Characterization of 3'-azido-3'-deoxythymidine inhibition of ricin and Pseudomonas exotoxin A toxicity in CHO and Vero cells

Treatments for Pulmonary Ricin Intoxication: Current Aspects and Future Prospects

Ricin, a plant-derived toxin originating from the seeds of Ricinus communis (castor beans), is one of the most lethal toxins known, particularly if inhaled. Ricin is considered a potential biological threat agent due to its high availability and ease of production. The clinical manifestation of pulmonary ricin intoxication in animal models is closely related to acute respiratory distress syndrome (ARDS), which involves pulmonary proinflammatory cytokine upregulation, massive neutrophil infiltration and severe edema. Currently, the only post-exposure measure that is effective against pulmonary ricinosis at clinically relevant time-points following intoxication in pre-clinical studies is passive immunization with anti-ricin neutralizing antibodies. The efficacy of this antitoxin treatment depends on antibody affinity and the time of treatment initiation within a limited therapeutic time window. Small-molecule compounds that interfere directly with the toxin or inhibit its intracellular trafficking may also be beneficial against ricinosis. Another approach relies on the co-administration of antitoxin antibodies with immunomodulatory drugs, thereby neutralizing the toxin while attenuating lung injury. Immunomodulators and other pharmacological-based treatment options should be tailored according to the particular pathogenesis pathways of pulmonary ricinosis. This review focuses on the current treatment options for pulmonary ricin intoxication using anti-ricin antibodies, disease-modifying countermeasures, anti-ricin small molecules and their various combinations.

Different in vitro toxicity of ribosome-inactivating proteins (RIPs) on sensory neurons and Schwann cells

OBJECTIVE

To study the neurotoxicity induced by Ricinus communis agglutinin (RCA), ricin A chain (RTA), and trichosanthin (TCS) in vitro.

METHODS

Rat neurons and Schwann cells were cultured and real-time up-take of RIPs was traced. TUNEL, Annexin V and DAPI were employed to study the mechanism.

RESULTS

The purity of both primary neuronal and Schwann cell cultures attained 80-90%. In neuritis, transport of FITC-RCA was demonstrated, but RTA and TCS were not detected. RCA elicited the strongest TUNEL and annexin V signals in both cultures. RTA evoked a stronger apoptotic signal than TCS in neurons. In contrast, compared with TCS, RTA elicited an attenuated apoptotic reaction in Schwann cells. All internalized RIPs were concentrated in the cytoplasm of the cells and their nuclei were not stained by DAPI.

CONCLUSION

The toxicity of these RIPs on neurons is different from that on Schwann cells. Although they enter cells by different mechanisms they all induce apoptosis. These results may find application in in vivo neural lesioning studies and clinical therapy.

Proteins pattern alteration in AZT-treated K562 cells detected by two-dimensional gel electrophoresis and peptide mass fingerprinting

In this study we report the effect of AZT on the whole protein expression profile both in the control and the AZT-treated K562 cells, evidenced by two-dimensional gel electrophoresis and peptide mass fingerprinting analysis. Two-dimensional gels computer digital image analysis showed two spots that appeared up-regulated in AZT-treated cells and one spot present only in the drug exposed samples. Upon extraction and analysis by peptide mass fingerprinting, the first two spots were identified as PDI-A3 and stathmin, while the third one was proved to be NDPK-A. Conversely, two protein spots were present only in the untreated K562 cells, and were identified as SOD1 and HSP-60, respectively.

The effect of AZT and chloroquine on the activities of ricin and a saporin-transferrin chimeric toxin

This study deals with the combination of chloroquine (CQ, an anti-malaric drug) and 3'-azido-3'-deoxythymidine (AZT, anti-human immuno-deficiency virus (HIV) drug) with a chimeric toxin (TS) obtained by chemical linking of saporin (a ribosome inactivating protein from the plant Saponaria officinalis) and human transferrin, in the intoxication of the human chronic myeloid leukaemia cells (K562). Our data demonstrate that AZT, at concentrations comparable to those reached in the blood of HIV-infected patients under pharmacological treatment with this drug, can increase the toxicity of TS in cooperation with CQ inducing an increased effect on protein synthesis in K562 cells ( approximately 50% inhibition of protein synthesis for TS alone, and TS with AZT and approximately 70% with both AZT and CQ). Furthermore, pre-treatment of cells with AZT alone can induce an increase of apoptosis in K562 cells intoxicated with TS. By comparing data obtained with the model toxin ricin, we get indications that the two toxins partially differ in their intracellular routes, also suggesting that chimeric constructs containing ricin-like toxins (i.e. immunotoxins) could be coupled with the use of common and cheap drugs for the treatment of cancer in HIV-infected patients.

