Expression of Bombyx mori 30Kc19 protein in Escherichia coli and its anti-apoptotic effect in Sf9 cell

The silkworm hemolymph has an anti-apoptotic activity in insect, mammalian, and human cell systems. The protein from silkworm hemolymph with the highest apoptosis inhibiting activity was found to be 30Kc19 protein, which was one of the ‘30K proteins’. In this study, 30Kc19 protein encoded by the 30Kc19 gene of the silkworm was expressed in Escherichia coli with (pET-22b(+)) and without (pET-3a) pelB leader sequence. 30Kc19 protein was over-expressed largely as a soluble form by pET-3a and both as soluble and insoluble forms by pET-22b(+). The medium was supplemented with each of the recombinant 30Kc19 proteins, and their presence was found to inhibit nuclear fragmentation and apoptotic body formation in actinomycin D-induced Sf9 cell apoptosis. Moreover, 30Kc19 protein repressed the activation of Sf-caspase-1. The 30Kc19 protein obtained from periplasm showed the most effective anti-apoptotic activity. This protein holds great potential for industrial and pharmaceutical applications since mass production and easy purification of this protein is possible.     

Inhibition of HeLa Cell apoptosis by storage-protein 2

Apoptosis is a barrier to maintaining high viable cell densities in animal cell culture. Silkworm hemolymph and its 30K protein have been reported to exhibit anti-apoptotic activity in various mammalian and insect cell systems. The 30K protein is thermally unstable at temperatures higher than 60 degrees C; however, the silkworm hemolymph heat-treated at 70-80 degrees C still exhibited anti-apoptotic activity. This indicates that silkworm hemolymph contains another anti-apoptotic compound other than 30K protein. In this article, the anti-apoptotic molecule other than 30K protein was found from the silkworm hemolymph and identified. This molecule was storage-protein 2 (SP2), which has no homology with any known anti-apoptotic protein. This molecule was heat-stable up to 80 degrees C, while 30K protein lost its activity at temperatures higher than 60 degrees C. When apoptosis was induced by staurosporine in HeLa cells, SP2 protein suppressed nuclear fragmentation and apoptotic body formation. Moreover, the generation of reactive oxygen species after apoptosis induction was inhibited, which means the inhibition occurred in an early step of the apoptotic process. Inhibition of apoptosis by the SP2 protein would lead to the minimization of cell death during commercial mammalian cell culture.     

Inhibition of human cell apoptosis by silkworm hemolymph

Many studies on preventing apoptosis have been carried out from the viewpoint of anti-apoptotic cloned-gene expressions inside cells, whereas in this study, we investigated the inhibition of apoptosis by the addition of silkworm hemolymph, a natural compound, from outside of the cells. In a previous study, we reported the inhibition effect of silkworm hemolymph on the baculovirus-induced insect cell apoptosis. Using the vaccinia virus-HeLa cell system as a model system in this study, we found that silkworm hemolymph, the insect serum, inhibits apoptosis not only in the insect cell system but also in the human cell system. The vaccinia virus-induced HeLa cell apoptosis was analyzed using DNA electrophoresis, TUNEL, and flow cytometry, and the resulting data confirmed that silkworm hemolymph inhibits human cell apoptosis. The inhibition of apoptosis due to silkworm hemolymph was not caused by an inhibition of virus binding and internalization steps, nor did silkworm hemolymph interfere with the virus production. The inhibition of apoptosis by silkworm hemolymph decreased the cell detachment from an adhering surface. With these characteristics, silkworm hemolymph can be effectively used to minimize cell death in commercial animal cell culture.     

