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 Table of Contents  
Year : 2021  |  Volume : 17  |  Issue : 4  |  Page : 943-950

Camptothecin enhances 131I-rituximab-induced G1-arrest and apoptosis in Burkitt lymphoma cells

1 Radiopharmaceuticals Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
2 Radiopharmaceutical Quality Control Program, Board of Radiation and Isotope Technology, Navi Mumbai, Maharashtra, India

Date of Submission22-Nov-2019
Date of Decision05-Feb-2020
Date of Acceptance07-Jan-2020
Date of Web Publication03-Aug-2021

Correspondence Address:
Chandan Kumar
Radiopharmaceuticals Division, Bhabha Atomic Research Centre, Mumbai - 400 085, Maharashtra
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jcrt.JCRT_1012_19

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 > Abstract 

Background: Rituximab is a chimeric monoclonal antibody against CD20. It is an established immunotherapeutic agent for non-Hodgkin's lymphoma. Even though rituximab has been used in clinics for decades, only 50% of the patients respond to rituximab therapy. To enhance the in vitro effect of rituximab, it was labeled with Iodine-131 (131I) and combined effect of 131I-rituximab and camptothecin (CPT) was studied on a tumor cell line expressing CD20.
Objective: The aim is to study the magnitude of cell killing and the underlying mechanism responsible for enhancing in vitro therapeutic efficacy.
Materials and Methods: Rituximab was labeled with 131I by the iodogen method. Raji cells were pretreated with CPT (250 nM) for an hour followed by 131I-rituximab (0.37 and 3.7 MBq) and incubated for 24 h in a humidified atmosphere of CO2 incubator at 37°C. Subsequently, Raji cells were harvested and thoroughly washed to carry out studies of cellular toxicity, apoptosis, cell cycle, and mitogen-activated protein kinase (MAPK) pathways.
Results: Maximal inhibition of cell proliferation and enhancement of apoptotic cell death was observed in the cells treated with the combination of CPT and 131I-rituximab, compared to controls of CPT-treated and 131I-rituximab-treated cells. Raji cells undergo G1 arrest after 131I-rituximab treatment, which leads to apoptosis and was confirmed by the downregulation of bclxl protein. Expression of p38 was decreased while an increase in phosphorylation of p38 was observed in the combination treatment of CPT and 131I-rituximab.
Conclusions: It was concluded from the findings that CPT enhanced 131I-rituximab-induced apoptosis, G1 cell cycle arrest and p38 MAPK phosphorylation in Raji cells.

Keywords: Apoptosis, camptothecin, G1-arrest, 131I-rituximab, mitogen-activated protein kinase, rituximab

How to cite this article:
Kumar C, Sharma R, Repaka KM, Pareri AU, Dash A. Camptothecin enhances 131I-rituximab-induced G1-arrest and apoptosis in Burkitt lymphoma cells. J Can Res Ther 2021;17:943-50

How to cite this URL:
Kumar C, Sharma R, Repaka KM, Pareri AU, Dash A. Camptothecin enhances 131I-rituximab-induced G1-arrest and apoptosis in Burkitt lymphoma cells. J Can Res Ther [serial online] 2021 [cited 2022 Nov 29];17:943-50. Available from: https://www.cancerjournal.net/text.asp?2021/17/4/943/322899

 > Introduction Top

B-cell lymphoma contributes to >90% of the hematological malignancies.[1] Errors during class switching of immunoglobulin, V(D)J recombination, translocation of the proto-oncogene, and infection of B-cell with viruses are some of the causes of B-cell malignancies.[2] Several therapeutic modalities are available for the B-cell malignancies and a combination of two or more different modes of therapy mitigates the limitations of individual modalities. Chemotherapy and radioimmunotherapy combined are known to enhance cell death.[3] In the present study, the focus is on the combined effect of camptothecin (CPT) and Iodine-131 (131I) labeled rituximab (131I-rituximab) on human Burkitt lymphoma cell line (Raji cells) to assess the magnitude of cell death and the underlying mechanism responsible for the enhancement of in vitro therapeutic efficacy.

