Radiation responses of Sf9, a highly radioresistant lepidopteran insect cell line

Effects of X-ray irradiation on the fitness of the established invasive pest fall armyworm Spodoptera frugiperda

BACKGROUND

Spodoptera frugiperda has spread to Africa, Asia, and Oceania, posing a serious threat to global agriculture. We estimated the appropriate dose of X-ray sterilization for S. frugiperda using an X-ray irradiation instrument to investigate environmentally acceptable control techniques, laying the framework for future applications of sterile insect technology (SIT) to manage the pest environmentally-friendly.

RESULTS

This study is the first to investigate the effects of X-ray irradiation on the growth, development, survival, reproduction, and flight of S. frugiperda. The results showed that irradiation with 50-400 Gy had no significant effect on pupal eclosion, but females were more sensitive than males in terms of reproductive parameters, especially when doses of radiation were > 350 Gy. After irradiation with a sub-sterilizing dose of 250 Gy, the parental sterility rate was > 85%, and the sterility traits could be passed on to their offspring, resulting in a continuous decrease in the population of F₁ and F₂ generations.

CONCLUSION

Our laboratory experiments theoretically confirmed the feasibility of SIT for controlling S. frugiperda in the field using X-ray radiation. This study provides a theoretical basis for future regional pest management strategies. © 2022 Society of Chemical Industry.

Advances and Challenges of Using the Sterile Insect Technique for the Management of Pest Lepidoptera

Over the past 30 years, the sterile insect technique (SIT) has become a regular component of area-wide integrated pest management (AW-IPM) programs against several major agricultural pests and vectors of severe diseases. The SIT-based programs have been especially successful against dipteran pests. However, the SIT applicability for controlling lepidopteran pests has been challenging, mainly due to their high resistance to the ionizing radiation that is used to induce sterility. Nevertheless, the results of extensive research and currently operating SIT programs show that most problems with the implementation of SIT against pest Lepidoptera have been successfully resolved. Here, we summarize the cytogenetic peculiarities of Lepidoptera that should be considered in the development and application of SIT for a particular pest species. We also discuss the high resistance of Lepidoptera to ionizing radiation, and present the principle of derived technology based on inherited sterility (IS). Furthermore, we present successful SIT/IS applications against five major lepidopteran pests, and summarize the results of research on the quality control of reared and released insects, which is of great importance for their field performance. In the light of new research findings, we also discuss options for the development of genetic sexing strains, which is a challenge to further improve the applicability of SIT/IS against selected lepidopteran pests.

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.

Gamma radiation tolerance in different life stages of the fruit fly Drosophila melanogaster

PURPOSE

Insects are known to have higher levels of radiation tolerance than mammals. The fruit fly Drosophila provides opportunities for genetic analysis of radiation tolerance in insects. A knowledge of stage-specific sensitivity is required to understand the mechanisms and test the existing hypothesis of insect radiation tolerance.

MATERIALS AND METHODS

Drosophila melanogaster were irradiated using gamma rays at different life stages. Irradiation doses were chosen to start from 100-2200 Gy with increments of 100 Gy, with a dose rate of 12.5 and 25 Gy/min. The threshold of mortality, LD₅₀ and LD₁₀₀ 1 h post-irradiation was recorded for larvae and adults and 24 h post-irradiation for eggs and after 2-3 days for early and late pupae. Total antioxidant capacity for all the life stages was measured using the phosphomolybdenum method.

RESULTS

Twenty-four hours post-irradiation, 100% mortality was recorded for eggs at 1000 Gy. One hour post irradiation 100% mortality was recorded at 1300 Gy for first instar larvae, 1700 Gy for second instar larvae, 1900 Gy for feeding third instar larvae and 2200 Gy for non-feeding third instar larvae. Post-irradiation complete failure of emergence (100% mortality) was observed at 130 Gy for early pupae and 1500 Gy for late pupae; 100% mortality was observed at 1500 Gy for adults. The values of LD₅₀ were recorded as 452 Gy for eggs, 1049 Gy for first instar larvae, 1350 Gy for second instar larvae, 1265 Gy for feeding third instar larvae, 1590 Gy for non-feeding third instar larvae, 50 Gy for early pupae, 969 Gy for late pupae, 1228 Gy for adult males and 1250 Gy for adult females.

