A calcium-insensitive attenuated nitrosative stress response contributes significantly in the radioresistance of Sf9 insect cells

Alterations in the expression pattern of RBC membrane associated proteins (RMAPs) in whole body γ-irradiated Sprague Dawley rats

PURPOSE

Peripheral blood serum/plasma proteins are frequently studied for their potential use as radiation exposure biomarkers. Here we report RBC membrane associated proteins (RMAPs), which show alterations in expression level following whole-body γ-irradiation of rats at sub-lethal/lethal doses.

MATERIALS AND METHODS

RBCs from peripheral blood of Sprague Dawley rats were segregated using the Ficoll-Hypaque method, and membrane fractions were hypotonically isolated at various time points (6 h, 24 h, 48 h) after γ-irradiation at 2 Gy, 5 Gy, and 7.5 Gy doses. Following purification of proteins from these fractions, two-dimensional electrophoresis (2-DE) was carried out. Treatment induced differentially expressed protein spots (≥2 fold increase/decrease) were picked up, trypsinized, and identified using LC-MS/MS analysis. Western immunoblots using protein specific antibodies were used to confirm the results. Gene ontology and interactions of these proteins were also studied.

RESULTS

From a number of differentially expressed radiation-responsive 2-DE protein spots detected, eight were identified unequivocally using LC-MS/MS. Out of these, actin, cytoplasmic 1 (ACTB) showed detectable yet insignificant variation (<50%) in expression. In contrast, peroxiredoxin-2 (PRDX2) and 26S proteasome regulatory subunit RPN11 (PSMD14) were the two most prominently over-expressed proteins. Five more proteins, namely tropomyosin alpha-3 chain (TPM3), exosome component 6 (EXOSC6), isoform 4 of tropomyosin alpha-1 chain (TPM1), serum albumin (ALB), and the 55 kDa erythrocyte membrane protein (P55) showed distinct alteration in their expression at different time-points and doses. ALB, EXOSC6, and PSMD14 were the most responsive at 2 Gy, albeit at different time-points. While EXOSC6 and PSMD14 showed maximum over-expression (5-12 fold) at 6 h post-irradiation, ALB expression increased progressively (4 up to 7 fold) from 6 h to 48 h. TPM1 showed over-expression (2-3 fold) at all doses and time-points tested. TPM3 showed a dose-dependent response at all time-points studied; with no variation at 2 Gy, ∼2 fold increase at 5 Gy, and 3-6 fold at the highest dose used (7.5 Gy). The p55 protein was over-expressed (∼2.5 fold) only transiently at 24 h following the lethal (7.5 Gy) dose.

CONCLUSION

This is the first study to report γ-radiation induced alterations in the RBC membrane associated proteins. We are further evaluating the potential of these proteins as radiation biomarkers. Due to the abundance and easy use of RBCs, this approach can prove very useful for detecting ionizing radiation exposure.

Intrinsic attenuation of post-irradiation calcium and ER stress imparts significant radioprotection to lepidopteran insect cells

Sf9 lepidopteran insect cells are 100-200 times more radioresistant than mammalian cells. This distinctive feature thus makes them suitable for studies exploring radioprotective molecular mechanisms. It has been established from previous studies of our group that downstream mitochondrial apoptotic signaling pathways in Sf9 cells are quite similar to mammalian cells, implicating the upstream signaling pathways in their extensive radioresistance. In the present study, intracellular and mitochondrial calcium levels remained unaltered in Sf9 cells in response to radiation, in sharp contrast to human (HEK293T) cells. The isolated mitochondria from Sf9 cells exhibited nearly 1.5 times greater calcium retention capacity than mammalian cells, highlighting their inherent stress resilience. Importantly, UPR/ER stress marker proteins (p-eIF2α, GRP4 and SERCA) remained unaltered by radiation and suggested highly attenuated ER and calcium stress. Lack of SERCA induction further corroborates the lack of radiation-induced calcium mobilization in these cells. The expression of CaMKII, an important effector molecule of calcium signaling, did not alter in response to radiation. Inhibiting CaMKII by KN-93 or suppressing CaM by siRNA failed to alter Sf9 cells response to radiation and suggests CaM-CaMKII independent radiation signaling. Therefore, this study suggests that attenuated calcium signaling/ER stress is an important determinant of lepidopteran cell radioresistance.

