Regulation of Glycosylphosphatidylinositol-Specific Phospholipase D Secretion from βTC3 cells*

Glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) is abundant in mammalian serum, but the source of the circulating enzyme is unknown. Pancreatic islets have been reported to contain and secrete GPI-PLD. In this report we examined the regulation of GPI-PLD secretion from beta TC3 cells, a mouse insulinoma cell line. In the absence of glucose, phorbol myristic acid (0.1 microM) stimulated insulin secretion by 2.5-fold and GPI-PLD secretion by 2-fold. Carbachol (5 microM), glucagon-like peptide I-(7-36) amide (0.1 microM), and isobutylmethylxanthine (0.1 mM) had no significant effect on insulin or GPI-PLD secretion in the absence of glucose. Glucose (16.7 mM) stimulated both GPI-PLD and insulin secretion from beta TC3 cells by 55% and 235%, respectively. In addition, glucose potentiated the secretagogue effect of isobutylmethylxanthine, phorbol myristic acid, and glucagon-like peptide I on both insulin and GPI-PLD secretion. By immunohistochemistry and confocal microscopy, beta TC3 cells contain both insulin and GPI-PLD, which generally colocalized intracellularly. However, GPI-PLD secretion differed from insulin secretion by a higher rate of basal release (2.8% vs. 0.23%/h), a lower magnitude of response to secretagogues, and a more prolonged period of increased secretion. These results demonstrate that beta TC3 cells secrete GPI-PLD in response to insulin secretagogues and suggest that GPI-PLD may be secreted via the regulated pathway in these cells.