CONCLUSIONS

Early pupae were found to be prone to radiation, whereas the non-feeding third instar larvae were most resistant among all stages. The chromosome number being constant and total antioxidant capacity being nearly constant in all stages, we suggest that high rate of cell division during early pupae makes this stage sensitive to radiation.

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.

The mitochondria-mediate apoptosis of Lepidopteran cells induced by azadirachtin

Mitochondria have been shown to play an important role in apoptosis using mammalian cell lines. However, this seems not to be the case in Drosophila, an insect model organism; thus more in-depth studies of insect cell apoptosis are necessary. In the present study, mitochondrial involvement during azadirachtin- and camptothecin-induced apoptosis in Spodoptera frugiperda Sf9 cells (isolated from Spodoptera frugiperda pupal ovarian tissue) was investigated. The results showed that both azadirachtin and camptothecin could induce apoptosis in Sf9 cells. Reactive oxygen species (ROS) generation, activation of mitochondrial permeability transition pores (MPTPs) and loss of mitochondrial membrane potential (MMP) were observed very early during apoptosis and were followed subsequently by the release of cytochrome-c from the mitochondria. Furthermore, the results also revealed that the opening of MPTPs and the loss of MMP induced by azadirachtin could be significantly inhibited by the permeability transition pore (PTP) inhibitor cyclosporin A (CsA), which was used to identify the key role of mitochondria in the apoptosis of Sf9 cells. However, in camptothecin-treated Sf9 cells, CsA could not suppress the opening of MPTPs and the loss of MMP when apoptosis was induced. The data from caspase-3 and caspase-9 activity assays and detection of apoptosis by morphological observation and flow cytometry also uncovered the different effect of CsA on the two botanical apoptosis inducers. Although different mechanisms of apoptosis induction exist, our study revealed that mitochondria play a crucial role in insect cell line apoptosis.

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.

An improved non-enzymatic "DNA ladder assay" for more sensitive and early detection of apoptosis

Conventional DNA ladder assay has certain shortcomings such as loss of DNA fragments during sample processing, involvement of multiple steps and requirement of expensive reagents. The present study demonstrates a rapid, easy-to-perform cost-effective method for detection of apoptotic DNA fragments with considerable improvement in the sensitivity by avoiding loss of DNA fragments. It involves a few minutes of procedure involving direct lysis of cells with dimethyl sulphoxide (DMSO), brief vortexing, addition of 2% SDS-TE buffer, and a single step of centrifugation. This cost- and time-efficient method reduces the assay time considerably and can be used for a large number of samples with excellent sensitivity.

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

Lepidopteran insects/insect cells display 50-100 times higher radioresistance than humans, and are evolutionarily closest to mammals amongst all radioresistant organisms known. Compared to mammalian cells, Lepidopteran cells (TN-368, Sf9) display more efficient antioxidant system and DNA repair and suffer considerably less radiation-induced DNA/cytogenetic damage and apoptosis. Recent studies indicate that a considerably lower radiation-induced oxidative stress may significantly reduce macromolecular damage in Lepidopteran cells. Since nitrosative stress contributes in radiation-induced cellular damage, we investigated its nature in the γ-irradiated Sf9 cells (derived from Spodoptera frugiperda; order Lepidoptera; family Noctuidae) and compared with BMG-1 human cell line having significant NOS expression. Radiation induced considerably less ROS/RNS in Sf9 cells, which remained unchanged on treatment with NOS inhibitor l-NMMA. Surprisingly, growth of Sf9 cultures or irradiation could not induce NO or its metabolites, indicating negligible basal/radiation-induced NOS activity that remained unchanged even after supplementation with arginine. Cytosolic calcium release following high-dose (1000-2000Gy at 61.1cGys(-1)) γ-irradiation or H(2)O(2) (250μM) treatment also failed to generate NO in Sf9 cells having high constitutive levels of calmodulin, whereas BMG-1 cells displayed considerable calcium-dependent NO generation even following 10Gy dose. These results strongly imply the lack of calcium-mediated NOS activity in Sf9 cells. Addition of exogenous NO from GSH-NO caused considerable increase in radiation-induced apoptosis, indicating significant contribution of constitutively attenuated nitrosative stress response into the radioresistance of Lepidopteran cells. Our study demonstrates for the first time that a calcium-insensitive, attenuated nitrosative stress response may contribute significantly in the unusual radioresistance displayed by Lepidopteran insect cells.

