- Open Access
Differential expression and role of p21cip/waf1 and p27kip1 in TNF-α-induced inhibition of proliferation in human glioma cells
© Kumar et al; licensee BioMed Central Ltd. 2007
- Received: 14 February 2007
- Accepted: 12 June 2007
- Published: 12 June 2007
The role of TNF-α in affecting the fate of tumors is controversial, while some studies have reported apoptotic or necrotic effects of TNF-α, others provide evidence that endogenous TNF-α promotes growth and development of tumors. Understanding the mechanism(s) of TNF-α mediated growth arrest will be important in unraveling the contribution of tissue associated macrophages in tumor resistance. The aim of this study was to investigate the role of Cyclin Dependent Kinase Inhibitors (CDKI) – p21cip/waf1 and p27kip1 in TNF-α mediated responses in context with p53 and activation of NF-κB and Akt pathways. The study was done with human glioma cell lines -LN-18 and LN-229 cells, using monolayer cultures and Multicellular Spheroids (MCS) as in vitro models.
TNF-α induced inhibition of proliferation and enhanced the expression of p21cip/waf1 and p27kip1 in LN-18 cells. p21 was induced on exposure to TNF-α, localized exclusively in the nucleus and functioned as an inhibitor of cell cycle but not as an antiapoptotic protein. In contrast, p27 was constitutively expressed, localized predominantly in the cytoplasm and was not involved in arrest of proliferation. Our data using IκBα mutant LN-18 cells and PI3K/Akt inhibitor-LY294002 revealed that the expression of p21 is regulated by NF-κB. Loss of IκBα function in LN-229 cells (p53 positive) did not influence TNF-α induced accumulation of pp53 (Ser-20 p53) suggesting that p53 was not down stream of NF-κB. Spheroidogenesis enhanced p27 expression and p21 induced by TNF-α was significantly increased in the MCS compared to monolayers.
This study demarcates the functional roles for CDKIs-p21cip/waf1 and p27kip1 during TNF-α stimulated responses in LN-18 glioma cells. Our findings provide evidence that TNF-α-induced p21 might be regulated by NF-κB or p53 independently. p21 functions as an inhibitor of cell proliferation and does not have a direct role in rendering the cells resistant to TNF-α mediated cytotoxicity.
- Human Glioma Cell Line
- Multicellular Spheroid
- Antiapoptotic Function
- Tumor Infiltrate Macrophage
- Tritiated Thymidine Incorporation Assay
Gliomas are the most common malignant brain tumors in adults . The median survival of the patients is less than two years and the prognosis remains poor despite extensive research and advances in radiation therapy and chemotherapeutic regimes. The factors responsible for the aggressive behavior of gliomas include marked local invasive growth, neovascularization and evasion to immune responses . Macrophages comprise an important component of the immune system against tumors. In gliomas, macrophages are recruited and remain at the site of tumor constituting a major proportion of the tumor mass . The ability of gliomas to escape the host immune system is thought to contribute to the malignant behavior of these tumors, raising speculation about the role of these tumor infiltrating macrophages in progression and aggressiveness of gliomas . Secretion of cytokines, particularly TNF-α, by activated macrophages is known to affect survival, growth and proliferation of the tumor . TNF-α mediates apoptotic or necrotic effects in many tumors depending on cell types, and causes growth inhibition in 40% of tumor cell lines [5–7]. Reports on the effects of TNF-α on glioma growth and progression are contradictory. TNF-α-induced responses range from cytotoxicity to cytostatic effects [8–10]. Earlier in vivo studies with tumors indicated that TNF-α was a death inducer and hence used in clinical trials for treatment of cancers . However recent reports provide evidence that endogenous TNF-α promotes growth and development of tumors [12, 13]. In gliomas, TNF-α is shown to reduce growth and prolong survival by enhancing macrophage recruitment and microcyst formation . Understanding the mechanism(s) of TNF-α mediated growth arrest will be important in unraveling the contribution of tissue associated macrophages in tumor resistance in gliomas.
