Regulation of the NF-kappaB and p53 pathways by the aryl hydrocarbon receptor nuclear translocator

Nuclear factor-[kappa]B (NF-[kappa]B) signaling is critical for the proper function of the immune system, and when deregulated, promotes the development of immune disorders and cancer. It is important to understand the complex regulatory mechanisms that govern NF-[kappa]B activity so that we can improve therapies for cancer and immune diseases. Notably, the aryl hydrocarbon receptor nuclear translocator (ARNT) has been shown to regulate the chromatin binding activity of the NF-[kappa]B subunit RelB, in turn promoting RelB-p50 DNA binding, a process that inhibits canonical NF-[kappa]B signaling. However, ARNT is expressed as two alternatively spliced isoforms, isoforms 1 and 3. Whether each isoform has a specific role in NF-κB signaling is unclear. In the first project, ARNT isoforms 1 and 3 are shown to contribute in distinctive ways to lymphoid cancer cell growth through regulating RelB and p53. Importantly, suppression of isoform 1 in lymphoid cancer cell lines triggered S-phase cell cycle arrest, spontaneous apoptosis, and sensitized cells to doxorubicin treatment. Furthermore, co-suppression of RelB or p53 with ARNT isoform 1 prevented cell cycle arrest and blocked doxorubicin-induced cell death. Together these findings reveal that certain hematological malignancies rely on ARNT isoform 1 to potentiate proliferation by antagonizing RelB and p53-dependent cell cycle arrest and apoptosis. In Chapter 4, the individual functions of ARNT isoforms 1 and 3 in NF-[kappa]B signaling were investigated. The results from this study point to ARNT as a regulator of RelB-p52/p100 nuclear import in a cell-type specific manner. Furthermore, while both ARNT isoform 1 and 3 were necessary for regulating RelB-p52/p100 nuclear import in Karpas 299 cells, isoform 3 was found to be a negative regulator of NF-[kappa]B target gene expression, whereas ARNT isoform 1 neither promoted nor inhibited NF-[kappa]B signaling. While in HEK 293T cells, the presence of both ARNT isoform 1 and 3 promoted p100 nuclear translocation and co-localization with RelB. Together, these data indicate that ARNT isoform 1 and 3 form a complex with RelB and p52/p100 to fine-tune NF-[kappa]B nuclear translocation and activity. Significantly, our results give insight into a novel regulatory mechanism of NF-[kappa]B activity and identify ARNT as a potential target for anticancer therapies.