Effect of chloroquine on biosynthesis, release and degradation of insulin in isolated islets of rat pancreas

Autophagy Differentially Regulates Insulin Production and Insulin Sensitivity

Autophagy, a stress-induced lysosomal degradative pathway, has been assumed to exert similar metabolic effects in different organs. Here, we establish a model where autophagy plays different roles in insulin-producing β cells versus insulin-responsive cells, utilizing knockin (Becn1F121A) mice manifesting constitutively active autophagy. With a high-fat-diet challenge, the autophagy-hyperactive mice unexpectedly show impaired glucose tolerance, but improved insulin sensitivity, compared to mice with normal autophagy. Autophagy hyperactivation enhances insulin signaling, via suppressing ER stress in insulin-responsive cells, but decreases insulin secretion by selectively sequestrating and degrading insulin granule vesicles in β cells, a process we term "vesicophagy." The reduction in insulin storage, insulin secretion, and glucose tolerance is reversed by transient treatment of autophagy inhibitors. Thus, β cells and insulin-responsive tissues require different autophagy levels for optimal function. To improve insulin sensitivity without hampering secretion, acute or intermittent, rather than chronic, activation of autophagy should be considered in diabetic therapy development.

Sulfatide promotes the folding of proinsulin, preserves insulin crystals, and mediates its monomerization

Sulfatide is a glycolipid that has been associated with insulin-dependent diabetes mellitus. It is present in the islets of Langerhans and follows the same intracellular route as insulin. However, the role of sulfatide in the beta cell has been unclear. Here we present evidence suggesting that sulfatide promotes the folding of reduced proinsulin, indicating that sulfatide possesses molecular chaperone activity. Sulfatide associates with insulin by binding to the insulin domain A8--A10 and most likely by interacting with the hydrophobic side chains of the dimer-forming part of the insulin B-chain. Sulfatide has a dual effect on insulin. It substantially reduces deterioration of insulin hexamer crystals at pH 5.5, conferring stability comparable to those in beta cell granules. Sulfatide also mediates the conversion of insulin hexamers to the biological active monomers at neutral pH, the pH at the beta-cell surface. Finally, we report that inhibition of sulfatide synthesis with chloroquine and fumonisine B1 leads to inhibition of insulin granule formation in vivo. Our observations suggest that sulfatide plays a key role in the folding of proinsulin, in the maintenance of insulin structure, and in the monomerization process.

Mode of action of chloroquine in patients with non-insulin-dependent diabetes mellitus

Clinical studies have demonstrated that chloroquine and hydroxychloroquine improve glucose metabolism in patients with insulin-resistant diabetes mellitus. The mechanism of action has not been determined. We undertook a randomized double-blind placebo-controlled trial of 3 days of oral chloroquine phosphate, 250 mg four times daily, in 20 patients with non-insulin-dependent diabetes mellitus controlled by diet. Rates of glucose appearance (Ra) and disappearance (Rd) were evaluated by infusion of stable isotopically labeled D-glucose ([6,6-2H2]glucose) during hyperinsulinemic euglycemic clamps before and after treatment with chloroquine or placebo. Chloroquine significantly improved fasting plasma glucose from 199.8 +/- 8.6 to 165.6 +/- 7.6 mg/dl (P less than 0.01). Total exogenous glucose infusion required to maintain euglycemia significantly increased (1,792.6-2,040.1 mg.kg-1.330 min-1, P less than 0.05) due to an increase in Rd (2,348.0-2,618.9 mg.kg-1.330 min-1, P less than 0.01) without change in Ra. Metabolic clearance rate of insulin decreased by 39% from 14.4 +/- 1.3 to 11.0 +/- 0.6 ml.kg-1.min-1 (P less than 0.01) at plasma insulin levels of 150-200 mU/l but not at levels of 2,000-3,000 mU/l. In addition, chloroquine increased fasting C-peptide secretion by 17% and reduced feedback inhibition of C-peptide by 9.1 and 10.6% during low- and high-dose insulin infusions, respectively.

Chronic chloroquine treatment enhances insulin release in rats

The effects of chronic chloroquine treatment on intravenous glucose tolerance, and the plasma insulin response to intravenous glucose were studied in rats. Plasma glucose disappearance constants (Kg) were significantly greater (6.2 +/- 0.5% min-1) than in corresponding controls (3.7 +/- 0.4% min-1, p less than 0.001). This improved glucose tolerance was associated with significantly higher plasma immunoreactive insulin levels in response to glucose injection. Islets isolated from rats treated with chloroquine showed significantly enhanced (P less than 0.05) insulin release when incubated with 16.7 mM glucose but not with lower glucose concentrations (3 and 8 mM). Pre-incubation of islets with streptozotocin (0.05-1.5 mg/ml) produced a dose-dependent reduction in glucose-stimulated insulin secretion which was not modified in islets from chloroquine-treated rats. The concentration of chloroquine in the pancreas increased rapidly during administration and reached a value of 20.2 +/- 0.7 micrograms/g (fresh weight) after 20 weeks of treatment. It is concluded that chronic chloroquine treatment results in an improved glucose tolerance associated with an enhanced glucose-induced insulin secretion. Although earlier work showed chloroquine to reduce the severity of diabetes induced subsequently with streptozotocin, the present study shows that such an amelioration was not due to a protective effect against the beta-cell cytotoxic action of streptozotocin.