The BUB3-BUB1 Complex Promotes Telomere DNA Replication

Mad1 destabilizes p53 by preventing PML from sequestering MDM2

Mitotic arrest deficient 1 (Mad1) plays a well-characterized role in the mitotic checkpoint. However, interphase roles of Mad1 that do not impact mitotic checkpoint function remain largely uncharacterized. Here we show that upregulation of Mad1, which is common in human breast cancer, prevents stress-induced stabilization of the tumor suppressor p53 in multiple cell types. Upregulated Mad1 localizes to ProMyelocytic Leukemia (PML) nuclear bodies in breast cancer and cultured cells. The C-terminus of Mad1 directly interacts with PML, and this interaction is enhanced by sumoylation. PML stabilizes p53 by sequestering MDM2, an E3 ubiquitin ligase that targets p53 for degradation, to the nucleolus. Upregulated Mad1 displaces MDM2 from PML, freeing it to ubiquitinate p53. Upregulation of Mad1 accelerates growth of orthotopic mammary tumors, which show decreased levels of p53 and its downstream effector p21. These results demonstrate an unexpected interphase role for Mad1 in tumor promotion via p53 destabilization.

Inhibition of BUB1 Kinase by BAY 1816032 Sensitizes Tumor Cells toward Taxanes, ATR, and PARP Inhibitors In Vitro and In Vivo

PURPOSE

The catalytic function of BUB1 is required for chromosome arm resolution and positioning of the chromosomal passenger complex for resolution of spindle attachment errors and plays only a minor role in spindle assembly checkpoint activation. Here, we present the identification and preclinical pharmacologic profile of the first BUB1 kinase inhibitor with good bioavailability.

EXPERIMENTAL DESIGN

The Bayer compound library was screened for BUB1 kinase inhibitors and medicinal chemistry efforts to improve target affinity and physicochemical and pharmacokinetic parameters resulting in the identification of BAY 1816032 were performed. BAY 1816032 was characterized for kinase selectivity, inhibition of BUB1 signaling, and inhibition of tumor cell proliferation alone and in combination with taxanes, ATR, and PARP inhibitors. Effects on tumor growth in vivo were evaluated using human triple-negative breast xenograft models.

RESULTS

The highly selective compound BAY 1816032 showed long target residence time and induced chromosome mis-segregation upon combination with low concentrations of paclitaxel. It was synergistic or additive in combination with paclitaxel or docetaxel, as well as with ATR or PARP inhibitors in cellular assays. Tumor xenograft studies demonstrated a strong and statistically significant reduction of tumor size and excellent tolerability upon combination of BAY 1816032 with paclitaxel or olaparib as compared with the respective monotherapies.

CONCLUSIONS

Our findings suggest clinical proof-of-concept studies evaluating BAY 1816032 in combination with taxanes or PARP inhibitors to enhance their efficacy and potentially overcome resistance.