curcumin, as indicated by the lack of plaque formation (Fig. 5B). However, dilutions of the virus-solvent control (DMSO) mixture restored the marginal inhibitory effects of DMSO. We then used the embryonic chicken egg, a potent amplification vessel for the influenza virus, to investigate whether curcumin treatment irreversibility inhibited the influenza virus. Viruses pre-treated with curcumin (for one hour) were unable to amplify in embryonic eggs. However, eggs innoculated with a high dose (5000 PFU) of PR8 treated with DMSO produced 25.5 HA units and 29.75 HA units of viral progeny at 18 h and 24 h after infection, respectively (Table 1 and Fig. 5C). These results indicated that the inhibitory effects of curcumin on influenza virus infectivity are irreversible.
Discussion
This study presents several novel findings. To our knowledge, it is the first to show that curcumin generally inhibits enveloped virus infectivity. In addition to inhibiting HA activity, a novel mech- anism was investigated; as evidenced in the liposome-based assay systems, we proposed that the integrity of membrane structure, e.g., viral envelope, could be affected by curcumin treatment. As for the four enveloped viruses analysed in the current study, the EC50 of curcumin on inhibition of plaque formation for larger viruses is greater than that for smaller viruses.
Previous studies reported that curcumin associates with membranes [26,27]. The hydrophobic properties of membranes favor the intercalation of curcumin into the lipid bilayer, such as in cellular membranes, where phenolic rings of curcumin are essential for interaction with hydrogen-bonding sites. Several studies also identified curcumin as a membrane-disturbing agent. Curcumin treatment induced alterations in membranous proper- ties, including morphological changes, and increased permeability and fluidity. The interactions between the cell membranes and curcumin might have caused these effects [26,28]. Jaruga et al. observed that treatment of erythrocytes with .100 mM curcumin concentrations induced changes in the integrity of their cell membranes [26,27]. Similarly, at a high treatment concentration (5 mM), curcumin increased lactate dehydrogenase (LDH) leakage in rat hepatocytes (25%) compared with the LDH leakage observed in untreated cells (15%) [29]. Research using a rat thymocyte model further showed that curcumin is able to penetrate the cytoplasm and accumulate in membranous struc- tures of intracellular organelles and the nuclear membrane. In addition to inducing morphological changes, curcumin treatment decreased mitochondrial membrane potentials [28]. In other studies, curcumin influenced the function of several proteins, such as epidermal growth factor receptor [30] and P-glycoprotein [31]. These results suggested that additional to curcumin’s effects on membrane structures, interactions between the phenolic groups of curcumin and hydrogen bonding sites in the cellular membranes
could influence membrane activities or membrane protein function.
In our previous study, curcumin effectively interfered with the HA activity of the influenza virus [10]. In this study, HA activity was inhibited significantly in NDV treated with curcumin concentrations higher than 30 mM. The hemagglutinin-neuramin- idase (HN) protein is responsible for the HA activity of NDV. Since the HN protein sequence and conformation are dissimilar from those of the HA protein of the influenza virus, inhibition of HA function in both viruses after curcumin treatment suggested that the HI effects might result from different mechanisms, or from a general disruptive effect on the viral envelope. Following the treatment of several enveloped viruses and one nonenveloped virus, EV71 (Piconaviridae) with curcumin, we observed that the effects of curcumin on inhibition of viral plaque formation are specific to enveloped viruses: the infectivity of EV71 remained unaffected by curcumin treatment. As the 7 enveloped viruses, including influenza A virus (H1 and H6 subtypes), pseudorabies virus, 2 flaviviruses (JEV and Dengue virus), vaccinia virus, and NDV, are classified into 5 families, it suggested that curcumin exerts a general inhibitory effect on viruses with an envelope structure. Given that pretreatment of the viruses with curcumin irreversibly abrogated plaque formation and HA activity, it indicated that curcumin could serve as a virucidal agent for enveloped viruses.
In this study, the concentration of curcumin required to inhibit viral HA activity (30 mM) is lower than that reported in our previous study [10], and those reported by other research groups as disruptive to erythrocyte membranes [26,27]. In HI assays, we observed hemolysis at curcumin concentrations higher than 500 mM. Pretreatment of RBC with 30 mM curcumin had no effects on the HA activity of influenza viruses, indicating that the erythrocyte membrane remains intact at curcumin concentrations effective for HA inhibition. At concentrations lower than 30 mM, we observed insignificant cellular toxic effects. However, EC50 required for inhibition of influenza virus was approximately 0.47 mM (with a selective index, CC50/EC50, of 92.5) [10]. These observations indicated that, despite viral envelopes and cellular membranes being composed of a phospholipid bilayer, curcumin (30 mM) selectively inhibits the infectivity of virus particles by disrupting the function of viral enveloped proteins, such as the HA protein of the influenza virus and the HN protein of the NDV. Cell viability, however, remains unaffected by curcumin treat- ment. Several factors might contribute to curcumin’s disrupting effects on different viruses and cells, such as the complexities of surface protein compositions and the sizes of viral particles. Numerous surface proteins attach to cellular membranes, whereas few proteins anchor to the surface of viral particles: Three envelope proteins (HA, NA, and M2) on the influenza virus, two envelope proteins (HN and F) on NDV, and two proteins (E and M) on flaviviruses. Considering the essential roles of each of these
curcumin, as indicated by the lack of plaque formation (Fig. 5B). However, dilutions of the virus-solvent control (DMSO) mixture restored the marginal inhibitory effects of DMSO. We then used the embryonic chicken egg, a potent amplification vessel for the influenza virus, to investigate whether curcumin treatment irreversibility inhibited the influenza virus. Viruses pre-treated with curcumin (for one hour) were unable to amplify in embryonic eggs. However, eggs innoculated with a high dose (5000 PFU) of PR8 treated with DMSO produced 25.5 HA units and 29.75 HA units of viral progeny at 18 h and 24 h after infection, respectively (Table 1 and Fig. 5C). These results indicated that the inhibitory effects of curcumin on influenza virus infectivity are irreversible.
