Abstract
RB1 loss (RB1null) or MYCN amplification (MYCNamp) in fetal human retina causes retinoblastoma. SKP2 loss kills RB1null cells, but small molecule SKP2 inhibitors remain unexplored therapeutically. Whether SKP2 is synthetic lethal in MYCNamp retinoblastoma is unclear. SKP2 is the substrate recognition component of two Cullin-RING Ligase complexes (CRL1SKP2/SCFSKP2, and CRL4SKP2), a family of multiprotein E3 ubiquitin ligases. NEDD8 activating enzyme (NAE) is required for Cullin neddylation and thus CRL activation. Here, we show that the NAE inhibitor, Pevonedistat (MLN4924), potently inhibits RB1null and MYCNamp tumors. Intravitreal MLN4924 suppressed multiple human xenografts with EC80s from 20 ng to 3.5 μg. Maximum tolerated dose (MTD) was 10–30 μg, highlighting a favorable therapeutic window. Inhibition of Cullin neddylation was similar in all cases, but cellular effects ranged from G1 arrest with apoptosis to G2/M arrest with endoreplication. However, even in less sensitive lines (EC50 ≈ 1 μM), prolonged exposure was lethal or induced persistent cytostasis. Mechanistically, depleting any single Cullin did not fully recapitulate drug phenotypes, but sensitivity to SKP2 loss correlated with that of drug. Thus, intravitreal MLN4924 is a promising new retinoblastoma therapy, mimicking the cancer-specific lethality of eliminating SKP2 complexes.
Highlights
Retinoblastoma (RB) is an aggressive intraocular childhood cancer that is lethal if not treated
We compared the efficacy of MLN4024 to Compound A, which blocks SKP2 binding to SCF26
Histological examination revealed no toxicity from 3 or 10 μg doses, but photoreceptor loss was detected at 30 μg (Fig. 1g), indicating the maximum tolerated dose (MTD) in murine vitreous is 10–30 μg, well above the therapeutic dose
Summary
Retinoblastoma (RB) is an aggressive intraocular childhood cancer that is lethal if not treated. In addition to RB1null cases, some tumors retain RB1, but amplify MYCN6. The extent to Cullin RING ligases (CRLs) regulate the ubiquitylation and turnover of many eukaryotic proteins[7] They have four components: one of eight Cullin proteins (CUL1, 2, 3, 4A, 4B, 5, 7, or 9) that are scaffolds, one of two ring finger proteins (RBX1 or 2) that recruit the ubiquitin-charged E2 protein; one of four adapters (SKP1, elongin B/C, BTB protein, or DDB1); and many substrate recognition proteins that form hundreds of CRL complexes[7]. CRL1 complexes utilize CUL1, of which a subset are the SCF (SKP1-CUL1-Fbox) complexes They utilize the E2 protein and RBX1 on one side of CUL1 to donate ubiquitin to a substrate binding protein pair on the other side (SKP1 bound to one of ~70 F-box proteins). SKP2 is essential for Official journal of the Cell Death Differentiation Association
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