Role of Rad18 in glioblastoma proliferation

The Rad18 E3 ubiquitin ligase, a key regulator of DNA damage tolerance,also functions in repair of DNA double strand breaks. Rad18 is overexpressed in the brain cancer glioblastoma (GBM) and its downregulation sensitizes GBM cells to DNA damaging agents. Here we show that Rad18 has an essential role in GBM cells proliferation in the absence of external damage, surprisingly independent of its catalytic activity. Rad18 downregulation leads to cell cycle arrest in the G1 phase with no apparent increase in DNA damage. We also founbd that Rad18 sustains GBM stem cells self-renewal and survival, as well as the growth of GBM orthotropic xenografts in mice. We also show that increased Rad18 expression enhances the growth of non-transformed cells and induces features of oncogenic transformation. Mechanistically, we show that Rad18 downregulation negatively regulates the Hippo pathway by interfering with the nuclear retention of the YAP1 transcription factor. Altogether, these data show that Rad18 has an essential, non-catalytic function, in GBM proliferation, and propose Rad18 as a key target to sensitize GBM to therapy. U87 cells treated with a control siRNA (siCTRL), with an siRNA targeting Rad18 expression (siRad18), with a control shRNA or with a shRNA targeting Rad18.