Inhibition of microRNA-14 contributes to actinomycin-D-induced apoptosis in the Sf9 insect cell line

Actinomycin-D (Act-D) and other inhibitors of RNA synthesis induce extensive and rapid apoptosis in the lepidopteran insect cells. Interestingly, a similar effect is not observed in the case of protein synthesis shutdown, implying that certain RNA species may be critically required for cell survival. In order to assess whether depletion of certain anti-apoptotic microRNAs may result in insect cell apoptosis induced by these transcriptional inhibitors, we inhibited two antiapoptotic microRNAs, viz. bantam and miR-14 (microRNA-14), with known functions in insect systems, by transfecting lepidopteran Sf9 cell line (derived from Spodoptera frugiperda) with sequence-specific inhibitory anti-miRs. Our results indicate that miR-14 is indeed required for constitutive cell survival as its inhibition caused considerable apoptosis. Importantly, exogenous supplementation with the mimics of miR-14 precursor molecules could partially inhibit the Act-D-induced Sf9 cell death. Further, our results indicate that miR-14 may function downstream of mitochondrial cytochrome c release in preventing Act-D-induced apoptosis, implying possible inhibitory interactions with caspases as reported previously in other organisms. While the microRNA species are known to regulate cell death in Drosophila, which belongs the insect order Diptera, the present study demonstrates a definitive antiapoptotic role of miR-14 in lepidopteran apoptosis as well. Our study also indicates that additional microRNA species may be regulating lepidopteran cell survival and death, thus warranting further in-depth investigations into these important mechanisms of cell death. Since lepidopteran cells are an excellent model for general stress resistance, this study presents important information about their stress response mechanisms.

Predictive role of mitochondrial genome in the stress resistance of insects and nematodes

Certain insects (e.g., moths and butterflies; order Lepidoptera) and nematodes are considered as excellent experimental models to study the cellular stress signaling mechanisms since these organisms are far more stress-resistant as compared to mammalian system. Multiple factors have been implicated in this unusual response, including the oxidative stress response mechanisms. Radiation or chemical-induced mitochondrial oxidative stress occurs through damage caused to the components of electron transport chain (ETC) leading to leakage of electrons and generation of superoxide radicals. This may be countered through quick replacement of damaged mitochondrial proteins by upregulated expression. Since the ETC comprises of various proteins coded by mitochondrial DNA, variation in the composition, expressivity and regulation of mitochondrial genome could greatly influence mitochondrial role under oxidative stress conditions. Therefore, we carried out in silico analysis of mitochondrial DNA in these organisms and compared it with that of the stress-sensitive humans/mammals. Parameters such as mitochondrial genome organization, codon bias, gene expressivity and GC₃ content were studied. Gene arrangement and Shine-Dalgarno (SD) sequence patterns indicating translational regulation were distinct in insect and nematodes as compared to humans. A higher codon bias (ENC≫35) and lower GC₃ content (≫0.20) were observed in mitochondrial genes of insect and nematodes as compared to humans (ENC>42; GC3>0.20), coupled with low codon adaptation index among insects. These features indeed favour higher expressivity of mitochondrial proteins and might help maintain the mitochondrial physiology under stress conditions. Therefore, our study indicates that mitochondrial genome organization may influence stress-resistance of insects and nematodes.

