The retinoblastoma gene family consisting of RB/p105, p107, and RB2/p130 cooperate to regulate cell-cycle progression through the G1 phase of the cell cycle. Previous data demonstrated an independent role for the reduction or loss of pRb2/p130 expression in the formation and/or progression of lung carcinoma. Rb2/p130 is mutated in a human cell line of lung small cell carcinoma as well as in primary lung tumors. To identify potential pRb2/p130 target genes in an unbiased manner, we have utilized an adenovirus-mediated expression system of pRb2/p130 in a non-small lung cancer cell line to identify specific genes that are regulated by pRb2/p130. Using oligonucleotide arrays, a number of Rb2/p130 downregulated genes were identified and their regulation was confirmed by semiquantitative reverse transcription–polymerase chain reaction (RT–PCR) and Western blot analysis. As a result, 40 genes showed greater than 2.0-fold modification in their expression level after the RB2/p130 viral transduction. In conclusion, coupling adenoviral overexpression with microarray and semiquantitative RT–PCR analyses proved to be a versatile strategy for identifying pRb2/p130 target genes and for better understanding the expression profiles of these genes. Our results may also contribute to identifying novel therapeutic biomarkers in lung carcinoma.
Lung cancer is the leading cause of cancer death worldwide and one of the most common malignancies diagnosed in the United States. The American Cancer Society estimated that 169,500 new cases of lung cancer would be diagnosed in 2001 and that 157,400 people would die of the disease that year. Data from the American Cancer Society for the year 2002 indicate that 169,400 new cases of lung cancer are estimated to occur and 154,900 patients are expected to die because of lung cancer (Greenlee et al., 2001). Additionally, lung cancer is usually diagnosed at an incurable stage.
The high mortality rate for lung cancer probably results from the absence of effective therapies as well as standard diagnostic procedures of early tumoral stages compared with colon, breast, and prostate cancers (Wiest et al., 1997). The majority of bronchogenic carcinomas can be classified into four histological types: small cell lung carcinomas, adenocarcinomas, squamous cell lung carcinomas, and large cell carcinomas. Small cell lung carcinomas are a separate entity, whereas the behavior of the other three histological subtypes is similar, for this reason these are grouped within the non-small cell lung cancer (NSCLC) type. NSCLC accounts for nearly 80% of lung malignant tumors and it is associated with a poor prognosis.
Lung cancer is the result of molecular changes in the cell, resulting in the deregulation of pathways that control normal cellular growth, differentiation, and apoptosis. In this scenario, proto-oncogenes and tumor suppressor genes are found to be mutated or have abnormal expression patterns in this disease. Although much is known about the natural history, predisposing factors, and outcome of NSCLC, the understanding of this disease is still unclear. Even though many molecular changes associated with NSCLC have been reported (Hibi et al., 1998; Forgacs et al., 2001), a fully understood mechanism associated with this type of cancer has not been described yet.
The retinoblastoma gene family consisting of RB, p107, and RB2/p130 cooperate to regulate cell-cycle progression through the G1 phase of the cell cycle. Products of RB, p107, and RB2/p130 are characterized by a peculiar steric conformation, the ‘pocket region’, which is responsible for most of the functional interactions that characterize the activity of these proteins in cell-cycle homeostasis (Paggi et al., 1996). Rb family members are nuclear proteins that are regulated in a cell-cycle-dependent manner by phosphorylation, exhibit growth suppressive properties in a cell-type-dependent manner, are implicated in various forms of differentiation, and are critical targets for inactivation by transforming oncoproteins of DNA tumor viruses.
During the past 6 years, the involvement of pRb2/p130 in lung cancer has been studied. In fact, immunohistochemical analysis of the expression patterns of the Rb family members (pRb/p105, p107, and pRb2/p130) in 235 specimens of lung cancer (Baldi et al., 1996) and the expression pattern of pRb2/p130 in 158 specimens of human lung cancer showed an inverse correlation between the histological grading of the tumors, the development of metastasis, and the level of expression of pRb2/p130 (Baldi et al., 1997). A statistically significant inverse relationship between the histological grading and the expression of pRb/p105, p107, and pRb2/p130 was found in fine needle aspiration biopsies of squamous cell carcinoma patients (Minimo et al., 1999).
