Beteiligung der Serinpeptidase HtrA2/Omi am Zelltodprogramm Parthanatos

Parthanatos is a regulated form of cell death. Preliminary work provided evidence that protease HtrA2/Omi may represent a key regulator of parthanatos. The aim of this study was to further characterize the role of HtrA2/Omi in parthanatos.

Confirming its importance, HtrA2/Omi-knockout cells were protected from parthanatos induction by MNNG. Interestingly, the reconstitution of knockout cells with either catalytically active or inactive HtrA2/Omi both restored the response to MNNG. As a result, HtrA2/Omi’s catalytic activity may not be necessary for parthanatos. Analyzing potential downstream mediators, a reduction of DBC1 and stathmin 1 could be observed in HtrA2/Omi expressing cells upon parthanatos induction. However, protein levels remained unchanged in deficient cells. By inhibition of the parthanatos-initiating enzyme PARP-1 cleavage was prevented. These results indicate that DBC1 and stathmin 1 are cleaved downstream of HtrA2/Omi and HtrA2/Omi itself acts downstream of PARP-1. In subsequent experiments, siRNA knockdowns of DBC1 and stathmin 1 did not protect from cell death. Hence, these proteins seem to be dispensable for parthanatos. For apoptosis, it is known that HtrA2/Omi translocates to the cytosol in order to deactivate inhibitor of apoptosis proteins. By immunofluorescence and Western Blot analysis, it was demonstrated that HtrA2/Omi does not leave the mitochondria during parthanatos. In further experiments, stimulation with SMAC mimetic birinapant could not overcome the protection of HtrA2/Omi knockout cells. Hence, HtrA2/Omi may act in an IAP-independent manner.

Further mitochondrial proteases were tested for a potential role in parthanatos. siRNA knockdowns or complete knockouts of neither LONP1, PMPCA nor PARL could protect from cell death. Finally, a mass-spectrometric screen could show that neither HtrA2/Omi-deficiency nor parthanatic cell death result in major alterations of the mitochondrial proteome.

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