Glucose, the principal regulator of endocrine pancreas, has several effects on pancreatic beta cells, including the regulation of insulin release, cell proliferation, apoptosis, differentiation, and gene expression. Although the sequence of events linking glycemia with insulin release is well described, the mechanism whereby glucose regulates nuclear function is still largely unknown. Here, we have shown that an ATP-sensitive K ؉ channel (KATP) with similar properties to that found on the plasma membrane is also present on the nuclear envelope of pancreatic beta cells. In isolated nuclei, blockade of the KATP channel with tolbutamide or diadenosine polyphosphates triggers nuclear Ca 2؉ transients and induces phosphorylation of the transcription factor cAMP response element binding protein. In whole cells, fluorescence in situ hybridization revealed that these Ca 2؉ signals may trigger c-myc expression. These results demonstrate a functional KATP channel in nuclei linking glucose metabolism, nuclear Ca 2؉ signals, and nuclear function.
Pancreatic beta cells play a critical role in maintaining a steady-state level of glucose in the blood and tissues. Increased levels of glucose stimulate beta cells to secrete insulin closing a well established feedback loop. Malfunction of beta cells causes the widespread pathology, diabetes mellitus. The signal transduction mechanism leading to insulin release involves the closure of plasma membrane K ATP channels as a result of glucose metabolism by increasing both the intracellular ATP͞ ADP ratio and diadenosine polyphosphates (DPs; refs. 1 and 2). Channel closure leads to membrane depolarization and the opening of voltage-activated Ca 2ϩ channels (3). The subsequent cytosolic Ca 2ϩ signal, which is oscillatory (4), triggers a pulsatile insulin secretion. In most cells, a single second messenger as Ca 2ϩ is able to provoke different responses depending on its route of entry, its localization, and a code of amplitude or frequency of Ca 2ϩ oscillations (4, 5). In pancreatic beta cells, Ca 2ϩ mediates not only insulin secretion but also a broad range of other processes such as gene expression (6, 7). Although it is well established that the nucleoplasmic concentration of free Ca 2ϩ regulates nuclear function (5), the mechanism whereby nuclear Ca 2ϩ signals are generated is still unclear. Here, we report confocal measurements of nuclear Ca 2ϩ concentration ([Ca 2ϩ ] n ) in intact beta cells exposed to glucose. Experiments in isolated nuclei revealed a K ATP channel present on the nuclear envelope whose blockade results in a [Ca 2ϩ ] n rise. This increased [Ca 2ϩ ] n induces phosphorylation of the transcription factor cAMP response element binding protein (CREB). We further demonstrate that [Ca 2ϩ ] n elevation may result in c-myc expression in whole cells.
Materials and MethodsCell Isolation, Culture, and Permeabilization. Islets from adult (8-10 weeks old) Swiss albino male mice (OF1) killed by cervical dislocation were isolated and then dispersed into single cells after a publ...