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Development of a chemoresistant orthotopic human nonsmall cell lung carcinoma model in nude mice
Analyses of tumor heterogeneity in relation to the immunohistochemical levels of expression of cyclooxygenase-2, ornithine decarboxylase, lung-related resistance protein, prostaglandin E synthetase, and glutathione-S-transferase (GST)-α, GST-μ, and GST-π
Version of Record online: 8 SEP 2004
Copyright © 2004 American Cancer Society
Volume 101, Issue 8, pages 1908–1918, 15 October 2004
How to Cite
Mathieu, A., Remmelink, M., D'Haene, N., Penant, S., Gaussin, J.-F., Van Ginckel, R., Darro, F., Kiss, R. and Salmon, I. (2004), Development of a chemoresistant orthotopic human nonsmall cell lung carcinoma model in nude mice. Cancer, 101: 1908–1918. doi: 10.1002/cncr.20571
- Issue online: 1 OCT 2004
- Version of Record online: 8 SEP 2004
- Manuscript Accepted: 8 JUL 2004
- Manuscript Revised: 22 APR 2004
- Manuscript Received: 12 JAN 2004
- Yvonne Boël Foundation (Brussels, Belgium)
- Region Bruxelles-Capitale (Brussels, Belgium)
- orthotopic model;
- nonsmall cell lung carcinoma;
- ornithine decarboxylase;
- prostaglandin E synthetase;
- lung-related resistance protein;
Nonsmall cell lung carcinomas (NSCLCs) are associated with very dismal prognoses, and adjuvant chemotherapy, including irinotecan, taxanes, platin, and vinca alkaloid derivatives, offer patients only slight clinical benefits. Part of the chemoresistance of NSCLC results from the expression in NSCLC cells of a very large set of endogenous proteins, which antagonize chemotherapy-mediated attacks on these tumor cells.
The authors set up an orthotopic model of a human NSCLC by grafting A549 cells into the lungs of nude mice. They tried treating these A549 NSCLC orthotopic xenograft–bearing nude mice on the basis of various chemotherapeutic protocols, including chronic administrations of taxol, oxaliplatin, and irinotecan. A cyclooxygenase-2 (COX-2) inhibitor (NS-398) also was assayed in combination with taxol. The immunohistochemical expression levels of COX-2, prostaglandin E synthetase (PGES), ornithine decarboxylase (ODC), the lung-related resistance protein (LRP), and glutathione-S-transferase-α (GST-α), GST-μ, and GST-π were quantitatively determined by means of computer-assisted microscopy in control and drug-treated NSCLC orthotopic xenografts.
The orthotopic A549 xenograft model developed in 100% of the grafted mice, leading to brain metastases in approximately 61% mice and to liver metastases in approximately 40% of mice. The model was resistant to taxol and oxaliplatin and was only weakly sensitive to irinotecan. High levels of chemoresistant markers (i.e., COX-2, PGES, ODC, LRP, GST-α, GST-μ, and GST-π) were observed in the nontreated A549 xenografts, although with dramatic variations in individual expression. Taxol and oxaliplatin significantly increased the levels of expression of COX-2, PGES, GST-μ, and GST-π in a number of different experimental protocols.
The A549 orthotopic xenograft model could be used to evaluate investigational chemotherapeutic agents to identify drugs rapidly that are more active than the drugs currently in use in hospitals. Cancer 2004. © 2004 American Cancer Society.