Point mutation in the RB2/p130 gene determining protein loss in a cell line of human small cell lung carcinoma (Helin et al., 1997) and identification of point mutations in primary lung cancer (Claudio et al., 2000) were found, indicating that pRb2/p130 may play a role in the pathogenesis and progression of certain lung cancers.
We have utilized an adenovirus-mediated expression of RB2/p130 in the H23 NSCLC cell line to identify specific genes that are regulated by pRb2/p130. Using oligonucleotide arrays, a number of downregulated genes by pRb2/p130 were identified and their modulation was confirmed by semiquantitative reverse transcription–polymerase chain reaction (RT–PCR) and Western blot analysis.
Materials and methods
The human lung adenocarcinoma cell line H23 has been described previously (Claudio et al., 2000). The packaging cell line 293 (primary embryonal human kidney cells) transformed by sheared human adenovirus type 5 has also been previously described (Claudio et al., 1999). H23 cells were maintained in DMEM supplemented with 10% fetal bovine serum, 2 mM L-glutamine. 293 cells were maintained in DMEM supplemented with 10% heat-inactivated fetal bovine serum, 2 mM L-glutamine.
Adenoviruses were generated by subcloning the full-length ORF of the RB2/p130 gene into the pAd.CMV-Link1 vector to form the Ad.CMV-RB2/p130 virus, as described previously (Davis et al., 1998; Claudio et al., 1999). The pAd.CMV-Link1 vector alone (to produce the Ad-CMV virus) was used as a negative control to assay the effects of viral infection alone without delivering a transgene. Adenoviruses were expanded, purified, and tittered as previously described (Claudio et al., 1999).
Flow cytometry analysis
Flow cytometry analysis (FACS) of exponentially growing H23 cells or H23 cells transduced with Ad-CMV or Ad-CMV-RB2/p130 were carried out as previously described (Claudio et al., 1996). Briefly, 5 × 105 cells were seeded and 24 h after the cells were transduced with 50 multiplicity of infection (MOI) of adenoviruses. At 48 h after transduction, cells were collected and analysed using a Coulter Flow cytometer.
Before submission of RNA samples for analysis, protein extracts prepared from replicate plates of the corresponding cell culture were analysed for expected enhanced expression of pRb2/p130 using Western blots. Oligonucleotide-based microarrays were purchased from Mergen (Mergen Ltd, San Leandro, CA, USA). The ExpressChip H05000 DNA microarray system was used for this study. This array contains more than 3200 genes that are involved in a variety of different processes. DNase-treated total RNA (20 μg) from H23 (parental cells), H23 cells transduced with Ad-CMV, or Ad-CMV-RB2/p130 cell lines 48 h after transduction were extracted using TRIzol (Life Technologies, Inc., Grand Island, NY, USA) according to the manufacturer's protocol. RNA integrity was verified for lack of degradation by formaldehyde gel electrophoresis. The biotin-labeled cRNA probes preparation, hybridization, and array scanning were performed using Mergen Labeling/Hybridization/Detection Service. Data acquisition and data analysis were performed using Imagene software (Biodiscovery Inc., Marina del Rey, CA, USA) and Mergen's ExpressData™ software (Mergen Ltd, San Leandro, CA, USA). Briefly, data were processed for local background correction and normalization. The raw spot for each gene was calculated as the mean signal of the spot values minus that of the local background. The Imax value was set to 65, 000, after local background removal. A normalization coefficient (N) was applied to either the control population or to sample spot raw values to compensate for slide-to-slide and probe-to-probe variations. The normalization coefficient was applied only if (Raw_spot/N)<Imax, otherwise values were set to Imax. Normalized values ⩽0 were excluded from the analysis. Genes regulated by adenovirus transduction (Ad-CMV) with respect to the parental cell line were removed from the analysis. Spots with mean intensities >45, 000 were excluded for the ratio analysis. The expression ratios calculated with corrected values less than mean of the local background on both channels were not used. Expression ratios of the analysed genes were calculated comparing genes’ expression values of H23 cells transduced with RB2/p130 with those of parental H23 or H23 cells transduced with Ad-CMV. A two-fold or higher levels of target genes’ expression ratio was considered significant, in accordance with most of the literature.
