Human papillomavirus type 16 E6 suppresses microRNA-23b expression in human cervical cancer cells through DNA methylation of the host gene C9orf3.

Oncogenic protein E6 of human papillomavirus type 16 (HPV-16) is believed to involve in the aberrant methylation in cervical cancer as it upregulates DNA methyltransferase 1 (DNMT1) through tumor suppressor p53. In addition, DNA demethylating agent induces the expression of one of the HPV-16 E6 regulated microRNAs (miRs), miR-23b, in human cervical carcinoma SiHa cells. Thus, the importance of DNA methylation and miR-23b in HPV-16 E6 associated cervical cancer development is investigated. In the present study, however, it is found that miR-23b is not embedded in any typical CpG island. Nevertheless, a functional CpG island is predicted in the promoter region of C9orf3, the host gene of miR-23b, and is validated by methylation-specific PCR and bisulfite genomic sequencing analyses. Besides, c-MET is confirmed to be a target gene of miR-23b. Silencing of HPV-16 E6 is found to increase the expression of miR-23b, decrease the expression of c-MET and thus induce the apoptosis of SiHa cells through the c-MET downstream signaling pathway. Taken together, the tumor suppressive miR-23b is epigenetically inactivated through its host gene C9orf3 and this is probably a critical pathway during HPV-16 E6 associated cervical cancer development.

Clitocine reversal of P-glycoprotein associated multi-drug resistance through down-regulation of transcription factor NF-κB in R-HepG2 cell line.

Multidrug resistance (MDR) is one of the major reasons for failure in cancer chemotherapy and its suppression may increase the efficacy of therapy. The human multidrug resistance 1 (MDR1) gene encodes the plasma membrane P-glycoprotein (P-gp) that pumps various anti-cancer agents out of the cancer cell. R-HepG2 and MES-SA/Dx5 cells are doxorubicin induced P-gp over-expressed MDR sublines of human hepatocellular carcinoma HepG2 cells and human uterine carcinoma MES-SA cells respectively. Herein, we observed that clitocine, a natural compound extracted from Leucopaxillus giganteus, presented similar cytotoxicity in multidrug resistant cell lines compared with their parental cell lines and significantly suppressed the expression of P-gp in R-HepG2 and MES-SA/Dx5 cells. Further study showed that the clitocine increased the sensitivity and intracellular accumulation of doxorubicin in R-HepG2 cells accompanying down-regulated MDR1 mRNA level and promoter activity, indicating the reversal effect of MDR by clitocine. A 5'-serial truncation analysis of the MDR1 promoter defined a region from position -450 to -193 to be critical for clitocine suppression of MDR1. Mutation of a consensus NF-κB binding site in the defined region and overexpression of NF-κB p65 could offset the suppression effect of clitocine on MDR1 promoter. By immunohistochemistry, clitocine was confirmed to suppress the protein levels of both P-gp and NF-κB p65 in R-HepG2 cells and tumors. Clitocine also inhibited the expression of NF-κB p65 in MES-SA/Dx5. More importantly, clitocine could suppress the NF-κB activation even in presence of doxorubicin. Taken together; our results suggested that clitocine could reverse P-gp associated MDR via down-regulation of NF-κB.

3,5-Dimethyl-H-furo[3,2-g]chromen-7-one as a potential anticancer drug by inducing p53-dependent apoptosis in human hepatoma HepG2 cells.

BACKGROUND/AIMS: Coumarins are natural compounds found in many plants that possess medical value by itself and its modified derivatives. METHOD: Six novel coumarin derivatives were synthesized and examined for their potential anticancer cytotoxicity. RESULT: Among the 6 derivatives, 3,5-dimethyl-(7)H-furo[3,2-g]chromen-7-one (DMFC) presented the strongest cytotoxicity against human hepatoma HepG2 cells in vitro with an IC(50) value of 8.46 ± 0.28 µM in a 48-hour treatment. Further experiments revealed that DMFC induced apoptosis in HepG2 cells through both extrinsic and intrinsic apoptotic pathways in a p53-dependent manner. Mechanistically, DMFC activated caspases 3, 8 and 9, depolarized mitochondrial membrane potential and induced cytochrome c and apoptosis-inducing factor release. DMFC-induced apoptosis was also characterized by DNA fragmentation, phosphatidylserine externalization and sub-G1 peak in DNA histograms. Moreover, both caspase 8 and 9 inhibitors suppressed the apoptosis induced by DMFC. Western blot analyses revealed that DMFC also significantly increased the expression levels of p53, Fas death receptor, Fas-associated death domain protein and proapoptotic Bcl-2 family members such as Bax, Bad and tBid, as well as decreased the levels of pro-survival members such as Bcl-2 and Bcl-xl. CONCLUSION: DMFC is potentially an effective therapeutic agent in liver cancer therapy.