Nonsmall cell lung carcinoma (NSCLC) is the leading cause of death from malignant disease in most developed nations.1, 2 In the United States and Japan, NSCLC accounts for more deaths each year than colorectal carcinoma, breast carcinoma, and prostate carcinoma combined.1, 2 Morphologic characteristics observable under light microscopy allow the classification of lung carcinomas into two major groups: SCLC and NSCLC.3, 4 NSCLC includes various histologic types, i.e., squamous cell carcinoma, adenocarcinoma, and large cell carcinoma,3, 4 and accounts for 80–85% of all lung carcinomas.3, 4 Distinguishing between SCLC and NSCLC is sufficient for most clinical needs.3, 4 Patients with NSCLC undergo surgical resection (curative therapy) whenever possible; however, this involves only approximately 20% of patients.3, 4 Manegold5 reported that, in patients with NSCLC who are in good clinical condition, platin-based combination chemotherapy currently is recommended to prolong survival, prevent or reduce tumor-related symptoms, and maintain the quality of life. Manegold5 and Greco6 reported that taxol-based, combination chemotherapy, particularly with platin-derived drugs, has become very popular in the United States. Oxaliplatin, vinorelbine, and irinotecan also have yielded promising data in various Phase II and Phase III studies.5–8 However, the fact remains that, with existing treatment protocols, the 5-year overall survival rates for patients with NSCLC are a dismal 15%.1, 2 This is due to the fact that 1) at the time of diagnosis, nearly 70% of patients with NSCLC have metastatic or locally advanced, nonresectable pathology1–4; and 2) NSCLCs contain a large set of cell mechanisms that render them chemoresistant. Among these mechanisms, cyclooxygenase-2 (COX-2), prostaglandin E synthetase (PGES), ornithine decarboxylase (ODC), the lung-related resistance protein (LRP), and several glutathione S-transferases (GSTs) play a major part.
COX-2 is a key isoenzyme in the conversion of arachidonic acid into prostaglandins (PGs),9 and it is believed that its contribution to carcinogenesis and the malignant tumor cell phenotype is related to its ability to 1) increase the production of prostaglandins, 2) convert procarcinogens into carcinogens, 3) inhibit apoptosis, 4) promote angiogenesis, 5) modulate inflammation and the immune function, and 6) increase tumor cell invasiveness.2, 9–11 COX-2 expression is increased in NSCLCs, and this expression is correlated with patient prognoses (for a review, see Xu9). Microtubule-interfering agents (including taxol and vinorelbine) stimulate microtubule-associated protein kinase signaling and activator protein-1 activity, which, in turn, lead to the up-regulation of COX-2 and the synthesis of PGE2.12, 13 Microsomal PGES (mPGES) functions downstream of COX-2 in the PGE2-biosynthetic pathway, and an aberrant expression of mPGES-1 in combination with COX-2 may contribute to tumorigenesis.14, 15 The immunohistochemical expression of GST-μ is greater in neoplastic lung tissue compared with normal lung tissue and varies in lung carcinomas in terms of the grade of differentiation and the tumor volume.16 The expression of GST-π in patients with NSCLC is related significantly to their response to cisplatin-based chemotherapy: In patients who are positive for GST-π expression, the response rate for chemotherapy is statistically lower.17, 18
LRP is a member of the class of membrane-transporter proteins that do not belong to the adenosine triphosphate-binding cassette superfamily of transporter proteins, such as P-glycoprotein and the multidrug resistance-associated proteins.19 LRP expression correlates inversely with response to chemotherapy in patients with NSCLC.20 Polyamines influence many cell processes associated with cell growth, and one of the features that characterize rapidly growing cells is their intracellular accumulation of polyamine, a process in which ODC plays a key role in the biosynthesis of the polyamine precursor putrescine.21
The objective of the current investigation was to develop an orthotopic model of human NSCLC cells grafted into the lungs of nude mice that would mimic the clinical development of NSCLC. Therefore, we focused our attention on a model with a weak to practically nonexistent response to taxol, oxaliplatin, and irinotecan; and we determined quantitatively the immunohistochemical levels of expression of COX-2, mPGES-1, ODC, LRP, GST-α, GST-μ, and GST-π in control and drug-treated NSCLC orthotopic xenografts. We chose the A549 NSCLC cell line, because it is known that this cell line strongly express the LRP protein,22 and because both PGES and COX-2 are coupled functionally in A549 cells.23
MATERIALS AND METHODS
Cell Lines and Culture Media
The A549 cells were obtained from the American Type Culture Collection (Manassas, VA) and were cultured at 37 °C in sealed (airtight) Falcon plastic dishes (Gibco, Nunc, Belgium) containing Eagle minimal essential medium (Gibco) supplemented with 10% fetal calf serum (FCS). All media were supplemented with a mixture of glutamine (0.6 mg/mL final concentration; Gibco), penicillin (200 IU/mL final concentration; Gibco), streptomycin (200 IU/mL final concentration; Gibco) and 0.1 mg/mL gentamycin (Gibco). The FCS was heat-inactivated for 1 hour at 56 °C.
