The contention that NURR1 plays an important role in the maintenance of terminal differentiation of epithelia is supported in the literature. Developmental animal models have shown that suppression of NURR1 is necessary for the maintenance of pluripotency of hematopoietic progenitor cells [
3]. Similarly, genetic models have demonstrated that the loss of NR4A-family receptors results in increased incidence of leukemia [
30]. NURR1 expression and activity is induced in response to several compounds with anti-neoplastic effects such as 6 mercaptopurine and 1,1-bis(3′-indolyl)-1-(aromatic)methane (C-DIM) analogs [
7,
10]. In contrast, several studies suggest that NURR1 is associated with increased proliferation of cancer cells. In HeLa cells, loss of NURR1 was associated with decreased anchorage independent growth and resulted in apoptosis, suggesting that NURR1 was necessary for the maintenance of a tumorigenic phenotype [
20]. In colon cancer, it has been demonstrated that prostaglandin E2 mediated proliferation is inhibited by expression of a dominant negative NURR1, demonstrating that NURR1 is indeed necessary for eicosanoid-mediated proliferation in colon cancer [
21]. It is feasible that NURR1, which is highly regulated at the transcriptional and post-translational levels, may have different roles in cancer based on the regulatory influences present within the cellular environment. To this point, NURR1 mislocalization to the cytoplasm is associated with poor clinical prognosis, yet pharmacological modulation of the transcriptional function of NURR1 is associated with compounds which induce apoptosis [
10,
19]. Together, these findings suggest that nuclear localization, and presumably transcriptional activity of NURR1 is associated with differentiation, whereas a cytosolic localization and a lack of transcriptional activation are supportive of tumorigenesis. Indeed, our own immunohistochemical studies suggest that in normal cells, NURR1 is strongly localized to the nuclear compartment, supporting the contention that NURR1 has differential roles in the normal and transformed breast. Therefore, expression of the receptor alone may not be indicative of its role in cancer, but its transcriptional activity may be the key to elucidating its role in the inhibition or promotion of breast cancer. Further elucidation of this potential mechanism is complicated by the fact that breast cancer cell lines are often intolerant of NURR1 overexpression (data not shown). Therefore, functional studies involving the transfection of wild-type and transcriptionally inactive variants of NURR1 will likely require a more nuanced approach, such as conditional overexpression models.