Björklund A, Grill V
Department of Molecular Medicine, Karolinska Hospital, Karolinska Institute, Stockholm, Sweden.
Diabetes. 1999 Jul;48(7):1409-14. doi: 10.2337/diabetes.48.7.1409.
Relative hypersecretion of proinsulin is a feature of type 2 diabetes. We investigated to what extent this feature can be induced in human pancreatic islets by elevated glucose or fatty acids, two major abnormalities of the diabetic state. A 48-h culture period with 27 mmol/l glucose increased the intraislet proinsulin-to-insulin (PI/I) ratio 5.0-fold, owing to preferential decrease of insulin. The PI/I ratio in culture medium was enhanced 1.9-fold versus islets cultured with 5.5 mmol/l glucose. This effect of elevated glucose persisted after normalization of glucose levels: during 60-min postculture 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-fold) after renewed postculture stimulation with 16.7 mmol/l glucose. Diazoxide was added to culture medium to block glucose-induced insulin secretion and thus investigate the importance of overstimulation. In cultures at 27 mmol/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 the release at 3.3 or 16.7 mmol/l glucose during postculture incubations by 85 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 media without affecting PI/I ratios. During postculture conditions, however, prior palmitate culture enhanced the PI/I ratio of release at 3.3 mmol/l glucose (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 glucose, prior palmitate culture further elevated the PI/I ratio of secretion (from 10.8 +/- 1.2 after previous 27 mmol/l glucose alone to 13.9 +/- 2.8% after palmitate and glucose, P < 0.05). We conclude that 1) long-term exposure of human islets to elevated glucose leads to preferential secretion of proinsulin, 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 mechanisms that are, in part, incongruous with an over-stimulation effect.
胰岛素原相对分泌过多是2型糖尿病的一个特征。我们研究了在人类胰岛中，糖尿病状态下的两个主要异常因素——高血糖或脂肪酸升高，能在多大程度上诱发这一特征。在27 mmol/l葡萄糖浓度下培养48小时，胰岛内胰岛素原与胰岛素（PI/I）的比值增加了5.0倍，这是由于胰岛素优先减少所致。与在5.5 mmol/l葡萄糖中培养的胰岛相比，培养基中的PI/I比值提高了1.9倍。高血糖的这种作用在血糖水平恢复正常后仍然存在：在基础葡萄糖浓度（3.3 mmol/l）下进行60分钟的培养后孵育期间，分泌的PI/I比值增加了4.9倍。在用16.7 mmol/l葡萄糖重新进行培养后刺激后，该比值也增加了（14倍）。向培养基中添加二氮嗪以阻断葡萄糖诱导的胰岛素分泌，从而研究过度刺激的重要性。在27 mmol/l葡萄糖的培养中，二氮嗪的存在使胰岛内容物的PI/I比值降低了76%，向培养基中累积分泌降低了70%，在培养后孵育期间，在3.3或16.7 mmol/l葡萄糖水平下的释放分别降低了85%和86%。在与5.5 mmol/l葡萄糖共培养期间或之后，二氮嗪的这些降低PI/I的作用均未重现。用0.2 mmol/l棕榈酸酯和5.5 mmol/l葡萄糖培养可降低胰岛中胰岛素原和胰岛素的含量，并增加培养基中的分泌产物，但不影响PI/I比值。然而，在培养后条件下，先前用棕榈酸酯培养可提高在3.3 mmol/l葡萄糖水平下释放的PI/I比值（从2.2±0.4%提高到5.4±0.9%，P<0.05）。用棕榈酸酯与2