CPT is a pentacyclic alkaloid isolated from Camptotheca acuminata used as an anticancer agent. It binds to topoisomerase I-DNA complexes, stabilizes the structure, and causes DNA strand breaks during replication.[4] However, CPT induces drug resistance, warranting the development of combination therapy with other modalities to achieve the desired therapeutic effect safely.[5]

Rituximab is an FDA approved chimeric monoclonal antibody against the extracellular surface protein of B-cell, commonly known as CD20.[6] Its expression during early B-cell development is tightly regulated to maintain cellular homeostasis. However, its overexpression in human B-cell malignancies, known as non-Hodgkin's lymphoma (NHL), makes it a possible target for the diagnosis and treatment.[7] Even though rituximab is routinely used for the treatment of NHL patients, clinical study reports have shown that only about 50% of the patients respond to rituximab therapy.[8] To enhance the therapeutic efficacy and to overcome the associated limitations in B-cell lymphoma patients, rituximab was administered in combination with other drugs such as paclitaxel, gemcitabine, vinorelbine, and cisplatin, resulting in enhanced cytotoxicity through the involvement of multiple signaling pathways.[9] Reports on radioimmunotherapy using 131I-rituximab are available in the literature.[10],[11] It has been reported that rituximab, along with X-rays triggers in-vitro apoptotic cell death.[12],[13] Earlier it was reported that 131I-rituximab enhanced cell killing in Raji cells in comparison to rituximab alone.[14] Cytotoxicity was further enhanced by combining 131I-rituximab with doxorubicin.[15] Herein, we report the studies of 131I-rituximab in combination with CPT on Raji cells and its possible role in enhancing in-vitro cell death and studies apoptosis, cell cycle arrest and associated mitogen-activated protein kinase (MAPK) signaling pathways.

 > Materials and Methods Top


All chemicals for different assays were purchased from Sigma Aldrich (St. Louis, MO, USA). PARP ELISA kit was purchased from Abcam Tokyo, Japan, while in-situ cell death detection ELISA kit was purchased from and Roche Diagnostics GmbH (Indianapolis, IN, USA). Guava cell cycle Reagent for flow cytometer was procured from Merck KGaA, Darmstadt, Germany. 131I was taken from the Radiochemical Section of Radiopharmaceuticals Division, Bhabha Atomic Research Centre Mumbai, India.

Cell culture

Raji (Burkitt lymphoma) cell line was obtained from the Cell Repository of National Center for Cell Sciences, Pune, India. These cells were grown in RPMI-1640 media supplemented with 10% fetal bovine serum (Invitrogen Carlsbad, CA, USA) and antibiotic solution (10 ml/L of 100 × solutions). Cells were grown at 37°C in humidified 5% CO2 atmosphere of an incubator.

Determination of the effect of camptothecin on Raji cell line

CPT was dissolved in dimethyl sulfoxide (DMSO) (27 mM) and the final DMSO concentration in cell samples was maintained at 0.1% or less. Raji cells (1 × 106) were plated in 24 well plates with different concentrations of CPT (10–1000 nM) and incubated for 24 h. Cells were harvested after the completion of incubation and washed thoroughly with PBS. About 1 × 103 cells were plated in each well of 96 well plates to carry out the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Briefly, MTT solution (10 μL of 5 mg/mL) was added in each well and incubated for 4 h at 37°C in humidified 5% CO2 atmosphere of an incubator in dark. Subsequently, solubilizing buffer (100 μL of 20% sodium dodecyl sulfate [SDS] in 50% dimethylformamide) was added, and the resulting color was quantified at 570 nm (with 630 nm reference) in BioTek Universal Microplate Reader (BioTek USA, Winooski, VT, USA). Cell viability was expressed as a percentage ratio of the optical density (OD) of treated sample to control sample and a graph of cell viability against CPT concentration was plotted. IC-25 was calculated from the graph and used for the assessment of combination treatment.

