Annals of Surgical Oncology 9:310-316 (2002)
© 2002 Society of Surgical Oncology
Phosphatase and Tensin Analog Gene Overexpression Engenders Cellular Death in Human Malignant Mesothelioma Cells via Inhibition of AKT Phosphorylation
Imran Mohiuddin, MD,
Xiaobo Cao, MD,
Mustafa K. Ozvaran, MD,
Lou Zumstein, PhD,
Sunil Chada, PhD and
W. Roy Smythe, MD
From the Section of Thoracic Molecular Oncology, Department of Thoracic and Cardiovascular Surgery, The University of Texas M. D. Anderson Cancer Center, Houston, Texas.
Correspondence: Address correspondence and reprint requests to: W. Roy Smythe, MD, Department of Thoracic and Cardiovascular Surgery, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Blvd., Box 109, Houston, TX 77030; Fax: 713-794-4669; E-mail: rsmythe{at}mdanderson.org
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ABSTRACT
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Background: Abnormal phosphatase and tensin analog (PTEN) gene expression has been noted in neoplasms. The PTEN protein cleaves phosphate groups from cellular growth kinases, inhibiting tumor propagation. A downstream target of PTEN is AKT, a serine-threonine kinase that when activated inhibits apoptosis. We sought to determine whether PTEN overexpression in mesothelioma cells would engender hypophosphorylation of AKT and apoptosis.
Methods: Human malignant mesothelioma cell lines REN and I-45 were transfected with adenoviral vectors AdPTEN and AdBgal (marker gene) at various multiplicities of infection (MOI). Cell viability was measured using a colorimetric assay, and apoptosis was assessed by morphology and subG1 fluorescence-activated cell sorter (FACS) analysis. PTEN protein and AKT phophorylation were evaluated by Western blot, and AKT kinase activity was evaluated by functional assay.
Results: Increased cellular killing was noted with AdPTEN gene transfer. The ratio of cell killing with AdPTEN to AdBgal widened with increasing MOI and was statistically significant at all MOI. Cells demonstrated apoptosis by morphologic and subG1 FACS analyses. Cells overexpressing PTEN demonstrated decreased phosphorylated (active) AKT and AKT kinase activity compared with controls.
Conclusions: Overexpression of PTEN engenders apoptosis in mesothelioma by AKT hypophosphorylation. The forced overexpression of PTEN may prove useful clinically in this treatment-resistant neoplasm.
Key Words: Mesothelioma • PTEN • AKT • Apoptosis
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INTRODUCTION
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Malignant mesothelioma (MPM) is a relatively uncommon neoplasm that arises from a variety of mesothelial lined and covered tissues, with the pleura most often affected.1 A definitive epidemiological link to asbestos has been established, and recent studies have implicated exposure to the SV40 tumor virus as well.1 Although uncommon, this tumor continues to be a frustrating clinical problem. Significant controversy currently exists regarding the conventional treatment of MPM, but little controversy exists regarding outcome. Although highly selected patients may derive benefit from aggressive therapy, the response to conventional treatment of all comers with MPM has been quite poor.2 Response rates to chemotherapy are generally3 <20%. This unresponsiveness to conventional therapy has led to an interest in more novel paradigms, such as photodynamic therapy, immunotherapy, and gene therapy approaches.4–7 The need to find effective treatment is given added urgency by the recent evidence that shows an increasing incidence of MPM in many Western countries. In Europe, the number of mesothelioma deaths in men is expected to double over the next 20 years, with a peak around the year 2020.8
Gene therapy for malignant pleural mesothelioma remains in its infancy. The main approach to date has been a prodrug approach, or gene therapeutics, whereby a protein, usually an enzyme, is delivered into the cell via a viral vector. This enzyme is then able to act on a subsequently delivered relatively inactive prodrug and metabolizes it to an active agent. Smythe et al.4,9 have described one such treatment with adenoviral transfer of the herpes simplex virus/thymidine kinase gene combined with exposure to ganciclovir.
