WEE1 opposes APC/C^CDH1 activity to promote S phase entry

WEE1 phosphorylates and inhibits the activity of cyclin-dependent kinases to prevent mitotic entry and establish a G2 DNA damage checkpoint^1-3. Consequently, WEE1 inhibitors have been developed to be synthetically lethal in TP53-mutated tumours that become reliant on this WEE1-mediated checkpoint, and such inhibitors are undergoing clinical trials. However, WEE1 inhibitors also have efficacy in TP53-wild-type tumours, and many trials have been terminated owing to high levels of toxic side effects, suggesting that WEE1 has unknown functions. Here we show that WEE1 promotes cell cycle re-entry from quiescence (G0) by opposing the activity of the E3 ubiquitin ligase, anaphase-promoting complex-cyclosome bound to CDH1 (APC/C^CDH1). WEE1 phosphorylates CDH1 (FZR1) at key residues that mediate the interaction between CDH1 and APC/C. Cells with loss of function of WEE1 during G0/G1 have delayed S phase entry, impaired G1/S transition, abnormal S phase accumulation of cyclin-dependent kinase inhibitor p21^Cip1/Kip1 and a p21-dependent G2 arrest. Reduced expression of APC/C^CDH1 or p21 renders cells more sensitive to acute WEE1 inhibition, and both pathways are downregulated in cells with acquired WEE1 inhibitor resistance. These results reveal a mechanism of cell cycle control that has implications for how WEE1 inhibitors should be used in the clinic.