The homeodomain transcription factor Nkx6.1 plays an important role in pancreatic islet ␤-cell development, but its effects on adult ␤-cell function, survival, and proliferation are not well understood. In the present study, we demonstrated that treatment of primary rat pancreatic islets with a cytomegalovirus promoter-driven recombinant adenovirus containing the Nkx6.1 cDNA (AdCMV-Nkx6.1) causes dramatic increases in [methyl-3 H] thymidine and 5-bromo-2-deoxyuridine (BrdU) incorporation and in the number of cells per islet relative to islets treated with a control adenovirus (AdCMV-␤GAL), whereas suppression of Nkx6.1 expression reduces thymidine incorporation. Immunocytochemical studies reveal that >80% of BrdU-positive cells in AdCMVNkx6.1-treated islets are ␤ cells. Microarray, real-time PCR, and immunoblot analyses reveal that overexpression of Nkx6.1 in rat islets causes concerted upregulation of a cadre of cell cycle control genes, including those encoding cyclins A, B, and E, and several regulatory kinases. Cyclin E is upregulated earlier than the other cyclins, and adenovirus-mediated overexpression of cyclin E is shown to be sufficient to activate islet cell proliferation. Moreover, chromatin immunoprecipitation assays demonstrate direct interaction of Nkx6.1 with the cyclin A2 and B1 genes. Overexpression of Nkx6.1 in rat islets caused a clear enhancement of glucosestimulated insulin secretion (GSIS), whereas overexpression of Nkx6.1 in human islets caused an increase in the level of [ 3 H]thymidine incorporation that was twice the control level, along with complete retention of GSIS. We conclude that Nkx6.1 is among the very rare factors capable of stimulating ␤-cell replication with retention or enhancement of function, properties that may be exploitable for expansion of ␤-cell mass in treatment of both major forms of diabetes.Type 1 diabetes results from autoimmune destruction of insulin-producing ␤ cells in the islets of Langerhans, whereas type 2 diabetes involves loss of glucose-stimulated insulin secretion (GSIS) and a gradual diminution of ␤-cell mass (45). Insulin injection therapy has been the standard treatment for type 1 diabetes since the discovery of the hormone more than 80 years ago. Islet transplantation has been investigated as an alternative to insulin injection, but a major obstacle to broad application of this approach has been an inadequate supply of human islets (21). Pharmacotherapy of type 2 diabetes includes administration of agents that enhance insulin secretion, but these drugs often lose efficacy over time and cause complications such as hypoglycemia (30). Moreover, no controlled strategy for restoration of ␤-cell mass has been identified for the type 2 disease. Thus, a more complete understanding of the mechanisms that control islet ␤-cell growth and function is required in order to develop more effective therapies for both major forms of diabetes.Several members of the homeodomain family of transcription factors, including Pdx1, Hb9/Hlxb9, Nkx2.2, Nkx6.1, Isl-1, Pax6, an...

Loss of functional β-cell mass is a hallmark of type 1 and type 2 diabetes, and methods for restoring these cells are needed. We have previously reported that overexpression of the homeodomain transcription factor NK6 homeobox 1 (Nkx6.1) in rat pancreatic islets induces β-cell proliferation and enhances glucosestimulated insulin secretion, but the pathway by which Nkx6.1 activates β-cell expansion has not been defined. Here, we demonstrate that Nkx6.1 induces expression of the nuclear receptor subfamily 4, group A, members 1 and 3 (Nr4a1 and Nr4a3) orphan nuclear receptors, and that these factors are both necessary and sufficient for Nkx6.1-mediated β-cell proliferation. Consistent with this finding, global knockout of Nr4a1 results in a decrease in β-cell area in neonatal and young mice. Overexpression of Nkx6.1 and the Nr4a receptors results in increased expression of key cell cycle inducers E2F transcription factor 1 and cyclin E1. Furthermore, Nkx6.1 and Nr4a receptors induce components of the anaphasepromoting complex, including ubiquitin-conjugating enzyme E2C, resulting in degradation of the cell cycle inhibitor p21. These studies identify a unique bipartite pathway for activation of β-cell proliferation, suggesting several unique targets for expansion of functional β-cell mass.

The Nr4a family of nuclear hormone receptors is composed of three members—Nr4a1/Nur77, Nr4a2/Nurr1 and Nr4a3/Nor1. While currently defined as ligandless, these transcription factors have been shown to regulate varied processes across a host of tissues. Of particular interest, the Nr4a family impinge, in a tissue dependent fashion, on cellular proliferation, apoptosis and fuel utilization. The regulation of these processes occurs through both nuclear and non-genomic pathways. The purpose of this review is to provide a balanced perspective of the tissue specific and Nr4a family member specific, effects on cellular proliferation, apoptosis and fuel utilization.