Northern blot analysis
H23 cells were grown to 60% confluency, then infected with 50 MOI of adenoviruses carrying the RB2/p130 gene or with the control Ad-CMV. After 14 h, the medium was changed, and cells were harvested at 48 h from the transduction. DNAse-treated total RNA from H23, H23-Ad-CMV, and H23-Ad-CMV-RB2/p130 transduced cells were extracted using TRIzol (Life Technologies, Inc., Grand Island, NY, USA) according to the manufacturer's protocol. Northern blot analysis was performed as previously described (Claudio et al., 1994).
RT–PCR was used to analyse target gene expression in the present study. A 2 μg aliquot of DNAse-treated total RNA from each sample was reverse transcribed for single-stranded cDNAs using M-MLV reverse transcriptase (Invitrogen, Carlsbad, CA, USA) according to the manufacturer's protocol. The same cDNA product obtained from each sample was used for subsequent PCR amplification with the primer sets prepared for the target gene and β-actin (Act-β)/HPRT housekeeping genes. The amplification of the β-actin and HPRT genes were used as double internal control. The ratio between the samples and each housekeeping gene was calculated to normalize for initial variations in sample concentration and as a control for reaction efficiency. Primer sequences were designed using the software Primer 3 (developed by Steve Rozen, Helen J Skaletsky) available on-line at http://www-genome.wi.mit.edu. Primer sequences can be provided upon request. PCR reaction conditions were individually optimized for each gene product studied and the number of PCR cycles was set up to be within the linear range of product amplification.
In each experiment, possible DNA contamination was determined by a control reaction in which reverse transcriptase was omitted from the reaction mixture. PCR products were loaded onto ethidium bromide stained 1.5% agarose gels. Densitometric analyses of the PCR products were performed using an Alpha Imager system (Alpha Innotech Corporation, San Leandro, CA, USA) and the ImageJ v1.29 software (developed by Wayne Rasband) available on-line at http://rsb.info.nih.gov/ij/. All PCR products were purified using QIAquick PCR purification kit (Qiagen, Santa Clarita, CA, USA) and their identities verified by automated DNA forward and reverse sequencing using a dideoxy terminator reaction chemistry for sequence analysis on the Applied biosystem Model 373A DNA sequencer.
Western blot analysis and antibodies
Western blot analysis of exponentially growing H23 cells or of H23 cells transduced with Ad-CMV or Ad-CMV-RB2/p130 were carried out as previously described (Claudio et al., 1999). Extracts were normalized for protein content by Bradford analysis (Bio-Rad Laboratories, Inc., Melville, NY, USA) and Coommasie blue gel staining.
Primary anti-B-MYB, E2F-1, KPNA2, MKK3, NIK, PCNA, PLK, RAF1 (Santa Cruz Biotechnology, Inc., Santa Cruz, CA, USA), anti-MAGE-A (Upstate, Lake Placid, NY, USA), and anti-HSP70 (Oncogene Science, Cambridge, MA, USA) were used following the manufacturer's instructions.
Effects of RB2/p130 adenoviral transduction on the H23 lung adenocarcinoma cell line
H23 cells were plated at a density of 5 × 105 in four 10-cm tissue culture dishes. Cells were transduced with 50 MOI of the control Ad-CMV or Ad-CMV-RB2/p130 and harvested after 48 h. Two tissue culture dishes were used to extract total RNA. One tissue culture dish was used to extract the proteins and one for FACS analysis.
Northern blot analysis of samples transduced with RB2/p130 showed an increased expression of the RB2/p130 transcript more than 20-fold with respect to the control (Figure 1a). Western blot analysis showed more than 100-fold enhanced expression of pRb2/p130 in the Ad-CMV-RB2/p130 transduced cells (Figure 1b).
