Enhancing effects of long-term elevated glucose and palmitate on stored and secreted proinsulin-to-insulin ratios in human pancreatic islets

Relative hypersecretion of proinsulin is a feature of type 2 diabetes. We i nvestigated to what extent this feature can be induced in human pancreatic islets by elevated glucose or fatty acids, two major abnormalities of the d iabetic state. A 48-h culture period with 27 mmol/l glucose increased the i ntraislet proinsulin-to-insulin (PI/I) ratio 5.0-fold, owing to preferentia l decrease of insulin. The PI/I ratio in culture medium was enhanced 1.9-fo ld versus islets cultured with 5.5 mmol/l glucose. This effect of elevated glucose persisted after normalization of glucose levels: during 60-min post culture incubations at a basal glucose concentration (3.3 mmol/l), the PI/I . ratio of secretion increased 4.9-fold, The ratio was also increased (14-f old) after renewed postculture stimulation with 16.7 mmol/l glucose. Diazox ide was added to culture medium to block glucose-induced insulin secretion and thus investigate the importance of overstimulation. In cultures at 27 m mol/l glucose, the presence of diazoxide decreased the PI/I ratio of islet contents by 76%, the accumulated secretion to culture medium by 70%, and th e release at 3.3 or 16.7 mmol/l glucose during postculture incubations by 8 5 and 86%, respectively. None of these PI/I-decreasing effects of diazoxide were reproduced during or after coculture with 5.5 mmol/l glucose. Culture with 0.2 mmol/l palmitate and 5.5 mmol/l glucose decreased islet contents of proinsulin and insulin and increased the secreted products in culture me dia without affecting PI/I ratios. During postculture conditions, however, prior palmitate culture enhanced the PI/I ratio of release at 3.3 mmol/l gl ucose (from 2.2 +/- 0.4 to 5.4 +/- 0.9%, P < 0.05), Culture with palmitate together with 27 mmol/l glucose decreased islet contents of proinsulin and insulin and further enhanced intraislet PI/I ratios (from 9.3 +/- 1.1 to 13 .4 +/- 2.5%, P < 0.05). However, palmitate failed to affect PI/I ratios in culture medium. In contrast, in postculture incubations at 3.3 mmol/l gluco se, prior palmitate culture further elevated the PI/I ratio of secretion (f rom 10.8 +/- 1.2 after previous 27 mmol/l glucose alone to 13.9 +/- 2.8% af ter palmitate and glucose, P < 0.05). We conclude that 1) long-term exposur e of human islets to elevated glucose leads to preferential secretion of pr oinsulin, and this effect persists also after glucose normalization; 2) the glucose effect appears secondary to depletion of mature insulin granules; and 3) elevated fatty acids influence PI/I ratios of secretion by mechanism s that are, in part, incongruous with an overstimulation effect.