The phosphatase and tensin analog (PTEN)/MMAC1/TEP1 tumor suppressor gene has recently been described in the literature to encode a protein that has a key role in both cell growth and integrin function. This gene has been localized to chromosome 10q23.10 PTEN dysfunction was initially linked to a group of inherited germline disorders such as Cowden’s syndrome.11 Since then, deleted or mutated PTEN genes have been described in various tumors, including gliomas; endometrial cancers; breast, thyroid, bladder, ovary, and small-cell lung cancer; and hematological malignancies.12 The N-terminal domain of PTEN shows extensive homology to tensin and auxillin, key cytoskeletal proteins. In another exon of the gene resides the signature motif of the dual specificity protein phosphatase, which dephosphorylates serine, threonine, and tyrosine residues. Mutations in PTEN are found to be almost exclusively in this phosphatase domain.13 One key protein that is dephosphorylated by wild-type PTEN is the serine-threonine kinase AKT. Phosphorylated AKT, the active form of this kinase, promotes cell survival through multiple pathways, many of which are associated with cell growth and survival. Levels of phosphorylated AKT have been reported to be increased in multiple tumors.14
This article describes work evaluating the effects of forced overexpression of the PTEN gene product in MPM cells. We hypothesized that this would engender apoptosis in these treatment-resistant cells via changes in the phosphorylation status of AKT. The results presented here indicate that such forced overexpression does indeed engender apoptotic cellular death, inhibition of AKT kinase, and subsequent inhibition of AKT phosphorylation. Although we noted these findings in both cell lines evaluated, they were attenuated somewhat in the p53 wild-type MPM cell line, I-45.
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MATERIALS AND METHODS
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Cell Lines and Adenoviral Vectors
To study the effect of PTEN transfection in mesothelioma, two well-characterized cell lines were used: I-45 and REN. I-45 is a human sarcomatous–type mesothelioma, p53 wild type (donated generously by Dr. Joseph Testa of the Fox Chase Institute). REN is a human epithelial–type mesothelioma and p53 mutant (developed by author W.R.S.). Cell lines were maintained in RPMI media containing 10% (v/v) fetal bovine serum, 1% nonessential amino acids, 100 U/ml penicillin, and 100 µg/ml of streptomycin. Cells were housed in a humidified incubator with 5% CO2 at 37°C. AdPTEN and Ad–ß-galactosidase (Bgal) are first-generation adenovirus type 5 vector constructs with E1 and E3 deletions. These vectors were created by use of homologous recombination in U937 cells. AdPTEN was provided by Introgen Therapeutics, Inc. (Houston, TX). AdBgal was synthesized in our laboratory. A standard cytomegalovirus promoter was used in both constructs.
Adenoviral Transfection
Cells were exposed to adenoviral vectors in 96-well and 6-cm plates. Cells were initially collected, counted manually, and then incubated overnight. Cells were then washed with phosphate-buffered saline (PBS) and transfected with varying multiplicities of infection (MOIs) of adenovirus diluted in serum-free RPMI. After 3 hours of exposure to vector, RPMI media supplemented with 20% fetal bovine serum was added in equal volumes to bring the final concentration to 10% serum.
Cell Viability
Cell viability studies were performed with the XTT assay kit (Roche Diagnostics Corporation, Indianapolis, IN). Cells were plated on 96-well plates at a concentration of 3000 cells per well. After overnight incubation, cells were transfected with either AdPTEN or AdBgal marker virus for 3 hours at a wide range of MOIs from 0 to 120. At each time point, 50 µl of XTT reagent was added to all wells and allowed to incubate with the cells for 4 hours. After this incubation period, all plates were read with a colorimetric plate reader system (Dynatech, Inc., Langley, VA). Analysis of data was performed with Microsoft Excel (Microsoft Corp., Redmond, WA). These experiments were performed in triplicate.