To confirm the effects of pRb2/p130-enhanced expression in H23 cells, we performed FACS analysis. Figure 2 shows that adenoviral RB2/p130 transduction resulted in 81.2% of the cells accumulated in the G0/G1 phase of the cell cycle when compared to the control (54%).
Oligonucleotide microarray assay following enhanced expression of pRb2/p130 in a human lung adenocarcinoma cell line
H23 cells were transduced with Ad-CMV or Ad-CMV-RB2/p130. After 48 h, 20 μg of DNA-free total RNA from H23, H23-Ad-CMV, or H23-Ad-CMV-RB2/p130 cells was reverse transcribed and the double-strand cDNA was used as a template to generate Cy3-labeled cRNA probes and then hybridized to the Mergen H05000 oligonucleotide-based microarray containing more than 3,200 genes that are involved in a variety of different processes. Analysis was performed by Mergen Ltd (San Leandro, CA, USA). Microarray experiments were performed comparing H23 vs H23-Ad-CMV, H23-Ad-CMV vs H23-Ad-CMV-RB2/p130, and H23 vs H23-Ad-CMV-RB2/p130 cells. Duplicate experiments were carried out on a single total RNA preparation from the cells.
In this study, 40 genes were downregulated more than 2.0-fold (Table 1). Figure 3 shows the plots of the differential expression of 3263 genes in H23-Ad-CMV vs H23-Ad-CMV-RB2/p130 cells and H23 vs H23-Ad-CMV-RB2/p130 cells. Overall, the expression of the majority of the spotted genes was not altered by RB2/p130. Modulated genes were classified in Table 2 on the basis of a well-documented and established biological or pathological function of the encoded protein. The genes downregulated by pRb2/p130-enhanced expression belong mainly to the following categories: cell division; signaling and communication; cell structure and motility; gene expression; metabolism; and disease.
Validation of the oligonucleotide microarray assay using semiquantitative RT–PCR and Western blot analysis
To determine the gene expression level of specific pRb2/p130 target genes, semiquantitative RT–PCR analysis was used. A panel of 11 genes, randomly selected among the 40 identified by microarray analysis, was analysed. We confirmed this by RT–PCR downregulation of B-MYB, Cyc B2, Cyc D1, GRPR, KPNA2, MKK3, NIK, PCNA, PIM, PLK, and RAF-1 (Figure 4). Genes highly downregulated (range between 6- and 17-fold) in microarray analysis such as PCNA, MKK3, B-MYB, and NIK showed a comparative downregulation in semiquantitative RT–PCR analysis between 7- and 3.5-fold. Genes still downregulated in microarray analysis, but at a lower extent such as RAF-1, PIM1, CycD1, GRPR, KPNA2, and CycB2, showed a comparable downregulation in semiquantitative RT–PCR analysis between 3.4- and 2.0-fold. PLK, which showed a high downregulation ratio in microarray analysis, failed to be validated by semiquantitative RT–PCR. In fact, PLK showed almost a two-fold difference expression level by RT–PCR.
Of the 11 transcriptionally downregulated genes that were studied by RT–PCR analysis, only seven genes (B-MYB, KPNA2, MKK3, NIK, PLK, and RAF-1) were found expressed by Western blot analysis at a lower level upon enhanced pRb2/p130 expression with a ratio between 1.9- and 3.0-fold (Figure 5). As the MAGE gene family is composed of 23 related genes divided into four clusters and the MAGE-A subfamily comprises 12 genes highly identical in their coding sequence, we were not able to perform RT–PCR on this gene family, but we could confirm by Western blot analysis the contingent downregulation of MAGEA-3/6 to enhanced pRb2/p130 expression. Surprisingly, PCNA that was highly downregulated in the microarray analysis, also appearing modulated in RT–PCR, showed no protein expression changes upon enhanced pRb2/p130 expression. However, it has been shown that PCNA has a relatively long half-life that can extend beyond the S phase into the M phase and beyond into the G0 phase of cells in rapidly proliferating tumors.