Silkworm hemolymph inhibits baculovirus-induced insect cell apoptosis

The effect of silkworm hemolymph on baculovirus-induced insect cell apoptosis was investigated. The addition of silkworm hemolymph into the culture medium either before or during the baculovirus infection increased the host cell longevity; however, its addition after the infection was less effective. This can be explained by the higher transfer rate of silkworm hemolymph which is caused by endocytosis during the virus internalization step. The delayed cell death due to silkworm hemolymph was not caused by an inhibition of the virus attachment and internalization steps. The apoptosis was analyzed using DNA fragmentation and TUNEL assays, and the resulting data confirm that silkworm hemolymph inhibits baculovirus-induced insect cell apoptosis.     

Purification and characterization of an anti-apoptotic protein isolated from Lonomia obliqua hemolymph

Previously it was reported that supplementation of insect cell culture with Lonomia obliqua hemolymph could extend culture longevity (Maranga et al. Biotechnol. Prog. 2003, 19, 58-63). In this work the anti-apoptotic properties of this hemolymph in Spodoptera frugiperda (Sf-9) cell culture were investigated. The presence or absence of apoptotic cells was characterized by light microscopy, flow cytometry, and agarose gel electrophoresis. Hemolymph was fractionated by several ion exchange and gel filtration chromatographic steps for identification of the compounds responsible for this effect. Fractions exhibiting a potent anti-apoptotic effect were isolated and tested in cell culture. A protein of about 51 kDa was identified, isolated, and tested for apoptosis inhibition. Addition of this purified protein to Sf-9 cultures was able to prevent apoptosis induced by nutrient depletion as well as by potent apoptosis chemical inducers such as Actinomycin D. This work confirms that the enhanced culture longevity obtained by supplementation with L. obliqua hemolymph is due to the presence of potent anti-apoptotic factors.     

Anti-oxidative effects of silkworm storage protein 1 in HeLa cell

Silkworm hemolymph contains unique proteins that exhibit anti-apoptotic activity in mammalian cells. Among them, 30 K protein, which is one of the major anti-apoptotic molecules in silkworm hemolymph, has been well investigated. However, little is known about the biological functions of storage protein 1 (SP1), another main protein in silkworm hemolymph. In this study, the anti-apoptotic and anti-oxidative activities of SP1 were analyzed. A stable cell line expressing SP1 was constructed, which showed strong anti-apoptotic effect induced by staurosporine treatment. In addition, the cell line exhibited resistance to oxidative stress caused by hydrogen peroxide. For practical applications of SP1, recombinant SP1 was produced in Escherichia coli, and the supplementation of recombinant SP1 into culture medium exhibited anti-apoptotic and anti-oxidative activities. In addition, SP1 was found to be a cell-penetrating protein and localized in the cytosol as well as on the plasma membrane. The findings showed that SP1 itself is not an anti-oxidant; rather, it mediates intracellular anti-oxidative activity. In conclusion, the cellular resistance of SP1 to apoptosis and oxidative stress will provide a new strategy that could be utilized in the bio-industry for the production of biologics as well as for the development of anti-aging cosmetics.     

Kinetic effect of silkworm hemolymph on the delayed host cell death in an insect cell-baculovirus system

The kinetic effect of silkworm hemolymph on host cell viability during a baculovirus-induced insect cell death process was investigated. Host cell viability after viral infection is important for replication of the baculovirus DNA containing a recombinant gene and expression of the cloned gene. The baculovirus-induced insect cell death process can be divided into a delay phase and a first-order death phase, which are characterized by a delay time (t(d)) and a specific death rate (k(d)), respectively. For 0-10% silkworm hemolymph in the media, higher concentrations resulted in longer delay times and lower specific death rates. By adding 10% silkworm hemolymph, the delay time increased from 72 to 164 h, and the specific death rate was reduced from 13.8 x 10(-)(3) to 6.0 x 10(-)(3) h(-)(1). In addition, host cell viability correlated with DNA fragmentation, which is the biochemical hallmark of apoptosis. This indicates that the silkworm hemolymph inhibits the baculovirus-induced insect cell apoptosis. However, the silkworm hemolymph did not affect the number of hypothetical targets, which represents host cell susceptibility to the baculovirus. The concentration of fetal bovine serum (FBS) in the medium did not affect the delay time, while lower concentrations of silkworm hemolymph resulted in shorter delay times. This means that the substance which increases the longevity of the host cell is not in the FBS but in the silkworm hemolymph.     