Evidence for a radiation-responsive 'p53 gateway' contributing significantly to the radioresistance of lepidopteran insect cells

Recently, we have demonstrated that microRNA-31 (miR-31) overexpression is inherent to radiation-induced cell death in the highly radioresistant Sf9 insect cells, and regulates pro-apoptotic Bax translocation to mitochondria. In the present study, we report that at sub-lethal radiation doses for Sf9 cells, miR-31 is significantly downregulated and is tightly regulated by an unusual mechanism involving p53. While ectopic overexpression of a well-conserved Sfp53 caused typical apoptosis, radiation-induced p53 accumulation observed selectively at sub-lethal doses failed to induce cell death. Further investigation of this paradoxical response revealed an intriguing phenomenon that sub-lethal radiation doses result in accumulation of a ‘hyper-phosphorylated’ Sfp53, which in turn binds to miR-31 genomic location and suppresses its expression to prevent cell death. Interestingly, priming cells with sub-lethal doses even prevented the apoptosis induced by lethal radiation or ectopic Sfp53 overexpression. On the other hand, silencing p53 increased radiation-induced cell death by inhibiting miR-31 downregulation. This study thus shows the existence of a unique radiation-responsive ‘p53 gateway’ preventing miR-31-mediated apoptosis in Sf9 cells. Since Sfp53 has a good functional homology with human p53, this study may have significant implications for effectively modulating the mammalian cell radioresistance.

Radioresistant Sf9 insect cells readily undergo an intrinsic mode of apoptosis in response to histone deacetylase (HDAC) inhibition

Insect cell lines have been utilized as an important higher eukaryotic model system to decipher stress responses and cell death mechanisms. Lepidopteran Sf9 cells (derived from the ovaries of Spodoptera frugiperda) display nearly 100 times higher resistance to ionizing radiation in contrast to mammalian cells, which is partly contributed by an unusually high HDAC activity. However, their response to HDAC inhibition remains to be evaluated. In the present study, the effects of HDAC inhibitor (NaBt) on Sf9 cellular/nuclear morphology, cell cycle progression, DNA damage/repair, redox status, and mitochondrial perturbations were evaluated. NaBt-induced apoptosis was evident at 18 h in Sf9 cells at 2 mM concentration, primarily through mitochondrial induction of oxidative stress and subsequent DNA damage. Cell cycle analysis revealed appearance of sub-G1 DNA content at 12 h onwards and DNA fragmentation by 18 h. Initial few hours of treatment caused significant loss in MMP through oxidation of mitochondrial inner membrane protein, i.e., cardiolipin. HDAC inhibition-mediated apoptosis was associated with increased Bax/Bcl2 ratio, mitochondrial cytochrome-c release, and caspase-3 activation. The study thus infers that Sf9 cells, which can withstand very high radiation doses, are quite sensitive to the increase in the chromatin acetylation levels. In addition, HDAC inhibition also sensitized Sf9 cells to radiation-induced DNA damage, further corroborating our recent finding that chromatin compactness contributes significantly to their radioresistance. Therefore, the study demonstrates prominence of prevailing DNA/chromatin protective mechanisms in Lepidopteran insect cells.

Constitutive hyperactivity of histone deacetylases enhances radioresistance in Lepidopteran Sf9 insect cells

BACKGROUND

Lepidopteran insect cells withstand multifold higher radiation doses and suffer far less DNA damage despite carrying numerous structural/functional homologies with mammalian cells. Since DNA-histone interactions significantly influence radiation-induced DNA damage, we investigated the role of histones in insect cell radioresistance.

METHODS

Modified comet assay was used to assess the γ-radiation-induced DNA damage following serial histone depletion by varied salt concentrations. Acid-Urea-Triton (AUT) gel analysis combined with in silico predictions was used to compare mammalian and insect histones and acetylation status while HDAC activity was assessed/modified for studying the latter's role in radioresistance. Cell death was measured by morphological analysis and flow cytometry.

RESULTS

High-salt extraction pattern from Sf9 nuclei suggested stronger DNA-histone affinity as the two core histones H2A/H2B could be extracted at much higher (2M) concentration as compared to 1.2M NaCl in mammalian (AA8) cells. Electrophoretic mobility of unirradiated Sf9 cells remained unaltered at all salt concentrations (0.14M-2M NaCl), and radiation-induced DNA damage increased only by 2M-NaCl pre-treatment. In silico analysis confirmed excellent conservation of Lepidopteran H2A/H2B sequence with human histones including comparable N-terminal lysine residues, yet these had ~60% lower acetylation. Importantly, insect cells showed ~70% higher histone deacetylase activity whose inhibition by Trichostatin-A reversed hypo-acetylation state and caused significant radiosensitization, thereby confirming the protective contribution of reduced acetylation.

