Nuclear localized Raf1 isoform alters DNA‐dependent protein kinase activity and the DNA damage response. Issue 1 (14th August 2018) - Nixon, Benjamin R.
ABSTRACT: Raf1/c‐Raf is a well‐characterized serine/threonine–protein kinase that links Ras family members with the MAPK/ERK signaling cascade. We have identified a novel splice isoform of human Raf1 that causes protein truncation and loss of the C‐terminal kinase domain (Raf1‐tr). We found that Raf1‐tr has increased nuclear localization compared with full‐length Raf1, and this finding was secondary to reduced binding of Raf1‐tr to the cytoplasmic chaperone FK506 binding protein 5. We show that Raf1‐tr has increased binding to DNA‐dependent protein kinase (DNA‐PK), which inhibits DNA‐PK function and causes amplification of irradiation‐ and bleomycin‐induced DNA damage. We found that the human colorectal cancer cell line, HCT‐116, displayed reduced expression of Raf1‐tr, and reintroduction of Raf1‐tr sensitized the cells to bleomycin‐induced apoptosis. Furthermore, we identified differential Raf1‐tr expression in breast cancer cell lines and showed that breast cancer cells with increased Raf1‐tr expression become sensitized to bleomycin‐induced apoptosis. Collectively, these results demonstrate a novel Raf1 isoform in humans that has a unique noncanonical role in regulating the double‐stranded DNA damage response pathway through modulation of DNA‐PK function.—Nixon, B. R., Sebag, S. C., Glennon, M. S., Hall, E. J., Kounlavong, E. S., Freeman, M. L., Becker, J. R. Nuclear localized Raf1 isoform alters DNA‐dependent protein kinase activity and the DNA damage response. FASEB J. 33, 1138–1150 (2019). www.fasebj.org