SfDredd,a Novel Initiator Caspase Possessing Activity on Effector Caspase Substrates in Spodoptera frugiperda

Sf9, a cell line derived from Spodoptera frugiperda, is an ideal model organism for studying insect apoptosis. The first notable study that attempted to identify the apoptotic pathway in Sf9 was performed in 1997 and included the discovery of Sf-caspase-1, an effector caspase of Sf9. However, it was not until 2013 that the first initiator caspase in Sf9, SfDronc, was discovered, and the apoptotic pathway in Sf9 became clearer. In this study, we report another caspase of Sf9, SfDredd. SfDredd is highly similar to insect initiator caspase Dredd homologs. Experimentally, recombinant SfDredd underwent autocleavage and exhibited different efficiencies in cleavage of synthetic caspase substrates. This was attributed to its caspase activity for the predicted active site mutation blocked the above autocleavage and synthetic caspase substrates cleavage activity. SfDredd was capable of not only cleaving Sf-caspase-1 in vitro but also cleaving Sf-caspase-1 and inducing apoptosis when it was co-expressed with Sf-caspase-1 in Sf9 cells. The protein level of SfDredd was increased when Sf9 cells were treated by Actinomycin D, whereas silencing of SfDredd reduced apoptosis and Sf-caspase-1 cleavage induced by Actinomycin D treatment. These results clearly indicate that SfDredd functioned as an apoptotic initiator caspase. Apoptosis induced in Sf9 cells by overexpression of SfDredd alone was not as obvious as that induced by SfDronc alone, and the cleavage sites of Sf-caspase-1 for SfDredd and SfDronc are different. In addition, despite sharing a sequence homology with initiator caspases and possessing weak activity on initiator caspase substrates, SfDredd showed strong activity on effector caspase substrates, making it the only insect caspase reported so far functioning similar to human caspase-2 in this aspect. We believe that the discovery of SfDredd, and its different properties from SfDronc, will improve the understanding of apoptosis pathway in Sf9 cells.     

Anti-apoptosis engineering

An increased understanding of apoptosis makes anti-apoptosis engineering possible, which is an approach used to inhibit apoptosis for the purpose of therapeutic, or industrial applications in the treatment of the diseases associated with increased apoptosis, or to improve the productivity of animal cell cultures, respectively. Some known anti-apoptotic proteins are the Bcl-2 family, IAP (inhibitor of apoptosis) and Hsps (heat shock proteins), with which anti-apoptosis engineering has progressed. This article reviews anti-apoptosis engineering using known anti-apoptotic compounds, and introduces a 30 K protein, isolated from silkworm hemolymph, as a novel anti-apoptotic protein, that shows no homology with other known anti-apoptotic proteins. The regulation of apoptosis, using anti-apoptotic proteins and genes originating from the silkworm,Bombyx mori, may provide a new strategy in this field.     

Isolation and characterization of an apoptosis-inhibiting component from the hemolymph of Bombyx mori.

Silkworm (Bombyx mori) hemolymph showed an apoptosis-inhibiting activity in insect cells (Sf 9) infected with baculovirus (AcNPV). The addition of silkworm hemolymph into the culture medium increased the host cell longevity due to its apoptosis-inhibition activity. Components with an apoptosis-inhibiting effect were purified from the silkworm hemolymph by heat treatment, gel-filtration chromatography, and ion-exchange chromatography. The component with highest activity was characterized by periodic acid-Schiff staining, isoelectric focusing, MALDI-TOF-mass spectrometry, and N-terminal sequencing and was found to be a nonglycosylated monomeric protein with a molecular weight of ca. 28,000 Da.     

