Comparative Binding of 125I-and 99mTc-Labeled Native and Glycated Low-Density Lipoprotein to Human Microvascular Endothelial Cells-Potential for Atherosclerosis Imaging?

Native (n), glycated (g), and glycoxidated (go) low-density lipoproteins (LDL) were labeled with ¹²⁵I or ^99mTc, and the labeling efficiency and binding were assessed for potential use of these LDL compounds in imaging analysis of atherosclerotic lesions (PPAR-γ receptors) by determining the number of specific receptors for nLDL, gLDL or goLDL on human microvascular endothelial cells as well as the K_D s using either ¹²⁵I-or ^99mTc-labeled LDLs. The specific activity of labeled gLDL and goLDL was much higher (for goLDL 20 times higher) than that of nLDL. Gel filtration of labeled LDLs revealed, however, that ^99mTc–g/goLDL is significantly degraded by the labeling reaction. No fragmentation was observed for ^99mTc-nLDL and all the ¹²⁵I-labeled LDL forms. Binding studies using both ¹²⁵I-and ^99mTc-nLDL indicated a weak binding affinity (K_D 10^{? 7}mol/L) to human microvascular endothelial cells. The binding affinity of ¹²⁵I-g/goLDL to these cells was significantly higher (K_D 10^{? 9}mol/L) and could be increased further by preactivation of the endothelial cells using TNFα. Incubation with ^99mTc-goLDL, however, did not result in specific binding of the ligand, possibly as a consequence of the fragmentation of the lipoprotein during the labeling. Scatchard transformation of the binding data with ^99mTc-gLDL revealed the presence of only a few binding sites. This was in contrast to the results obtained with ¹²⁵I-labeled gLDL, which revealed a much higher membrane density of scavenger receptors for this ligand. We conclude that for in vitro binding studies as well as for potential in vivo imaging, only ¹²⁵I-labeled goLDL should be used, whereas nLDL may be applied as ¹²⁵I-or ^99mTc-labeled ligand.