Synthesis and antiproliferative activities of novel 5'-Schiff-base group substituted psoralen derivatives.

It was found that psoralen derivative could perform a Friedel-Crafts acylation smoothly with acetic anhydride to give 5'-acetylpsoralen in a 73% yield. In the presence of boron trifluoride etherate, 5'-acetylpsoralen reacted with both aromatic amines and aliphatic amine smoothly to afford 5'-Schiff-base group substituted psoralen derivatives in 72%-92% yields. The novel synthetic method has the advantages of cheap materials, mild reaction conditions, good yields and high regioselectivity in the Friedel-Crafts acylation. Cell viability assay by MTT demonstrates that some of the psoralen derivatives 6 have antiproliferative activities.

P-glycoprotein enhances radiation-induced apoptotic cell death through the regulation of miR-16 and Bcl-2 expressions in hepatocellular carcinoma cells.

P-glycoprotein (Pgp), an efflux pump, was confirmed the first time to regulate the expressions of miR/gene in cells. Pgp is known to be associated with multidrug resistance. RHepG2 cells, the multidrug resistant subline of human hepatocellular carcinoma HepG2 cells, expressed higher levels of Pgp as well as miR-16, and lower level of Bcl-2 than the parental cells. In addition, RHepG2 cells were more radiation sensitive and showed more pronounced radiation-induced apoptotic cell death than the parental cells. Mechanistic analysis revealed that transfection with mdr1 specific antisense oligos suppressed radiation-induced apoptosis in HepG2 cells. On the other hand, ectopic mdr1 expression enhanced radiation-induced apoptosis in HepG2 cells, SK-HEP-1 cells, MiHa cells, and furthermore, induced miR-16 and suppressed its target gene Bcl-2 in HepG2 cells. Moreover, the enhancement effects of Pgp and miR-16 on radiation-induced apoptosis were counteracted by overexpression of Bcl-2. The Pgp effect on miR-16/Bcl-2 was suppressed by Pgp blocker verapamil indicating the importance of the efflux of Pgp substrates. The present study is the first to reveal the role of Pgp in regulation of miRNA/gene expressions. The findings may provide new perspective in understanding the biological function of Pgp.

HPV-16 E6 upregulation of DNMT1 through repression of tumor suppressor p53.

The HPV-16 early proteins E6 and E7 are considered to function as oncoproteins in cervical cancer. DNA methyltransferase 1 (DNMT1) is one of the enzymes involved in epigenetic silencing of tumor suppressor genes. In the present study, the functional role and regulation of DNMT1 in HPV-16 E6 associated cervical cancer development were examined. Knockdown of E6 in HPV-16 positive human cervical carcinoma SiHa and CaSki cells led to the increase in p53, repression of DNMT1 protein and promoter activity. Moreover, p53 knockdown increased the DNMT1 protein as well as promoter activity, indicating that p53 may mediate E6 upregulation of DNMT1. In addition, E6 knockdown induced growth retardation in SiHa cells, and the effect was partially reverted by DNMT1 overexpression. The results suggest that HPV-16 E6 may act through p53/ DNMT1 to regulate the development of cervical cancer.

AF1q enhancement of gamma irradiation-induced apoptosis by up-regulation of BAD expression via NF-kappaB in human squamous carcinoma A431 cells.

BAD (BCL-2 antagonist of cell death) is a pro-apoptotic BCL-2 family protein that plays a critical role in the regulation of apoptotic response. This study presents direct evidence that AF1q increased the radiation-induced apoptosis through up-regulation of BAD in human squamous carcinoma A431 cells and the key transcription factor involved is NF-kappaB. The minimal promoter sequence of BAD was identified; the activity was increased in AF1q stable transfectants and decreased upon AF1q siRNA transfection. The NF-kappaB consensus binding sequence is detected on BAD promoter. Inactivation of NF-kappaB by NF-kappaB inhibitor Bay 11-7082 or NF-kappaB p65 siRNA suppressed the expression and promoter activity of BAD; the suppression is more obvious in AF1q stable transfectants which also have an elevated NF-kappaB level. Mutation of putative NF-kappaB motif decreased the BAD promoter activity. The binding of NF-kappaB to the BAD promoter was confirmed by chromatin-immunoprecipitation. These findings indicate that AF1q up-regulation of BAD is through its effect on NF-kappaB and this may hint of its oncogenic mechanism in cancer.