In Vivo Orthotopic Grafting of Human A549 NSCLC Cells
In vivo nude mice xenografts were obtained by grafting 2.106 A549 cells through the thorax into the left part of the lungs of 8-week-old female nu/nu mice (weight, 21–23 g; BioServices, The Netherlands). The grafts were performed under anesthesia (saline; Rompun®; Bayer, Leverkusen, Germany; Imalgene®; Merial, Lyon, France; 5 volume/1 volume/1 volume). Highly reproducible tumor developments (100%) were obtained in each experiment.
Four drugs, i.e. taxol (Paclitaxel; S.A. Bristol-Myers Squibb, Brussels, Belgium), oxaliplatin (Oxaliplatin; Inter-Chemical Ltd, ShenZhen, China), irinotecan (Campto; 100 mg/5 mL; Aventis, Brussels, Belgium), and NS-398 (no. 70590; Cayman Chemical, Ann Arbor, MI) were assayed on the A549 NSCLC orthotopic model. The maximum tolerated dose (MTD) index was determined for each drug, except NS-398, which was assayed at 36 mg/kg according to previous data published in the literature in which significant NS-398-mediated effects were observed in vivo.11 The MTD index was determined by administering the drug intraperitoneally to groups of 3 healthy nude mice 3 times per week (on Mondays, Wednesdays, and Fridays) for over 3 consecutive weeks.
Taxol was assayed on the A549 NSCLC orthotopic models at MTD/2, MTD/3, MTD/4, MTD/8, MTD/16, and MTD/32; oxaliplatin was assayed at MTD/2 and MTD/4; and irinotecan was assayed at MTD/2. The endpoint of each experiment was the recording of the survival period for all A549 NSCLC–bearing nude mice. Each animal was killed (with CO2 atmosphere) when it had lost 20% of its weight compared with its weight at the time of the tumor graft. Autopsy was then performed. The lungs, livers, and brains were removed, fixed in buffered formalin, and embedded in paraffin for immunohistochemical analysis. All in vivo experiments described in the current study were performed with the authorization of the Animal Ethics Committee of the Faculty of Medicine at the Université Libre de Bruxelles (agreement no. 55/LA 1230342).
Immunocytochemical Procedures and Markers
|COX-2||Cat. no. 160112||M||1/50||Human||Amino acids 580–599||Cayman Chemical (Ann Arbor, MI)|
|mPGES-1||Cat. no. 160140||P||1/50||Human||Amino acids 59–75||Cayman Chemical|
|GST-α||NCL-GST-α||P||1/100||Human||Novocastra Laboratories (Newcastle, United Kingdom)|
|LRP||MAB 4126||M||1/20||Human||Internal epitope of the LRP/major vault protein (P110)||Chemicon International Inc. (Temecula, CA)|
Quantitative Immunohistochemistry with Computer-Assisted Microscopic Analysis
Two variables were computed for each immunostaining submitted to the computer-assisted microscope (SAMBA 2005 system; SAMBA Technologies, Grenoble, France), which was equipped with a × 20 (aperture 0.40) magnification lens. The labeling index (LI) refers to the percentage of cells specifically stained by the antibody. The mean optical density (MOD) denotes the staining intensity (its concentration). The way in which we used the computer-assisted system to quantify the immunostaining is detailed elsewhere.24, 25
Statistical comparisons between the control mice and the treated mice were made first by carrying out the Kruskal–Wallis test (a nonparametric one-way analysis of variance). In cases where this test revealed significant differences, we investigated whether any of the groups treated differed from the control group. For this purpose, we applied the Dunn multiple comparison procedure (a 2-sided test) adapted to the special case of effective comparisons between treatments and controls, i.e., where only (k − 1) comparisons were made in the k groups tested by the Kruskal–Wallis test (in place of the possible k[k − 1]/2 comparisons considered under the general procedure).