The progression of cells through cell cycle depends on the activation of cyclins and cyclin dependent kinases – CDKs which function together in the G1 phase for initiating S and progression to G2/M phases. The activity of the complexes is regulated by two families of cyclin dependent kinase inhibitors (CDKIs)-including INK4 proteins that bind only to CDK4/CDK6 and are specific for G1 phase and the cip/kip proteins which include p21cip/waf1 and p27 kip1 and are not specific for any particular cell cycle phase [15, 16]. p21 is an universal inhibitor of cell cycle progression and arrests in G1 as well as G2 phases . p21 negatively regulates cell cycle progression by inhibiting CDK2 and CDK4 and blocks DNA replication by binding to PCNA . p21 is also implicated in terminal differentiation, replicative senescence and protection from p53 dependent and p53-independent apoptosis [19, 20]. p27 arrests at G1 phase of the cell cycle by contact inhibition and controls the G1/S transition by inhibiting the activity of a wide variety of cyclin/CDK complexes . It is expressed in many tumors and may be partly responsible for growth arrest in tumor cells [22, 23]. Though mutations in p27 are rare, a negative correlation between p27 expression and tumor progression is documented in many cancers and loss of p27 in tumors is associated with poor prognosis .
Elucidation of cellular and molecular mechanisms involved in survival/death signaling in response to chemotherapeutic agents has been a focus of recent studies for understanding drug resistance in tumors. However, such studies have been hampered by limitations in the conventional in vitro models as they do not reflect the complexities of solid tumors. Multicellular spheroids (MCS) closely resemble the in vivo situation with regard to cell shape and environment, which in turn can affect the gene expression and behavior of the cells . The biological significance and clinical relevance of MCS have been well documented and these experimental systems have been used for studying micro-environmental effects on basic mechanisms, such as regulation of proliferation, metabolism, differentiation, cell death, invasion, angiogenesis and immune response [26–28]. Though the MCS show greater resistance compared to the two dimensional-monolayer cultures, the mechanisms that confer greater resistance to radiotherapy, chemotherapy in the MCS are not well understood . In this study, we analyzed the effect of TNF-α on proliferation and survival of human glioma cells and investigated the role of CDKIs-p21cip/waf1 and p27kip1 in context with p53 and activation of NF-κB and Akt pathways using monolayer cultures and MCS as in vitro models.
TNF-α-induced inhibition of proliferation is mediated by TNFR1
TNF-α up regulates p21 and p27 expression
Cyclin dependent kinase inhibitors (CDKI) p21cip/waf1 and p27 kip1 are important in TNF-α mediated signaling . We studied the effect of TNF-α on the expression of these CDKI proteins in the two culture models of LN-18 cells by immunoblotting. As shown in Figure 2B, the expression of p21 was below the level of detection in the both the culture systems. On stimulation with TNF-α, the level in monolayers was marginally increased while the expression was strongly enhanced in the spheroids. Our data also revealed that LN-18 cells constitutively expressed p27 and the levels increased drastically with spheroidogenesis. Stimulation with TNF-α resulted in further upregulation of p27 expression in both the models (Fig. 2B). We further performed time course experiments by Confocal Laser scanning Microscopy (CLSM) to study the expression and localization of p21 and p27 in response to TNF-α. We found that p21 was not detectable in the control cells confirming the data from immunoblotting experiments. p21 was induced from 3 hr post-treatment with TNF-α with further increase in the number of p21 positive cells at 6 hr. The level decreased at 12 hr and complete loss of expression was observed at 24 hr. Using an antibody that recognizes p21 in the nucleus as well as cytoplasm it was found that p21 was restricted to the nucleus (Fig. 2C). The cells were highly positive for p27 (>95%) with predominantly cytoplasmic staining though nuclear staining was also seen in some cells. TNF-α treatment marginally enhanced the p27 expression from 3 hr onwards (Fig. 2C).