Discussion
This study presents several novel findings. To our knowledge, it is the first to show that curcumin generally inhibits enveloped virus infectivity. In addition to inhibiting HA activity, a novel mech- anism was investigated; as evidenced in the liposome-based assay systems, we proposed that the integrity of membrane structure, e.g., viral envelope, could be affected by curcumin treatment. As for the four enveloped viruses analysed in the current study, the EC50 of curcumin on inhibition of plaque formation for larger viruses is greater than that for smaller viruses.
Previous studies reported that curcumin associates with membranes [26,27]. The hydrophobic properties of membranes favor the intercalation of curcumin into the lipid bilayer, such as in cellular membranes, where phenolic rings of curcumin are essential for interaction with hydrogen-bonding sites. Several studies also identified curcumin as a membrane-disturbing agent. Curcumin treatment induced alterations in membranous proper- ties, including morphological changes, and increased permeability and fluidity. The interactions between the cell membranes and curcumin might have caused these effects [26,28]. Jaruga et al. observed that treatment of erythrocytes with .100 mM curcumin concentrations induced changes in the integrity of their cell membranes [26,27]. Similarly, at a high treatment concentration (5 mM), curcumin increased lactate dehydrogenase (LDH) leakage in rat hepatocytes (25%) compared with the LDH leakage observed in untreated cells (15%) [29]. Research using a rat thymocyte model further showed that curcumin is able to penetrate the cytoplasm and accumulate in membranous struc- tures of intracellular organelles and the nuclear membrane. In addition to inducing morphological changes, curcumin treatment decreased mitochondrial membrane potentials [28]. In other studies, curcumin influenced the function of several proteins, such as epidermal growth factor receptor [30] and P-glycoprotein [31]. These results suggested that additional to curcumin’s effects on membrane structures, interactions between the phenolic groups of curcumin and hydrogen bonding sites in the cellular membranes
could influence membrane activities or membrane protein function.
In our previous study, curcumin effectively interfered with the HA activity of the influenza virus [10]. In this study, HA activity was inhibited significantly in NDV treated with curcumin concentrations higher than 30 mM. The hemagglutinin-neuramin- idase (HN) protein is responsible for the HA activity of NDV. Since the HN protein sequence and conformation are dissimilar from those of the HA protein of the influenza virus, inhibition of HA function in both viruses after curcumin treatment suggested that the HI effects might result from different mechanisms, or from a general disruptive effect on the viral envelope. Following the treatment of several enveloped viruses and one nonenveloped virus, EV71 (Piconaviridae) with curcumin, we observed that the effects of curcumin on inhibition of viral plaque formation are specific to enveloped viruses: the infectivity of EV71 remained unaffected by curcumin treatment. As the 7 enveloped viruses, including influenza A virus (H1 and H6 subtypes), pseudorabies virus, 2 flaviviruses (JEV and Dengue virus), vaccinia virus, and NDV, are classified into 5 families, it suggested that curcumin exerts a general inhibitory effect on viruses with an envelope structure. Given that pretreatment of the viruses with curcumin irreversibly abrogated plaque formation and HA activity, it indicated that curcumin could serve as a virucidal agent for enveloped viruses.
In this study, the concentration of curcumin required to inhibit viral HA activity (30 mM) is lower than that reported in our previous study [10], and those reported by other research groups as disruptive to erythrocyte membranes [26,27]. In HI assays, we observed hemolysis at curcumin concentrations higher than 500 mM. Pretreatment of RBC with 30 mM curcumin had no effects on the HA activity of influenza viruses, indicating that the erythrocyte membrane remains intact at curcumin concentrations effective for HA inhibition. At concentrations lower than 30 mM, we observed insignificant cellular toxic effects. However, EC50 required for inhibition of influenza virus was approximately 0.47 mM (with a selective index, CC50/EC50, of 92.5) [10]. These observations indicated that, despite viral envelopes and cellular membranes being composed of a phospholipid bilayer, curcumin (30 mM) selectively inhibits the infectivity of virus particles by disrupting the function of viral enveloped proteins, such as the HA protein of the influenza virus and the HN protein of the NDV. Cell viability, however, remains unaffected by curcumin treat- ment. Several factors might contribute to curcumin’s disrupting effects on different viruses and cells, such as the complexities of surface protein compositions and the sizes of viral particles. Numerous surface proteins attach to cellular membranes, whereas few proteins anchor to the surface of viral particles: Three envelope proteins (HA, NA, and M2) on the influenza virus, two envelope proteins (HN and F) on NDV, and two proteins (E and M) on flaviviruses. Considering the essential roles of each of these
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