Sos1 disruption impairs cellular proliferation and viability through an increase in mitochondrial oxidative stress in primary MEFs.

Using a 4-hydroxytamoxifen (4OHT)-inducible, conditional Sos1-null mutation, we analyzed wild-type (WT), single Sos1-KO, Sos2-KO and double Sos1/2 KO primary mouse embryonic fibroblasts (MEF) with an aim at evaluating the functional specificity or redundancy of the Sos1 and Sos2 alleles at the cellular level. The 4OHT-induced Sos1-KO and Sos1/2-DKO MEFs exhibited distinct flat morphology, enlarged cell perimeter and altered cytoskeletal organization that were not observed in the WT and Sos2-KO counterparts. The Sos1-KO and Sos1/2-DKO MEFs also displayed significant accumulation, in comparison with WT and Sos2-KO MEFs, of cytoplasmic vesicular bodies identified as autophagosomes containing degraded mitochondria by means of electron microscopy and specific markers. Cellular proliferation and migration were impaired in Sos1-KO and Sos1/2-DKO MEFs in comparison with WT and Sos2-KO MEFs, whereas cell adhesion was only impaired upon depletion of both Sos isoforms. RasGTP formation was practically absent in Sos1/2-DKO MEFs as compared with the other genotypes and extracellular signal-regulated kinase phosphorylation showed only significant reduction after combined Sos1/2 depletion. Consistent with a mitophagic phenotype, in vivo labeling with specific fluorophores uncovered increased levels of oxidative stress (elevated intracellular reactive oxygen species and mitochondrial superoxide and loss of mitochondrial membrane potential) in the Sos1-KO and the Sos1/2-DKO cells as compared with Sos2-KO and WT MEFs. Interestingly, treatment of the MEF cultures with antioxidants corrected the altered phenotypes of Sos1-KO and Sos1/2-DKO MEFs by restoring their altered perimeter size and proliferative rate to levels similar to those of WT and Sos2-KO MEFs. Our data uncover a direct mechanistic link between Sos1 and control of intracellular oxidative stress, and demonstrate functional prevalence of Sos1 over Sos2 with regards to cellular proliferation and viability.

WEE1 accumulation and deregulation of S-phase proteins mediate MLN4924 potent inhibitory effect on Ewing sarcoma cells.

Ewing sarcoma (ES) is an aggressive bone and soft tissue tumor of children and young adults in which finding effective new targeted therapies is imperative. Here, we report an in-depth preclinical study of the investigational cullin-RING ubiquitin ligase (CRL) inhibitor MLN4924 in ES, as we have recently demonstrated the implication of a CRL component in the ES pathogenesis. First, our results support a high sensitivity of ES cells to MLN4924 growth inhibition both in vitro (14 ES cell lines tested, median IC50=81 nM) and in tumor xenografts (tumor regression achieved with 60 mg/kg BID, subcutaneously, n=9). Second, we report a dual mechanism of action of MLN4924 in ES cells: while a wide range of MLN4924 concentrations (∼30-300 nM) trigger a G2 arrest that can only be rescued by WEE1 kinase inhibition or depletion, saturating doses of the drug (>300 nM) cause a delay in S-phase progression concomitant with unbalanced CDK2-Cyclin E and CDK2-Cyclin A relative levels (accumulation of the first and depletion of the latter). The aberrant presence of CDC6 in the nucleus at late S-phase cell cycle stage confirmed the loss of CDK2-Cyclin A-specific functions. Remarkably, other mechanisms explored (P27 accumulation and DNA damage signaling pathways) were found unable to explain MLN4924 effects, strengthening the specificity of our findings and suggesting the absence of functionality of some CRL substrates accumulated in response to MLN4924. This study renders a rationale for clinical trials and contributes molecular mechanisms for a better understanding of this promising antitumoral agent.