TNF-α-induced p21 is regulated by NF-κB and p53 independently
p21 siRNA did not induce cell death in LN-18 cell line
p21 but not p27 was involved in the inhibition of proliferation
In this report, we established that though p21cip/waf1 and p27kip1 are upregulated in response to TNF-α, p21 but not p27 is involved in the arrest of proliferation in TNF-α stimulated LN-18 glioma cells. TNF-α-induced inhibition of proliferation was mediated via TNFR1. This observation is significant as most aggressive gliomas express high levels of TNFR1. We also demonstrated that p21 and p27 differed in their localization and expression levels in the experimental models represented by monolayers and spheroids. While p27 was constitutively present, p21 was induced on stimulation with TNF-α and the upregulation in the spheroids was markedly higher compared to monolayers.
p27 is expressed abundantly in human malignant gliomas and is associated with better prognosis [39, 40]. p27 is normally a nuclear protein but cytoplasmic localization has been demonstrated in breast and colon cancers and correlates with poor survival in adenocarcinoma . Relocalization of p27 in the cytoplasm is the result of phosphorylation of p27 nuclear localization domain induced by Akt kinase  and the presence of cytoplasmic p27 correlates with activated Akt in breast tumors . It is likely that such a correlation exists in gliomas also as high basal levels and activated Akt were detected in LN-18 cells (our unpublished data) and the cells were highly positive for p27 with predominant cytoplasmic localization. Interestingly, PI3K/Akt inhibitor had no effect on the TNF-α-induced p27 upregulation implying that unlike the nuclear p27, cytoplasmic protein may not be regulated via PI3K/Akt pathway. Our findings also rule out the possibility of the involvement NF-κB in regulation of p27 as there was no difference in the expression levels between the parent and the IκBα mutant LN-18 cell lines, which essentially differed in the NF-κB activity. It is believed that p27 translocated to the cytoplasm initiates its degradation and activates CDK2 and can enhance import of cyclin D to the nucleus  resulting in the loss of functionality of p27 in cytoplasm. On these lines, it is reasonable to conjecture that the antiproliferative effect of p27 may be functionally impaired in many cancers by sequestration in cyclinD-CDK4/6 complexes and mislocalization in the cytoplasm away from the nuclear targets [16, 42]. LN-18 cells were highly positive for p27 and the expression was significantly enhanced on spheroidogenesis. This finding is consistent with the reports that expression of p27 is regulated by contact inhibition and increased level of p27 has been reported in tumor cell lines maintained as spheroids as opposed to cell cultured as monolayers . Though p27 expression was enhanced remarkably on stimulation with TNF-α, experiments with p27-antisense oligonucleotides indicated that p27 did not contribute to the inhibition of proliferation, suggesting the loss of functionality of p27 as an inhibitor of cell cycle. However, the role of cytoplasmic p27 during spheroidogenesis and in response to TNF-α stimulation remains unclear.
Elevated levels of p21 is reported in aggressive gliomas and is used as a prognostic marker , surprisingly, p21 was not detectable in LN-18 cells. Evidence for the antiapoptotic function of p21 has been provided in TNF-α induced cytotoxicity in Ewing tumor and MCF-7 breast carcinoma cell line , in differentiation induced apoptosis of monocytes  and CD95 mediated cell death in human glioma cell lines . In our study, p21 siRNA did not induce cell death in TNF-α-stimulated LN-18 cells, indicating that p21 did not have an antiapoptotic function.
Activation of NF-κB during TNF-α stimulation is believed to one of the major factors contributing to resistance in most tumors. It is speculated that constitutive activation of NF-κB in glioblastoma multiforme (GBM) may be associated with tumor resistance to TNF-α immunotherapy . To investigate the role of p21 in resistance to TNF-α mediated cytotoxicity, we used a stable IκBα mutant LN-18 cell line that was rendered sensitive to TNF-α induced death by transfection with a double mutant construct pcDNA3-IκBα in which both serines (32 and 36) at the amino-terminal are replaced with alanine that are essential for phosphorylation of IκBα during NF-κB activation . In contrast to the parental LN-18 cells, which showed upregulation of p21 in response to TNF-α treatment, p21 was not detectable in the IκBα mutant cells suggesting an association between p21 expression and NF-κB activity. This observation is in accordance with the report demonstrating a correlation between p21 expression and NF-κB activity in Ewing tumor cells . However, other studies demonstrate that p21 induction in daunomycin treated breast and colon carcinoma cell lines does not involve NF-κB though the p21 promoter harbors p53 responsive elements and a functional NF-κB binding site in these cells .
