Insulin internalization into monocytes is decreased in patients with type II diabetes mellitus.

We studied the internalization of [125I]insulin into circulating human monocytes, a cell type widely used for insulin binding studies. The internalization of [125I]insulin was assessed by both an acid extraction technique, which removes surface-bound insulin but not intracellular insulin, and by a trypsinization technique, which removes cell surface-bound hormone. After 5 h of incubation at 22 C, over 40% of the total cell-associated [125I]insulin was internalized into monocytes of normal subjects. This internalization was temperature dependent; the fraction of internalized hormone was progressively decreased when the incubation temperature was reduced from 37 to 4 C. Treatment of monocytes with increasing concentrations of 2,4-dinitrophenol also decreased [125I]insulin internalization, whereas dansylcadaverine, an inhibitor of transglutaminase, had no effect. Analysis by gel filtration of the internalized labeled hormone after 4 h of incubation at 22 C indicated that 50-60% of the label was degraded insulin, but detectable intact insulin was still present. Internalization of insulin was then studied in monocytes from eight obese patients (161% of ideal body weight) with type II diabetes mellitus. After 4 h of incubation at 22 C, the specific total monocyte-associated [125I]insulin was decreased compared to that in cells from 7 normal subjects [6.02 +/- 0.38% (+/- SE) vs. 3.91 +/- 0.31% of the total; P less than 0.001]. Moreover, the percentage of hormone that was internalized was also decreased from 41.4 +/- 1.2% of the total to 28.9 +/- 1.8% (P less than 0.001). In 20 nondiabetic obese subjects, specific cell-associated [125I]insulin was reduced to 3.9 +/- 0.3% (P less than 0.001). However, compared to that in normal subjects, the percentage of hormone that was internalized was not decreased (39.7 +/- 3.51% of the total). The present findings indicate that human circulating monocytes internalize [125I]insulin; this process is temperature and energy dependent; and monocytes from obese type II diabetic patients have a significantly decreased ability to internalize insulin. This decreased internalization may play a role in the cellular resistance to insulin that occurs in these patients.