New Roles of the Transcription Factor NKX6.1 in Beta Cell Biology
Schisler, Jonathan Cummings
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Pancreatic islet beta cells play a critical role in the maintenance of metabolic fuel homeostasis. Type 1 diabetes results from autoimmune destruction of the beta cells, whereas type 2 diabetes involves loss of beta cell function, particularly glucose-stimulated insulin secretion (GSIS), and a gradual diminution of beta cell mass. To identify genes that are involved in GSIS and contribute to the mature beta cell phenotype, a panel of beta cell lines with varying capacities for GSIS was employed for a candidate gene screen. Expression of the homeodomain transcription factor Nkx6.1 was found to be positively correlated with the capacity for GSIS in these lines. Although previously identified as an essential factor for beta cell differentiation, little was known about the function of Nkx6.1 in mature beta cells. The purpose of this dissertation research was to investigate the role of Nkx6.1 in the biology of mature beta cells via manipulation of its expression in beta cell lines and primary pancreatic islets and measurement of a variety of functional outcomes. These studies led to identification of three novel roles of Nkx6.1 in the mature beta cell. First, Nkx6.1 was found to suppress expression of the glucagon gene via a direct interaction with its promoter, although the studies also demonstrate a role for Pdx1 and Mafbeta in mediating this suppression. Second, the silencing of Nkx6.1 expression in pancreatic beta cells results in a severe impairment in GSIS, suggesting that Nkx6.1 target genes are critical for robust GSIS. Third, Nkx6.1 is shown to regulate beta cell proliferation, in part via direct interaction with the cyclin B1 gene and stimulation of its expression. Thus, whereas the majority of primary beta cells are quiescent, overexpression of Nkx6.1 is sufficient to initiate DNA synthesis and cell division. Importantly, and different from the common experience, this manipulation also enhances GSIS. These studies suggest that Nkx6.1 should be further investigated as a gene involved in development of diabetes and as a potential new therapeutic target.