Apoptosis in motion. An apical, P35-insensitive caspase mediates programmed cell death in insect cells

Activation of caspases by proteolytic processing is a critical step during apoptosis in metazoans. Here we use high resolution time lapse microscopy to show a tight link between caspase activation and the morphological events delineating apoptosis in cultured SF21 cells from the moth Spodoptera frugiperda, a model insect system. The principal effector caspase, Sf-caspase-1, is proteolytically activated during SF21 apoptosis. To define the potential role of initiator caspases in vivo, we tested the effect of cell-permeable peptide inhibitors on pro-Sf-caspase-1 processing. Anti-caspase peptide analogues prevented apoptosis induced by diverse signals, including UV radiation and baculovirus infection. IETD-fmk potently inhibited the initial processing of pro-Sf-caspase-1 at the junction (TETD-G) of the large and small subunit, a cleavage that is blocked by inhibitor of apoptosis Op-IAP but not pancaspase inhibitor P35. Because Sf-caspase-1 was inhibited poorly by IETD-CHO, our data indicated that the protease responsible for the first step in pro-Sf-caspase-1 activation is a distinct apical caspase. Thus, Sf-caspase-1 activation is mediated by a novel, P35-resistant caspase. These findings support the hypothesis that apoptosis in insects, like that in mammals, involves a cascade of caspase activations.     

Silkworm hemolymph as a substitute for fetal bovine serum in insect cell culture

Summary The effectiveness of silkworm hemolymph was investigated as a substitute for fetal bovine serum (FBS) in insect cell culture. Cells were adapted to grow in reduced FBS medium supplemented with silkworm hemolymph through a gradual adaptation process. FBS concentration in the medium could be reduced to 1% without decrease in cell growth rate and maximum cell concentration by adding 5% silkworm hemolymph.     

Induction of apoptosis in SF21 cell line by conditioned medium of the entomopathogenic fungus, Nomuraea rileyi, through Sf-caspase-1 signaling pathway

The apoptosis in SF-21 cell line can be induced by the conditioned medium (CM) of the entomopathogenic fungus, Nomuraea rileyi, based on changes in morphology and formation of apoptotic bodies in cultured cells, and with the onset of DNA fragmentation as shown by TUNEL staining and agarose electrophoresis. Moreover, the induction of apoptosis in SF-21 cells was inhibited by adding the inhibitor of effector caspase, viz. z-DEVD-fmk, to the CM, indicating that Sf-caspase-1 is involved in this apoptosis. Similarly, the inhibitor of initiator caspase, viz., z-VAD-fmk, inhibited apoptosis. Therefore, both initiator and effector caspases are possibly involved in the apoptosis of SF-21 cells. In addition, we detected Sf-caspase-1 activity in the process of apoptosis in SF-21 cells, suggesting that the effector caspase in SF-21 is similar to that found in mammalian cells. Our results also indicated that the apoptosis found in this line is accomplished through a Sf-caspase-1 signaling pathway.     

Enhancement of recombinant protein production in Chinese hamster ovary cells through anti-apoptosis engineering using 30Kc6 gene

It was previously reported that silkworm hemolymph (SH) inhibits apoptosis and increases the production of recombinant human erythropoietin (EPO) in Chinese hamster ovary (CHO) cells. The apoptosis-inhibiting component in SH is a member of 30K protein family. In this study, the CHO cell line producing EPO was manipulated genetically to express the 30Kc6 gene encoding a 30K protein in the hemolymph of the silkworm, Bombyx mori. The transient expression of 30Kc6 significantly suppressed the cell death induced by serum deprivation. A stable cell line expressing 30Kc6 with an anti-apoptotic property was established. The stable expression of 30Kc6 inhibited serum-deprivation-induced apoptosis and increased the cell density and EPO titer by 5- and 10-fold, respectively. The positive effects of the 30Kc6 expression on cell viability and productivity were due to the stable maintenance of the mitochondrial activity. The 30Kc6 expression efficiently suppressed the depolarization of the mitochondrial membrane and subsequently balanced the generation/consumption of ATP. The use of the 30Kc6 gene is expected to provide a new method of host cell engineering for improving the productivity of the recombinant protein.     

