Comment on: Torii et al. (2009) Gene Silencing of Phogrin Unveils Its Essential Role in Glucose-Responsive Pancreatic β-Cell Growth. Diabetes 58:682–692

In the March 2009 issue of Diabetes, Torii et al. (1) reported that neither the knockdown nor the overexpression in MIN6 cells of insulinoma-associated protein (IA)-2β (also known as phogrin), a transmembrane dense core vesicle (DCV) protein, had any effect on glucose-stimulated insulin secretion (GSIS). These findings, however, differ from those of Doi et al. (2), who reported that the overexpression of IA-2β in MIN6 cells inhibited GSIS, whereas the knockdown of IA-2β in MIN6 cells had little effect on GSIS but seemed to ameliorate the inhibitory effect of ghrelin on GSIS. The findings of Torii et al. also contrast with those of Kubosaki et al. (3), who showed that the knockout of IA-2β in mice resulted in glucose intolerance and an ∼40% decrease in GSIS from isolated mouse β-cells. These latter findings are consistent with recent observations in our laboratory that showed that transient knockdown of IA-2β in MIN6 cells produced a 40–50% decrease in GSIS and that tetracycline-induced overexpression of IA-2β in MIN6 cells resulted in a twofold increase in GSIS. These observations are similar to earlier reports with regard to IA-2, a transmembrane DCV protein closely related to IA-2β (4).

Although one would have expected similar results from the three laboratories because each knocked down and overexpressed IA-2β in MIN6 cells, there were, in fact, a number of technical differences. These include the source and age of the MIN6 cells, the use of transient versus stably transfected cells, the extent of the knockdown and overexpression of IA-2β, and the time of collecting and method of evaluating GSIS.

At least two steps could be taken to resolve the inconsistent findings in MIN6 cells. First, the experiments could be repeated by different laboratories using the same protocols, cells, and reagents. Second, other methods and cell lines with DCV that secrete insulin or other hormones could be evaluated by knocking down and overexpressing IA-2β. Such studies could go a long way to helping explain the disparate GSIS findings in MIN6 cells.