Intracellular insulin processing is altered in monocytes from patients with type II diabetes mellitus

We studied total cell-associated A14-[125I]insulin radioactivity (including surface-bound and internalized radioactivity), insulin internalization, and its intracellular degradation at 37 C in monocytes from nonobese type II untreated diabetic patients (n = 9) and normal subjects (n = 7). Total cell-associated radioactivity was decreased in diabetic patients [2.65 +/- 1.21% (+/- SD) vs. 4.47 +/- 1.04% of total radioactivity; P less than 0.01]. Insulin internalization was also reduced in diabetic patients (34.0 +/- 6.8% vs. 59.0 +/- 11.3% of cell-associated radioactivity; P less than 0.01). Using high performance liquid chromatography six intracellular forms of radioactivity derived from A14-[125I] insulin were identified; 10-20% of intracellular radioactivity had approximately 300,000 mol wt and was identified as radioactivity bound to the insulin receptor, and the remaining intracellular radioactivity included intact A14-[125I]insulin, [125I]iodide, or [125I]tyrosine, and three intermediate compounds. A progressive reduction of intact insulin and a corresponding increase in iodine were found when the incubation time was prolonged. Intracellular insulin degradation was reduced in monocytes from diabetic patients; intracellular intact insulin was 65.6 +/- 18.1% vs. 37.4 +/- 18.0% of intracellular radioactivity (P less than 0.01) after 2 min and 23.6 +/- 22.3% vs. 3.9 +/- 2.3% (P less than 0.01) after 60 min in diabetic patients vs. normal subjects, respectively. In conclusion, 1) human monocytes internalize and degrade insulin in the intracellular compartment in a stepwise time-dependent manner; and 2) in monocytes from type II diabetic patients total cell-associated radioactivity, insulin internalization, and insulin degradation are significantly reduced. These defects may be related to the cellular insulin resistance present in these patients.