BUB1B MITOTIC KINASE DEPENDENCY REVEALS CENP-E INHIBITION AS A TRACTABLE APPROACH FOR CASTRATION AND ENZALUTAMIDE RESISTANT PROSTATE CANCER

While persistent androgen receptor (AR) signaling is a mechanism of CRPC progression, BUB1B promotion of CRPC was not dependent on AR as assessed through AR knockdown and PROTAC-mediated AR degradation after PROTAC treatment. Ectopic expression of BUB1B also rendered PC cells resistant to enzalutamide in vitro and in vivo. We showed that BUB1B kinase activity was essential sufficient for CRPC progression, as only wild type (wtWT) BUB1B and not two kinase-dead mutants promoted castration-resistant growth of androgen-dependent PCADPC cells.

Rescue experiments with BUB1B wt WT or kinase-dead mutants showed that BUB1B kinase activity was required to maintain castration resistance. Expression of BUB1B’s knownprimary substrate, CENP-E, wild typeWT or a phosphomimic mutant of the BUB1B phosphorylation site, promoted progression to CRPC. In contrast, in a similar extent thancompared the corresponding CENP-E phosphodeficient mutant was not sufficient for CRPC growth to BUB1B cells.

FurthermoreConsistent with the importance of the BUB1B/CENP-E axis, only the phosphomimic mutant rescued growth after BUB1B depletion in CRPC cells, demonstratingindicating that BUB1B phosphorylation of CENP-E iswas requiredsufficient to progression to sustainnot only for progression to but also to sustainand necessary to sustain CRPC growth. Targeting BUB1B substrate, CENP-E, with the clinically tested, small-molecule inhibitor GSK-923295 phenocopied BUB1B knockdown, sensitized resistant cells to enzalutamide and suppressed growth of enzalutamide-resistant xenografts in vivo.

These data support BUB1B kinase as a key driver of CRPC progression and enzalutamide resistance thereby positioning CENP-E inhibition as a rapidly attainable approach for treatment-resistant disease.