Survival analysis was performed by using Kaplan–Meier curves and the Gehan generalized Wilcoxon test. All statistical analyses were carried out using Statistica software (Statsoft, Tulsa, OK).
Histopathologic Analyses of the A549 NSCLC Orthotopic Model
The intrapulmonary engraftment of human A549 tumor cells into the nude mice induced a large primary tumor at the injection site in the left lung and a subsequent metastatic spread to the ipsilateral and contralateral lung (Fig. 1A,B). Histopathologic features showed poorly differentiated tumors appearing as solid groups of relatively small, uniform cells (Fig. 1A,D,E,G). The nuclei appeared ovoid and moderately pleomorphic (Fig. D,E,G). Mitotic figures were frequent and atypical. Areas of necrosis could be identified. The tumors invaded the visceral pleura and the mediastinal lymph nodes with lymphatic and vascular embolisms. Brain metastases with histopathologic characteristics similar to those of the primary tumors (Fig. 1C) were present in about two-thirds of the grafted mice (Table 2). Micrometastases and macrometastases were present in the livers of ≈ 40% of the mice (Table 2), and 80% of them exhibited either brain or liver metastases (Table 2). Periodic acid–Schiff (PAS)/PAS-digested staining revealed the presence of mucosecretions, a feature that could indicate that the A549 tumor xenograft model (Fig. 1D) is of adenocarcinomatous origin. Cytokeratin 7 (CK7), CK20, and the thyroid transcription factor-1 (TTF-1) often are used in surgical pathology to analyze adenocarcinomas from different histologic sites. Figure 1E indicates that CK7 is locally and slightly positive in A549 orthotopic xenografts. Figure 1F illustrates the pattern of CK7 immunohistochemical expression in a human lung adenocarcinoma. The CK20 immunohistochemical expression was negative in the A549 orthotopic xenograft model (data not shown). TTF-1 shows a cytoplasmic expression in the A549 xenografts (Fig. 1G), in contrast to the nuclear location in human lung adenocarcinomas (Fig. 1H).
|Mouse no.||Primary tumor (lung)||Metastases||Mouse no.||Primary tumor (lung)||Metastases|
Characterization of the A549 NSCLC Orthotopic Model Sensitivity to Taxol, Oxaliplatin, and Irinotecan
The data in Figure 2A show that the protocol involving 9 administrations of 10 mg/kg taxol was toxic, whereas the Kaplan-Meier analyses revealed that the remaining 3 protocols did not induce any significant increases in the survival periods. Figure 2B concerns the A549 NSCLC–bearing mice treated with 100 mg/kg irinotecan once per week for 3 consecutive weeks. Because of the toxic side effects, as revealed by body weight measurements (data not shown), we injected irinotecan at MTD/2 only once per week. The data show that irinotecan induced a weak but significant increase in the survival of the A549 NSCLC–bearing mice.
Because neither taxol nor oxaliplatin induced any significant increase in the survival of the A549 NSCLC–bearing nude mice when treatment began 21 days postgraft, we repeated the same set of experiments that are illustrated in Figure 2A, but we started the treatments on Day 14 instead of Day 21 after the tumor grafts. The purpose of this initiative was to treat the nude mice that potentially were less sick in terms of the A549 NSCLC cell dissemination. With respect to taxol, we also reduced the highest dose, i.e., 10 mg/kg (MTD/2) (Fig. 2A, solid circles) to 7.5 mg/kg (MTD/3) (Fig. 2C, solid circles). The data illustrated in Figure 2C show that neither 7.5 mg/kg taxol nor 5.0 mg/kg taxol (Fig 2C, open circles) significantly increased the survival of these A549 NSCLC–bearing nude mice. We also performed the same experiments that are illustrated in Figure 2A with respect to oxaliplatin, but we started the treatments on Day 14 rather than Day 21 after the tumor grafts. Again, we obtained no statistically significant taxol-induced or oxaliplatin-induced increase in the survival of the A549 NSCLC–bearing nude mice (data not shown).
