Ctenopharyngodon idella PKZ facilitates cell apoptosis through phosphorylating eIF2α

PKZ, protein kinase containing Z-DNA binding domains, is a novel member of eIF2α kinase in fish. PKZ can be up-regulated in response to Poly I:C or heat stress, and it has a typical eIF2α kinase activity. However, the relationship between fish PKZ and apoptosis remains unclear. In the present study, effects of PKZ on apoptosis were explored. Initially, we found that PKZ and PKR were up-regulated by poly I:C in a time-dependent manner. Q-PCR analysis showed that the change of Caspase-3 mRNA was consistent with that of PKZ under the stimulation of poly I:C or transfection with PKZ siRNA in CIK cells. It suggested that PKZ might mediate the induction of apoptosis. Knockdown and overexpression assays indicated that a significant increase of apoptotic cell number was observed in the wild type PKZ (PKZ-wt) transfected PKZ-deficient cells, while mutant PKZ-K198R lost this ability. MTT also showed that overexpression of PKZ-wt in CIK cells resulted in a striking decrease of cell viability rate to about 32.5%, whereas the viability of cell with PKZ-K198R was about 88.1%. Likewise, when transfected eIF2α-wt or phosphomimetic eIF2α (S51D), CIK cells displayed higher apoptotic rate than the controls. In contrast, overexpression of phosphodeficient eIF2α (S51A) blocked induction of apoptosis. In addition, levels of eIF2α phosphorylation were significantly related to apoptosis in these CIK cells. Furthermore, when mutant eIF2α (S51A) was transiently transfected into the PKZ/PKR knockdown cells that transfected with wildtype PKZ previously, the apoptotic rates and levels of eIF2α phosphorylation were dramatically decreased. Overall, these results suggested that PKZ could facilitate apoptosis via eIF2α phosphorylation.