Radiolabeling of rituximab and the treatment of tumor cells with camptothecin and 131I-rituximab

Rituximab was labeled with 131I and characterized following the protocol reported by the author in the previous studies.[14] Raji cells (1 × 106) were added in 24 well plates and treated with CPT (250 nM) for an hour followed by addition of 0.37 and 3.7 MBq of 131I-rituximab for 24 h. For comparative studies similar experiments were carried out where the same concentrations of Raji cells were separately treated with an equivalent amount of unlabeled rituximab (1 and 10 μg/mL) present in the 0.37 and 3.7 MBq of 131I-rituximab, 0.1% DMSO and CPT (250 nM). MTT assay was carried to assess cell viability.

Study of apoptosis by estimation of DNA fragmentation induced by camptothecin and 131I-rituximab

Apoptotic DNA fragmentation was estimated by following a previously reported procedure.[15] In brief, Raji cells (1 × 105) were lysed and the supernatant was added into a streptavidin-coated microplate well. Subsequently, anti-histone-biotin and anti-DNA-peroxidase solutions were added in each microplate well with gentle shaking. Thereafter, each well was washed and incubated with substrate solution. The intensity of the resulting color was quantified at 405 nm. Apoptotic DNA fragmentation was expressed as an enrichment factor (EF), which is the ratio of OD of treated to control sample.

Study of apoptosis by estimation of poly ADP ribose polymerase cleavage induced by camptothecin and 131I-rituximab

Poly ADP-ribose polymerase (PARP) cleavage was estimated by following the procedure previously described by us.[15] In brief, Raji cells (1 × 105) were lysed and protein concentration was estimated using Bio-Rad Protein Assay (Bio-Rad Lab Inc., Hercules CA, USA). Around 60 μg of protein was added to each well of anti-PARP coated ELISA microplate and the assay were performed as per the protocol provided in the kit. EF of PARP cleavage was calculated as the ratio of OD of treated to control sample.

Study of the cell cycle in Raji cells treated with camptothecin and 131I-rituximab

Synchronized Raji cells were treated with different concentrations of rituximab, CPT, 131I-rituximab, and its combinations. These cells were harvested after the completion of treatment and washed with PBS. To fix the cells, chilled ethanol (70%) was added in the tubes containing Raji cells and incubated at 4°C for 2 h. Cells were washed with PBS and pellets were resuspended in the residual PBS. Around 200 μL of Guava cell cycle reagent was added in the tubes and incubated at room temperature for 20 min. Cell cycle analysis was carried out in the Guava EasyCyte flowcytometer.

Study of cell-death-related proteins expression in Raji cells treated with camptothecin and 131I-rituximab

Raji cells were harvested after different treatments, washed thoroughly with PBS and lysed in cell lysis buffer spiked with protease inhibitor cocktail. The concentration of proteins was determined by the Bio-Rad Protein Assay kit. Around 40 μg of protein was loaded onto SDS polyacrylamide gel electrophoresis gel and subjected to electrophoresis. Electroblotting was carried out to transfer the resolved protein bands onto a nitrocellulose membrane. Nonfat milk protein (5%) was used to block nitrocellulose membrane before incubation of primary antibodies for bclxl, beta-actin, p38 MAPK, and its phospho-protein separately for 1.5 h. The primary antibody bound nitrocellulose membrane was washed with 0.1% of Tween-20 containing tris buffer saline followed by incubation of secondary antibody. Subsequently, the nitrocellulose membrane was submerged in the ECL reagents (Cell Signaling Tech., USA) and exposed to Hyper film, which was processed using Kodak developer and fixer solutions.

Data analysis

Results were calculated as the mean ± standard deviation of at least three independent experiments, where one-way ANOVA was used to determine statistically significant differences between treated and control samples (P ≤ 0.05).