Starting on Day 14 postgraft, we assayed the taxol daily at 2.5 mg/kg (open squares), 1.25 ng/kg (open circles), and 0.63 mg/kg (open triangles) mg/kg 5 times per week (from Monday through Friday) for 3 consecutive weeks, as illustrated in Figure 2D. These taxol-related protocols did not show any statistically significant increase in the survival of the A549 NSCLC–bearing nude mice.
Quantitative Determination of the Immunohistochemical Levels of Expression of Chemoresistance Markers in the A549 NSCLC Orthotopic Model
Figure 3A shows that > 60% of A549 NSCLC cells (Fig. 3A, hatched bars) expressed significant amounts (Fit. 3A, black bars) of COX-2 (an immunohistochemical pattern of expression of which is illustrated in Fig. 3B) in the 5 experimental groups illustrated in Figure 2A. The data in Figure 3A indicate that neither taxol nor oxaliplatin significantly modified the immunohistochemical levels of expression of COX-2 in the A549 NSCLC orthotopic xenografts; this may be due to the fact that the basic levels of COX-2 expression already were very high. A very similar pattern of immunohistochemical expression was observed with respect to ODC (Fig. 3C,D). Almost all A549 NSCLC cells expressed PGES, and the 10-mg/kg taxol treatment significantly increased the cell concentration of PGES (Fig. 3E,F). Both 10 mg/kg taxol and 5 mg/kg oxaliplatin significantly increased the percentage of LRP-immunopositive A549 NSCLC cells (Fig. 3G,H).
Although taxol did not modify the levels of expression of GST-α significantly (Fig. 4A,B), it did significantly increase both the LI and the cellular concentrations of GST-μ (Fig. 4C,D) and GST-π (Fig. 4E,F) at 10 mg/kg. Oxaliplatin significantly increased the percentage of immunohistochemically positive GST-μ (at 10 mg/kg) (Fig. 4C) and GST-π (at 5 mg/kg and 10 mg/kg) (Fig. 4F) A549 cells; whereas, like taxol, it remained without any apparent (P > 0.05) effect on the immunohistochemical levels of GST-α expression (Fig. 4A).
Characterization of COX-2, ODC, LRP, GST-α, GST-μ, and GST-π Immunohistochemical Expression in Individual A549 NSCLC Orthotopic Xenografts
Figure 5 illustrates the variations in the immunohistochemical levels of COX-2, ODC, LRP (Fig. 5A,B), GST-α, GST-μ, and GST-π (Fig. 5C,D) in 5 of the 10 A549 NSCLC xenografts in the control group from the experiment detailed in Figure 2A. These 5 NSCLC xenografts were obtained from Mouse 3 (M3), M4, M5, M7, and M10 in this group. For the sake of clarity, we have illustrated the heterogeneity of the protein expression levels for only 5 of the 10 xenografts.
Figure 5A shows that, although the percentages of A549 cells that were immunohistochemically positive for ODC (Fig. 5A, hatched bars) were relatively similar in the 5 A549 xenografts illustrated, the percentages of A549 cells that were immunohistochemically positive for COX-2 (Fig. 5A, dotted bars) and for LRP (Fig. 5A, black bars) differed markedly from one xenograft to another. The cell concentrations of COX-2, ODC, and LRP also varied markedly from one xenograft to another (Fig. 5C). Similar observations were made with respect to PGES (data not shown). Both the percentages (Fig. 5B) and the cellular concentrations (Fig. 5D) of GST-α (Fig. 5B,D, dotted bars), GST-μ (Fig. 5B,D, hatched bars), and GST-π (Fig. 5B,D, black bars) varied markedly between the xenografts.
