Inhibition of host cell apoptosis by Toxoplasma gondii is accompanied by reduced activation of the caspase cascade and alterations of poly(ADP-ribose) polymerase expression.
The obligate intracellular protozoan parasite Toxoplasma gondii has been shown to protect different cell types from apoptosis induced by a variety of pro-apoptotic treatments. However, the precise cell biological mechanisms of this inhibition remained unknown. As shown in this study, apoptosis in human-derived HL-60 and U937 cells induced by treatment with actinomycin D or TNF-alpha in combination with cycloheximide, respectively, was indeed dose-dependently downregulated by prior infection with T. gondii, as determined by DNA fragmentation assays. Cleavage of caspase 3 and caspase 9 after treatment with pro-apoptotic stimuli was considerably diminished by T. gondii. Furthermore, release of mitochondrial cytochrome c during apoptosis in HL-60 cells was prevented by intracellular parasites and this was correlated with the absence of DNA strand breaks on the single cell level. Inhibition of cytochrome c release coincided with a twofold upregulation of Mcl-1 protein levels in HL-60 and U937 cells, while Bcl-2 expression did not increase after infection. Parasitic interference with the caspase cascade led to a reduced proteolytic cleavage of the nuclear target molecule protein kinase C delta. In parallel, poly(ADP-ribose) polymerase protein levels were prominently downregulated by T. gondii, irrespective of whether HL-60 and U937 cells had been treated with pro-apototic stimuli or left untreated. However, poly(ADP-ribose) polymerase mRNA levels remained unchanged after infection as determined by RT-PCR analyses. These observations suggest that T. gondii has evolved different mechanisms that may contribute to downregulation of host cell apoptosis, namely inhibition of cytochrome c release and subsequent caspase activation as well as downregulation of poly(ADP-ribose) polymerase protein levels.
Goebel
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S
Department of Bacteriology, Georg-August-University Göttingen, Kreuzbergring 57, D-37075 Göttingen, Germany.
Gross
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U
Lüder
C G
CG
D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000502:physiology*; D053148:Caspase 3; D053453:Caspase 9; D020169:Caspases / Q000378:metabolism*; D003574:Cytochrome c Group / Q000378:metabolism; D003609:Dactinomycin / Q000494:pharmacology; D018922:HL-60 Cells; D006790:Host-Parasite Interactions / Q000502:physiology; D006801:Humans; D008928:Mitochondria / Q000378:metabolism; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D009363:Neoplasm Proteins / Q000096:biosynthesis; D009687:Nuclear Proteins / Q000378:metabolism; D011065:Poly(ADP-ribose) Polymerases / Q000096:biosynthesis*; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D019253:Proto-Oncogene Proteins c-bcl-2; D015398:Signal Transduction / Q000502:physiology; D014122:Toxoplasma / Q000502:physiology*; D014123:Toxoplasmosis / Q000503:physiopathology*; D020298:U937 Cells
D003574:Cytochrome c Group; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D009363:Neoplasm Proteins; D009687:Nuclear Proteins; D011500:Protein Synthesis Inhibitors; D019253:Proto-Oncogene Proteins c-bcl-2; D003609:Dactinomycin; D011065:Poly(ADP-ribose) Polymerases; C505407:CASP3 protein, human; C506433:CASP9 protein, human; D053148:Caspase 3; D053453:Caspase 9; D020169:Caspases