Promyelocytic leukemia-nuclear body formation is an early event leading to retinoic acid-induced differentiation of neuroblastoma cells

Differentiating Neuroblastoma: A Systematic Review of the Retinoic Acid, Its Derivatives, and Synergistic Interactions

A neuroblastoma (NB) is a solid paediatric tumour arising from undifferentiated neuronal cells. Despite the recent advances in disease management and treatment, it remains one of the leading causes of childhood cancer deaths, thereby necessitating the development of new therapeutic agents and regimens. Retinoic acid (RA), a vitamin A derivative, is a promising agent that can induce differentiation in NB cells. Its isoform, 13-cis RA or isotretinoin, is used in NB therapy; however, its effectiveness is limited to treating a minimal residual disease as maintenance therapy. As such, research focuses on RA derivatives that might increase the anti-NB action or explores the potential synergy between RA and other classes of drugs, such as cellular processes mediators, epigenetic modifiers, and immune modulators. This review summarises the in vitro, in vivo, and clinical data of RA, its derivatives, and synergising compounds, thereby establishing the most promising RA derivatives and combinations of RA for further investigation.

Characterization of nuclear localization and SUMOylation of the ATBF1 transcription factor in epithelial cells

ATBF1/ZFHX3 is a large transcription factor that functions in development, tumorigenesis and other biological processes. ATBF1 is normally localized in the nucleus, but is often mislocalized in the cytoplasm in cancer cells. The mechanism underlying the mislocalization of ATBF1 is unknown. In this study, we analyzed the nuclear localization of ATBF1, and found that ectopically expressed ATBF1 formed nuclear body (NB)-like dots in the nucleus, some of which indeed physically associated with promyelocytic leukemia (PML) NBs. We also defined a 3-amino acid motif, KRK^2615-2617, as the nuclear localization signal (NLS) for ATBF1. Interestingly, diffusely distributed nuclear SUMO1 proteins were sequestered into ATBF1 dots, which could be related to ATBF1's physical association with PML NBs, known SUMOylation hotspots. Furthermore, ATBF1 itself was SUMOylated. ATBF1 SUMOylation occurred at more than 3 lysine residues including K²³⁴⁹, K²⁸⁰⁶ and K³²⁵⁸ and was nuclear specific. Finally, the PIAS3 SUMO1 E3 ligase, which interacts with ATBF1 directly, diminished rather than enhanced ATBF1 SUMOylation, preventing the co-localization of ATBF1 with SUMO1 in the nucleus. These findings suggest that nuclear localization and SUMOylation are important for the transcription factor function of ATBF1, and that ATBF1 could cooperate with PML NBs to regulate protein SUMOylation in different biological processes.

50nm-scale localization of single unmodified, isotopically enriched, proteins in cells

Imaging single proteins within cells is challenging if the possibility of artefacts due to tagging or to recognition by antibodies is to be avoided. It is generally believed that the biological properties of proteins remain unaltered when ¹⁴N isotopes are replaced with ¹⁵N. ¹⁵N-enriched proteins can be localised by dynamic Secondary Ion Mass Spectrometry (D-SIMS). We describe here a novel imaging analysis algorithm to detect a few ¹⁵N-enriched proteins - and even a single protein - within a cell using D-SIMS. The algorithm distinguishes statistically between a low local increase in ¹⁵N isotopic fraction due to an enriched protein and a stochastic increase due to the background. To determine the number of enriched proteins responsible for the increase in the isotopic fraction, we use sequential D-SIMS images in which we compare the measured isotopic fractions to those expected if 1, 2 or more enriched proteins are present. The number of enriched proteins is the one that gives the best fit between the measured and the expected values. We used our method to localise ¹⁵N-enriched thymine DNA glycosylase (TDG) and retinoid X receptor α (RXRα) proteins delivered to COS-7 cells. We show that both a single TDG and a single RXRα can be detected. After 4 h incubation, both proteins were found mainly in the nucleus; RXRα as a monomer or dimer and TDG only as a monomer. After 7 h, RXRα was found in the nucleus as a monomer, dimer or tetramer, whilst TDG was no longer in the nucleus and instead formed clusters in the cytoplasm. After 24 h, RXRα formed clusters in the cytoplasm, and TDG was no longer detectable. In conclusion, single unmodified proteins in cells can be counted and localised with 50 nm resolution by combining D-SIMS with our method of analysis.

NF1 is a tumor suppressor in neuroblastoma that determines retinoic acid response and disease outcome

Retinoic acid (RA) induces differentiation of neuroblastoma cells in vitro and is used with variable success to treat aggressive forms of this disease. This variability in clinical response to RA is enigmatic, as no mutations in components of the RA signaling cascade have been found. Using a large-scale RNAi genetic screen, we identify crosstalk between the tumor suppressor NF1 and retinoic acid-induced differentiation in neuroblastoma. Loss of NF1 activates RAS-MEK signaling, which in turn represses ZNF423, a critical transcriptional coactivator of the retinoic acid receptors. Neuroblastomas with low levels of both NF1 and ZNF423 have extremely poor outcome. We find NF1 mutations in neuroblastoma cell lines and in primary tumors. Inhibition of MEK signaling downstream of NF1 restores responsiveness to RA, suggesting a therapeutic strategy to overcome RA resistance in NF1-deficient neuroblastomas.