Evaluation of Apoptosis
Apoptotic cell death was examined by cell morphology and cell cycle (sub-G1) flow cytometry. Fluorescence-activated cell sorter analysis was performed as follows: cells were plated in 6-cm plates at a concentration of 1 x 106 cells per plate, incubated for 24 hours, and transfected with AdPTEN at an MOI of 60 for 3 hours. Seventy-two hours after transfection, cells were trypsinized, collected by centrifugation, resuspended in PBS, and fixed in 70% ethanol at -20°C overnight. After fixation, the cells were washed in PBS, followed by the addition of propridium iodide and RNase (Boehringer Mannheim Co., Indianapolis, IN). Controls with AdBgal were prepared in a similar fashion. Cells were then heated in a 37°C water bath for 10 minutes and then kept in the dark. The specimens were then analyzed with the use of an EPICS Profile II flow cytometer (Beckman Coulter, Inc., Miami, FL). An analysis region was set on the basis of the negative controls, and the percentage of labeled cells was calculated from this region. Attention was concentrated on the sub-G1 population of cells. The area under the curve at this point was used to determine the amount of apoptosis.
PTEN Expression
Western blot analysis was performed to confirm overexpression of PTEN after adenoviral transfer. Cells were plated in 6-cm plates at a concentration of 1 x 106 cells per plate, incubated overnight, and then transfected with AdPTEN at an MOI of 60. Forty-eight hours after transfection, total cell lysates were prepared by lysing plated cell monolayers with a sodium dodecyl sulfate–polyacrylamide gel electrophoresis sample buffer. The protein content of the lysates was then determined by bicinchoninic acid (BCA) protein assay (Pierce, Rockford, IL). Then, each lane on an sodium dodecyl sulfate–polyacrylamide gradient (8%–12%) gel was loaded with 5 µg of cell lysate and electrophoresed to separate proteins by using a monoclonal antibody directed against PTEN (Chemicon, Temecula, CA). After electrophoresis at 100 V for 2 hours, the proteins were transferred to high-bond electrogenerated chemiluminescence membranes. The membranes were then incubated with the primary and secondary antibodies and developed according to the Amersham (Uppsala, Sweden) electrogenerated chemiluminescence protocol. These experiments were repeated in triplicate.
AKT Phosphorylation Assay
To confirm expression of AKT and discern the amount of phosphorylated AKT, an AKT antibody assay was performed with an assay kit (New England Biolabs, Beverly, MA). Briefly, cells were plated at concentration of 1 x 106 cells per plate in a 6-cm plate, incubated for 24 hours, transfected with either AdPTEN or AdBgal at an MOI of 60, and collected at 48 hours; then, by use of antibodies directed against AKT and phosphorylated AKT provided in the kit, they were electrophoresed as previously described. A commercially available inhibitor to AKT kinase from Cell Signaling Technology (Beverly, MA) was used as a positive control at a dose of 50 µM. By using Optimas software (Media Cybernetics, Inc., Silver Spring, MD), the intensity of bands from each cell line was compared. These experiments were repeated in triplicate.
AKT Kinase Activity
AKT kinase activity was detected by using an enzyme assay kit (New England Biolabs). Briefly, cells were plated in concentrations of 1 x 106 cells per plate in 6-cm plates, incubated for 24 hours, and transfected with either AdPTEN or AdBgal at an MOI of 60. Forty-eight hours after transfection, cells were collected and lysed. An antibody to AKT was used to selectively immunoprecipitate AKT from cell lysates. The resulting immunoprecipitate was then incubated with a glycogen synthetase kinase (GSK)-3 fusion protein (substrate) in the presence of adenosine triphosphate and kinase buffer; this allows immunoprecipitated AKT to phosphorylate GSK-3. Phosphorylation of GSK-3 was measured by Western blotting by using a phospho-GSK-3a/b (Ser21/9) antibody. The intensity of the band is representative of the amount of AKT kinase activity. These experiments were repeated in triplicate.
