Abstract A59: Bach1 is critical for transformation induced by RasV12
Authors: Nakanome Ayako , Ota Kazushige , Brydun Andrey , Shiga Kiyoto , Kobayashi Toshimitsu , Igarashi Kazuhiko 
Published: May 2012

Apoptosis, transient cell cycle arrest, and cellular senescence mediated by the tumor suppressor protein p53 are the key strategies to suppress expansion of cells with mutations. Our group has reported that transcription factor Bach1 is a crucial negative regulator of oxidative stress-induced cellular senescence (Dohi et al., Nature Struct. Mol. Biol., 2008). Bach1 forms a complex that contains p53, histone deacetylase 1 (HDAC-1) and nuclear corepressor N-coR. Bach1 is recruited to a subset of p53 target genes and contributes to impeding p53 action by promoting histone deacetylation of p53 target genes. Therefore, Bach1 may play a role in cancer transformation by inhibiting p53 activity.

We have explored the possibility of Bach1 being as a positive regulator for cancer. First, we compared the timing of immortalization of wild-type and Bach1-deficient mouse embryonic fibroblasts (MEFs). Immortalization is an important step in cellular transformation and tumorigenesis, conferring upon cells the ability to grow indefinitely by bypassing cellular senescence. Bach1-deficient MEFs entered into the p53-dependent senescence state more rapidly than wild-types. However, they became immortalized in a similar kinetics as wild-types when cultured by 3T3 protocol. Thus, while Bach1-deficient MEFs were kept in senescence state longer than wild-type cells, they ultimately bypassed senescence as wild-type cells did. Inactivation of tumor suppressor genes such as p53 or p19 was observed in both types of MEFs, indicating that the p19–p53 pathway was critical for the maintenance of senescence in these cells. Next, we compared transformation of immortalized wild-type and Bach1-deficient MEFs by the activated oncogene H-RasV12. We found that formation of transformed colonies on dish and in soft agar was markedly reduced in Bach1-deficient cells as compared with wild type cells. RasV12-transduced wild-type MEFs produced apparent tumors in all athymic nude mice when injected subcutaneously, whereas Bach1-deficient cells formed significantly smaller tumors at best. Based on these observations, we suggest that Bach1 plays important roles in freeing from contact inhibition and acquiring anchorage-independent growth.

To understand how Bach1 contribute to the carcinogenesis, we carried out following experiments using RasV12-expressing immortalized MEFs. Reactive oxygen species (ROS) levels were very low in Bach1-deficients. Phospholylation of ERK1/2 and H2AX was markedly reduced in Bach1-deficients. These data suggest that some intra-cellular signal pathways or DNA-repairing pathways are blocked in Bach1-deficients. We are currently identifying Bach1-target genes which are involved in these processes.

Based on these observations, we concluded that Bach1 is critical for transformation induced by RasV12. Bach1 is necessary for transformation of MEFs by not only impeding the function of p53 but also acting through other pathways for transformation. We will discuss about the mechanism of transformation supposed by Bach1.

Citation Information: Cancer Res 2009;69(23 Suppl):A59.