Rationale for the use of immunotoxins in the treatment of HIV-infected humans

The first step in the replication of human immunodeficiency virus (HIV) is selective binding of the envelope glycoprotein (gp120) to CD4 receptors on T cells or macrophages. After penetration in these cells, the genome of the virus is integrated in the human genome. HIV-infection causes depletion of CD4-positive cells resulting in a severe immunosuppression. It is believed that eliminating HIV-infected cells is crucial in limiting further reduction of CD4-positive cells and thus, preventing disease progression. The most commonly used drugs, such as zidovudine (AZT), appeared to be not completely effective. Therefore many investigators are searching for alternative treatment modalities. The use of immunotoxins (ITs) to eliminate HIV-infected cells is discussed. ITs are chimeric molecules in which cell-binding ligands are coupled to toxins and can specifically eliminate undesired cells. The cell-binding carriers of anti-HIV ITs have been directed against different regions of the HIV envelope glycoprotein (gp120 and gp41) and surface antigens (e.g CD4, CD25). The ITs have been composed of different ribosome-inactivating proteins (RIPs) like pokeweed antiviral protein (PAP), Pseudomonas exotoxin (PE), Diphtheria toxin (DT), or ricin. In in vitro studies, several of these ITs have been shown to be effective and specific in killing acute and persistently HIV-infected cells. The ITs were effective at concentrations (ID50 range from 10(-9) M to 10(-12) M) that were not toxic to uninfected cells or cells without the antigen. The IT CD4(178)PE40, a fusion protein directed against the CD4 binding site of gp120, has been investigated in two in vivo trials. The results were disappointing considering the antiviral activity in vitro. This was thought to be due to the rapid clearance of the IT and the differential resistance of clinical HIV isolates. Use of a panel of ITs is likely to be more effective because multiple approaches cover the intrinsic variability of HIV and the presence of IT-resistant or latently infected cells, as well as the blocking presence of neutralizing anti-HIV antibodies and the immunogenicity of most ITs. It may be possible to control the virus completely with a panel of ITs in combination with other antiviral or immunosuppressive agents such as RT inhibitors (e.g AZT), interferon alpha, or cyclosporine. More research will be necessary to develop such a combined therapy.

In vitro fusion of endocytic vesicles: effects of reagents that alter endosomal pH

Ricin, a plant toxin that binds to galactose-terminated glycoproteins and glycolipids on the cell surface, is internalized into endosomes before reaching the cytosol where it exerts its toxic activity. Fusion of early endosomes containing ricin or transferrin was demonstrated by using postnuclear supernatant fractions from K-562 cells. For both ligands, fusion depended on time, temperature, and ATP and was blocked by preincubation with N-ethylmaleimide. Some reagents that increase endosomal pH, the ionophores monensin and nigericin and the weak base chloroquine, stimulated the rate of fusion. However, bafilomycin A1, a specific inhibitor of vacuolar H(+)-ATPases, did not alter the rate of fusion. Moreover, it reduced or eliminated stimulation caused by monensin, nigericin, or chloroquine. Thus, the increased rate of fusion did not correlate with the higher lumenal pH of the endosome. The results suggest instead that fusion was stimulated by reagents that promoted accumulation of cations within the vesicles.

Drugs that show protective effects from ricin toxicity in in vitro protein synthesis assays

We used an in-vitro, inhibition of protein synthesis assay (PSI) to test a wide variety of drugs for possible therapeutic use against ricin, a toxic glycoprotein that causes death in animals by inhibiting protein synthesis. Selection of test drugs was based on possible interference with ricin activity at different stages of the toxic process. Most of the drugs tested had no effect on ricin-induced PSI, were toxic when tested alone, or enhanced the toxicity of ricin. The only ones showing protection were galactose, lactose, and several derivatives of these sugars, Brefeldin A (BFA), 3'-azido-3'-deoxythymidine (AZT), and a purine derivative (BM33203). THe sugar derivatives provided 50% protection against a PSI ED99 of ricin (0.1 micrograms/ml). Concentrations of BFA greater than 0.5 micro M caused about 50% PSI by itself, but blocked any further inhibitory effects of ricin. AZT, at optimum concentrations, reached a maximum protection level of about 40% in the presence of an ED99 dose of ricin, while the nucleoside derivative, BM33203 and AZT appeared to have an additive effect, showing up to 80% protection from an ED99 dose of ricin. Drugs showing protection in the PSI cell assay showed no protection from ricin in a cell-free translation assay used to determine if they would block ricin at the protein synthesis site.