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RESULTS
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AdPTEN Transfection Results in Overexpression of PTEN Protein
To confirm that exposure to our adenoviral construct was effectively leading to PTEN overexpression at the posttranslational level, we evaluated both cell lines for PTEN protein expression after transfection by use of a monoclonal PTEN antibody and Western analysis. A small amount of baseline PTEN expression was noted; however, as seen in Fig. 1, there was a dense PTEN band in both cell lines that corresponded with PTEN overexpression when compared with marker virus transfection after exposure to AdPTEN.
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FIG. 1. Cells from both cell lines were transfected with either Ad–phosphatase and tensin analog (PTEN) or Ad–ß-galactosidase (first-generation adenovirus type 5 vector constructs with E1 and E3 deletions). By using monoclonal antibody directed against PTEN, cell lysates were analyzed by Western blot for PTEN expression at 48 hours after transfection. Overexpression of PTEN was seen with both cell lines after AdPTEN transfection.
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Transfection of PTEN Engenders Apoptotic Cellular Death in Mesothelioma Cell Lines
To asses the ability of overexpression of the PTEN gene to engender cell death in our mesothelioma cell lines, we transfected both with variable MOIs of AdPTEN and compared it with marker virus transfection. We observed that as early as 48 hours after transfection, there was a significant amount of cellular death in both cell lines. REN seemed to be much more sensitive to PTEN-induced cell death than I-45 (P = .024). At an MOI of 20, there was a 32.9% ± 5.5% and 16.22% ± 3.5% increase in cell killing when compared with marker virus for REN and I-45, respectively, at 72 hours. Both cell lines showed a statistically significant increase in cell killing with PTEN when compared with AdBgal (P = .011 and P = .019, respectively). After studying a time course of cellular death over 5 days at an MOI of 60, both cell lines seemed to reach peak cellular death at 72 hours (Fig. 2A and 2B).
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FIG. 2. (A) Cells from both cell lines were transfected with either Ad–phosphatase and tensin analog (PTEN) or Ad–ß-galactosidase (Bgal) (first-generation adenovirus type 5 vector constructs with E1 and E3 deletions). Cell viability was measured at 72 hours by XTT assay. Cell killing was calculated from controls and was normalized by subtracting baseline AdBgal cell killing from AdPTEN cell killing for each cell line. The increase in the amount of cell killing with AdPTEN from AdBgal was plotted against multiplicity of infection (MOI) of AdPTEN for each cell line. AdPTEN-induced cell killing was seen in both cell lines. However, cell killing in REN was greater than that seen in I-45. (B) Cells from both cell lines were transfected with either AdPTEN or AdBgal. Cell viability was measured at 24, 48, 72, 96, and 120 hours by XTT assay. Cell killing was calculated from controls and was normalized by subtracting baseline AdBgal cell killing from AdPTEN cell killing for each cell line. The increase in the amount of cell killing with AdPTEN from AdBgal was plotted against hours after inspection. Cell killing increased until 72 hours and then did not increase afterward. For all time points, there was a greater amount of AdPTEN cell killing in REN than in I-45 (P = .024).
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After confirming that PTEN did induce cellular death, we next investigated the amount of apoptosis. Morphological changes (nuclear condensation and fragmentation) associated with apoptosis were noted in both lines after transfection with AdPTEN, but not AdBgal. We quantified the amount of apoptosis by measuring the area under the curve denoting the sub-G1 population for propridium iodide fluorescence-activated cell sorter analysis. Our results confirmed that indeed there was a significant amount of PTEN-induced apoptosis when compared with marker virus. A higher degree of apoptosis was noted in the REN cell line when compared with I-45 (P = .003). At an MOI of 60, the apoptotic fraction by sub-G1 quantification in REN was 25.4% ± 1.04%, compared with 14.1% ± 1.55% for I-45. When compared with marker virus AdBgal, there was a 4.6-fold and 3.2-fold increase, respectively, for REN and I-45. Levels of apoptosis in both cell lines were statistically significant with respect to marker virus (P = .002 and P = .038 for REN and I-45, respectively; Fig. 3).
