No abstract

In order to use the LDL receptor pathway to target radionuclides to cancer sites for imaging and diagnostic purposes, a labeling procedure of LDL with 111In using the DTPA-bis(stearylamide) (L) has been developed. This bifunctional ligand is intended to be incorporated into the phospholipid monolayer of LDL and to specifically chelate the In3+ cation at the surface. The ligand was incorporated into LDL in buffered medium with a 65-80% yield. The L-LDL samples are stable over a 24 h period when examined by dialysis, allowing their storage before indium-111 radiolabeling. In vitro studies of In-L-LDL particles show that indium labeling is rapidly achieved (1 h). More than 85% of the indium atoms are bound to the chelating functions of the incorporated DTPA derivatives and less than 10% to the nonspecific complexation sites of LDL (e.g., protein residues). After incubation in human serum, the indium activity recovered in the LDL fraction of In-L-LDL samples (95%) is much higher than in In-LDL samples (35%), pointing out the strong stabilizing chelating effect of the ligand. Competitive binding studies show that In-L-LDL are recognized by LDL receptors of A549 cells like native LDL when the In-L/LDL ratio varies from 5 to 30. All these in vitro experiments demonstrate that the In-L-LDL conjugates possess properties suitable for further work with in vivo experiments.

HMG-CoA reductase catalyzes the synthesis of mevalonate, a crucial intermediate in the biosynthesis of cholesterol and non-sterol isoprenoid compounds essential for cell growth. The HMG-CoA reductase activity of the A549 tumor cell line is higher than that of normal human fibroblasts. This deregulation in mevalonate needs was not due to an alteration in the activated state of the enzyme by short-term regulation. We show that the HMG-CoA reductase in A549 cell line was subject to a multivalent feedback control. A high fraction (40%) of the reductase activity was devoted to non-sterol products. In contrast, normal fibroblasts had only 15-20% of the reductase activity that generated non-sterol products. We also show that cholesterol and at least one of the non-sterol products are necessary for optimal cell growth of A549 cells. Our data strongly suggest that A549 cells produce more non-sterol substances which may be related to increased requirements of mevalonate for upregulated cell growth.o 1993 Wzley-Liss, Inc.