Predictive inference on cytoplasmic and mitochondrial thioredoxin peroxidases in the highly radioresistant Lepidopteran insect Spodoptera frugiperda

Lepidopteran insects show remarkable resistance to radiation and chemical stress than insects of other orders. Despite this, the antioxidant machinery of insects of this order is poorly understood. Recently we demonstrated the significance of cytoplasmic NOS and a stronger mitochondrial antioxidant enzyme system in the stress-resistance of Lepidopteran insects. In the present study, we hypothesize two thioredoxin peroxidase orthologues (Sf-TPx1 and Sf-TPx2) in Lepidopteran insect Spodoptera frugiperda and demonstrate their structural/functional features important for cellular antioxidant activity and stress resistance. Results show a higher mitochondrial localization score (WoLFPSORT) of Sf-TPx2 (mitochondria-18.0, cytoplasm-7.0, nucleus-4.0) than its Drosophila orthologue Jafrac2 (secretory-30.0; mitochondria/nucleus/cytoplasm-no signal), which is important for antioxidant activity, and a higher cytoplasmic localization score of Sf-TPx1 (mitochondria-no signal; cytoplasm-22.0; nucleus-3.5) than the Drosophila Jafrac1 (mitochondria-17; nucleus- 11; cytoplasm-no signal). Structural modeling data show certain motifs present in Jafrac1 and Jafrac2 that affect active site conformation and separate cysteine residues at distances not suitable for disulphide bridge formation (5.21Å; 5.73Å). These motifs are absent in Sf-TPx1 and Sf-TPx2, yielding shorter distance (2.01Å; 2.05Å) between the cysteine residues suitable for disulphide bridge formation. Taken together, the disulphide bridge as well as mitochondrial and cytoplasmic localization are crucial for peroxidatic activity of TPx's. Therefore,we hypothesize that the Spodoptera TPx's offer potentially stronger anti-oxidant activity than that of Drosophila orthologues, and may contribute in the high radioresistance of Lepidopteran insects.

Mitochondrial antioxidant defence in radio-resistant Lepidopteran insect cells

Cells isolated from Lepidopteran insects (butterfly and moths) display very high radioresistance as compared to mammals and other insect species. Since free radical induced mitochondrial damage under stress conditions is very crucial for cellular fate determination, antioxidant system is the major protective modality required to minimize stress-induced damage and to modulate cellular sensitivity. In this study, we predict the mitochondrial localization potential and co-existence of important antioxidant enzymes in insect cells and compare with other radiosensitive (mammals, Dipteran insects) and radioresistant (nematodes) species. Our study clearly demonstrates the inter-species variation in then localization potential of various antioxidant enzymes. A higher mitochondrial localization potential as a function of mitoprot score was evident for all important antioxidant enzymes in the lepidopteran insect Bombyx mori (Mn-SOD, 0.694; GPx, 0.862; TRPx, 0.997; TR, 0.9), besides an unusual mitochondrial localization prediction for catalase (0.453). We further found coexistence of glutathione and thioredoxin system in the mitochondria of lepidopteran insects as also reported in various plant species. On the basis of above observations, we hypothesize that a strong mitochondrial antioxidant enzyme system including the unusual coexistence of catalase, glutathione and thioredoxin system may help minimize the free radical mediated damage to mitochondria and can contribute to the intrinsic radioresistance of lepidopteran insects.

