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. 2005 Jun;25(12):5040–5051. doi: 10.1128/MCB.25.12.5040-5051.2005

DNA Topoisomerase I Is a Cofactor for c-Jun in the Regulation of Epidermal Growth Factor Receptor Expression and Cancer Cell Proliferation

Antoine Mialon 1,2,†, Matti Sankinen 1,2,†, Henrik Söderström 1,2, Teemu T Junttila 2,3,5, Tim Holmström 1, Riku Koivusalo 3, Anastassios C Papageorgiou 1, Randall S Johnson 6, Sakari Hietanen 3,4, Klaus Elenius 2,3, Jukka Westermarck 1,2,*
PMCID: PMC1140586  PMID: 15923621

Abstract

DNA topoisomerase I (Topo I) is a molecular target for the anticancer agent topotecan in the treatment of small cell lung cancer and ovarian carcinomas. However, the molecular mechanisms by which topotecan treatment inhibits cancer cell proliferation are unclear. We describe here the identification of Topo I as a novel endogenous interaction partner for transcription factor c-Jun. Reciprocal coimmunoprecipitation analysis showed that Topo I and c-Jun interact in transformed human cells in a manner that is dependent on JNK activity. c-Jun target gene epidermal growth factor receptor (EGFR) was identified as a novel gene whose expression was specifically inhibited by topotecan. Moreover, Topo I overexpression supported c-Jun-mediated reporter gene activation and both genetic and chemical inhibition of c-Jun converted cells resistant to topotecan-elicited EGFR downregulation. Topotecan-elicited suppression of proliferation was rescued by exogenously expressed EGFR. Furthermore, we demonstrate the cooperation of the JNK-c-Jun pathway, Topo I, and EGFR in the positive regulation of HT-1080 cell proliferation. Together, these results have identified transcriptional coactivator Topo I as a first endogenous cofactor for c-Jun in the regulation of cell proliferation. In addition, the results of the present study strongly suggest that inhibition of EGFR expression is a novel mechanism by which topotecan inhibits cell proliferation in cancer therapy.

Several transcription factors (e.g., c-Myc, NF-κB, and c-Jun) have been implicated as crucial regulators of cancer progression. This is based on their ability to stimulate the expression of genes that promote cell growth and survival (6). The AP-1 transcription factor c-Jun was initially discovered as a human counterpart of the viral oncogene v-jun (36). Overexpression of c-Jun causes transformation in rat and chicken cells, and it regulates expression of genes involved in cell proliferation and tumorigenesis (7, 36). Transcriptional activities of c-Jun are stimulated by N-terminal phosphorylation of the protein, mainly by JNK group of mitogen-activated protein kinases (24, 32). In concert with the tumorigenic role of c-Jun, inhibition of JNK proteins has been recently identified as a potential approach for cancer therapy (7, 24).

Recent studies have indicated that the regulation of expression of growth factors and their cell surface receptors is an important mechanism by which intracellular signaling pathways regulate cell proliferation (6, 7, 24). Many growth regulatory signaling pathways converge on epidermal growth factor receptor (EGFR) which is overexpressed in several types of malignancies (25, 38). Recently, the EGFR gene was shown to be a direct transcriptional target for c-Jun (13, 39). Moreover, epidermis-specific deletion of c-Jun in mouse led to eyes-open-at-birth phenotype similar to the one observed in EGFR knockout mice and resulted in reduced keratinocyte proliferation and tumor growth (21, 39). However, no information has yet been available about the molecular mechanisms involved in c-Jun-mediated stimulation of EGFR expression.

The role of c-Jun as an oncoprotein has been supported by studies using genetically engineered mouse models (reviewed in reference 7). However, in contrast to c-Myc and NF-κB, neither activating mutations, amplifications, nor constant altered expression patterns of c-Jun have been observed in human malignancies (6, 7, 36). This suggests that the role of c-Jun as an oncoprotein in humans might be regulated by alternative mechanisms that are not revealed by conventional expression or mutational analysis. One possible mechanism could be the JNK-dependent interaction of c-Jun with cofactors that are preferentially expressed and/or activated in cancerous cells. However, endogenous cofactors that would, together with c-Jun, regulate the expression of cancer relevant genes have not yet been identified.

DNA topoisomerase I (Topo I) is a nuclear phosphoprotein capable of releasing torsional stress of supercoiled DNA by sequential cleavage and rejoining of the DNA backbone (18, 22). Topo I expression and activity is increased in several malignancies, and it is a molecular target for anticancer agent topotecan in the treatment of small cell lung cancer and ovarian carcinomas (12, 18, 22). However, the molecular mechanisms underlying the requirement of DNA topoisomerase I activity for cancer cell growth are not clear (18, 22). Topo I was identified as an activity required for transcription factor-mediated activation of RNA polymerase II (RNApolII) (17, 26, 34). Mechanistically, Topo I has been shown to both promote TFIID-TFIIA complex assembly during transcription activation and to facilitate transcription elongation by reversing the superhelical tension of the chromatinized DNA (17, 23, 26-28, 34). In vivo, Topo I colocalizes with active RNApolII in chromosome puffs in Drosophila (9). Importantly, inhibition of Topo I by camptothecin was shown to specifically inhibit transcription factor Tat-mediated transcription of human immunodeficiency virus type 1 (20) and Topo I-dependent transcription on a chromatin template in vitro (28). These results suggested that the antitumor effects of topotecan could, at least partly, stem from the inhibition of the transcriptional coactivator function of Topo I. Regardless of the compelling evidence demonstrating the role of Topo I on RNApolII-mediated transcription in vitro, mammalian transcription factors that would bind to and functionally cooperate with Topo I in the regulation of gene expression in vivo have not been identified yet. In addition, the role of transcriptional coactivator function of Topo I in the regulation of cell proliferation in unknown.

In the present study, we have identified Topo I as the first endogenous interaction partner for c-Jun in the regulation of cancer cell proliferation. We show that Topo I and JNK-c-Jun pathway cooperate in the regulation of EGFR gene expression and in the proliferation of HT-1080 cancer cells. Importantly, the results presented here provide novel evidence for the transcriptional regulation of gene expression by Topo I in vivo and strongly suggest that inhibition of EGFR expression is a previously unidentified mechanism by which Topo I inhibiting drugs inhibit cancer cell proliferation in cancer therapy.

MATERIALS AND METHODS

Reagents and antibodies.

