RBL2/p130 is a direct AKT target and is required to induce apoptosis upon AKT inhibition in lung cancer and mesothelioma cell lines.

The retinoblastoma (RB) protein family includes RB1/p105, RBL1/p107, and RBL2/p130, which are key factors in cell-cycle regulation and stand at the crossroads of multiple pathways dictating cell fate decisions. The role of RB proteins in apoptosis is controversial because they can inhibit or promote apoptosis depending on the context, on the apoptotic stimuli and on their intrinsic status, impacting on the response to antitumoral treatments. Here we identified RBL2/p130 as a direct substrate of the AKT kinase, a key antiapoptotic factor hyperactive in multiple cancer types. We showed that RBL2/p130 and AKT1 physically interact and AKT phosphorylates RBL2/p130 Ser941, located in the pocket domain, but not when this residue is mutated into Ala. We found that pharmacological inhibition of AKT, through the highly selective AKT inhibitor VIII (AKTiVIII), impairs RBL2/p130 Ser941 phosphorylation and increases RBL2/p130 stability, mRNA expression and nuclear levels in both lung cancer and mesothelioma cell lines, mirroring the more extensively studied effects on the p27 cell-cycle inhibitor. Consistently, AKT inhibition reduced cell viability, induced cell accumulation in G0/G1, and triggered apoptosis, which proved to be largely dependent on RBL2/p130 itself, as shown upon RBL2/p130 silencing. AKT inhibition induced RBL2/p130-dependent apoptosis also in HEK-293 cells, in which re-expression of a short hairpin-resistant RBL2/p130 was able to rescue AKTiVIII-induced apoptosis upon RBL2/p130 silencing. Our data also showed that the combination of AKT and cyclin-dependent kinases (CDK) inhibitors, which converge on the re-activation of RBL2/p130 antitumoral potential, could be a promising anticancer strategy.

NONO ubiquitination is mediated by FBW7 and GSK3 ß via a degron lost upon chromosomal rearrangement in cancer.

NONO is an RNA-binding protein involved in transcription, mRNA splicing, DNA repair, and checkpoint activation in response to UV radiation. NONO expression has been found altered in several tumor types, including prostate, colon, breast, melanoma, and in papillary renal carcinoma, in which an X chromosome inversion generates a NONO-TFE3 fusion protein. Upon such rearrangement, NONO loses its C-terminal domain. Through bioinformatics analysis, we identified a putative degron motif, known to be recognized by the Skp1-Cul1-F-box-protein (SCF) complex. Here, we evaluated how this domain could affect NONO protein biology. We showed that NONO interacts with the nuclear FBW7α isoform and its ubiquitination is regulated following modulation of the GSK3ß kinase. Mutation of T428A/T432A within the degron impaired polyubiquitination upon FBW7α and GSK3ß overexpression. Overall, our data suggest that NONO is likely subjected to proteasome-mediated degradation and add NONO to the list of proteins targeted by FBW7, which is itself often deregulated in cancer.