The purine anti-metabolite 6-mercaptopurine is one of the most widely used drugs for the treatment of acute childhood leukemia and chronic myelocytic leukemia. Developed in the 1950s, the drug is also being used as a treatment for inflammatory diseases such as Crohn's disease. The antiproliferative mechanism of action of this drug and other purine anti-metabolites has been demonstrated to be through inhibition of de novo purine synthesis and incorporation into nucleic acids. Despite the extensive clinical use and study of 6-mercaptopurine and other purine analogues, the cellular effects of these compounds remain relatively unknown. More recently, purine anti-metabolites have been shown to function as protein kinase inhibitors and to regulate gene expression. In an attempt to find small molecule regulators of the orphan nuclear receptor Nurr1, interestingly, we identified 6-mercaptopurine as a specific activator of this receptor. A detailed analysis of 6-mercaptopurine regulation of Nurr1 demonstrates that 6-mercaptopurine regulates Nurr1 through a region in the amino terminus. This activity can be inhibited by components of the purine biosynthesis pathway. These findings indicate that Nurr1 may play a role in mediating some of the antiproliferative effects of 6-mercaptopurine and potentially implicate Nurr1 as a molecular target for treatment of leukemias.The Nobel prize-winning work of Elion et al.(1) demonstrated that differences in nucleic acid metabolism between cancerous cells and normal cells or between cells from different organisms led to the design and development of nucleic acid analogs that would effectively and selectively block nucleic acid synthesis in the desired target cells. Among the drugs that emanated from this work are 6-mercaptopurine (6-MP), 1 6-thioguanine (6-TG), azathioprine, allopurinol, and acyclovir (2, 3). These drugs are still in use for the treatment of leukemias (6-MP and 6-TG) and autoimmune disorders and the prevention of organ transplant rejection (azathioprine), gout (allopurinol), and herpes virus infections (acyclovir). Additional nucleic acid anti-metabolites that were developed are effective in bacterial infections and malaria.The clinical efficacy of 6-MP is due in part to antiproliferative and cytotoxic effects resulting primarily from the inhibition of purine biosynthesis at multiple steps and incorporation into nucleic acids as thioguanine nucleotides (2-7). More recent work has expanded the function of purine anti-metabolites by demonstrating that compounds such as 6-thioguanine or 6-mercaptopurine can target biological activities outside of the purine biosynthesis pathway including telomerase (5), protein kinase N (6, 7), axon growth and regeneration (8, 9), and apoptosis in B cells through the regulation of the Bcl-2/Bax ratio (10).In this study, we have identified, from a high throughput screen, 6-mercaptopurine as a regulator of the transcriptional activity of the orphan nuclear hormone receptor Nurr1. There are three members of the NGFI-B group, including Nurr1 ...