CONCLUSION

Our study reveals that the hypo-acetylated state of well-conserved core histones, maintained by considerable HDAC activity, contributes significantly in Lepidopteran radioresistance.

GENERAL SIGNIFICANCE

This investigation shows constitutively high activity of HDACs as a potential radioprotective mechanism existing in insect cells.

Evidence for microRNA-31 dependent Bim-Bax interaction preceding mitochondrial Bax translocation during radiation-induced apoptosis

Downregulation of microRNA-31 has been linked with enhanced stress resistance, while its overexpression leads to cell death. In this study, we found mediatory role of microRNA-31 in γ-radiation-induced apoptosis in a model insect cell line Sf9 carrying well-conserved apoptotic machinery. Mature microRNA-31 is perfectly conserved amongst insects; hence we used biotinylated probes designed from Bombyx mori sequence for its successful detection in Sf9 cells. Target identification using Bombyx mori 3′UTRs predicted miR-31′s potential role in Lepidopteran apoptosis, which prompted us to investigate alterations in its expression during radiation-induced cell death. We found significant overexpression of Sf-miR-31 following lethal dose (1,000Gy–3,000Gy) irradiation. Its mediatory role was finally confirmed as antisense-microRNA-31 could successfully inhibit radiation-induced cytochrome-c release, caspase-3 activation and apoptosis. While Bax/Bcl-2 expression remained unchanged, lethal radiation doses induced Bim overexpression and direct Bim-Bax interaction (co-immunoprecipitation) which is not yet unequivocally demonstrated during apoptosis. Quite important, these events were found to be dependent on radiation-induced miR-31 overexpression, as antisense-miR-31 inhibited both the responses and resulted in significant inhibition of cell death. Pro-apoptotic role of miR-31 was further confirmed when miR-31 mimic induced apoptosis involving similar Bim/Bax alterations. Therefore, our study reveals an important mediatory role of miR-31 in radiation-induced cell death.

Radioresistant Sf9 insect cells display efficient antioxidant defence against high dose γ-radiation

PURPOSE

To investigate the effect of gamma radiation-induced alterations in antioxidant defence of radioresistant Sf9 insect cells.

MATERIALS AND METHODS

Sf9 cells were irradiated at doses ranging from 0.5-200 Gy. Lipid peroxidation and protein carbonylation levels were observed at 4 h post-exposure along with reduced glutathione/oxidized glutathione (GSH/GSSG) profile as well as specific activities of redox active enzymes superoxide dismutase (SOD), catalase, ascorbate peroxidase (APOx), and glutathione reductase (GR). Human brain malignant glioma (BMG-1) cells were used for comparing radiation response of mammalian cells.

RESULTS

Sf9 cells displayed significantly less radiation-induced reactive oxygen/nitrogen species (ROS/RNS) generation, protein carbonylation and growth inhibition as compared to mammalian cells. Sf9 cells have higher basal APOx (∼4-fold), catalase (∼1.7-fold), SOD (∼1.3-fold) activity and GSH level (∼2.2-fold) compared to mammalian cells. A radiation dose-dependent increase in SOD, Catalase and APOx activity was found in Sf9 cells at least up to 100 Gy dose, while maximum activity in mammalian cells was achieved by 10 Gy.

CONCLUSION

The present study suggests that Lepidopteran insect cells carry a stronger antioxidant system that protects against radiation-induced macromolecular damage, growth inhibition and cell death.

Radioresistant Sf9 insect cells undergo an atypical form of Bax-dependent apoptosis at very high doses of γ-radiation

PURPOSE

To investigate the underlying mechanisms of cell-death at extremely high doses of radiation in radioresistant Spodoptera frugiperda-9 (Sf9) insect cells.

MATERIALS AND METHODS

Morphology, cell proliferation and DNA-fragmentation analysis was performed at 500-2000 Gy. Changes in intracellular reactive oxygen species (ROS), mitochondrial membrane potential (MMP), cardiolipin oxidation and Annexin-V externalization were studied using flow-cytometry. Cytochrome-c release was measured using immunofluorescence microscopy. Inhibitors of apoptosis, i.e., Bongkrekic acid (BKA), Caspase-9 inhibitor (C9i), 5-(4-fluorosulfonylbenzoyl) adenosine hydrochloride (FSBA) and Cyclosporin-A (CsA) were used to dissect apoptotic mechanism at many classical steps. Caspase-3 activity was measured using a caspase-activity assay kit.