 > Results Top

Effect of camptothecin on cell proliferation of Raji cells

[Figure 1] shows the effect of CPT alone on the proliferation of Raji cells. A dose-dependent decrease in Raji cell proliferation was observed, and IC-25 (250 ± 10.5 nM) was calculated which was used for the combination treatment of 131I-rituximab.
Figure 1: Determination of inhibitory concentration of camptothecin in Raji cells by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay

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Effect of camptothecin and 131I-rituximab on the proliferation of Raji cells

[Figure 2] depicts the efficacy of the combination therapy of CPT and 131I-rituximab, as estimated by cell proliferation assay. A decrease in cell proliferation was observed in Raji cells when they were treated with 131I-rituximab in combination with CPT compared to their respective controls. The magnitude of cell proliferation in the case of CPT in combination with 0.37 and 3.7 MBq of 131I-rituximab treated Raji cells was 53.9% ± 1.5% and 46.5 ± 1.2%, respectively, compared to separate exposure to CPT (76.2% ± 1.7%) or 131I-rituximab (80.6% ± 1.06% for 0.37 MBq and 73.17% ± 3.08% for 3.7 MBq) which are statistically significant at P ≤ 0.05. A significant difference was also observed between the combination treatments of CPT with 0.37 MBq (53.9% ± 1.5%) compared to CPT with 3.7 MBq (46.5% ± 1.2%) of 131I-rituximab.
Figure 2: Estimation of cell proliferation by 3-(4,5-dimethylthiazol-2-yl) -2,5-diphenyltetrazolium bromide assay in Raji cells treated with camptothecin, rituximab, 131I-rituximab and its combination(t-test was performed to find statistical significance at P < 0.05)

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Estimation of apoptotic cell death by DNA fragmentation

DNA fragmentation is a marker for apoptotic cell death. Nuclear DNA of apoptotic cells cleaves in multiples of 180 base pairs, which may be detected precisely by ELISA methods. Results of DNA fragmentation studies showed that CPT enhanced 131I-rituximab induced apoptotic cell death; however, there is no significant difference in EF between the different concentrations of 131I-rituximab used in combination treatment (EF 6.88 ± 0.32 for 0.37 MBq and 6.65 ± 0.2 for 3.7 MBq of 131I-rituximab). However, there is a statistically significant (P ≤ 0.05) increase of EF showing DNA fragmentation in the case of combination treatment compared to the corresponding individual controls of CPT (EF-3.3 ± 0.1) and the two tested concentrations of 131I-rituximab (EF-2.2 ± 0.09 for 0.37 MBq and 3.1 ± 0.08 for 3.7 MBq), as shown in [Figure 3].
Figure 3: Estimation of apoptotic DNA fragmentation of Raji cells treated with camptothecin, rituximab, 131I-rituximab and its combination (t-test was performed to find statistical significance at P < 0.05)

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Estimation of apoptotic cell death by PARP cleavage

The extent of cell death occurred through apoptosis was determined by estimating the cleavage of PARP protein. Results depicted in [Figure 4] show that there was a significant increase (P ≤ 0.05) of PARP cleavage in cells treated with CPT in combination with 131I-rituximab (EF 4.5 ± 0.12 for 0.37 MBq, 4.86 ± 0.12 for 3.7 MBq) compared to their corresponding controls, i.e., CPT (EF 2.28 ± 0.075) and 131I-rituximab (EF 1.8 ± 0.019 for 0.37 MBq, 2.1 ± 0.038 for 3.7 MBq). However, no significant difference was observed for the PARP cleavage between the two sets of combination therapy.
Figure 4: Estimation of PARP cleavage of Raji cells treated with camptothecin, rituximab, 131I-rituximab and its combination (t-test was performed to find statistical significance at P < 0.05)

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Effect of camptothecin and 131I-rituximab on cell cycle