In light of a recent report by Schumm et al, demonstrating the critical role of p52/p100 NF-κB subunit in p53-dependent p21 regulation in U-2 0S cells , it was of interest to investigate the role of p53 in TNF-α-induced p21 in glioma cells. As LN-18 cells have mutant p53, we used another human glioma cell line – LN-229, expressing wild type p53 protein. The tumor suppressor p53 is post-translationally activated by different mechanisms including phosphorylation on serine residues within the C-terminus or N-terminal regions [51, 52]. Phosphorylation of p53 at ser20 is an important event in modulation of p53 stability and activity in response to UV and irradiation  and is shown to activate p21 in response to DNA damaging agents by enhancing transactivating function of p53 . In this study, we found that TNF-α-enhanced accumulation of pp53 was associated with down regulation of p21 expression in LN-229 cells. Upregulation of p53 accompanied by decreased p21 expression is a typical feature of inhibition of transcription and has been demonstrated in LNCap cells treated with flavopiridol . Negative expression of p21 and overexpression of p53 together is associated with aggressive behavior in gastric tumors . Furthermore, the data from IκBα dominant negative construct transfected LN-229 cells exposed to TNF-α suggested that p53 might not be downstream of NF-κB and p21 could be regulated independently by the p53. Our findings differ from the study with MCF-7 cells reporting that p21 induced on stimulation with TNF-α is not regulated by p53 but by NF-κB . It is note worthy that in LN-229 cells, dexamethosone does not induce p53 expression or alter camptothecin induced p53 expression implying that while regulation of p21 by p53 independent pathway in glioma cells is believed to cell type specific , the response may vary also depending on stimuli. Another important finding of this study is that, p53 expression in LN-229 cells was not influenced by the absence of functional IκBα substantiating the conclusion that p53 is not regulated by NF-κB. This observation is differs from the report that IκBα inhibits p53 by binding to it, resulting in cytoplasm p53 and hence prevents nuclear translocation .
Recent studies suggest an antiapoptotic function for p21 . To exert protective effect against damage induced by apoptotic agents, it is essential that p21 is phosphorylated and is localized in the cytoplasm . We found that TNF-α-induced p21 was restricted to the nucleus suggesting that it might not function as an antiapoptotic protein. Our conclusion that TNF-α-induced p21 was not phosphorylated and was restricted to nuclear localization is based on our findings using two different antibodies to examine the phosphorylation status of p21, an antibody that was immunoreactive with nuclear as well as cytoplasmic p21 and a second antibody recognizing exclusively phosphorylated p21 (details in materials and methods). Though the molecular mechanisms in compartmentalization of p21 are not clear, it is reported that Akt might regulate the subcellular changes . Phosphotidylinositol 3-kinase (PI3K) and its downstream target, the Akt/PKB serine threonine kinase  are activated on TNF-TNFR binding. Akt activity is elevated in most GBM cells particularly those with mutant form of PTEN and is important in glioma formation and progression . Activation of Akt is paralleled with increased p21 stability and correlated with resistance to taxol-induced apoptosis in glioblastoma cell lines . High grade gliomas are known to express high levels of Akt  and enhanced Akt activity is demonstrated in spheroids from Ewing tumors . We observed increased levels and activity of Akt in spheroids of LN-18 cells (our unpublished data). These findings and our data depicting increased expression of TNF-α-induced p21 in spheroids compared to monolayer cells prompted us to examine the involvement of Akt in the regulation of p21. We found that PI3K/Akt inhibitor-LY290042 did not affect the expression of p21 suggesting that it was not regulated by this pathway. Experiments with p21 siRNA reversed the inhibition of proliferation confirming its role as a regulator of cell cycle progression. These results led us to conclude that p21 functioned as an inhibitor of proliferation but not as an antiapoptotic protein in TNF-α mediated response in LN-18 cells. Furthermore, our studies with IκBα mutant LN-18 cells and pharmacological inhibitor-LY290042 revealed that p21 expression was regulated by NF-κB and not Akt pathway.
In conclusion, this study demarcates the functions of CDKIs-p21cip/waf1 and p27kip1 during TNF-α stimulated responses in LN-18 glioma cells. p21 induced on stimulation with TNF-α may be regulated by NF-κB or p53, probably depending on the status of p53. Our findings do not support a role for p27 in regulation of proliferation or rendering resistance in TNF-α induced cytotoxicity, which could be attributed to its cytoplasmic localization. However questions concerning the relevance of enhanced cytoplasmic p27 expression during spheroidogenesis and in response to TNF-α stimulation remain to be answered.
