PAR bZIP-bik is a novel transcriptional pathway that mediates oxidative stress-induced apoptosis in fibroblasts

Abstract

PAR bZIP (cells knockout for PAR bZIP transcription factors) proteins, thyrotroph embryonic factor (TEF), albumin D-site-binding protein (DBP), and hepatic leukemia factor (HLF), are a family of transcription factors that have been shown to contribute to the expression of genes involved in detoxification and drug metabolism. Recently, we showed that PAR bZIP proteins were able to regulate the BH3-only gene bcl-gS in tumor cells. Here, we have extended the role of these transcription factors in the control of apoptosis executors by analyzing the expression of BH3-only genes in PAR bZIP triple knockout mouse fibroblasts. We found that bik was the only BH3-only gene downregulated in knockout cells. Consistently, transfection of TEF or DBP induces the expression of endogenous bik, regardless of the presence of active p53. Moreover, both promoter-reporter and chromatin immunoprecipitation assays indicate that PAR bZIP proteins activate the bik promoter directly. Treatment with different stress stimuli reveals a higher survival of knockout fibroblasts compared with that of wild-type cells, especially after incubation with H 2 O 2 , which suggest that PAR bZIP proteins participate in oxidative stress-induced apoptosis. Furthermore, the apoptotic cell death promoted by treatment with H 2 O 2 can be impaired by reducing the expression of Bik in wild-type fibroblasts or enhanced by the overexpression of Bik in knockout cells. These findings reveal a novel transcriptional pathway relevant in transducing the apoptotic response to oxidative stress.

The Bcl-2 proteins Noxa and Bcl-xL co-ordinately regulate oxidative stress-induced apoptosis

“…It has been demonstrated that, in the presence of an apoptotic stimulus, such as oxidative stress, one or more BH3-only Bcl-2 proteins are activated to mediate apoptotic signalling [18,20]. To systematically explore which BH3-only Bcl-2 protein is activated upon exposure to H 2 O 2 , we examined a panel of ten wellestablished BH3-only Bcl-2 proteins to identify the BH3-only protein whose expression would reflect the effects of H 2 O 2 on cell viability.…”

“…Overexpressing Bcl-2 has been shown to suppress oxidantinduced apoptosis, probably by inhibiting lysosomal membrane rupture [16,17]. In addition, several BH3-only Bcl-2 proteins have also been found to be involved in H 2 O 2 -induced cell death [18][19][20]. Nonetheless, how anti-apoptotic Bcl-2 proteins coordinate with pro-apoptotic Bcl-2 proteins to modulate oxidative stress-induced cell death remains unclear.…”

The role of endogenous Bcl-xL in regulation of apoptotic signaling pathways.

“…Systematic examination of the involvement of pro-apoptotic BH3-only BcI-2 proteins in H202-induced apoptosis revealed that Noxa was the major mediator of In fibroblasts, Bik expression is crucial for H202-triggered cell death [127]. In agreement with our observations, Noxa is largely responsible for the cytotoxicity of H202 in T leukemia Jurkat cells [128].…”

“…Overexpressing Bcl-2 has been shown to suppress oxidant-induced apoptosis probably by inhibiting lysosomal membrane rupture [124,125]. Several BH3-only BcI-2 proteins have also been found to be involved in H 2 0 2 -induced apoptosis [126][127][128]. Nonetheless, how anti-apoptotic BcI-2 proteins coordinate with pro-apoptotic Bcl-2 proteins to regulate apoptosis remains unclear.…”

“…Thus, the involvement of a particular BH3-only Bcl-2 protein in H202-induced apoptosis appears to depend on cell type, which could be attributed to cell type-specific transcriptional responses to cell death stimuli. While the proline-and acid-rich (PAR) subfamily of basic leucine-zipper (bZIP) transcriptional factor family is responsible for Bik expression increase in H202-exposed MEFs [127], the activating transcription factor 4 (ATF4) has been implicated in the induction of Noxa expression in Jurkat cells upon H 2 0 2 exposure [128]. It is still unclear how oxidative stress-induced apoptosis signalling is transduced into the nucleus to induce the transcription of BH3-only Bcl-2 proteins.…”