Epigallocatechin gallate up-regulation of miR-16 and induction of apoptosis in human cancer cells.

Epigallocatechin gallate (EGCG) is a major type of green tea polyphenols and is known to have cancer prevention effect. MicroRNAs (miRNAs) are 19 to 25 nucleotides and are believed to be important in gene regulation. In the present study, the influence of EGCG on the expressions of miRNAs in human cancer cells was investigated as this has not yet been reported. By miRNA microarray analysis, EGCG treatment was found to modify the expressions of some of the miRNAs in human hepatocellular carcinoma HepG2 cells, 13 were up-regulated and 48 were down-regulated. miR-16 is one of the miRNAs up-regulated by EGCG and one of its target genes is confirmed to be the anti-apoptotic protein Bcl-2. EGCG treatment induced apoptosis and down-regulated Bcl-2 in HepG2 cells. Transfection with anti-miR-16 inhibitor suppressed miR-16 expression and counteracted the EGCG effects on Bcl-2 down-regulation and also induction of apoptosis in cells. Results from the present study confirm the role of miR-16 in mediating the apoptotic effect of EGCG and also support the importance of miRNAs in the regulation of the biological activity of EGCG.

Oncofetal H19-derived miR-675 regulates tumor suppressor RB in human colorectal cancer.

H19 is an imprinted oncofetal non-coding RNA recently shown to be the precursor of miR-675. The pathophysiological roles of H19 and its mature product miR-675 to carcinogenesis have, however, not been defined. By quantitative reverse transcription-polymerase chain reaction, both H19 and miR-675 were found to be upregulated in human colon cancer cell lines and primary human colorectal cancer (CRC) tissues compared with adjacent non-cancerous tissues. Subsequently, the tumor suppressor retinoblastoma (RB) was confirmed to be a direct target of miR-675 as the microRNA suppressed the activity of the luciferase reporter carrying the 3'-untranslated region of RB messenger RNA that contains the miR-675-binding site. Suppression of miR-675 by transfection with anti-miR-675 increased RB expression and at the same time, decreased cell growth and soft agar colony formation in human colon cancer cells. Reciprocally, enhanced miR-675 expression by transfection with miR-675 precursor decreased RB expression, increased tumor cell growth and soft agar colony formation. Moreover, the inverse relationship between the expressions of RB and H19/miR-675 was also revealed in human CRC tissues and colon cancer cell lines. Our findings demonstrate that H19-derived miR-675, through downregulation of its target RB, regulates the CRC development and thus may serve as a potential target for CRC therapy.

Facilitation of drug resistance development by gamma-irradiation in human cancer cells.

Hepatocellular carcinoma HepG2 cells (G cells) were subjected to selection first with gamma-radiation and then doxorubicin (Dox). The radiation treatment consisted of 2 Gy for 10 days (G2) or 10 Gy for 2 days (G10) and the Dox treatment was continuous exposure for up to 10 microM. Compared with respective parental G, G2, G10 cells, the Dox-selected cells showed mdr1 amplification/P-glycoprotein overexpression, Dox resistance and also less intracellular Dox accumulation. Verapamil reversed the drug resistance and increased the Dox accumulation in all cells. Decay in drug resistance and reduction in mdr1 amplification/P-glycoprotein overexpression were observed in the Dox-selected cells culturing in Dox-free condition. Among the Dox-selected cells, G2R cells showed the highest levels of drug resistance, mdr1 amplification, but the least resistance decay. Results from the study indicate the possible influence of radiation treatment on the development of drug resistance in cancer cells and it may even lead to a highly resistant phenotype.

Mechanistic study on growth suppression and apoptosis induction by targeting hepatoma-derived growth factor in human hepatocellular carcinoma HepG2 cells.