During the last 6 years, different studies demonstrated the involvement of RB2/p130 in lung cancer. We have shown an inverse correlation between histological grading of lung cancer and the expression pattern of the tumor suppressor gene pRb2/p130 (Baldi et al., 1996,1997; Minimo et al., 1999). Nevertheless, the molecular mechanisms by which Rb2/p130 participate in lung cancer are not yet clear.
In the recent past, our group demonstrated that virally enhanced expression of pRb2/p130 caused about a 3.2-fold or 69% tumor mass reduction in nude mice. These data strongly suggest the gene therapeutical potential of RB2/p130 in lung cancer. In this view, the identification of genes modulated by enhanced pRb2/p130 expression could serve as a valuable tool in identifying new targets for gene therapy in lung cancer as well as to understand the molecular mechanisms behind this approach better. For these reasons, we decided to analyse the NSCLC H23 cells transduced with RB2/p130.
In our study, we induced an overexpression on the RB2/p130 gene in H23 cells (human lung adenocarcinoma) using an adenoviral system and performed an oligonucleotide microarray analysis to measure and evaluate the gene's expression profile. We identified 40 genes that were downregulated more than 2.0-fold by pRb2/p130 adenovirus-mediated overexpression. Regulated genes were summarized in 24 categories as shown in Table 2. We applied a cutoff ratio of 2.0 that has been commonly used in many microarray data analyses previously developed. We confirmed the modulation of 11 selected genes by semiquantitative RT–PCR and of six by Western blot analysis (Figures 4 and 5).
B-MYB in our microarray analysis showed an almost 10-fold reduction of expression with respect to the control 48 h after pRb2/p130 transduction. Downregulation of B-MYB was confirmed by RT–PCR and Western blot analysis. B-MYB is a member of a family of transcription factors, which include c-MYB and A-MYB, that interact with the promoters of different genes involved in differentiation as well as cell cycle (Lyon et al., 1994). It has been demonstrated that B-Myb can activate promoters, both via DNA binding-dependent and -independent mechanisms, following phosphorylation mediated by cyclin A/E-cdk2 kinase (Lane et al., 1997; Sala et al., 1997). B-MYB is a positive regulator of cell proliferation and its expression is required for the growth and survival of cells of different origins. B-MYB expression is stringently coupled to the proliferative state of the cell, and its overexpression promotes cell proliferation (Sala et al., 1996). It has been reported that pRb2/p130 increases during neuroblastoma differentiation with a parallel decrease of B-Myb (Raschella et al., 1997). Additionally, there is a large body of evidence that the negative regulation of B-MYB takes place, at least in part, at a transcriptional level (Raschella et al., 1996), through an E2F site in the promoter region of B-MYB when RB family proteins form complexes with E2F factors bound to DNA (Zwicker et al., 1996).
Recently, it was also demonstrated that B-Myb is abundantly expressed in primary lung cancers, suggesting that its overexpression may contribute to lung tumorigenesis (Hibi et al., 1998). Our microarray data corroborating those of RT–PCR and Western blotting are in accordance with previously published studies.
Cyclin B2 (Cyc B2) in our microarray analysis showed a 4.7-fold reduction of expression with respect to the control 48 h after pRb2/p130 transduction. Downregulation of Cyc B2 was confirmed by RT–PCR. Cyc B2 is a member of the cyclin family and a component of the B type. Recent data suggest that overexpression of human cyc B2, not due to mere gene amplification, could contribute through an alteration of the spindle checkpoint and chromosomal segregation, and to chromosomal instability of cancer cells, which is a phenotype persistently present in human cancers including lung cancer (Masuda and Takahashi, 2002).