Characterization of the Response of the A549 NSCLC Orthotopic Model to a Combined Treatment that Included NS-398 and Taxol
Whereas Figures 3 and 5 show that COX-2 was expressed markedly in the A549 NSCLC orthotopic xenografts, Figure 2 reveals that this model did not respond to taxol. Therefore, we tried to reduce COX-2 expression by means of NS-398 in an attempt to render the A549 model sensitive to taxol. Figure 6A shows that the combined use of NS-398 and taxol did not induce any significant increase in the survival of the A549 NSCLC–bearing nude mice. In fact, Figure 6B shows that taxol significantly increased the levels of expression of COX-2 in terms of COX-2-immunopositive A549 cells. It should be emphasized that the mean basal levels of COX-2-immunopositive A549 cells in the control group illustrated in Figure 6B were lower than the levels that were observed in the previous set of experiments detailed in Figure 3. However, the data illustrated in Figure 5 indicate the dramatic heterogeneity in COX-2 expression observed from one A549 NSCLC orthotopic xenograft to another. Figure 6B shows that NS-398 significantly antagonized the taxol-induced increase in the percentages of COX-2-immunopositive cells. However, this COX-2-antagonistic effect displayed by NS-398 against taxol was not sufficient to increase the survival of the A549 NSCLC orthotopic xenograft–bearing nude mice by taxol treatment (Fig. 6A).
Although lung carcinoma is the fourth most common cancer type—after breast, prostate, and colorectal carcinomas—in Western Europe, the United States, and Japan, it is the leading cause of cancer mortality. Paclitaxel, vinorelbine, irinotecan, and oxaliplatin are the most promising drugs for the treatment of patients with NSCLC.5–8 Nevertheless, the fact remains that > 50% of patients with NSCLC are diagnosed at an advanced stage of the pathology, and only 1% of those patients remain alive 5 years after diagnosis.6
Numerous experimental models have been developed to understand better the pathophysiology of human NSCLCs or to test new investigational therapies (for a review, see Hoffman26). Experimental NSCLCs can be developed from either rodent27 or human tumors and, in the latter case, either from fresh materials obtained by surgery28–30 or from permanent in vitro cell lines.31–33 When they are developed from human permanent carcinoma cell lines, higher metastatic potential is obtained, for example, with A549 cells when histologically intact human A549 adenocarcinoma lung tumors are transplanted into the left lung of nude mice rather than with direct implantation of A549 cell suspensions.34
When human NSCLC cell suspensions are implanted directly into the left pleural cavity of nude mice,31–33 tumors grow extensively in the pleural cavity and infiltrate the lung parenchyma directly; furthermore, tumors metastasize to the mediastinum and the contralateral pleural cavity through lymphatic routes.31–33 All of these features were observed in the current study. Hastings et al.35 reported that, in orthotopic models of human lung carcinoma, bone and lymph node metastases were observed with great frequency after periods of a few weeks, but metastases to other organs were rare. Here, we observed that ≈ 60% of the A549-grafted nude mice developed brain metastases, and ≈ 40% developed liver metastases. The A549 orthotopic xenograft model has been used extensively and successfully in various experiments, including not only chemotherapy26, 32 but also more sophisticated approaches, such as gene therapy36, 37 or the use of thymidine-phosphorylase inhibitors.38 Among the most widely used, permanent human NSCLC cell lines orthotopically xenografted into nude mice are the H460 and A549 models,26 with the H460 model appearing to be much more chemosensitive than the A549 model.32 Part of the biologic aggressiveness of the A549 NSCLC orthotopic model can be explained by high expression levels of matrix metalloproteinase 2 (MMP-2), MMP-9, and their activators, i.e. membrane-type 1-MMP and urokinase-type plasminogen activator, all of which are believed to be associated with the growth, invasion, and metastasis of the A549 cells.39