The findings in this study assume importance from the point of TNF-α-TNFR1 interaction in the in vivo milieu in gliomas wherein TNF-α is secreted by the tumor infiltrating macrophages and TNF receptors are abundantly expressed by the tumor cells. The stimulation leads to activation of various pathways including NF-κB pathway which contributes to chemoresistance. The resistance may be further enhanced by induction of p21 by TNF-α in NF-κB dependent manner leading to the arrest of cells in the G1 phase. The regulation of p21 by p53 and NF-κB in wild type p53 and mutant p53 tumors in response to TNF-α deserves further in-depth studies Additionally, heightened induction of p21 to TNF-α stimulation in MCS representing the cells in low proliferating phase underscore the MCS as more realistic model for studying the molecular mechanisms in resistance to chemotherapeutic agents in solid tumors.
Cell cultures and generation of multicellular spheroids
The human glioma cell lines LN-18, LN-229 and U87MG were procured from American Type Culture Collection (ATCC, U.S.A). Cells were cultured in Dulbecco' s modified eagle's medium (DMEM) supplemented with 1.5 gm/L sodium bicarbonate, 4 mM glutamine and 5%-10% fetal bovine serum (Gibco, U.S.A) at 37°C in an atmosphere of 5% CO2/95% air. Multicellular spheroids were generated by the liquid overlay technique as described previously  with a slight modification. All experiments were done using 24 hr cultures. Single cell suspensions were prepared from spheroids by dissociation with 0.25% trypsin in PBS at 37°C for 30 min.
Cell viability assay
The effect of TNF-α on cell viability was assessed by MTT (3-(4, 5-Dimethyl-2-thiazolyl)-2, 5-diphenyl-2H-tetrazollium bromide) assay. Cells were treated with TNF-α (1–100 ng/ml) for 24 hr. During the last 4 hr of incubation, 10 μl of MTT (5 mg/ml) (Amersham Biosciences, U.S.A) were added. The crystals were dissolved in 10%SDS-0.01N HCl and absorbance was measured at 570 nm with reference to 650 nm. The percent viability was calculated considering controls as 100%. Cell death was also confirmed by trypan blue dye exclusion method.
Tritiated thymidine incorporation assay
Cell proliferation was measured by tritiated thymidine incorporation assay. Cells were grown as monolayers or spheroids in 96 well plates and treated with serial concentrations of TNF-α (1 to 100 ng/ml – Peprotech, U.S.A) for 24 hr, during the last 18 hr before termination of the assay, 1 μCi tritiated thymidine (Specific activity 240 Gbq/mmole, from Board of Radiation and Isotope Technology-BRIT, India) were added to wells. Monolayer cells were trypsinized in the same plate while spheroids were transferred to fresh plate and harvested using cell harvester (Nunc, Denmark). The radio activity-Counts per Minute (CPM) were measured using a liquid scintillation counter (Packard, U.S.A). The percent inhibition was calculated, considering incorporation in control (untreated) cells as 100%.
Cell cycle analysis
Cells were plated at 2 × 106/100 mm dish in duplicates and treated with TNF-α (10 ng/ml) up to 72 hr. Cells were harvested at 24 hr interval post-treatment, stained with propidium iodide (50 μg/ml) and DNA content was determined on the FL-2A channel with flowcytometer equipped with 488 nm argon laser (FACS Vantage – BD Sciences, U.S.A). The data was analyzed using Modfit software to determine population in different phases of cell cycle.
Cells were lysed in RIPA buffer (50 mM Tris-HCl, pH 7.4, 150 mM NaCl, 1% TritonX-100, 1% NP-40, 1% Sodium deoxycholate, 0.1% Sodium dodecyl sulphate, 2 mM Phenylmethylsulfonylfluoride, 50 mM Sodium fluoride, 5 mM iodoacetamide, 2 mM β-glycero-phosphate, 1 mM benzamidine, 1 mM sodium orthovanadate) and protease inhibitor cocktail (Roche, Germany). Protein samples were separated by SDS-PAGE (10–15%) and electro-transferred onto PVDF membrane (Amersham BioSciences, U.S.A). The expression of proteins was analyzed using antibodies to p21 and p27 (Cell Signaling and Santacruz Biotechnology, U.S.A), TNFR1 (R&D Systems, U.S.A). The blots were incubated with appropriate secondary antibodies conjugated to horse radish peroxidase (Biorad, U.S.A), and developed using ECL PLUS kit (Amersham Biosciences, U.S.A). The protein levels were normalized by reprobing the blots with antibody to β-actin (ICN, U.S.A).