Topotecan was purchased from GlaxoSmithKline, AG1478 was purchased from Calbiochem, SP600125 was purchased from BIOMOL Research Laboratories, and Herceptin was obtained from Roche. Antibodies to Topo I, c-Jun, TBP, cyclin D1, cyclin E, JNK2, p-JNK, EGFR, His, hemagglutinin (HA), glutathione S-transferase (GST), CDK4, PARP, and PCNA were purchased from Santa Cruz Biotechnology. MEK1,2, ERK1,2, p-EGFR, and p-Jun antibodies were purchased from Cell Signaling Technology. Thymidine and actin antibody were purchased from Sigma. green fluorescent protein (GFP) antibody was kindly provided by Francis Barr (Max-Planck-Institute of Biochemistry, Martinsried, Germany). Establishment of the c-Jun−/− cell line have been described previously (14).

Protein complex purification, Western blotting, and immunoprecipitation analysis.

The use of the tandem affinity purification method for the identification of c-Jun interacting protein complex and methods for coimmunoprecipitation and Western blot analysis have been described earlier in detail (37).

Reporter assays.

HT-1080 cells on 96-well plate were transfected at 50% confluency using FuGene liposomal reagent (Roche), with reporter plasmids pf2Luc, together with CMVGalDBD, CMVJunGal (Stratagene), pEGFP, or Topo-GFP (31) (a generous gift from Peter D'Arpa) expression constructs. Transfection efficiency was controlled by cotransfection of the ubiquitin promoter-driven Renilla luciferase reporter. At 24 h after transfection, both firefly luciferase and Renilla luciferase activities were measured by using the Dual-Glo luciferase assay system (Promega). The mean values ± the standard deviations (SD) of three independent experiments, each done with four parallel datum points, were determined.

Transfection and siRNA experiments.

Subconfluent HT-1080 cells were transiently transfected with Junwt, JunAla, or JunbZIP expression constructs (19, 35) or with EGFR expression construct (16) (a generous gift from Y. Daaka). FuGENE (Roche)-assisted transfection resulted in 80 to 90% transfection efficiency as determined by fluorescence microscopy analysis of cells transfected with pEGFP construct. To generate SiJun1 and SiJun2 constructs, double-stranded hairpin oligonucleotides corresponding to 17 to 37 and 853 to 873 regions of c-Jun, respectively, were inserted into pSuper vector (4) according to the manufacturer's instructions (OligoEngine). For control scrambled small interfering RNA (siRNA) vector, we used previously a published oligonucleotide sequence (4). Cells were lysed 24 or 48 h after transfection, and cell extracts were analyzed by Western blotting.

Proliferation assays.

To determine the effects of topotecan, SP600125, AG1478, and Herceptin on cell proliferation, cells were seeded on 96-well plate and cultured in 1% fetal calf serum-Dulbecco modified Eagle medium for 12 h. Afterward, medium was changed, and cells were supplied with medium containing the indicated concentrations of topotecan, SP600125, AG1478, and Herceptin for 24 h. Cell proliferation was thereafter analyzed by a cell proliferation ELISA/BrdU assay (Roche).

Immunofluorescence.

At 24 h after transfection with TopoGFP and HA-c-Jun, HT-1080 cells cultured on glass coverslips were permeabilized and fixed for 10 min in PTEMF (100 mM PIPES [pH 6.8], 10 mM EGTA, 1 mM MgCl2, 0.2% Triton X-100, 4% formaldehyde). After three washes with phosphate-buffered saline (PBS) nonspecific antibody binding was blocked for 30 min with 3% bovine serum albumin in PBS. Incubations with a mouse anti-HA antibody were carried out for 1 h at room temperature. Bound antibody was visualized by incubation with Cy3-conjugated secondary antibody (Jackson Immunoresearch) for 1 h. After three washes with PBS, the coverslips were mounted in 50% glycerol, PBS, and 2% (wt/vol) DABCO (Sigma-Aldrich). For analysis of colocalization images were acquired by using a confocal laser scanning microscope (LSM 510; Carl Zeiss, Inc.).

Real-time PCR analysis.

Real-time PCR analysis of cDNA samples was performed with specific primers and fluorescent probes designed by using Primer Express software (PE Biosystems). Primers and probes were designed to target unconserved regions of the cDNAs. The specificity of EGFR and ErbB primers has been demonstrated earlier (15). For the other primers, the data demonstrating the specificity are available upon request. To obtain an internal control for experimentation, probes and primers annealing to β-actin mRNA were also synthesized. PCR was carried out in a solution containing 300 nM concentrations of primers (Medprobe), a 200 nM concentration of 5′ 6-FAM- or VIC-labeled probe (PE Biosystems), 12.5 μl of TaqMan universal PCR Master Mix (PE Biosystems), and 0.5 μl of template cDNA in a final volume of 25 μl. Thermal cycling was performed with ABI Prism 7700 Sequence Detector (PE Biosystems). Cycling was initiated with 2 min at 50°C and 10 min at 95°C, followed by 40 cycles of 15 s at 95°C and 1 min at 60°C.

Accumulation of the specific PCR products was detected real-time as an increase in fluorescence. Observed fluorescence was plotted against cycle number to generate amplification plots and to determine threshold cycle (CT) values, i.e., the cycle numbers at which the fluorescence signal exceeded a threshold value of 0.05 relative fluorescence units. Each determination of a CT value was done in duplicate and normalized with the CT values of simultaneous duplicate measurements of β-actin expression from the same samples. The range between two parallel CT values was <5% of the mean in all measurements. The relative expression of the gene analyzed (target gene) was calculated as follows: relative expression = 2−ΔCT, where ΔCT = CT (target gene) − CT (β-actin). Expression of each ErbB transcript was presented as the percentage of ErbB mRNA expression relative to the control β-actin mRNA expression.

RESULTS

Identification of Topo I as a novel c-Jun-interacting protein.

In order to gain understanding about the mechanisms by which c-Jun regulate gene expression in human cells, we searched for proteins that interact with the amino-terminal transactivation domain of c-Jun in vivo. For that purpose, we developed a mammalian expression construct coding for fusion protein consisting of amino acids 1 to 223 of c-Jun linked to the tandem affinity purification (TAP) domain (c-Jun1-223TAP) (Fig. 1A). TAP strategy (30, 37), combined with silver staining analysis and mass spectrometric peptide sequencing, was thereafter used to identify c-Jun interacting proteins. Several proteins with confirmed or predicted functions in gene regulation were found to copurify with c-Jun1-223TAP from HEK293 cell nuclear extracts, but these proteins were absent in eluates from mock-transfected cells (Fig. 1B and Table 1).

