Low Density Lipoprotein Binding and Uptake by Human and Rat Islet   Cells

Abnormalities in lipoprotein metabolism are common in diabetes. It is unknown whether variations in form or concentration of lipoproteins influence the function of pancreatic β cells. This study investigates whether low density lipoproteins (LDL) exhibit specific interactions with islet β cells. Radioactively labeled LDL (125I-LDL) and fluorescently labeled LDL (DiI-LDL) were used as tracers. Rat islet cells express high affinity LDL binding sites (Kd = 9 nm), which are also recognized by very low density lipoproteins and which are down-regulated by LDL. Binding of LDL appears restricted to the β cells, as it was not detected on islet endocrine non-β cells. At 37 C, LDL is taken up and lysosomally degraded by islet β cells but not by islet non-β cells. Human islet cells were also found to present LDL binding, uptake, and degradation. Compared with rat islet cells, human islet cells exhibit 10-fold less binding sites (2.107vs. 2.108 per 103 cells) with a 2-fold lower Kd value (5 nm) and an equal sensitivity to LDL-induced down-regulation. In conclusion, human and rat islet β cells express LDL receptors that can internalize the lipoprotein. This pathway should be examined for its potential role in (dys)regulating pancreatic β cell functions.