Multiple factors conferring high radioresistance in insect Sf9 cells

Sf9, a lepidopteran cell line isolated from the fall armyworm, Spodoptera frugiperda, was shown to be significantly more resistant to growth inhibition and apoptosis induction effects of x-ray irradiation than several human cell lines of different origins. The single-cell electrophoresis technique revealed that Sf9 cells showed lower x-ray irradiation-induced DNA damage as well as better efficiency at repairing these damages. In addition, Sf9 cells were lower in both background and x-ray irradiation-induced intracellular oxidative stress, in which the higher intracellular level of reduced glutathione seemed to play a major role. The significance of oxidative stress in determining the radioresistance of Sf9 cells was confirmed by their being more resistant to hydrogen peroxide while equally susceptible to other non-reactive oxygen species of N-nitroso alkylating agents when compared with a human cell line. Although the Sf9 and human cell lines were equally susceptible to the lethal effects of N-nitroso alkylating agents, the components of DNA damage-induced and the repair enzymes involved significantly differ. This phenomenon is also discussed in this report.

Mitochondrial regulation of insect cell apoptosis: evidence for permeability transition pore-independent cytochrome-c release in the Lepidopteran Sf9 cells

Role of cytochrome-c in insect cell apoptosis is highly controversial, with many earlier reports suggesting lack of involvement of mitochondrial factors in Drosophila while more recent studies have indicated otherwise, thus warranting more in-depth studies of insect cell apoptosis. In the present study, we investigated mitochondrial involvement during actinomycin-D induced apoptosis in Sf9 Lepidopteran cells. Cytochrome-c was released from mitochondria very early during apoptosis, and was preceded quickly by ROS generation and cardiolipin peroxidation. Albeit cytochrome-c release and apoptosis induction were inhibited by bongkrkicacid (BKA) it appears that the release is independent of permeability transition pore (PTP) as it preceded mitochondrial Ca(2+) buildup and mitochondrial membrane potential (MMP) loss. Further, the release was found to be unaffected by PTP inhibitor cyclosporin-A. Bax inhibitory peptide BiP-P5 could effectively block both cytochrome-c release and apoptosis induction indicating dependence on Bax-channel formation. Inhibition of apoptosis by FSBA, a nucleotide analog that inhibits apoptosome formation through Apaf1 binding, suggested activity of apoptosome similar to mammalian cells. Mitochondria isolated from treated cells activated caspases in the cytosolic fraction of untreated cells while mitochondrial lysates of treated or untreated cells had similar effect. Sequestering cytochrome-c in mitochondrial lysates inhibited DEVDase activity, and addition of purified cytochrome-c and dATP to Sf9 cytosolic fraction induced DEVDase activity, suggesting that cytochrome-c may be exclusively required for Lepidopteran apoptosis. This is the first detailed study demonstrating mitochondrial regulation of Lepidopteran insect cell apoptosis, and reiterates its homology with mammalian cell apoptosis while showing distinctive differences from earlier reports in Drosophila.

Role of nitric oxide synthase in insect cell radioresistance: an in-silico analysis

Previous studies on various insect cell lines have displayed very high radioresistance in Lepidoptera (butterflies and moths) as compared to mammals as well as other orders of Insecta including Diptera. Since NOS is known to modulate cellular radiation sensitivity, we carried out in silico analysis of Lepidopteran NOS and compared its structural and functional features including the sequence homology, predicted tertiary structure, post-translational phosphorylation and intracellular localization with the other species. Our study demonstrates that Lepidopteran NOS, while carrying significant sequence homology with mammalian nNOS, has structural/ functional features that may enhance resistance to radiation and other stress agents. A higher phosphorylation score of Lepidopteran NOS (0.885+/-0.02 as against 0.694+/-0.094 of mammalian NOS; predicted using Net Phos 2.0) was observed at many well-conserved phosphorylation sites, which may reduce NOS activation by stress agents including radiation. Further, the primarily cytoplasmic localization of Lepidopteran NOS (score 23 against 10 of mammalian NOS, derived using WoLFPSORT), aided by higher phosphorylation scores as well as sequence-driven cytoplasmic localizing signals, may significantly reduce amplification of extraneous oxidative damage. Based on these findings, we hypothesize that a primarily cytosolic and less responsive NOS could significantly contribute to radioresistance of Lepidopteran insects as well as their cultured cell lines.