Expression of CD38 with intracellular enzymatic activity: a possible explanation for the insulin release induced by intracellular cADPR

CD38 is a transmembrane glycoprotein expressed in multiple cell types, including pancreatic beta cells. It can serve as an enzyme that catalyzes the metabolism of two different Ca(2+)-mobilizing compounds, cyclic adenosine diphosphoribose (cADPR) and nicotinic acid adenine dinucleotide phosphate. One of these metabolites, cADPR, is known to be involved in glucose-induced insulin secretion from pancreatic beta cells. Although the essential role of CD38 for endogenous cADPR synthesis has been established, the relationship between the proposed extracellular enzymatic activity of CD38 and the intracellular Ca(2+) modulation caused by the intracellular cADPR accumulation has not yet been fully explained. For a better understanding of the role of CD38 in the insulin secretion machinery, analysis of the intracellular localization of this molecule in pancreatic beta cells is essential. In an attempt to provide a method to probe the N-terminal and C-terminal of CD38 separately, we generated an insulin-secreting MIN6 murine pancreatic beta cell line expressing a human CD38 bearing an N-terminal FLAG epitope tag. We found a weak but consistent expression of the FLAG epitope outside of the cells, indicating the presence of a small amount of CD38 with cytoplasmic enzymatic activity. MIN6 cells transfected with human CD38 exhibited increased glucose-induced insulin release. In addition, anti-FLAG cross-linking further enhanced the insulin release, suggesting that the N-terminal of CD38 expressed on the cell surface functions as a receptor for an unknown ligand and triggers positive signals for insulin secretion.