FIG. 1.

FIG. 1.

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Identification of novel c-Jun-interacting protein complex. (A) Structure of c-Jun and c-Jun1-223TAP fusion proteins. DBD, DNA-binding domain; ZIP, leucine zipper; CBP, calmodulin binding peptide; TEV, tobacco etch virus protease cleavage site. (B) HEK293 cells were transiently transfected with an mammalian expression construct coding for Jun1-223TAP fusion protein or left untransfected (Mock). At 24 h after transfection, nuclear proteins from both cell cultures were subjected to tandem affinity purification, and proteins in the final eluates were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and silver staining. The indicated proteins from Jun1-23TAP eluates were identified thereafter by tandem mass spectrometric analysis. (C) c-Jun and Topo I colocalize in the perinucleolar region. HT-1080 cells were transiently transfected with HA-c-Jun and TopoGFP expression constructs. After 24 h cells were fixed and stained for HA epitope, and the colocalization of proteins was then analyzed by confocal microscopy. The white arrowheads in the merged panel indicate sites for colocalization between c-Jun and Topo I.

TABLE 1.

Functions of c-Jun-interacting proteins in transcriptional regulation

Sequence Protein name Function(s) Reference
Q08211 RNA helicase A DNA/RNA helicase
Nuclear DNA helicase Mediates binding of transcription factor CREB to RNApolII 28a
P11387 DNA topoisomerase I DNA topoisomerase
Involved in both repression and activation of transcription 26
Enhances TFIID-TFIIA complex assembly during activation of transcription 34
Reverses transcriptional repression by chromatin 28
Q13436 RNA helicase II/Gu RNA helicase
Cofactor for c-Jun-activated transcription 37
P52272 hnRNPM Associates with the pre-mRNA at early stages of spliceosome assembly 15a
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We have previously confirmed the interaction between c-Jun, RNA helicase A, RNA helicase RHII/Gu, and hnRNPM (37). In the present study, we report identification of DNA topoisomerase I (Topo I) as a novel endogenous interaction partner for c-Jun. In order to further explore the putative interaction between Topo I and c-Jun, the colocalization of these proteins was studied in human HT-1080 fibrosarcoma cells transfected with TopoGFP and HA-c-Jun constructs. As expected, Topo I was predominantly localized in the nucleolus with punctate staining in nucleoplasm (Fig. 1C) (31). c-Jun staining was observed throughout the nucleoplasm, but a fraction of c-Jun was clearly concentrated in perinucleolar domains (Fig. 1C). Importantly, a clear colocalization of c-Jun and Topo I was observed in several of these perinucleolar domains (Fig. 1C). Perinucleolar localization of c-Jun has been observed also in earlier studies (8), and it provides a plausible explanation for the interaction between c-Jun and several nucleolar proteins (RHII/Gu, RNA helicase A, and Topo I) identified in the c-Jun protein complex (37).

Interaction between endogenous c-Jun and Topo I is regulated by JNK pathway.

As yet, RHII/Gu is the only transcriptional cofactor of c-Jun for which the interaction between endogenous proteins has been demonstrated (37). To study whether endogenous Topo I and c-Jun interact, we performed coimmunoprecipitation analysis on nuclear extracts of HT-1080 fibrosarcoma cells by using a c-Jun specific antibody raised in a goat. As shown in Fig. 2A, endogenous Topo I coimmunoprecipitated with endogenous c-Jun, and no significant amounts of these proteins were immunoprecipitated with the preimmune goat serum used as a control. TATA-binding protein (TBP) is a major component of TFIID complex and mediates binding of the complex to the promoter DNA. Both c-Jun and Topo I have been demonstrated to bind to TBP in vitro (10, 17, 26). Importantly, endogenous TBP was detected in c-Jun immunoprecipitate from HT-1080 cells (Fig. 2A). However, cyclin D1 was not present in the complex, demonstrating that Topo I, c-Jun, and TBP do not immunoprecipitate together due to the antibody aggregate or other completely nonspecific means (Fig. 2A). In order to further substantiate the support of a physical interaction between endogenous Topo I and c-Jun, we immunoprecipitated c-Jun with an independent antibody raised in rabbit, with GST antibody as a negative control. As shown in Fig. 2B, Topo I and c-Jun were seen to coimmunoprecipitate specifically also in these conditions. Importantly, in a reciprocal experiment, endogenous c-Jun coimmunoprecipitated with overexpressed TopoGFP protein but not with GFP alone, from HT-1080 nuclear extracts (Fig. 2C).

FIG. 2.

FIG. 2.

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Topo I is a novel endogenous interaction partner for c-Jun. (A and B) HT-1080 cell nuclear lysates were subjected to immunoprecipitation (IP) with either c-Jun goat antibody and preimmune goat sera (PI) (A) or with c-Jun rabbit antibody and GST antibody (B). After washing, immunoprecipitates and 5% of the nuclear proteins (NP 5%) used as a starting material for the assay were analyzed by Western blotting with the indicated antibodies. (C) Reciprocal immunoprecipitation analysis of c-Jun-Topo I interaction. HT-1080 cells were transiently transfected with GFP or TopoGFP expression construct. At 24 h after transfection, nuclear proteins of each samples were subjected to immunoprecipitation analysis by GFP antibody. Immunoprecipitates and 5% of the nuclear proteins (NP 5%) used as a starting material were analyzed for endogenous c-Jun and for TopoGFP by Western blotting. (D) Topo I preferentially interacts with phosphorylated c-Jun. HT-1080 cells were pretreated with JNK inhibitor SP600125 for 2 h, and the interaction between endogenous c-Jun and Topo I was studied by coimmunoprecipitation with c-Jun rabbit antibody. Immunoprecipitates were analyzed for endogenous total and phosphorylated c-Jun and for Topo I by Western blotting with the indicated antibodies. (E) HT-1080 cells were transiently transfected with HAc-Junwt, with HAc-JunAla, or with empty expression plasmid (Mock). At 24 h after the transfections, cellular lysates were subjected to coimmunoprecipitation analyses with HA antibody. Immunoprecipitates (IP) and 5% of the nuclear proteins (NP 5%) used as a starting material were analyzed for endogenous Topo I and for HA-tagged c-Jun proteins by Western blotting. (A to E) All panels show representative examples of two or three experiments with similar results. (F) HT-1080 cells were transiently transfected with 5xGal luciferase reporter plasmid, JunGal, or DBDGal expression constructs, together with the indicated amounts of pEGFP or TopoGFP constructs. The luciferase activity in cell lysates was measured 24 h after transfection. Transfection efficiency was monitored by cotransfecting the cell with Ubi-Renilla luciferase construct. Shown are mean values + the SD of three experiments done with four parallel samples.

