Neuroblastoma (NB) is the most common extracranial solid tumor in childhood cancers. The 5-year survival of patients with advanced disease is <40%. Relapse occurs in over 50% of patients. Evidence suggests that most cancers contain a “stem-like” population that is resistant to current therapeutics and can repopulate the tumor bulk after treatment. This cell population is thought to be an underlying cause for cancer relapse. Therefore, targeting the cancer stem-like cell (CSC) fraction can result in treatment improvements. Previous proteomic work identified that UCHL1 (ubiquitin carboxyl-terminal esterase L1) was differentially expressed in the enriched CSC fraction of human NB cell lines, suggesting a possible link between UCHL1 and CSCs. UCHL1 is part of the ubiquitin proteasome system (UPS), which is responsible for degradation of proteins involved in biological processes such as cell cycle control and the breakdown of transcription factors. Deregulation of UCHL1 expression has been shown to be associated with oncogenic properties; however, the function of UCHL1 in cancer progression is still not fully understood. We blocked the hydrolase activity of UCHL1 using a small molecule inhibitor LDN-57444. We treated the CSC fraction of NB cell line, SK-N-BE(2), with the inhibitor and observed an impairment of sphere formation (read-out for self-renewal). We then explored whether this was due to apoptosis. We found through Annexin V staining that the percentage of apoptotic cells was not significant between treated and control CSC cells. We also observed caspase 3/7 activity was not significant between treated and control groups. We performed cell cycle studies on inhibitor treated CSCs, which showed an increase in the percentage of cells in the G1 phase. The cell cycle is an important process in self-renewal, division, and cell differentiation. Global gene expression analysis on inhibitor treated CSCs also indicated that several cell cycle regulator genes and members of the Wnt/β-catenin signaling pathway were deregulated. Among these genes were MYC, CCND2, LEF-1, TCF7L2, and CDKN2A. Likely, UCHL1 may be functioning through the Wnt/β-catenin pathway to affect MYC expression, which would indirectly regulate the cell cycle. MYC has also been shown to be a difficult therapeutic target; therefore, UCHL1 will be a more viable target. We also performed in silico analysis to determine existing associations between these genes using Sidekick. Several of the cell cycle regulators were connected with components of the Wnt/β-catenin network. Targeting the UPS, which functions in cell development and survival, has been shown to be successful in some cancers. These observations suggest that UCHL1 functions in the maintenance of CSCs, and targeting UCHL1 could lead to better design of therapeutic intervention that improves quality of life and long-term survival of NB patients.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5195. doi:1538-7445.AM2012-5195