Hepatoma-derived growth factor (HDGF) is frequently overexpressed in human cancer. The growth factor was previously demonstrated to be a survival factor as knock-down of HDGF suppresses the growth and induces apoptosis in human cancer cells through the Bad-mediated intrinsic apoptotic pathway. However, inactivation of Bad cannot completely repress the apoptosis induced upon HDGF knock-down, indicating the presence of other unidentified pathways. In the present study, HDGF knock-down was shown to trigger the Fas-mediated extrinsic apoptotic pathway in human hepatocellular carcinoma HepG2 cells through NF-kappaB signaling pathway. Increases in Fas expression and fas promoter activity were detected upon HDGF knock-down by Western blot analysis and luciferase reporter assay. Knock-down of fas inhibited HDGF knock-down effect on apoptosis induction and growth suppression as revealed by annexin V binding assay and soft agar assay. Down-regulation of IkappaBalpha was also observed upon HDGF knock-down. Overexpression of IkappaBalpha by transient transfection or inhibition of NF-kappaB by BAY11-7082 suppressed HDGF knock-down effect on fas promoter activation, Fas up-regulation, apoptosis induction and growth suppression. Furthermore, the interaction of Fas-mediated extrinsic and Bad-mediated intrinsic apoptotic pathways was demonstrated as a stronger inhibition on apoptosis induction and growth suppression upon HDGF knock-down was observed when both pathways were inactivated. The results therefore suggested that, through both intrinsic and extrinsic apoptotic pathways, HDGF may function as a survival factor and be a potential target for cancer therapy.

Suillin from the mushroom Suillus placidus as potent apoptosis inducer in human hepatoma HepG2 cells.

During the search of new anti-cancer agent from high fungi, the ethyl acetate extract of the mushroom Suillus placidus was found to exhibit a significant cytotoxic activity against human hepatoma HepG2 cells. With bioassay-guided fractionation, a cytotoxic component suillin was isolated from the extract. The anti-cancer effect of suillin was subsequently examined in 8 human cancer cell lines by using MTT assay. It is of interest to note that human liver cancer cells (HepG2 cells, Hep3B cells, and SK-Hep-1) were preferentially killed by suillin with an IC(50) of approximately 2microM in a 48h treatment. Mechanistically. suillin was found for the first time to induce apoptosis in HepG2 cells as characterized by DNA fragmentation, phosphatidyl-serine (PS) externalization, activation of caspase-3, -8, -9, depolarization of mitochondrial membrane potential, as well as release of cytochrome c into the cytosol. Moreover, the apoptosis induced by suillin was suppressed by both caspase-8 and -9 inhibitors. Western blot analysis revealed significant increases in the protein levels of Fas death receptor, adaptor FADD protein, pro-apoptotic protein Bad and a decline of Bid. These results suggest that the induction of apoptosis by suillin is through both death receptor and mitochondrial pathways. Taken together, our results suggest that suillin might be an effective agent to treat liver cancer.

Estrogen-related receptor alpha (ERRalpha) inverse agonist XCT-790 induces cell death in chemotherapeutic resistant cancer cells.

Estrogen-related receptor alpha (ERRalpha) is primarily thought to regulate energy homeostasis through interacting with peroxisome proliferator-activated receptor gamma coactivator-1alpha and -1beta (PGC-1alpha and -1beta). They coordinately control the transcription of genes in the oxidative phosphorylation pathway. In addition to its role in energy metabolism, ERRalpha has also been implicated as a prognostic marker for breast, ovarian, colon and prostate cancers. In this study, we found that an ERRalpha inverse agonist XCT-790 induced cell death in HepG2 hepatocarcinoma and its multi-drug resistance (MDR) sub-line R-HepG2. Using a dye Mitotracker Green which stains mitochondrion independent of mitochondrial membrane potential (DeltaPsi(m)), we found that XCT-790 dose-dependently decreased mitochondrial mass. Intriguingly, XCT-790 increased DeltaPsi(m) upon short term treatment but decreased DeltaPsi(m) upon longer term treatment. The changes of DeltaPsi(m) in turn promoted the production of reactive oxygen species (ROS) and led to ROS-mediated caspases 3/7, 8, 9 activation and cell death. Importantly, we established that an anti-oxidative compound Mn(III) Tetra(4-benzoic acid) porphyrin chloride (MnTBAP) blocked the caspases activities and cell death increased by XCT-790 treatment. Finally, we found that XCT-790 synergized with paclitaxel to induce cell death in multi-drug resistance sub-line R-HepG2. Our results provide a conceptual framework for further developing chemotherapeutics based on suppressing ERRalpha activity.

The miR-18a* microRNA functions as a potential tumor suppressor by targeting on K-Ras.