Cyclin D1 (Cyc D1) showed a 2.1-fold reduction of expression with respect to the control 48 h after pRb2/p130 transduction by microarray analysis. Downregulation of Cyc D1 was confirmed by RT–PCR. Cyc D1 is a cell-cycle regulator essential for G1 phase progression and a candidate proto-oncogene implicated in the pathogenesis of several human tumor types. The Cyc D1 gene maps to one of the most frequently amplified chromosomal regions (11q13) in human carcinomas (Motokura and Arnold, 1993) and it is frequently amplified in breast and lung cancer (Keyomarsi and Pardee, 1993). Additionally, Cyc D1 is frequently overexpressed in the absence of genetic amplification in breast cancers (Gillett et al., 1994) and its overexpression is associated with advanced local invasion and the presence of lymph node metastases in head and neck carcinomas (Muller et al., 1997). Cyc D1 is amplified and overexpressed in NSCLC (Reissmann et al., 1999) and its expression has been shown to be a negative prognostic marker in lung cancer (Caputi et al., 1999), confirming the hypothesis of Schauer et al. (1994). In fact, it has been suggested that lung tumors may evade cell-cycle control through abnormal expression of cyc D1. Additionally, cyc D1 overexpression and lack of expression of retinoblastoma protein is frequently seen in lung cancer.
In our microarray, E2F-1 was one of the genes that, unfortunately, resulted in not being statistically significant. However, because recently it was shown that E2F-1 can act either as an oncogene or as a tumor suppressor gene and that E2F-1 overexpression may contribute to the development of NSCLCs by promoting proliferation (Gorgoulis et al., 2002), we performed a promoter analysis of all the 11 selected genes using the TESS software searching for E2F-1 and B-Myb DNA-binding regulatory regions. We found that the GRP-R, MAGE-A 3/6, MKK3, RAF1, B-MYB, PLK, PCNA, PIM1, Cyc B2 and Cyc D1 genes contain E2F-1 and/or B-Myb DNA-binding sites in their promoter sequences. All these data strongly validate and explain the downregulation of GRPR, MAGE-A 3/6, MKK3, RAF1, B-MYB, PLK, PCNA, PIM1, Cyc B2, and Cyc D1 in our system. In fact, the enhanced expression of pRb2/p130 downregulates E2F-1 and B-MYB expression (Figure 5). Interestingly, the promoter sequence of the NIK gene that does not contain E2F-1 and/or B-Myb DNA-binding sites is also downregulated by enhanced pRb2/p130. No data are available for the human promoter sequence of the KPNA2 gene, instead.
The gastrin-releasing factor receptor gene (GRP-R) showed a 2.7-fold reduction of expression with respect to the control 48 h after pRb2/p130 transduction by microarray analysis. Downregulation of GRP-R was confirmed by RT–PCR. GRP-R is a member of the G protein-coupled receptor family and mediates important physiological actions of its specific ligand, the gastrointestinal hormone GRP. Many studies have found that GRP increases tumor cellular proliferation, leading to the hypothesis that this peptide hormone is a mitogen important for the growth of various cancers (Rozengurt, 1988). GRP has been implicated in the development of lung epithelium (Hoyt et al., 1993) and it is thought to be a growth factor involved in human lung carcinoma (Fathi et al., 1996). GRP-R has mitogenic activity and it is produced in an autocrine fashion in small and non-small lung carcinoma (Siegfried et al., 1999). Therefore, the preliminary data that pRb2/p130 downregulates the transcription of this mitogen in lung cancer is an important hint that many and different pathways may be regulated by this member of the retinoblastoma family.
The karyopherin KPNA2 showed a 3.8-fold reduction of expression with respect to the control 48 h after pRb2/p130 transduction by microarray analysis. Downregulation of KPNA2 was confirmed by RT–PCR and Western blot analyses. The active transport of proteins into the nucleus requires an array of proteins, including karyopherin. KPNA2 is one of six forms of karyopherin alpha proteins. Recently, it was shown that KPNA2 is able to interact with the p65 subunit of nuclear factor-κB (NF-κB) (Cunningham et al., 2003). NF-κB activation requires removal and degradation of its inhibitor κB, an event that occurs after phosphorylation of inhibitor κB by a complex of inhibitor κB kinases. These events allow NF-κB to translocate into the nucleus, to bind DNA, and regulate gene transcription. NF-κB nuclear translocation plays an important role in preventing apoptotic cell death in some cancers. Recently, it was shown that inhibition of NF-κB enhances apoptosis in human lung adenocarcinoma cells in vitro (Milligan and Nopajaroonsri, 2001). Therefore, it is possible to hypothesize that downregulation of KPNA2 could evade, in part, NF-κB nuclear translocation and subsequently, its antiapoptotic effect.
