Targeting the p27 E3 ligase SCFSkp2 results in p27-and Skp2-mediated cell-cycle arrest and activation of autophagy

Decreased P27(KiP1) levels are a poor prognostic factor in many malignancies, and can occur through up-regulation of SCFSkp2 E3 ligase function, resulting in enhanced p27 ubiquitination and proteasome-mediated degradation. While proteasome inhibitors stabilize P27(Kip1), agents inhibiting SCFSkp2 may represent more directly targeted drugs with the promise of enhanced efficacy and reduced toxicity. Using high-throughput screening, we identified Compound A (CpdA), which interfered with SCFSkP2 ligase function in vitro, and induced specific accumulation of p21 and other SCFSkp2 substrates in cells without activating a heat-shock protein response. CpdA prevented incorporation of Skp2 into the SCFSkp2 ligase, and induced G(1)/S cell-cycle arrest as well as SCFSkp2- and p27-dependent cell killing. This programmed cell death was caspase-independent, and instead occurred through activation of autophagy. In models of multiple myeloma, CpdA overcame resistance to dexamethasone, doxorubicin, and melphalan, as well as to bortezomib, and also acted synergistically with this proteasome inhibitor. Importantly, CpdA was active against patient-derived plasma cells and both myeloid and lymphoblastoid leukemia blasts, and showed preferential activity against neoplastic cells while relatively sparing other marrow components. These findings provide a rational framework for further development of SCFSkP2 inhibitors as a novel class of antitumor agents.