Glucose regulates islet amyloid polypeptide gene transcription in a PDX1-and calcium-dependent manner

Islet amyloid polypeptide (IAPP) and insulin are expressed in the p-cells o f the islets of Langerhans, They are co-secreted in response to changes in glucose concentration, and their mRNA levels are also regulated by glucose. The promoters of both genes share similar cis-acting sequence elements, an d both bind the homeodomain transcription factor PDX1, which plays an impor tant role in the regulation of the insulin promoter and insulin mRNA levels by glucose. Here we examine the role of PDX1 in the regulation of the huma n IAPP promoter by glucose. The experiments were facilitated by the availab ility of a human p-cell line (NES2Y) that lacks PDX1, NES2Y cells also lack operational K-ATP channels, resulting in a loss of control of calcium sign aling. We have previously used these cells to show that glucose regulation of the insulin gene is dependent on PDX1, but not calcium. In the mouse p-c ell line Mine, glucose (16 mM) Stimulated a 3.5-4-fold increase in the acti vity of a -222 to +450 IAPP promoter construct compared with values observe d in 0.5 mM glucose. In NES2Y cells, glucose failed to stimulate transcript ional activation of the IAPP promoter. Overexpression of PDX1 in NES2Y cell s failed to reinstate glucose-responsive control of the IAPP promoter, Gluc ose effects on the IAPP promoter were observed only in the presence of PDX1 when normal calcium signaling was restored by overexpression of the two K- ATP channel subunits SUR1 and Kir6.2. The importance of calcium was further emphasized by an experiment in which glucose-stimulated LAPP promoter acti vity was inhibited by the calcium channel blocker verapamil (50 mu M). Vera pamil was further shown to inhibit the stimulatory effect of glucose on IAP P mRNA levels, These results demonstrate that like the insulin promoter, gl ucose regulation of the IAPP promoter is dependent on the activity of PDX1, but unlike the insulin promoter, it additionally requires the activity of another, as yet uncharacterized factor(s), the activity of which is calcium -dependent.