Enhancement of Sf-9 cell growth and longevity through supplementation of culture medium with hemolymph

The benefits of insect cell culture medium supplementation with hemolymph of Lonomia obliqua were investigated. The addition of hemolymph to the medium induced high levels of cell growth, and the viability was maintained for longer periods. The maximum cell yield increased almost 3-fold after hemolymph supplementation. Cultures in their stationary phase were rescued through hemolymph supplementation, also reaching high cell concentrations. These actions were much dependent on the concentration of hemolymph; low hemolymph concentration had a positive effect in cell growth, whereas high hemolymph concentration showed a deleterious effect. Fractionation of hemolymph by gel filtration chromatography showed the presence of three factors with different activity in insect cell culture: an potential anti-apoptotic factor, a growth-promoting factor, and an enzyme that hydrolyzes sucrose. Addition of hemolymph to the medium induced high levels of glucose production. The sucrose to glucose conversion was also linearly dependent upon the hemolymph concentration. Therefore, we conclude that cell growth and longevity can be increased by supplementation of the culture medium with hemolymph.     

Beneficial effect of silkworm hemolymph on a CHO cell system: Inhibition of apoptosis and increase of EPO production

To produce erythropoietin (EPO), Chinese hamster ovary (CHO) cells were first cultured in a medium containing FBS (growth medium) and then in a serum-free medium containing sodium butyrate (production medium). Sodium butyrate increases recombinant protein production, but also induces apoptosis, which reduces cell viability and productivity. In a previous study, we found that silkworm hemolymph (SH), an insect serum, inhibits the apoptosis of insect and mammalian cells. To overcome sodium butyrate-induced apoptosis, we added SH to growth medium. This pretreatment with SH inhibited the sodium butyrate-induced apoptosis of CHO cells and consequently increased their longevity and their ability to produce EPO. As a result, the volumetric productivity of EPO was increased five-fold. SH was found to inhibit cytochrome c release from mitochondria into the cytosol, and prevented the activation of caspase-3 and other subsequent caspase reactions.     

Protective Effect of the Silkworm Protein 30Kc6 on Human Vascular Endothelial Cells Damaged by Oxidized Low Density Lipoprotein (Ox-LDL)

Although the 30K family proteins are important anti-apoptotic molecules in silkworm hemolymph, the underlying mechanism remains to be investigated. This is especially the case in human vascular endothelial cells (HUVECs). In this study, a 30K protein, 30Kc6, was successfully expressed and purified using the Bac-to-Bac baculovirus expression system in silkworm cells. Furthermore, the 30Kc6 expressed in Escherichia coli was used to generate a polyclonal antibody. Western blot analysis revealed that the antibody could react specifically with the purified 30Kc6 expressed in silkworm cells. The In vitro cell apoptosis model of HUVEC that was induced by oxidized low density lipoprotein (Ox-LDL) and in vivo atherosclerosis rabbit model were constructed and were employed to analyze the protective effects of the silkworm protein 30Kc6 on these models. The results demonstrated that the silkworm protein 30Kc6 significantly enhanced the cell viability in HUVEC cells treated with Ox-LDL, decreased the degree of DNA fragmentation and markedly reduced the level of 8-isoprostane. This could be indicative of the silkworm protein 30Kc6 antagonizing the Ox-LDL-induced cell apoptosis by inhibiting the intracellular reactive oxygen species (ROS) generation. Furthermore, Ox-LDL activated the cell mitogen activated protein kinases (MAPK), especially JNK and p38. As demonstrated with Western analysis, 30Kc6 inhibited Ox-LDL-induced cell apoptosis in HUVEC cells by preventing the MAPK signaling pathways. In vivo data have demonstrated that oral feeding of the silkworm protein 30Kc6 dramatically improved the conditions of the atherosclerotic rabbits by decreasing serum levels of total triglyceride (TG), high density lipoprotein cholesterol (HDL-C), low density lipoprotein cholesterol (LDL-C) and total cholesterol (TC). Furthermore, 30Kc6 alleviated the extent of lesions in aorta and liver in the atherosclerotic rabbits. These data are not only helpful in understanding the anti-apoptotic mechanism of the 30K family proteins, but also provide important information on prevention and treatment of human cardiovascular diseases.     