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FIG. 3. Cells from both cell lines were transfected with either Ad–phosphatase and tensin analog (PTEN) or Ad–ß-galactosidase (Bgal) (first-generation adenovirus type 5 vector constructs with E1 and E3 deletions). Apoptosis was measured at 72 hours by propridium iodide flow cytometry analysis. The percentage of cells in the sub-G1 population was equated to apoptosis. Percentage apoptosis was plotted against each cell line for AdBgal and AdPTEN. There was a greater amount of apoptosis in the REN cell line (P = .003).
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The Level of Unphosphorylated AKT Is Increased After AdPTEN Exposure, and This Is Related to a Decrease in AKT Kinase Activity
Previous studies have revealed that one of the downstream effectors of PTEN is the phosphate kinase AKT. Wild-type PTEN normally dephosphorylates AKT. We decided to investigate whether the level of AKT phosphorylation was altered in our cell lines after exposure to AdPTEN and overexpression of PTEN protein. After transfection with AdPTEN, both cell lines displayed a decrease in the amount of phosphorylated AKT when compared with marker virus transfection. This decrease was less prominent in the I-45 cell line (Fig. 4).
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FIG. 4. Cells from both cell lines were transfected with either Ad–phosphatase and tensin analog (PTEN) or Ad–ß-galactosidase (Bgal) (first-generation adenovirus type 5 vector constructs with E1 and E3 deletions). A separate group of cells was treated with kinase inhibitor and used as a positive control. By using polyclonal antibody directed against phosphorylated and unphosphorylated AKT, Western blots were performed at 48 hours after transfection. The ratio of intensity between phosphorylated and unphosphorylated AKT bands was calculated for each experimental group. There was a significant decrease in the amount of phosphorylated to unphosphorylated AKT with PTEN transfection or inhibitor, but not with Bgal transfection. REN was more sensitive to AdPTEN- or inhibitor-induced dephosphorylation than I-45 (P, AdPTEN; B, AdBgal; I, kinase inhibitor).
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After demonstrating PTEN overexpression–driven hypophosphorylation of AKT, we assessed the amount of activity of the actual AKT phosphorylating enzyme by using a substrate-driven immunoprecipitation assay. As noted in Fig. 5, after AdPTEN transfection, there was a decrease in the amount of AKT kinase activity when compared with marker vector (Fig. 5). However, as in the previous assay, the decrease in activity of the AKT kinase after PTEN transfection was greater in the REN cell line than in the I-45 line.
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FIG. 5. AKT kinase activity. Cells from both cell lines were transfected with either Ad–phosphatase and tensin analog (PTEN) or Ad–ß-galactosidase (Bgal) (first-generation adenovirus type 5 vector constructs with E1 and E3 deletions). By using a nonradioactive substrate-driven immunoprecipitation assay, AKT kinase activity was measured at 48 hours after transfection in both experimental groups. AKT kinase activity was decreased after AdPTEN transfection in both groups. The decrease in REN was much more significant than in I-45.
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DISCUSSION
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Although uncommon, MPM continues to be a frustrating clinical problem with no definitively effective conventional therapy. In the past decade, however, advances have been made on several fronts that indicate that novel forms of therapy may eventually prove promising. Both experimental and preliminary clinical trial data suggest that gene therapy approaches for MPM have emerged as a class of potentially effective contemporary treatments, and this paradigm certainly deserves additional investigation.15
The overexpression of PTEN in the MPM cell lines examined here led to the development of apoptotic cellular death. The forced induction of programmed cellular death has not been carefully studied in mesothelioma, but by clinical behavior, it seems somewhat resistant. One group evaluated a number of conventional inducers of apoptosis in mesothelioma cells, including calcium ionophore and hydrogen peroxide, but found the cells to be resistant.16 Several unconventional treatments have been shown to lead to apoptosis in mesothelioma cells, such as talc, asbestos, amphotericin, and the anticholesterol drug lovastatin17–20; however, no discrete mechanism has been revealed in any of these studies.