The part of c-Jun that is present in c-Jun1-223TAP carries regulatory phosphorylation sites for JNK, serines 63 and 73, and threonines 91 and 93 (Fig. 1A). Interestingly, in a subsequent TAP purification experiment we identified, by mass spectrometric peptide sequencing, Topo I as a protein that selectively copurified with c-Jun1-223-AspTAP but not with c-Jun1-223-AlaTAP (data not shown). Since substitution of JNK phosphorylation sites with aspartic acid in c-Jun1-223-AspTAP mimics phosphorylated c-Jun and alanine substitutions converts c-Jun to inactive form (35), these results indicate that Topo I might selectively interact with transcriptionally active c-Jun. In order to further study the role of c-Jun phosphorylation in Topo I interaction in a more physiological context, HT-1080 cells were treated with a specific chemical JNK inhibitor SP600125 for 2 h, and interaction between endogenous c-Jun and Topo I was thereafter studied by coimmunoprecipitation analysis. As shown in Fig. 2D, SP600125 treatment potently inhibited c-Jun phosphorylation and clearly decreased the amount of Topo I in c-Jun immunoprecipitate. Importantly, SP600125 treatment did not have any effect on the levels of Topo I or c-Jun proteins in nuclear proteins (NP) used as an starting material for immunoprecipitation, nor did it affect immunoprecipitation efficiency of c-Jun (Fig. 2D). To further substantiate the evidence for the requirement of c-Jun phosphorylation for Topo I interaction, we transfected HT-1080 cells with HA-tagged wild-type or Ala-substituted c-Jun mutant (19, 35) and studied the interaction of endogenous Topo I with these proteins by coimmunoprecipitation analysis with HA antibody. As shown in Fig. 2E, both forms of HA-tagged c-Jun proteins were equally expressed and immunoprecipitated from HT-1080 cells, whereas Topo I was observed only in c-Jun wild-type immunoprecipitates. These results together demonstrate that endogenous c-Jun and Topo I interact in HT-1080 fibrosarcoma cells and that c-Jun-Topo I interaction is positively regulated by JNK-mediated c-Jun amino-terminal phosphorylation.

In order to study whether c-Jun amino-terminal transactivation domain can recruit Topo I to interact on gene regulation, HT-1080 cells were transiently transfected with expression construct coding for a fusion protein in which the transactivation domain of c-Jun (amino acids 1 to 223) was fused to the Gal4 DNA-binding domain (JunGal). Cells were cotransfected also with Topo I expression construct (TopoGFP) or with GFP control vector, and cotransfected Gal4-Luciferase reporter was used to monitor functional interaction between JunGal and TopoGFP. Overexpression of JunGal stimulated Gal4-Luciferase reporter activity by four- to sixfold compared to cells transfected with Gal4 DNA-binding domain alone (DBDGal) (Fig. 2F). Importantly, cotransfection of TopoGFP enhanced JunGal-mediated promoter activation by two- to threefold compared to cells cotransfected with a GFP expression construct, whereas TopoGFP expression did not have any effect on DBDGal-driven basal transcription (Fig. 2F).

Identification of EGFR as an endogenous target gene for c-Jun and Topo I.

The results presented above show that Topo I preferentially interacts with phosphorylated form of c-Jun and supports c-Jun-mediated reporter gene activation. Based on these observations, it is plausible that Topo I is involved in RNApolII-mediated transcriptional activation of endogenous c-Jun target genes. EGFR (ErbB1) has been recently identified as a direct transcriptional target for c-Jun (13, 39). In order to study whether EGFR is a target gene for c-Jun also in HT-1080 cells, we transiently transfected cells with His-tagged c-Jun mutant lacking the amino acids 1 to 195 of the transactivation domain (JunbZIP) (19). EGFR expression levels were thereafter analyzed by Western blotting. As shown in Fig. 3A, EGFR was highly expressed in HT-1080 cells, and overexpression of wild-type c-Jun did not further increase EGFR expression. However, overexpression of JunbZIP dramatically reduced EGFR expression after 24 h (Fig. 3A). The role of c-Jun in the positive regulation of EGFR expression was further confirmed by depleting endogenous c-Jun protein from the cells by siRNA construct. Transient transfection of siRNA construct (SiJun1) effectively inhibited endogenous c-Jun expression and caused downregulation of EGFR expression compared to nontransfected or scrambled siRNA transfected cells (Fig. 3B). Together, these results show that HT-1080 cells display high constitutive EGFR expression that is dependent on c-Jun.

FIG. 3.

FIG. 3.

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Topo I activity is selectively required for EGFR expression (A and B) HT-1080 cells were transiently transfected with dominant-negative c-Jun expression construct (A) or with c-Jun specific siRNA construct (B). Expression levels of His-tagged recombinant c-Jun (His), endogenous c-Jun, EGFR, and β-actin proteins were studied 24 h (A) or 72 h (B) after transfection by Western blotting. (C) Concentration-dependent inhibition of EGFR expression by topotecan. HT-1080 cells were treated with indicated concentrations of topotecan for 24 h, and the expression levels of EGFR and β-actin were studied by Western blotting. The relative expression levels of EGFR are indicated below the actin panel. Quantitation has been done by MCID imaging analysis software and correlated to actin expression from the same sample. (D) HT-1080 cells were treated with topotecan (1 μM) for 12 h, and the amounts of total and phosphorylated EGFR were determined by Western blotting with EGFR and p-EGFR antibodies. (E and F) HT-1080 cells were treated with topotecan (1 μM) for 12 h, and the expression levels of the indicated proteins were determined by Western blotting. (A to F) All panels show representative examples of two or three experiments with similar results.