Confocal laser scanning microscopy
Cells grown for 24 hr on cover slips were treated with TNF-α (10 ng/ml) for different time points. Cells were washed twice with PBS, fixed with 3.7% paraformaldehyde for 10 min at RT. Cells were permeabilized with 0.2% Triton-X 100 for 5 min and blocked with PBS containing 1% BSA and 10% goat serum for 30 min. Cells were incubated with appropriately diluted antibodies to pp53, p65, and p27 (SantaCruz Biotechnology, U.S.A) p21, pp21(Cell Signaling Technology, U.S.A) overnight, washed and incubated with corresponding secondary antibodies labeled with Cy-3 for I hr. Cells were washed with PBS, stained with DAPI (0.5 μg/ml) and mounted in mounting media (Oncogene, U.S.A) and viewed under Confocal microscope (Zeiss LSM 510, Germany) equipped with argon laser and helium lasers. The controls comprised of cells processed with isocontrol antibody or without primary antibody. For assessing the role of PI3K/Akt pathway in the regulation of p27 and p21, cells were incubated for 2 hr with LY290042 (Calbiochem, U.S.A), a pharmacological inhibitor of PI3K/Akt prior to TNF-α treatment.
Cell transfections and generation of IκBα mutant stable LN-18 cell line
IκBα mutant stable cell line was generated using a double mutant construct in which both serines (32 and 36) at the amino-terminal of IκBα, are mutated to alanine. DNA was mixed with 10 μl lipofectamine 2000 (Invitrogen, U.S.A) in 500 μl of serum free medium for 30 min and layered over the LN-18 cells grown at 50% confluence and after 6 hr the medium was changed. After incubation for 24 hr, the cells were grown in selection medium (G418, 600 μg/ml) and clones were picked and plated into 96 well plates. Single cell colonies were grown in G418 (400 μg/ml). Transfections done with empty vectors comprised as controls. The mutant status in the cell line was confirmed by analyzing the response to TNF-α by IκBα degradation and p65 nuclear localization. Transient transfections were done with IκBα dominant negative construct in LN-229 cells.
Antisense oligonucleotide transfection
LN-18 cells were transfected with 200 nM of p27 phosphorothioate labeled antisense oligonucleotides synthesized by Life technologies, U.S.A. The sequences were as follows – p27 antisense-5'-gacactctcacgtttgacat-3', scrambled-5'-ttgccgctgctaagttcg-3', using oligofectamine (Invitrogen, U.S.A) in serum free medium. After 6 hr, the medium was supplemented with serum, cultured for 24 hr and treated with TNF-α (10 ng/ml). Western blot analysis for confirming p27 expression and tritiated thymidine assays were performed as described above.
Transfection with p21 siRNA
Cells were cultured for 24 hr and transfected with p21 siRNA oligonucleotides (50 nM) according to the manufacturer's instructions (Cell Signaling Technology, U.S.A). After 24 hr, medium was replaced and cells were treated with TNF-α (10 ng/ml) for 3 hr and processed for p21 staining, proliferation and viability assays as described above.
Statistical analysis was performed using one-way analysis of variance (ANOVA) followed by students t-test or Fisher's exact test. A p-value < 0.05 was considered to be statistically significant.
We are thankful to Dr. Sudhir Krishna, NCBS, for providing IκBα double mutant construct, and Dr. Karunagaran Devarajan, Rajiv Gandhi Centre for Biotechnology, for providing IκBα dominant negative construct. The authors acknowledge the technical help by Ms. Ashwini Atre in confocal microscopy and Ms. Hemangini Shikhave for flowcytometry. The study was partly funded by Indian Council of Medical (ICMR). P. Sudheer Kumar was supported by ICMR fellowship.
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