In the current study, the A549 orthotopic NSCLC model appeared not only biologically aggressive but also chemoresistant. The data from the current study show that it did not respond to paclitaxel or to oxaliplatin and responded only weakly to irinotecan. The absence of any clear sensitivity on the part of the A549 NSCLC orthotopic model to these xenobiotics may relate, at least in part, to the presence of a large set of proteins involved in chemoresistance mechanisms. Chemoresistance remains a major problem in the chemotherapy of NSCLCs, and it is believed that several mechanisms are involved in drug resistance, including those associated with apoptosis, drug transport, and detoxification.21 Among these proteins with biologic functions that can be associated, at least in part, to chemoresistance, we observed that COX-2 and mPGES were expressed highly in A549 orthotopic xenografts and that paclitaxel significantly increased the levels of expressions of these proteins. Cells that overexpress COX-2 tend to be resistant to apoptosis, and COX-2 inhibitors were capable of inducing apoptosis in these types of cells (for a review, see Pyo et al.11). We did not observe any additive or synergistic effects involving paclitaxel and NS-398 (a COX-2 inhibitor) on the survival of the A549 NSCLC–bearing nude mice. High levels of both COX-2 and PGE2 are found in NSCLCs, and this results in local immune suppression, a condition that favors tumor growth.40
A number of studies have demonstrated a direct relation between GST-π expression and response to chemotherapy in NSCLC.16–18, 41 In contrast, no such correlation has been evidenced clearly, at least to our knowledge, with respect to GST-μ. The data from the current study show that paclitaxel and oxaliplatin significantly increased the percentages of immunohistochemically positive GST-π and GST-μ cells, but not GST-α cells, in the A549 NSCLC orthotopic xenografts. Paclitaxel also increased both the GST-μ and GST-π cellular concentrations (see Fig. 4). In addition, paclitaxel and oxaliplatin significantly increased the percentage of LRP immunohistochemically positive A549 cells (see Fig. 3).
High ODC activity—the first rate-limiting step in polyamine biosynthesis—and increased levels of intracellular polyamines are known to occur in rapidly proliferating cells or in cells undergoing differentiation and transformation.42 The transport of polyamine analogues into cells disrupts the normal function of the natural polyamines by decreasing ODC activity and increasing the activity of the catabolic enzyme SSAT and polyamine oxidase.42, 43 It already has been shown that the growth status of NSCLC cell populations can affect significantly their response to antitumor polyamine analogues.43
The current results show that ODC is expressed highly in A549 NSCLC orthotopic xenografts. When analyzing the levels of expression of various markers relating to chemoresistance, we observed highly variable expression of these markers from one A549 xenograft to another. We tried to clone A549 cell subpopulations according to a methodology detailed elsewhere,44 but the distinct clones that we obtained continued to express the same dramatic biologic heterogeneity, at least with respect to the levels of immunohistochemical expression of the chemoresistance markers under study (data not shown).
The results of the current study show that the A549 NSCLC orthotopic model in nude mice is resistant to paclitaxel and oxaliplatin is and weakly sensitive to irinotecan. The high levels of chemoresistance observed in the A549 NSCLC orthotopic xenografts can be explained at least in part by the high levels of expression of COX-2, PGES, LRP, and GSTs. This NSCLC orthotopic model will now be used to screen the potential synergic combinations of emergent cytotoxic and biologic therapies that are still in the pharmaceutical industry pipelines.
- 4Histological typing of lung and pleural tumours. World Health Organization International histological classification of tumours, 3rd ed. New York: Springer Verlag, 1999., , , , .
- 24Evaluation of the efficiency of chemotherapy in in vivo orthotopic models of human glioma cells with and without 1p19q deletions and in C6 rat orthotopic allografts serving for the evaluation of surgery combined with chemotherapy. Cancer. 2002; 95: 641–655., , , et al.
- 26Metastatic orthotopic mouse models of lung cancer. Methods Mol Med. 2003; 74: 457–464..