Anti-apoptotic mechanism of silkworm hemolymph in HeLa cell apoptosis

Silkworm hemolymph (SH) was found to exhibit anti-apoptotic activities in mammalian and insect cell systems. An anti-apoptotic mechanism of SH was investigated in a staurosporine-induced HeLa cell using flow cytometry, caspase assay, Immunoblot, and Immunochemistry. The addition of 5% SH to the medium resulted in lower intracellular activities of caspase-3 and caspase-9 after 0.6 μM of staurosporine treatment; however, SH did not directly inhibit the activities of those enzymes. This suggests SH inhibits the event upstream of these caspase activation steps, such as mitochondrial level events. We found from Immunoblot and Immunochemistry that cytochrome c release from the mitochondria was blocked by SH. SH also inhibited Bax translocation to the mitochondria. On the contrary, SH did not block the apoptosis when Bax is not involved in promoting apoptosis. With these results, we propose that SH protects mitochondria from apoptosis signal via blocking Bax translocation, and the subsequent apoptotic events are then inhibited. The inhibition of apoptosis using SH and its components may lead to new approaches for the minimization of cell death during commercial animal cell cultures.     

Crystal structure of an invertebrate caspase

Caspases play an essential role in the execution of apoptosis. These cysteine proteases are highly conserved among metazoans and are translated as inactive zymogens, which are activated by proteolytic cleavages to generate the large and small subunits and remove the N-terminal prodomain. The 2.3 A resolution crystal structure of active Sf-caspase-1, the principal effector caspase of the insect Spodoptera frugiperda, is presented here. The structure represents the first nonhuman caspase to be resolved. The structure of the cleaved and active protease was determined with the tetrapeptide inhibitor N-acetyl-Asp-Glu-Val-Asp-chloromethylketone covalently bonded to the active site cysteine. As expected, the overall fold of Sf-caspase-1 is exceedingly similar to that of the five active caspases from humans solved to date. The overall structure and active site arrangement of Sf-caspase-1 is most comparable with that of the human effector caspases, with which it shares highest sequence homology. The most prominent structural difference with Sf-caspase-1 is the position of the N-terminal region of the large subunit. Unlike the N terminus of human caspases, the N terminus of Sf-caspase-1 originates from the active site side where it interacts with active site loop L2 and then extends to the backside of the heterodimer. This unusual structural arrangement raises the possibility that the N-terminal prodomain plays a regulatory role during effector caspase activation or enzyme activity in insects.     

Inhibition of apoptosis by a Bombyx mori gene

An apoptosis-inhibiting component of silkworm hemolymph, isolated and characterized in our previous study, showed 95% N-terminal amino acid sequence homology with one of the 30K proteins, a group of structurally related proteins. The 30K protein was expressed in mammalian HEK293 cells and CHOK1 cells by transfection with 30Kc6. The expression of 30Kc6 inhibited apoptosis comparably to that of whole silkworm hemolymph, indicating that both intracellular expression and external supplementation inhibited apoptosis. The expression of 30Kc6 resulted in lower intracellular activity for caspase 3. However, the results of in vitro assay of caspase 3 show that the 30Kc6 protein does not inhibit caspase 3 activity. This indicates that the 30Kc6 protein inhibits the apoptosis by working in a further upstream event than caspase 3 activation.