The Ras proto-oncogene mediates a wide variety of cellular events and is frequently mutated in cancer. MicroRNAs (miRNAs) may regulate the development of cancer through their effect on the target genes. In the search of miRNAs that target on Ras, miR-18a* is the first time confirmed to target on K-Ras and furthermore not on N- and H-Ras. miR-18a* repression by transfection with anti-miR-18a* inhibitor increased the K-Ras expression as well as the luciferase activity of a reporter construct containing the 3'-untranslated region of K-Ras messenger RNA. Furthermore, the miR-18a* repression also increased the cell proliferation and promoted the anchorage-independent growth in soft agar of human squamous carcinoma A431 cells, colon adenocarcinoma HT-29 cells and fetal hepatic WRL-68 cells. On the other hand, ectopic expression of miR-18a* by transfection with miR-18a* precursor suppressed K-Ras expression, cell proliferation and anchorage-independent growth of A431 cells. The increase in cell proliferation and anchorage-independent growth upon miR-18a* repression was, however, rendered by the Ras inhibitor farnesylthiosalicylic acid. In conclusion, miR-18a* may function as a tumor suppressor by targeting on K-Ras. Therefore, the miRNA may also be a potential therapeutic agent or target for cancer therapy.

Oncogene AF1q enhances doxorubicin-induced apoptosis through BAD-mediated mitochondrial apoptotic pathway.

AF1q is an oncogenic factor involved in leukemia development, thyroid tumorigenesis, and breast cancer metastasis. In the present study, AF1q was found to be down-regulated in a doxorubicin-resistant subline of human squamous carcinoma A431 cells. Knockdown of AF1q decreased the apoptosis induced by doxorubicin, Taxol, gamma-radiation, IFN-alpha, and IFN-gamma in A431 cells. On the other hand, overexpression of AF1q increased the doxorubicin-induced apoptosis in A431 cells as well as in HepG2 and HL60 cells. Both exogenous and ectopic expression of AF1q in A431 cells increased the mRNA and protein levels of BAD, a proapoptotic BCL-2 family protein. Gene silencing of BAD by small interfering RNA suppressed the AF1q enhancement of apoptosis, suggesting that BAD is downstream of AF1q in regulation of apoptosis. Furthermore, AF1q enhanced the mitochondrial membrane depolarization, mitochondrial cytochrome c release, and activation of caspase-9 and caspase-3 on doxorubicin treatment. Collectively, AF1q increases doxorubicin-induced apoptosis in cells through activation of BAD-mediated apoptotic pathway. The study provides the first evidence that AF1q plays a critical role in the regulation of apoptosis and drug resistance.

Let-7a microRNA suppresses therapeutics-induced cancer cell death by targeting caspase-3.

MicroRNAs (miRNA) are endogenously expressed non-coding RNAs that regulate gene expression post-transcriptionally. Let-7a miRNA is a founding member in the let-7 family and its down-regulation in association with over-expression of RAS and HMGA2 oncogenes has previously been reported. In the present study, caspase-3, the executioner caspase, was confirmed to be the target of let-7a as ectopic expression of let-7a decreased the luciferase activity of a reporter construct containing the 3' untranslated region of caspase-3 and at the same time repressed the enzyme expression in human squamous carcinoma A431 cells and hepatocellular carcinoma HepG2 cells. Moreover, let-7a was over-expressed while caspase-3 was down-regulated in A10A cells, a doxorubicin-resistant A431 subline. Enforced let-7a expression increased the resistance in A431 cells and HepG2 cells to apoptosis induced by therapeutic drugs such as interferon-gamma, doxorubicin and paclitaxel. On the other hand, down-regulation of let-7a by the anti-let-7a inhibitor increased the doxorubicin-induced apoptosis in A431 parent cells, A10A cells and HepG2 cells while the increase was suppressed by caspase-3 inhibitor. Both anti-let-7a inhibitor and caspase-3 inhibitor however failed to affect the drug-induced apoptosis in human breast cancer MCF7 cells, the cells that do not express caspase-3. Therefore, let-7a by targeting caspase-3 may play a functional role in modulating drug-induced cell death in human cancer cells.

Downregulation of hepatoma-derived growth factor activates the Bad-mediated apoptotic pathway in human cancer cells.