Topotecan and camptothecin are highly specific chemical inhibitors of Topo I, and the effect of these drugs on cell growth and viability depend on Topo I inhibition (18, 22). Camptothecin has earlier been shown to specifically inhibit HIV-Tat induced reporter gene activity without an effect in general transcription or in cellular viability (20). Mechanistically, camptothecin treatment has been shown to inhibit RNApolII transcription elongation on chromatinized DNA in vitro and in vivo (23, 28). Interestingly, expression of only a subset of cellular genes is inhibited by camptothecins (5, 27). This can be explained by selective stalling of the elongating RNApolII by topotecan only in genes to whose promoters Topo I has been recruited by a transcription factor (27, 28, 34). Together, these studies validate the use of camptothecins as tools to study Topo I-mediated transcriptional responses.

In order to study whether Topo I is involved in the regulation of EGFR expression, HT-1080 cells were treated with increasing concentrations of topotecan and analyzed after 24 h for EGFR protein levels by Western blotting. Topotecan shows antitumor effects in mouse tumor models with submicromolar plasma concentrations (0.044 to 0.074 μM 30 min after drug administration) (2). A clear inhibition of EGFR expression in HT-1080 cells was detected already with 0.05 μM topotecan, whereas cells treated with 0.5 or 1 μM topotecan displayed dramatic further inhibition of EGFR expression (Fig. 3C). EGFR activity is positively regulated by phosphorylation on intracellular regulatory tyrosines (38). Using an antibody that specifically recognizes EGFR phosphorylated on tyrosine 1068, constitutive phosphorylation of EGFR was detected in nontreated HT-1080 cells (Fig. 3D). Importantly amount of phosphorylated EGFR decreased proportionally to total EGFR after topotecan treatment (Fig. 3D). These results identify EGFR as an novel protein whose expression is regulated by Topo I and demonstrate that topotecan treatment negatively regulates EGFR activity in HT-1080 cells.

Earlier studies have shown that camptothecins selectively inhibit transcription of certain genes without an effect on RNApolII-mediated transcription in general (5, 27). In order to confirm that suppression of EGFR expression by topotecan was not due to general suppression of transcription, we examined expression of other proliferation related proteins after topotecan treatment. As shown in Fig. 3E, treatment of HT-1080 cells for 12 h with 1 μM topotecan inhibited EGFR expression, whereas it did not have any effect on expression of MEK1,2 or ERK1,2. To further establish the selectivity of Topo I effects on gene expression, we also studied whether expression of other c-Jun target genes was inhibited by topotecan. To this end, the cellular lysates from the experiment shown in Fig. 3D (exhibiting clear EGFR downregulation) were reanalyzed for expression of CDK4 and c-Jun (11). Interestingly, neither CDK4 nor c-Jun expression was inhibited by topotecan treatment (Fig. 3F).

Taken together, these results clearly demonstrate that both c-Jun and Topo I positively regulate EGFR expression and that inhibition of EGFR expression by topotecan is not due to the suppression of transcription in general (see also Fig. 5D).

FIG. 5.

FIG. 5.

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Topo I promotes EGFR gene expression at the mRNA level in both transformed and nontransformed cells. (A) JNK pathway stimulates EGFR mRNA expression. HT-1080 cells were treated with chemical JNK inhibitor SP600125 for 3 and 6 h, and the EGFR mRNA expression levels were determined by real-time PCR analysis. (B) Low micromolar concentrations of topotecan inhibits EGFR mRNA expression in HT-1080 cells. HT-1080 cells were treated with indicated concentrations of topotecan for 6 h, and EGFR mRNA expression levels were determined by real-time PCR analysis. (C) HT-1080 cells were treated with topotecan (1 μM) for the indicated periods of time, and the expression levels of EGFR and ErbB2 mRNAs were determined by real-time PCR analysis. (D) Topo I is selectively required for EGFR mRNA expression. HT-1080 cells were treated with topotecan (1 μM) for 6 h, and the expression levels of indicated mRNAs were determined by real-time PCR analysis. In panels A to D, the data are presented as mean values + the SD of two to three independent experiments. (E) Inhibition of EGFR gene expression by topotecan is not specific for HT-1080 cells. The indicated cancer cell lines and normal human skin fibroblast cultures (NSF) were incubated with 1 μM topotecan for 3 or 6 h, and the expression levels of EGFR and ErbB2 mRNAs were determined by real-time PCR analysis. In all panels, each sample was measured in duplicate, and the expression of each transcript is presented as the percentage of indicated mRNA expression relative to control β-actin mRNA expression.

Inhibition of EGFR expression by topotecan is dependent on c-Jun.

Results above demonstrate that both c-Jun and Topo I are involved in the positive regulation of EGFR expression. In order to determine whether c-Jun and Topo I cooperate on EGFR gene regulation, we next studied whether Topo I inhibition was able to downregulate EGFR expression in cells lacking c-Jun. To do that, c-Jun−/− mouse embryonal fibroblasts (MEFs) (14) and their wild-type controls were treated with 1 μM topotecan for 12 h, and the EGFR levels were then studied by Western blotting. The positive role of c-Jun in the regulation of EGFR expression was supported by the significantly lower basal expression of EGFR protein in c-Jun−/− MEFs compared to wild-type controls (Fig. 4A and B). Moreover, treatment of wild-type MEFs with topotecan resulted in suppression of EGFR expression that was comparable to that observed by deletion of c-Jun (Fig. 4A and B). Importantly, no inhibition of EGFR expression by topotecan treatment was seen in c-Jun−/− MEFs (Fig. 4A and B). Figure 4B shows the quantitation of EGFR expression compared to actin expression from three separate experiments and the means and standard deviations of all three experiments.

FIG. 4.

FIG. 4.

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Inhibition of EGFR expression by topotecan is dependent on c-Jun. (A) MEFs lacking c-Jun (Jun−/−) and their wild-type counterparts (WT) were treated with topotecan (1 μM) for 12 h, and the expression levels of EGFR and actin were studied by Western blotting. (B) Quantification of the EGFR protein expression levels in Jun−/− and WT cells in response to topotecan treatment. Quantitation has been done by MCID imaging analysis software and correlated to actin expression from the same sample. Shown are actual values for three separate experiments and mean values + the SD of these experiments taken together. (C) Inhibition of JNK activity converts HT-1080 cells resistant to topotecan. HT-1080 cells were pretreated for 2 h with SP600125 and treated thereafter with topotecan. After 12 h the expression levels of EGFR and actin were determined by Western blotting. Quantitation of EGFR expression was done by MCID imaging analysis software and correlated to actin expression from the same sample. Mean values + the SD of two separate experiments done in duplicate are shown.