Encouraging results of cell toxicity and apoptosis provided an impetus to carry out cell cycle analysis. The cell population exhibited a decreasing trend in G2/M while an increasing trend in G1 phases compared to the corresponding controls [Figure 5] and [Table 1]. The G1 phase arrest observed in Raji cells in combination with 131I-rituximab treatment revealed the cause of apoptotic cell death.
Figure 5: Analysis of Raji cell cycle by Flowcytometer where, (a) control (b) cells treated with 0.1% DMSO (c) cells treated with 250 nM camptothecin (d) Cells treated with 1 μg of rituximab (e) cells treated with 10μg of rituximab (f) cells treated with 0.37 MBq of 131I-rituximab (g) cells treated with 3.7 MBq of 131I-rituximab (h) cells treated with camptothecin and 0.37 MBq of 131I-rituximab, and (i) cells treated with camptothecin and 3.7 MBq of 131I-rituximab

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Table 1: Cell cycle analysis of camptothecin, rituximab, 131I-rituximab treated Raji cells

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Effect of camptothecin and 131I-rituximab on proteins involved in apoptosis and mitogen-activated protein kinase signaling pathway

The results from cell proliferation, cell cycle, and apoptosis studies showed the improved efficacy of the combinatorial approach of CPT and 131I-rituximab in Raji cell lines. To investigate further and illustrate the mechanism of cell death, expression study of bclxl and p38 MAPK pathways proteins was carried out. In these studies, the anti-apoptotic protein (bclxl) expression was seen to be downregulated in treated cells compared to the controls. Maximum downregulation of bclxl protein was observed in the cells treated with a combination of CPT and 131I-rituximab [Figure 6]a and [Figure 6]b.
Figure 6: (a) Expressions of antiapoptotic protein in Raji cells treated with camptothecin, rituximab, 131I-rituximab and its combination. (b) Densitometry analysis of protein bands (t-test was performed to find statistical significance at P < 0.05)

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It was observed an increase in the expression of p38 MAPK protein in 131I-rituximab-treated cells compared to those treated in combination with CPT or CPT alone [Figure 7]a and [Figure 6]b. Similarly, it was recorded strong phosphorylation of p38 MAPK protein in cells treated with 131I-rituximab in combinations of CPT as well as CPT alone compared to the 131I-rituximab treated cells.
Figure 7: (a) Expressions of mitogen activated protein kinase proteins in Raji cells treated with camptothecin, rituximab, 131I-rituximab and its combination. (b) Densitometry analysis of p38 and phospho p-38 protein bands (t-test was performed to find statistical significance at P < 0.05, where *showed no significant difference)

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 > Discussion Top

Rituximab is emerging as a promising immunotherapeutic agent for B-cell malignancies,[6] inducing various signaling pathways governing cell death.[8] The tumor cell resistance observed with rituximab therapy limits its long-term repeated administration in clinical conditions.[8] Rituximab is known to induce tumor cell death, and tagging it with a beta-emitting radionuclide such as 131I (β-max0.6 MeV) further enhances the killing of cells.[14] Studies reported with rituximab labeled with various radionuclides such as 131I, 177Lu, 90Y, 188Re, and 227Th. indicate an enhancement in therapeutic efficacy.[16],[17],[18],[19] In addition, an increase in cell killing was also reported by combining 131I-rituximab and doxorubicin.[15] To further explore the effect of other chemotherapeutic drugs, CPT was used in combination with 131I-rituximab, and cell toxicity and mechanism of cell death were studied. In general, to study the combinatorial effect, the drug concentrations should be 2–3 times lesser than the IC50 values. Hence, IC-25 value of CPT in a combination of 131I-rituximab was considered to explore the effect on Raji cell lines.

