Activator Protein-1 (AP-1) Signaling Inhibits the Growth of Ewing Sarcoma Cells in Response to DNA Replication Stress.

Ribonucleotide reductase (RNR) catalyzes the rate-limiting step in the synthesis of deoxyribonucleosides and is required for DNA replication. Multiple types of cancer, including Ewing sarcoma tumors, are sensitive to RNR inhibitors or a reduction in the levels of either the RRM1 or RRM2 subunits of RNR. However, the polypharmacology and off-target effects of RNR inhibitors have complicated the identification of the mechanisms that regulate sensitivity and resistance to this class of drugs. Consequently, we used a conditional knockout (CRISPR/Cas9) and rescue approach to target RRM1 in Ewing sarcoma cells and identified that loss of the RRM1 protein results in the upregulation of the expression of multiple members of the activator protein-1 (AP-1) transcription factor complex, including c-Jun and c-Fos, and downregulation of c-Myc. Notably, overexpression of c-Jun and c-Fos in Ewing sarcoma cells is sufficient to inhibit cell growth and downregulate the expression of the c-Myc oncogene. We also identified that the upregulation of AP-1 is mediated, in part, by SLFN11, which is a replication stress response protein that is expressed at high levels in Ewing sarcoma. In addition, small-molecule inhibitors of RNR, including gemcitabine, and histone deacetylase inhibitors, which reduce the level of the RRM1 protein, also activate AP-1 signaling and downregulate the level of c-Myc in Ewing sarcoma. Overall, these results provide novel insight into the critical pathways activated by loss of RNR activity and the mechanisms of action of inhibitors of RNR. Significance: RNR is the rate-limiting enzyme in the synthesis of deoxyribonucleotides. Although RNR is the target of multiple chemotherapy drugs, polypharmacology and off-target effects have complicated the identification of the precise mechanism of action of these drugs. In this work, using a knockout-rescue approach, we identified that inhibition of RNR upregulates AP-1 signaling and downregulates the level of c-Myc in Ewing sarcoma tumors.

Olfactory dysfunction in the 3xTg-AD model of Alzheimer's disease.

Alzheimer's disease (AD) is an incurable neurodegenerative disease in which the risk of development increases with age. People with AD are plagued with deficits in their cognition, memory, and basic social skills. Many of these deficits are believed to be caused by the formation of amyloid-ß plaques and neurofibrillary tangles in regions of the brain associated with memory, such as the hippocampus. However, one of the early, preclinical symptoms of AD is the loss of olfactory detection and discrimination. To determine if a mouse model of AD expresses the same olfactory dysfunction seen in human AD, 3xTg-AD mice were given a buried food test and, unlike previous studies, compared to their background and parental strains. Results showed that over 52 weeks, the 3xTg-AD mice took significantly longer to find the buried food than the control strains. The olfactory bulbs of the 3xTg-AD mice were removed, sliced, and stained using Congo red for histological analysis. Amyloid deposits were observed predominantly in the granule layer of the olfactory bulb beginning at 13 weeks of age in 3xTg-AD mice, but not in the control strains of mice. Further examination of the buried food test data revealed that 3xTg-AD females had a significantly longer latency to detect the buried food than males beginning at 26 weeks of age. Overall, this study provides further validation of the 3xTg-AD mouse model of AD and supports the idea that simple olfactory testing could be part of the diagnostic process for human AD.