PDF《HtrA2/Omi》

Letter to the Editor The protease inhibitor Ucf-101 induces cellular responses independently of its known target, HtrA2/Omi Cell Death and Differentiation (2006) 13, 2157C2159.

doi:10.1038/sj.cdd.4401955;

published online

12 May

2006 Dear Editor, HtrA2 (also known as Omi) is a mammalian serine protease with high homology to bacterial HtrA proteases.1,2 HtrA2 is localized in the mitochondrial intermembrane space and is released in response to apoptotic stimuli. HtrA2 can induce cell death in a caspase-dependent manner by interacting with the inhibitor of apoptosis proteins as well as in a caspase-independent manner that relies on its own protease activity.3C6 However, mice in which the gene encoding HtrA2 has been deleted or inactivated by point mutation in the protease domain show no evidence of reduced rates of cell death, but on the contrary suffer loss of a population of neurons in the striatum resulting in a Parkinsonian syndrome.7,8 This phenotype suggests that the predominant physiological role of HtrA2 is a cell-protective protease function, probably in the mitochondria, and not a proapoptotic action in the cytosol. Recently, two HtrA2 point mutations have been identi?ed in Parkinson'

s disease (PD) patients.9 These mutations seem to result in partial loss of proteolytic activity, possibly contributing to the aetiology of PD in these patients. Mammalian HtrA2 may therefore function in vivo in a manner similar to its bacterial homologues DegS and DegP, which are involved in protection against cell stress.10,11 DegS senses unfolded proteins in the bacterial periplasm, activating a proteolytic cascade that results in the transcriptional upregulation of stress response genes. DegP degrades unfolded proteins at elevated temperatures, whereas it acts as a chaperone at low temperatures. A mitochondrial-speci?c stress response has been reported to exist in mammalian cells.12 Accumulation of unfolded proteins in the mitochondrial matrix results in the transcrip- tional upregulation of nuclear genes encoding mitochondrial stress proteins, via a mechanism involving the transcrip- tion factor C/EBP homologous protein (CHOP), also known as growth arrest and DNA-damage-inducible gene

153 (GADD153). Although HtrA2 is located in the intermembrane space of the mitochondria, it is possible that it might be involved in transmitting the stress signal from the matrix out of the mitochondria. Thus, we were interested in studying the induction of CHOP in response to a variety of stresses in wild- type and HtrA2-knockout mouse embryonic ?broblasts (MEFs). In addition, we used Ucf-101, a cell-permeable, furfurylidine-thiobarbituric acid compound that competitively and reversibly inhibits HtrA2 protease activity13 (Figure 1a). Ucf-101 shows very little activity against various other serine proteases and, when tested in caspase-9 null ?broblasts, was found to inhibit HtrA2 overexpression-induced cell death.13 On the assumption that it is a speci?c inhibitor of HtrA2, Ucf-101 has been employed to identify potential substrates of this protease14,15 and to study its role in cell death.16C19 Ucf-101 provides at least partial protection from cell death induction by cisplatin,14,16 myocardial ischaemia and reperfu- sion,17 TNFa19 and staurosporine (our unpublished data), leading to the assumption that HtrA2 plays a role in promoting cell death following these treatments. If Ucf-101 is a speci?c inhibitor of HtrA2, then the effect of using Ucf-101 should be similar to the deletion of HtrA2 in MEFs. The induction of CHOP is well studied in response to endoplasmic reticulum (ER) stress.20,21 Tunicamycin, which inhibits N-linked glycosylation in the ER, increases the CHOP mRNA level potently in wild-type and HtrA2-knockout MEFs (Figure 1b). Thapsigargin, an inhibitor of the ER Ca2 ? -ATPases, also induces CHOP in both cell lines (data not shown). To our surprise, however, Ucf-101 by itself induced CHOP mRNA, both in wild-type and HtrA2-knockout MEFs. After only