understood. In this study, the protein synthesis inhibitor cycloheximide was used as a tool to understand Pgp gene expression and regulation in animal tissues. The sensitive RNase protection assay was used to detect changes in Pgp mRNA levels and nuclear run-on assay was used to determine whether transcription or post-transcription is important. The results showed that cycloheximide significantly induced class II Pgp expression in all tissues examined. This was predominantly through post-transcriptional effect. In contrast, the relatively modest increase in class I Pgp expression by cycloheximide was found to be mainly due to increased transcriptional activity. On the other hand, cycloheximide induced class III Pgp expression in some tissues while caused decay of class III Pgp mRNA in other tissues. The transcriptional and post-transcriptional mechanisms exerted by cycloheximide on Pgp genes are discussed. These findings have implications for our understanding of gene regulation in animal tissues and MDR reversal strategies in vivo.

Cell-cycle-dependent turnover of P-glycoprotein in multidrug-resistant cells

Regulation of P-glycoprotein (Pgp) expression occurs not only at the DNA and mRNA level but also at the protein level. We showed previously that Pgp was stabilized when multidrug-resistant CH(R)C5 and SKVCR 2.0 ovarian cell lines were subjected to serum-starved or high-cell-density growth conditions, whereas Pgp turnover in a leukemic multidrug-resistant cell line, CEMVLB0.1, was not affected by serum starvation (Muller et al., 1995). On further analysis, we have observed that the majority of the CH(R)C5 and SKVCR 2.0 cells under these conditions were in the G1/G0 phase of the cell cycle, whereas the cell cycle of CEMVLB0.1 cells was not affected. Pgp in CEMVLB0.1 cells was stabilized only when the cell cycle was delayed in the G1/G0 phase by using amino acid-deficient growth medium. In CH(R)C5 cells, Pgp half-life was also considerably increased when the cell cycle of these ovary-derived cells was delayed in the G1/G0 phase by using high concentrations of progesterone under normal serum growth conditions. In contrast, Pgp stability was not greatly affected if these cells were delayed in the S or G2/M phase of the cell cycle with Ara-C, cisplatin, or colchicine under the same conditions. Insulin-like growth factors could release the serum-starved CH(R)C5 and SKVCR2.0 cells from the G1/G0 phase and destabilized Pgp. These results indicate that Pgp turnover is a cell-cycle-related process in MDR cells.

The AU-rich 3' untranslated region of human MDR1 mRNA is an inefficient mRNA destabilizer

The human multidrug resistance gene MDR1 encodes a membrane-bound protein, referred to as P-glycoprotein, that acts as a pump to extrude toxins from cells. The 3' untranslated region (3'UTR) of the human MDR1 mRNA is very AU-rich (70%) and contains AU-rich sequences similar to those shown to confer rapid decay on c-myc, c-fos, and lymphokine mRNAs. We tested the ability of the MDR1 3'UTR to act as an mRNA destabilizing element in the human hepatoma cell line HepG2. The MDR1 mRNA has an intermediate half-life of 8 h in HepG2 cells compared to a half-life of 30 min for c-myc mRNA. The MDR1 mRNA half-life was prolonged to >20 h upon treatment with the protein synthesis inhibitor cycloheximide. We constructed expression vectors containing the human beta-globin coding region with the 3'UTR from either MDR1 or c-myc. The c-myc 3'UTR increased the decay of the chimeric mRNA, but the MDR1 3'UTR had no effect. We tested the ability of MDR1 3'UTR sequences to compete for interaction with AU-binding proteins in cell extracts; MDR1 RNA probes had a fivefold lower affinity for AU-binding proteins that interact with the c-myc AU-rich 3'UTR. Overall, our data suggest that the MDR1 3'UTR does not behave as an active destabilizing element in HepG2 cells.