In order to study whether c-Jun dependency of topotecan effects in the regulation of EGFR expression was specific to genetically modified MEFs used in the previous experiment, we next inhibited c-Jun phosphorylation in HT-1080 cells by SP600125 treatment (Fig. 2D) and sought to determine whether in these conditions cells were sensitive to topotecan-elicited suppression of EGFR expression. As shown in Fig. 4C, treatment of HT-1080 cells with topotecan or with SP600125 alone inhibited EGFR expression similarly, and no further potentiation of EGFR downregulation was detected in cells treated with these compounds in combination.

These results recapitulate the findings obtained in c-Jun−/− cells that inhibition of c-Jun function converts cells resistant to topotecan-elicited inhibition of EGFR expression (Fig. 4A and B). Together, these results demonstrate that Topo I and c-Jun cooperate in the positive regulation of EGFR expression.

Topotecan inhibits EGFR mRNA expression in both transformed and nontransformed cells.

To verify that inhibition of JNK pathway activity and Topo I downregulates EGFR expression at the mRNA level, EGFR mRNA expression was studied in cells treated with SP600125 and with different concentrations of topotecan for 3 and 6 h. As measured by real-time PCR analysis with β-actin mRNA as an internal control (15), SP600125 (20 μM) reduced EGFR mRNA expression by 50%, and topotecan treatment resulted in concentration-dependent downregulation of EGFR mRNA levels (Fig. 5A and B). Next, we wanted to study the time course of topotecan effects on EGFR mRNA regulation. Treatment of HT-1080 cells with 1 μM topotecan caused downregulation of EGFR mRNA already after 3 h, and the maximal inhibition was detected after 6 h (Fig. 5C). Another EGFR family member, ErbB2, was expressed in significantly lower level than EGFR, whereas ErbB3 and ErbB4 mRNAs were nearly undetectable in these cells (data not shown). Importantly, ErbB2 mRNA levels were not downregulated by topotecan treatment at any of the time points or concentrations studied (Fig. 5C and data not shown). Also, the expression of glucose transporter 1 (Glut-1) or two c-Jun target genes (HB-EGF and MMP-1) was not inhibited by topotecan treatment after 6 h (Fig. 5D).

In order to study whether the requirement of Topo I for EGFR expression was specific to HT-1080 cells, we next treated two other human cancer cell lines (HeLa and HaCat) and normal human skin fibroblasts (NSF) with topotecan (1 μM) for 3 and 6 h and thereafter analyzed EGFR mRNA levels by real-time PCR analysis. Again, topotecan treatment potently inhibited EGFR mRNA expression in HT-1080 cells with the maximal inhibition detected at the 6-h time point (Fig. 5E). Downregulation of EGFR expression was also detected in other cell lines, although to a lesser extent than in HT-1080 cells (Fig. 5E). Importantly, inhibition of EGFR expression was not due to the general suppression of transcription in any of the cell lines studied, since topotecan treatment did not inhibit ErbB2 expression (Fig. 5E).

Together, these results show that the JNK pathway and Topo I regulate EGFR expression at the mRNA level. Furthermore, these findings confirm that topotecan treatment selectively inhibited EGFR gene expression and not transcription in general in both transformed and nontransformed cells.

The JNK-c-Jun pathway and Topo I cooperate on positive regulation of HT-1080 cell proliferation.

JNK-c-Jun signaling is required for cell proliferation and for EGFR-dependent cellular transformation (3, 7, 24). In order to evaluate the activity and the functional role of the JNK-c-Jun pathway in HT-1080 cells, we first analyzed JNK phosphorylation status by Western blotting in nontreated and in UVC-treated cells. As shown in Fig. 6A, prominent 54-kDa band was detected from untreated cells with an antibody that recognizes the phosphorylated forms of both JNK1 and JNK2. Reblotting of the membrane with a JNK2 specific control antibody revealed immunoreactivity only at the 54-kDa size, confirming that JNK2 is the constitutively active form of JNK in HT-1080 cells (Fig. 6A). Treatment of cells with UVC for an hour induced phosphorylation of 46-kDa form of JNK (JNK1) and of c-Jun (Fig. 6A). Importantly, pretreatment of cells for 30 min with 1 μΜ topotecan, did not have any effect on basal or UVC-induced JNK or c-Jun phosphorylation (Fig. 6A). These results show that HT-1080 cells display constitutive JNK2 activity. Moreover, together with our evidence that topotecan treatment does not influence c-Jun protein expression (Fig. 3F), these results demonstrate that effects seen by topotecan treatment on EGFR expression, or on proliferation of HT-1080 cells (see below) are not due to interference with function of JNK-c-Jun pathway per se.

FIG. 6.

FIG. 6.

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JNK-c-Jun pathway and Topo I cooperate on positive regulation of HT-1080 cell proliferation. (A) Analysis of JNK phosphorylation status in HT-1080 cells. HT-1080 cells were left untreated or were pretreated with 1 μM topotecan for 30 min before treatment with UVC for 1 h as indicated. The JNK1 (p46), JNK2 (p54), and c-Jun phosphorylation status was then determined by Western blotting with phospho-specific antibodies. (B and C) c-Jun positively regulates HT-1080 cell proliferation. HT-1080 cells were transiently transfected with dominant-negative c-Jun expression construct (B) or with c-Jun specific siRNA construct (C). The expression levels of His-tagged recombinant c-Jun (His), EGFR, β-actin, and PCNA as an indicator of proliferation were studied 24 h (B) or 72 h (C) after transfection by Western blotting. (D) The JNK-c-Jun pathway and Topo I cooperatively regulate HT-1080 cell proliferation. HT-1080 cells were treated with the indicated concentrations of JNK inhibitor SP600125 or topotecan alone or in combination. After 24 h the rate of proliferation was determined by using a BrdU incorporation assay. Mean values + the SD of three independent experiments are shown each with four parallel samples and datum points.

In order to evaluate the functional role of c-Jun in HT-1080 cells, we transfected the cells with dominant-negative c-Jun mutant or with construct coding for c-Jun siRNA. The expression of proliferating cell nuclear antigen (PCNA) was thereafter studied as an indicator of proliferation. A clear decrease in PCNA expression was detected after inhibition of c-Jun by both means, clearly demonstrating that the proliferation of these cells was dependent on c-Jun (Fig. 6B,C). The results in Fig. 2D showed that the inhibition of JNK activity by SP600125 potently inhibited c-Jun phosphorylation. To study whether inhibition of c-Jun phosphorylation resulted in the suppression of proliferation, HT-1080 cells were treated with SP600125 and analyzed after 24 h by bromodeoxyuridine (BrdU) incorporation assay. As expected, SP600125 treatment resulted in clear suppression of BrdU incorporation (Fig. 6D), further demonstrating that the JNK-c-Jun pathway positively regulates the proliferation of HT-1080 cells.