Hepatoma-derived growth factor (HDGF) is highly expressed in human cancer and its expression is correlated with poor prognosis of cancer. The growth factor is known to stimulate cell growth while the underlying mechanism is however not clear. Transfection with HDGF cDNA stimulated while its specific antisense oligonucleotides repressed the growth of human hepatocellular carcinoma HepG2 cells. Furthermore, knock-down of HDGF by antisense oligos also induced apoptosis in HepG2 cells and in other human cancer cells, e.g. human squamous carcinoma A431 cells. HDGF knock-down was found to induce the expression of the pro-apoptotic protein Bad and also inactivate ERK and Akt, which in turn led to dephosphorylation of Bad at Ser-112, Ser-136, and activation of the intrinsic apoptotic pathway, i.e. depolarization of the mitochondrial membrane, release of mitochondrial cytochrome c, increase in the processing of caspase 9 and 3. As HDGF knock-down not only suppresses the growth but also induces apoptosis in human cancer cells, HDGF may therefore serve as a survival factor for human cancer cells and a potential target for cancer therapy.

Induction of drug resistance and transformation in human cancer cells by the noncoding RNA CUDR.

Refractory to apoptosis induced by anticancer drugs is one of the major causes of drug resistance in human cancers. The involvement of noncoding RNA (ncRNA) in cancer cell drug resistance has not yet been reported. By using the technique of RT-PCR-based differential display, a novel gene, cancer up-regulated drug resistant (CUDR) gene, was found to be overexpressed in a doxorubicin-resistant subline of human squamous carcinoma A431 and A10A cells, which were also more resistant to drug-induced apoptosis. The full-length CUDR mRNA transcript is approximately 2.2 kb as detected by Northern blot analysis and has no sequence homology with other genes identified so far. Interestingly, no distinct open reading frame was found throughout the CUDR cDNA sequence, and no recombinant protein was detected from in vitro translation or from a protein lysate of human cancer cells after CUDR transfection. Therefore, CUDR is likely to exert its function as a noncoding RNA. Stable transfection with the CUDR gene was found to induce resistance to doxorubicin and etoposide as well as drug-induced apoptosis in A431 cells. By Western blot analysis, down-regulations of caspase 3 were observed in CUDR transfectants. On the other hand, overexpression of CUDR promoted anchorage-independent growth in A431 cells. Results from the present study suggest that CUDR may likely regulate the drug sensitivity and promote cellular transformation at least through caspase 3-dependent apoptosis.

p53-R273H gains new function in induction of drug resistance through down-regulation of procaspase-3.

Development of drug resistance is one of the major obstacles in cancer chemotherapy. The molecular mechanism leading to drug resistance is still not fully understood. A10A cells, a doxorubicin-resistant subline of human squamous cell carcinoma A431 cells, showed cross-resistance to methotrexate and also resistance to the drug-induced apoptosis. The cells also showed overexpression of a mutated form of p53, p53-R273H (Arg to His at codon 273), and down-regulation of procaspase-3. Knockdown of p53-R273H by p53 small interfering RNA in A431 cells increased procaspase-3 level and sensitized the cells to drug-induced apoptosis. On the other hand, transfection of p53-R273H into p53 null human osteosarcoma Saos-2 cells down-regulated procaspase-3 level and induced resistance to the drug toxicity and drug-induced apoptosis. The results support the idea that p53-R273H may gain new functions in induction of drug resistance and impairment in drug-induced apoptosis through down-regulation of procaspase-3 level. The study sheds new light on the understanding of the gain of function and drug resistance mechanisms associated with mutant p53.

Epidermal growth factor induction of resistance to topoisomerase II toxins in human squamous carcinoma A431 cells.

Alteration of the epidermal growth factor (EGF) signaling pathway occurs frequently in human cancer cells and may subsequently affect the cell survival towards anti-cancer agents. To elucidate the effect of long-term EGF treatment on the chemo-sensitivity of human cancer cells, human squamous carcinoma A431 cells (AP) were incubated continuously with 50 ng/ml EGF for 30 weeks and these cells were designated as the AC cells. The long-term EGF treatment did not alter the EGFR level and the EGF-induced protein tyrosine phosphorylation pattern in the AC cells. By MTT assay, the AC cells were shown to be more resistant than the AP cells to doxorubicin, etoposide and amsacrine but not to cisplatin. Among the drug-resistant proteins, topoisomerase IIalpha (topoII) was downregulated in the AC cells while there was no apparent change in the levels of P-glycoprotein, MRP-1 or glutathione- S-transferase-pi as compared to the AP cells. Furthermore, knockdown of topoII by antisense topoII oligonucleotide transfection decreased the sensitivity to doxorubicin, etoposide and amsacrine in the A431 cells. Results from the present study support an idea that long-term treatment with EGF may induce drug resistance in cells through the downregulation of topoII.