RESULTS

A dose-dependent induction of typical apoptosis was observed at extremely high doses, marked by extensive apoptotic body formation. However, certain atypical responses such as cellular hypertrophy and the lack of phosphatidylserine-externalization were observed during the initial hours after radiation. Loss of mitochondrial membrane potential observed at 48 h following a 2000 Gy dose was accompanied by an increase in ROS that caused significant cardiolipin oxidation leading to cytochrome-c release, caspase activation and internucleosomal DNA fragmentation. Inhibitors of B-cell lymphoma-2 (Bcl-2)-associated X protein (Bax)-mediated cytochrome-c release, apoptosome formation and caspase-9 effectively prevented radiation-induced apoptosis, strongly suggesting the role of Bax-dependent cell death mechanism.

CONCLUSIONS

Our study demonstrates that the Sf9 insect cells display good homology with human cells in the mitochondria-dependent events during radiation-induced apoptosis, although doses eliciting similar responses were 50-200 times higher than human cells. Factors upstream to mitochondrial damage remain pertinent for a thorough understanding of this extreme radioresistance displayed by lepidopteran cells.

Evidence for involvement of cytosolic thioredoxin peroxidase in the excessive resistance of Sf9 Lepidopteran insect cells against radiation-induced apoptosis

Lepidopteran insect cells display 50–100 times higher radioresistance compared to human cells, and reportedly have more efficient antioxidant system that can significantly reduce radiation-induced oxidative stress and cell death. However, the antioxidant mechanisms that contribute substantially to this excessive resistance still need to be understood thoroughly. In this study, we investigated the role of thioredoxin peroxidase (TPx) in high-dose γ-radiation response of Sf9 cell line derived from Spodoptera frugiperda, the Fall armyworm. We identified a TPx orthologue (Sf-TPx) in Spodoptera system, with primarily cytosolic localization. Gamma-irradiation at 500 Gy dose significantly up-regulated Sf-TPx, while higher doses (1000 Gy–2000 Gy) had no such effect. G2/M checkpoint induced following 500 Gy was associated with transition of Sf-TPx decamer into enzymatically active dimer. Same effect was observed during G2/M block induced by 5 nM okadaic acid or 10 µM CDK1 (cycline dependent kinase-1) inhibitor roscovitine, thus indicating that radiation-induced Sf-TPx activity is mediated by CDKs. Accumulation of TPx dimer form during G2/M checkpoint might favour higher peroxidase activity facilitating efficient survival at this dose. Confirming this, higher lethal doses (1000 Gy–2000 Gy) caused significantly less accumulation of dimer form and induced dose-dependent apoptosis. A ∼50% knock-down of Sf-TPx by siRNA caused remarkable increase in radiation-induced ROS as well as caspase-3 dependent radiation-induced apoptosis, clearly implying TPx role in the radioresistance of Sf9 cells. Quite importantly, our study demonstrates for the first time that thioredoxin peroxidase contributes significantly in the radioresistance of Lepidopteran Sf9 insect cells, especially in their exemplary resistance against radiation-induced apoptosis. This is an important insight into the antioxidant mechanisms existing in this highly stress-resistant model cell system.

Radioresistant Sf9 insect cells display moderate resistance against cumene hydroperoxide

Lepidopteran insect cells serve as excellent model to study stress responses and are known to display resistance against DNA damaging agents including ionizing radiation; however, limited information is available on the effects of membrane damaging agents in these cells. In this study, we investigated the response of Sf9 cells (derived from ovaries of Spodoptera frugiperda; order Lepidoptera) to cumene hydroperoxide (CHPx), compared to human BMG-1 cells. CHPx treatment at doses lethal for human cells also caused typical necrosis in Sf9. Severe necrosis in human BMG-1 cells was observed at 125 μM, whereas similar effect in Sf9 cells was observed at 250 μM. In Sf9 cells, CHPx (250 μM) induced negligible changes in mitochondrial membrane potential and intracellular reactive oxygen species, while moderate effect was observed on intracellular calcium distribution. Reduced DNA damage and lipid (including cardiolipin) oxidation was observed in Sf9 cells that could be due to moderate total antioxidant status and constitutive/induced glutathione S-transferase activity. This study importantly demonstrates that Lepidopteran insect cells having extensive resistance towards DNA damaging agents show only moderately higher resistance to membrane damaging agents. A stronger reducing environment involving efficient antioxidant system seems to contribute significantly in this response.