Subcellular Distribution and Function of Bcl-2 and BcI-xl in the β-cell

Abstract

Bcl-2 family proteins control β-cell apoptosis and emerging evidence also suggests important roles in β-cell physiology. Their function is regulated by the interaction of pro- and antiapoptotic family members at several intracellular organelles. Bcl- 2 and Bcl-xL are both believed to be important for β-cell survival, but their relative abundance and localization within the β-cell are not well characterized. Here we examined the subcellular distribution of endogenous and overexpressed Bcl-2 and Bcl-xL in β-cells to clarify their organelle-specific functions. In both cultured mouse β-cells and pancreatic sections, immunofluorescent staining with monoclonal and polyclonal antibodies to Bcl-2 revealed clear nuclear localization with no detectable cytosolic signal. Overexpressed Bcl-2:GFP on the other hand was observed exclusively in the cytoplasm with prominent mitochondrial co-localization. Endogenous Bcl-xL was found by both immunostaining and cell fractionation to be primarily mitochondrial and non-nuclear, and this distribution was also seen in cells overexpressing Bcl-xL:YFP. Small molecule inhibitors of Bcl-xL/Bcl-2 caused immediate mitochondrial hyperpolarization, KATP-channel-dependent cytosolic Ca2+ signals, and had immediate effects on glucose-stimulated insulin secretion. In conclusion, endogenous Bcl-2 and Bcl-xL differ markedly in their localization within the pancreatic β-cell. Taken together with the prominent mitochondrial effects of Bcl-2/Bcl-xL inhibitors, these findings suggest that Bcl-xL specifically, may act as an important regulator of β-cell mitochondrial apoptosis and physiology.