To determine whether Topo I is involved in the c-Jun-mediated positive regulation of HT-1080 cell proliferation, cells were first treated with topotecan for 24 h and then analyzed by BrdU assay. As shown in Fig. 6D, treatment of cells with 0.05 μM topotecan caused clear inhibition of proliferation. If topotecan-elicited suppression of proliferation was dependent on c-Jun-Topo I interaction, topotecan should not potentiate the effects of SP600125 on proliferation. This was based on the reasoning that SP600125 treatment prevents Topo I binding to c-Jun (Fig. 2D). Importantly, no additive effects of SP600125 and topotecan treatments were detected in the BrdU incorporation assay (Fig. 6D), indicating cooperation of c-Jun and Topo I in the regulation of proliferation.

Taken together, these results demonstrate that the constitutive activity of JNK-c-Jun pathway stimulates the proliferation of HT-1080 cells. Moreover, these results strongly suggest that, in addition to cooperation in the regulation of EGFR expression (Fig. 4), Topo I and c-Jun cooperate in the regulation of cancer cell proliferation.

Inhibition of EGFR expression is a novel mechanisms by which topotecan inhibits cancer cell proliferation.

The molecular mechanisms by which Topo I-inhibiting drugs inhibit cancer cell proliferation are not known. The results presented above strongly suggest that c-Jun and Topo I cooperate in the regulation of EGFR expression and HT-1080 cell proliferation. In order to examine whether the inhibition of EGFR expression is a novel mechanism by which topotecan inhibits cancer cell proliferation, we first compared the effects of chemical inhibitor of EGFR tyrosine kinase activity (AG1478) or topotecan on the expression of proliferation marker proteins and by BrdU assay. As shown in Fig. 7A, both AG1478 and topotecan potently inhibited EGFR phosphorylation and the expression of PCNA and cyclin E, markers for EGFR-induced G1/S cell cycle transition (25, 33). As expected, thymidine block also reduced PCNA and cyclin E expression but did not inhibit EGFR phosphorylation (Fig. 7A). Importantly, AG1478 or topotecan treatments did not decrease the expression of MEK1,2, ERK1,2, or actin, demonstrating that the inhibition of EGFR signaling by either of these treatments specifically inhibited expression of PCNA and cyclin E without effects on gene expression in general (Fig. 7A). In addition, treatment of cells for 24 h with AG1478 or with topotecan resulted in the inhibition of BrdU incorporation (Fig. 7B). However, in agreement with the low ErbB2 expression levels, treatment of HT-1080 cells with specific ErbB2 inhibitor Herceptin (10 μg/ml) did not have any effect on cell proliferation (Fig. 7B). In order to study whether the growth suppressive effects of Topo I and EGFR inhibition were two independent phenomena or relied on the shared mechanism, cells were next treated in combination with topotecan and AG1478. Importantly, no additive inhibitory effect of these inhibitors in cell proliferation was observed (Fig. 7E), demonstrating that under the conditions in which EGFR receptor activity is inhibited, topotecan treatment has no effects on cell proliferation.

FIG. 7.

FIG. 7.

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Inhibition of EGFR expression is a novel mechanism by which topotecan inhibits cancer cell proliferation. (A) EGFR activity supports HT-1080 cell proliferation. HT-1080 cells were treated with topotecan (1 μM), AG1478 (20 μM), or thymidine (2 mM) for 12 h, and EGFR phosphorylation and expression of cyclin E, PCNA, MEK1,2, ERK1,2, and actin were determined by Western blotting. A representative figure of two experiments with similar results is presented. (B) HT-1080 cells were treated with AG1478 (20 μM), topotecan (0.05 μM), or Herceptin (10 μg/ml) alone or in combinations for 24 h, and proliferation was thereafter determined by BrdU incorporation assay. Mean values + the SD of two to three independent experiments are presented, each with four parallel samples or datum points. (C) Overexpression of EGFR desensitizes HT-1080 cells to topotecan-elicited proliferation inhibition. HT-1080 cells were transiently transfected with empty expression vector (pcDNA) or with EGFR expression vector (pcDNAEGFR) or left untransfected. At 24 h after transfection cells were treated with topotecan, and the rate of proliferation was determined 24 h later by BrdU assay. The mean + the SD of relative rate of proliferation compared to untreated cultures from three independent experiments is presented, each with four parallel samples or datum points. (D) Analysis of EGFR, cyclin E, and actin expression levels from the cells transfected parallel with BrdU assay shown in panel C. (F) Proposed model for regulation of cancer cell proliferation through cooperation of Topo I and c-Jun on positive regulation of EGFR gene expression.

The results presented above strongly suggest that topotecan-elicited inhibition of cell proliferation is mediated by inhibition of EGFR expression. According to this model, cells expressing EGFR from heterologous promoter should be resistant to topotecan-induced inhibition of proliferation. To test this assumption, we transiently transfected HT-1080 cells with expression construct coding for EGFR and treated these cells together with control transfected or untransfected cells with topotecan and measured inhibition of proliferation by BrdU assay. As shown in Fig. 7C, topotecan at 0.05 μM inhibited proliferation in untransfected and control vector transfected cells, whereas no inhibition of proliferation was detected in EGFR-overexpressing cells. Cells transfected with EGFR expression construct displayed clearly higher levels of EGFR expression in both untreated and topotecan-treated cells (Fig. 7D). This analysis clearly support the idea that EGFR overexpression converted HT-1080 cells resistant to topotecan by retaining the EGFR expression levels in transfected cells above the endogenous levels. In order to further confirm that forced expression of EGFR resulted in resistance to topotecan-elicited growth suppression, we studied the expression levels of cyclin E from the same samples. In accordance with other results, topotecan treatment induced inhibition of cyclin E expression in control transfected cells, whereas no inhibition was observed in cells overexpressing EGFR (Fig. 7D).