MAGE genes were initially identified because they encode tumor antigens that are recognized by cytolytic T lymphocytes derived from blood lymphocytes of cancer patients (Van den Eynde and van der Bruggen, 1997). The MAGE gene family is composed of 23 related genes divided into four clusters. The MAGE-A subfamily is very appealing for its potential as antitumor immunotherapic because MAGE-A proteins are strictly tumor specific. Seven MAGE-A genes (MAGE-A1, -A2, -A3, -A4, -A6, -A10, and -A12) have been found highly transcribed in a large proportion of tumors of various histological origins, such as metastatic melanomas, testicular germ cell tumors, lung cancer, sarcoma, mammary tumors, and colon carcinomas (Ohman Forslund and Nordqvist, 2001). Recently, MAGE-A1, 3, 6, 12, and 4b subfamily members have also been proposed as candidate biomarkers in lung carcinoma (Sugita et al., 2002). Our microarray analysis revealed that MAGE-A 3/6 genes are downregulated with a ratio of 2.5 and these data were confirmed by Western blot analysis, suggesting an interesting link between MAGE-A 3/6 genes, RB2/p130, and lung cancer.
MKK3 showed a 15.8-fold reduction of expression with respect to the control 48 h after pRb2/p130 transduction by microarray analysis. Downregulation of MKK3 was confirmed by RT–PCR and Western blot analyses. MKK3 specifically activates p38-MAPK that regulates inflammation, apoptosis, and development (Schaeffer and Weber, 1999). Very recently, it has been shown that p38-MAPK is activated in NSCLC, suggesting an additional role of this kinase in malignant cell growth or transformation (Greenberg et al., 2002). Additionally, MKK3 has also been found to be homozygously deleted in a lung tumor cell line. All these data suggest that there is a close relation between this gene and lung carcinogenesis (Teng et al., 2001).
NIK showed a 6.6-fold reduction of expression with respect to the control 48 h after pRb2/p130 transduction by microarray analysis. Downregulation of NIK was confirmed by RT–PCR and Western blot analyses. NIK is a protein kinase able to upregulate NF-κB (Malinin et al., 1997) that is highly expressed in NSCLC (Mukhopadhyay et al., 1995), playing an important role in anchorage-independent and metastatic growth of lung carcinoma cells (Jiang et al., 2001).
The proto-oncogene Pim-1 (PIM-1) showed a 2.2-fold reduction of expression with respect to the control 48 h after pRb2/p130 transduction by microarray analysis. Downregulation of PIM-1 was confirmed by RT–PCR analysis. PIM-1 is a serine/threonine kinase that when overexpressed is involved in lymphomagenesis. PIM-1 also associates with protein complexes necessary for mitosis and maps to an area on chromosome 6, which is close to a region that is amplified in NSCLC (Bhattacharya et al., 2002).
Proliferating cell nuclear antigen (PCNA) showed a 16.9-fold reduction of expression with respect to the control 48 h after pRb2/p130 transduction by microarray analysis. Downregulation of PCNA was validated only by RT–PCR. PCNA is a nuclear protein acting as a cofactor for DNA polymerase delta (Mathews et al., 1984). PCNA has been used as a proliferation and prognostic marker in a variety of malignancies, including lung tumors (Kawai et al., 1994) and it is an important prognostic factor of clinical outcome in patients with NSCLC (Volm and Koomagi, 2000).
Unfortunately, PCNA has not lived up to its early promise as a cellular proliferation marker. The relatively long half-life of the protein extends its expression beyond the S phase into the M phase and beyond again into the G0 phase of cells in rapidly proliferating tumors. This could in part explain why we did not find protein downregulation when compared to its transcriptional modulation following RB2/p130-enhanced expression. In fact, recently Markey et al. (2002) found that in a doxocycline-inducible RB1 system, PCNA does not change at the protein level after 48 h from induced RB1 expression, suggesting that the failure to change could reflect a relatively long PCNA half-life.
