Immunocytochemical investigation of insulin secretion by pancreatic β- cells in control and diabetic Psammomys obesus

Abstract

Hyperproinsulinemia is a characteristic feature of non-insulin-dependent diabetes mellitus (NIDDM) caused by pancreatic β-cell dysfunction through a secretion-related alteration or impaired proinsulin processing. We have investigated the insulin processing and secretion in Psammomys obesus fed with low- and high-energy diets, which represent a model for diet-induced NIDDM. With a high-energy diet the animals develop hyperglycemia and hyperinsulinemia, whereas those maintained on a low-energy diet remain normoglycemic. Although a large amount of insulin immunoreactivity was detected in β-cells of the normoglycemic compared to hyperglycemic animals, in situ hybridization for insulin mRNA demonstrated a particularly high signal in the β-cells of the hyperglycemic animals. By electron microscopy, the β-cells of normoglycemic animals displayed large accumulations of secretory granules, whereas those of the hyperglycemic animals contained very few granules and large deposits of glycogen. These results reflect a secretory testing condition for the cells of the normoglycemic animals in contrast to stimulated synthetic and secretory activities in the cells of the hyperglycemic ones. Using colloidal gold immunocytochemistry at the electron microscopic level, we have examined subcellular proinsulin processing in relation to the convertases PC1 and PC2. Immunolabeling of proinsulin, insulin, C-peptide, PC1, and PC2 in different cell compartments involved in β-cell secretion were evaluated. Both PC1 and PC2 antigenic sites were detected in β-cells of hyperglycemic Psammomys, but their labeling intensity was weak compared to the cells of normoglycemic animals. In both groups of animals, higher levels of PC2 were found in the Golgi apparatus than in the immature granules. Major decreases in proinsulin, insulin, PC1, and PC2 immunoreactivity were recorded in β-cells of the hyperglycemic Psammomys. In addition, all these antigenic sites were detected in lysosome-like structures, revealing a major degradation process. These results suggest that the insulin-secreting cells in hyperglycemic Psammomys obesus are in a chronic secretory state during which impaired processing of proinsulin appears to take place.