Since its discovery, several lines of evidence have confirmed the PTEN gene’s role as a tumor suppressor both in vitro and in vivo in various types of malignancy. The PTEN gene (located at 10q23) was discovered and characterized in 1997 by Li et al.21 This group had determined that a loss of heterozygosity (LOH) at 10q23 was common and that the wild-type 10q23 locus was capable of suppression of tumorigenicity of glioblastoma cells in mice, and they were therefore looking for a candidate tumor suppressor gene. Germline mutations of this gene have been identified and are related to a number of genetic diseases, such as Cowden’s disease and the Bannayan-Zonana syndrome.11,22 Mutations and LOH for this gene are relatively frequent in glioblastoma and small-cell carcinoma but seem to be relatively uncommon in non–small-cell lung carcinoma.23–25 Despite the large number of data available regarding PTEN expression in various tumors, its activity has not been evaluated in mesothelioma. The mutation status in our mesothelioma cell lines is unknown, but an LOH is unlikely, because at least the portion of the PTEN protein that is recognized by our antibody via Western blot detects baseline expression in both of our cell lines.
PTEN has been demonstrated to be involved in the regulation of cell proliferation and signal transduction through the AKT pathway.26 By PTEN’s ability to act as a phosphatase, it is able to downregulate phosphorylated or active AKT, thereby inhibiting the tumor cell’s ability to communicate and proliferate. We have investigated the effect of forced overexpression of PTEN in two different human MPM cell lines. We demonstrated significantly increased induction of apoptotic cellular death after AdPTEN exposure and PTEN protein overexpression in both of these cell lines when compared with exposure to the AdBgal marker gene vector. We noted that a decrease in phosphorylation of AKT occurred in both cell lines and that this seems to be related to an inhibition of AKT kinase. Both of these changes were less marked in the I-45 (p53 wild-type) cell line, and these findings were also correlated with a less impressive rate of apoptotic cell death. One possible reason for the attenuated effect of PTEN on cell killing and apoptosis seen with the I-45 cell line may be its wild-type p53 status, but reports in the literature regarding this explanation are conflicting. Cheng et al.27 have demonstrated that inactivation of both alleles of p53 and PTEN tumor were found in >91% of nine human glioma cell lines. In another study, by Zhou et al.,28 it was demonstrated that there was no correlation between PTEN and p53 mutations in >88 brain tumor cell lines. Another unusual finding in this study was the inability of the kinase inhibitor to downregulate phosphorylated AKT in the I-45 line (Fig. 4). This finding raises the possibility of a mutated form of AKT or AKT kinase in this cell line that could also account for the relative resistance to the effects of AdPTEN.
Although this experiment does not unequivocally prove the relationship between overexpression of PTEN, hypophosphorylation of AKT, and apoptosis, what is known about the PTEN/AKT pathway is supportive. The use of an adenoviral vector-mediated dominant negative form of AKT has been compared with PTEN overexpression in pancreatic carcinoma cell lines, with identical results in regard to both downregulation of phosphorylated AKT and the induction of apoptosis.29 The PTEN regulatory pathway in mesothelioma deserves further study, because the forced overexpression of this protein, or other means of AKT kinase inhibition, may prove useful clinically in this apoptosis- and conventional therapy–resistant neoplasm.
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Acknowledgments
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Supported by an unrestricted Physician-Scientist award by the University of Texas M. D. Anderson Cancer Center (W.R.S.) and by the Wm. Keck Center for Gene Therapy.
Received for publication March 16, 2001.
Accepted for publication September 20, 2001.
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ABSTRACT
INTRODUCTION