PLK1 showed a seven-fold reduction of expression with respect to the control 48 h after pRb2/p130 transduction by microarray analysis. Downregulation of PLK1 was confirmed by RT–PCR and Western blot analyses. PLK1 is a member of PLKs serine–threonine kinases that are highly conserved during evolution. More evidence supports the concept that Plks regulate different cell-cycle stages throughout mitosis, including its initiation by activating Cdc2 through Cdc25 and direct phosphorylation of cyclin B1 targeting Cdc2/cyclin B1 to the nucleus. Moreover, Plks are key regulators of cytokinesis (Glover et al., 1998; Nigg, 1998). PLK1 is involved in centrosome maturation, DNA damage checkpoint adaptation, bipolar spindle formation, and activation of Cdc16, Cdc27 as components of the anaphase-promoting complex for mitotic exit. PLK1 is overexpressed in various human tumors, for example, NSCLC, squamous cell carcinomas of the head and neck, melanomas, endometrial, and ovarian carcinomas (Yuan et al., 1997,2002). It has also been recently shown that the constitutive expression of PLK1 may contribute to cancer progression (Smith et al., 1997). PLK1 overexpression is also a negative prognostic factor in patients suffering from NSCLC (Wolf et al., 1997). All these data, taken together, suggest for the first time a link between a member of the PLK family, RB2/p130, and lung cancer.
RAF1 showed a 2.1-fold reduction of expression with respect to the control 48 h after pRb2/p130 transduction by microarray analysis. Downregulation of RAF-1 was validated by RT–PCR and Western blot analyses. The serine/threonine kinase Raf-1 is the first downstream effector of Ras to be identified. Activation of this kinase is necessary and sufficient for the activation of a kinase cascade consisting of MEK1,2 and ERK 1,2 (Marshall, 1994). Activation of the pathway RAF1/MEK/ERK plays an important role in gene expression control during cell cycle, apoptosis, cell differentiation, and migration and is frequently associated with transformation of primary cells and tumor progression. In fact, recent studies suggest a possible role of Raf-1 in lung tumorigenesis (Yano et al., 1999; Ramakrishna et al., 2002).
In a typical cell, it has been estimated that most of the expressed genes can be expressed in abundance class at (or below) the detection threshold of microarray technology. This includes many genes that do not need to be expressed at extremely high levels in order to be biologically active, such as growth factors and transcriptional regulators (Fambrough et al., 1999). Therefore, a more sensitive method such as semiquantitative RT–PCR analysis could be an acceptable strategy, not only to validate microarray data but also to identify significantly induced genes whose transcripts belong to lower abundance classes.
Our microarray analysis indicates that adenovirus-mediated enhanced expression of the tumor suppressor gene RB2/p130 in the H23 human lung cancer cell line downregulated a variety of genes involved in many cellular processes including cell division, cell signaling/cell communication, cell structure/motility, gene expression, and metabolism. Adenovirus-mediated expression of pRb2/p130 upregulated a cluster of 28 genes with an average ratio between 2.0 and 8.5 (data not shown), and some of them have been found to be previously linked to lung cancer. Further studies are needed to investigate the significance of these genes’ regulation by pRb2/p130 better.
This study identifies a cluster of genes that are modulated by pRb2/p130 expression. Although part of these genes could not be the target of pRb2/p130 at its physiological level, most of them might mediate new potential therapeutical effects of RB2/p130 in lung cancer. Therefore, we feel that these studies, by bringing about a better understanding of ample spectra of proteins’ expression, are essential because they could identify new cancer biomarkers and could facilitate matching the appropriate therapies to lung cancer, thereby maximizing therapeutic efficacy and minimizing toxicity.
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This work was supported by NIH grants and the Sbarro Health Organization to AG. G.R. acknowledges the Ph.D. program: “Diagnostic, quantitative, and molecular pathology” of the University of Siena, Italy.
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