Radiation is known to inhibit cell proliferation and induce apoptotic cell death.[20] Beta radiation (average penetration range is 2–3 mm) emitted from 131I is not only more potent in the killing of cancer cells as compared to the gamma rays, but it also follows the radiobiological principles of cell killing.[21],[22] Radiation induces DNA damage and CPT triggers strand break during replication, which likely causes amplification in DNA damage. Since activation of PARP protein is very much essential for the recruitment of NAD+which is necessary for the repair of DNA damage.[23] Hence, there will be depletion of NAD+in the cells. The NAD+starved cells undergo severe stress and opt for necrosis, which is a later stage of apoptotic cell death characterized by DNA fragmentation and cleavage of PARP protein into 24 kD and 89 kD by caspases.[24] Our results are consistent with the reported observation of PARP cleavage and DNA fragmentation in apoptotic cell death. In the present study, we have also observed G1 arrest and a higher magnitude of apoptosis in the combination treatment [Figure 5] and [Table 1]. Protein p53 is activated soon after DNA damage, which triggers activation of p21 expression that causes cell cycle arrest at either G1 or G2/M phase to repair DNA damage.[25] Cells unable to repair the DNA damage undergo apoptosis.[25],[26],[27] Similarly, G1 or G2/M arrest and apoptosis in the combination treatment of drugs and radiation is reported in detail.[27] In spite of the 10-fold increase in 131I-rituximab radioactivity, the extent of apoptotic cell death was similar in the combination treatment of CPT with 0.37 and 3.7MBq of 131I-rituximab. This may be due to the low dose rate effect of beta radiation emitted from 131I-rituximab, which needs to be explored in future.[28]

Bclxl is an important antiapoptotic protein of Bcl-2 family, and its downregulation is a known marker for apoptotic cell death.[29] Our results confirm the downregulation of bclxl [Figure 6]a and [Figure 6]b, which is in agreement with the observation that rituximab and other chemotherapeutic drugs,[8],[9],[30] as well as 131I-rituximab and doxorubicin, enhanced cytotoxicity in B-cell lines.[15] This study confirms the apoptotic cell death in Raji cells.

Radiation plays a significant role in the rapid activation of MAPK pathways in tumor cells.[31],[32] Activation of p38 MAPK is strongly correlated with an increase in the inflammatory response and other cellular stress through phosphorylation of the same.[33] Expression of p38 protein was decreased in the combination treatment of CPT and 131I-rituximab; which might be due to the radiation-induced stress. The strong phosphorylation of p38 was observed in CPT alone and combination treatment of CPT and 131I-rituximab-treated cells while relatively weak phosphorylation was observed in others. This strong phosphorylation indicates a preponderance of apoptotic cell death. Similar results have been reported for rituximab treatment in B-cell lines where p38 played a role in apoptosis[8],[34] and also in the case of 131I-rituximab and doxorubicin.[15] Even though the role of the MAPK pathway in apoptosis is well established, the underlying detailed mechanisms of MAPK pathways in relation to G1 and G2/M cell cycle arrest are not clearly understood in the case of radionuclide therapy. Wang et al. reported that p38 MAPK induces G2 arrest in the cell cycle during radiation stress,[35] a previously reported, whereas Yu et al. showed the G1 or G2/M cell cycle arrest and apoptosis in the combination treatment of drugs and radiation.[27] Meng et al. demonstrated that combination treatment of drugs induced apoptosis in A549 lung adenocarcinoma through MAPK pathways.[36] However, it has been observed that various MAPK pathways have an opposing role in different cancer cell lines.[37],[38] Our observation of G1 arrest in Raji cells with combined treatment of CPT and 131I rituximab is useful in this background.

 > Conclusions Top

CPT at even IC-25 concentration plays a significant role in the combination therapy with 131I-rituximab. The G1 cell cycle phase arrest, anti-apoptotic protein downregulation, and MAPK signaling pathways are the major cellular mechanism operated for enhancing the in vitro therapeutic efficacy. Our findings suggest that combination therapy of CPT and 131I-rituximab has the potential to enhance in-vitro apoptotic cell death in Raji cells, and thus, it may overcome the limitations observed with immunotherapy. This data warrant further in vivo preclinical studies for tumor regression in B-cell lymphoma bearing nude mice model.

Financial support and sponsorship

Research activities of BARC are fully supported by the funding of Department of Atomic Energy, Government of India.

Conflicts of interest

There are no conflicts of interest.

 > References Top

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]

  [Table 1]

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