Regulation of Glycosylphosphatidylinositol-Specific Phospholipase D Secretion from TC3 cells

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Abstract
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 bTC3 cells, a mouse insulinoma cell line. In the absence of glucose, phorbol myristic acid (0.1 mM) stimulated insulin secretion by 2.5-fold and GPI-PLD secretion by 2-fold. Car- bachol (5 mM), glucagon-like peptide I-(7-36) amide (0.1 mM), 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 bTC3 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, bTC3 cells con- tain both insulin and GPI-PLD, which generally colocalized intracel- lularly. However, GPI-PLD secretion differed from insulin secretion byahigherrateofbasalrelease(2.8%vs.0.23%/h),alowermagnitude ofresponsetosecretagogues,andamoreprolongedperiodofincreased secretion. These results demonstrate that bTC3 cells secrete GPI- PLD in response to insulin secretagogues and suggest that GPI-PLD may be secreted via the regulated pathway in these cells. (Endocri- nology 138: 819-826, 1997)

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