Taken together, these results demonstrate that inhibition of EGFR expression is a novel mechanism by which topotecan inhibits cancer cell proliferation. Moreover, these results strongly suggest that Topo I is involved in JNK-c-Jun pathway-mediated regulation of cell proliferation through stimulation of EGFR expression. Figure 7E illustrates the proposed novel mechanism of Topo I-mediated regulation of cancer cell proliferation and mechanism of action for topotecan in cancer therapy.

DISCUSSION

Topo I expression is increased in several malignancies, and it is a molecular target for anticancer agent topotecan in the treatment of small cell lung cancer and ovarian carcinomas. However, the molecular mechanisms behind Topo I-mediated regulation of cancer cell proliferation have been obscure. The transcriptional coactivator function of Topo I has been proposed to be a potential target for Topo I inhibiting drugs in cancer therapy (20, 27). The results of the present study show that Topo I binds to and cooperates with c-Jun, hence providing the first evidence about the physical and functional interaction of Topo I with a mammalian transcription factor in vivo. Moreover, the results presented here show that transcriptional stimulation of EGFR expression is a novel mechanisms by which Topo I promotes cancer cell proliferation. In that regard, these results provide a proof-of-principle for the requirement of Topo I-mediated transcription for the proliferation of cancer cells and novel mechanistic basis for the use of Topo I inhibiting drugs in the cancer therapy.

In our model EGFR was the only gene whose expression was significantly suppressed by topotecan treatment. Moreover, recent microarray study showed that inhibition of Topo I with low micromolar concentration of topotecan inhibits expression of only a small number of genes (5). Together, these results clearly indicate that the transcriptional effects of Topo I inhibition on EGFR gene expression are not due to a general suppression of transcription. It is, however, likely that, in addition to EGFR, Topo I regulates other, as-yet-unidentified genes that are involved in the positive regulation of proliferation.

Importantly, all of the data presented here concerning the role of c-Jun and Topo I on EGFR gene expression have been obtained by interfering with endogenous Topo I and c-Jun and by studying expression of genes in their proper chromatin context. Earlier in vitro studies have shown that Topo I is involved in transcription only if it is recruited to the promoter by a transcription factor (28, 34). Our findings with c-Jun−/− cells show that Topo I supports EGFR transcription only in c-Jun positive cells (Fig. 4A and B), providing the first in vivo evidence that an interacting transcription factor is required for Topo I function on gene regulation. Moreover, our results show that constitutive JNK2 activity in HT-1080 cells stimulates c-Jun-Topo I interaction and is required for topotecan-elicited inhibition of EGFR expression (Fig. 2D and 4C). The specific requirement for constitutive JNK-c-Jun pathway activity for cooperation of c-Jun and Topo I on EGFR gene regulation may explain why EGFR has not been identified as a target gene for topotecan in other studies (5). On the other hand, our results show that many genes regulated by c-Jun are not inhibited by topotecan, suggesting that the role of Topo I on gene regulation is not solely determined by its interaction with c-Jun. This may be explained by requirement of other, as-yet-unidentified regulatory proteins that are selectively involved in c-Jun-mediated regulation of the EGFR promoter. In addition to its functions on chromatin (28), Topo I exerts its effects on transcription through assembly of the TFIID-TFIIA complex (17, 26, 34). Interestingly, TFIID was shown to predominantly deliver TBP to TATA-less promoters, whereas SAGA predominantly delivered TBP to TATA-containing promoters (1). EGFR gene promoter is TATA-less (13). Both c-Jun and Topo I has been demonstrated to bind to TBP in vitro (10, 17, 26), and in the present study we provide evidence for a physical in vivo interaction between TBP, Topo I, and c-Jun (Fig. 2A). Based on these considerations, it is tempting to speculate that EGFR might serve as a prototypical example of a gene whose TATA-less promoter requires Topo I activity for TFIID complex assembly and for TBP binding. In the future, it would be of great interest to determine whether the specificity of Topo I in gene regulation is determined by the promoter structure and/or by other, as-yet-unidentified proteins.

Increased JNK-c-Jun pathway activity has been detected in various malignancies (24). The results of our study are in accordance with recent studies indicating that the molecular mechanism by which JNK-c-Jun signaling promotes cancer cell growth involves transcriptional stimulation of EGFR expression (3, 7, 13, 39). Our results show that constant JNK activity in HT-1080 cells is required for c-Jun-Topo I interaction and for the EGFR-dependent proliferation of these cells. Importantly, the results of the present study indicate that the activity of JNK-c-Jun signaling pathway determines the sensitivity of cancer cells to Topo I-inhibiting chemotherapy. In addition to elucidating the mechanisms by which the sensitivity of cancer cells to topotecan is regulated, these results may provide important information for the future development of cancer therapies aimed at inhibition of JNK-c-Jun pathway activity (24). In the future it would be of great interest to study whether the selective effects of Topo I-inhibiting drugs in different cancer types in vivo can be correlated with the activity of the JNK-c-Jun pathway on those tumors.

In conclusion, we have identified transcriptional coactivator Topo I as a first endogenous interaction partner for c-Jun in the regulation of cell proliferation. Moreover, these results demonstrate that stimulation of EGFR expression is a novel mechanism by which Topo I stimulates cancer cell proliferation. Together, these results indicate that reevaluation of the function of Topo I in cancer progression in vivo might be needed and that further consideration should be given to the oncogenic properties of Topo I that are due to Topo I's role as a transcriptional coactivator. It is plausible that development of more specific approaches to inhibit specifically transcriptional activities of Topo I could be beneficial for future cancer therapies. Finally, recent studies have indicated that the lack of response to chemical inhibitors of EGFR tyrosine kinase activity in lung cancer patients is due to the fact that only a mutated form of EGFR is the target for drug action in vivo (29). In that respect, our results suggest that inhibition of transcriptional activities of Topo I by camptothecins or other approaches might hold promises for the treatment of diseases in the pathogenesis of which overexpression of EGFR plays an important role.

Acknowledgments

Dirk Bohmann, Johanna Ivaska, Panu Jaakkola, and Carsten Weiss are acknowledged for valuable comments on the manuscript. We also thank D. Bohmann for his contributions in the original identification of c-Jun complex. We thank P. D'Arpa, F. Barr, Y. Daaka, V.-M. Kähäri, S. Leppä, and P. Jaakkola for reagents. We thank also N. E. Fusenig and V.-M. Kähäri for the HaCat and NSF cell lines.

This project was supported by funding from the Academy of Finland (projects 53692 and 8212695), the Sigrid Juselius Foundation, the Turku University Foundation, and the Finnish Cancer Research Society.

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