Under these conditions, the VLDL receptor has a mobility of 115 to 120 kd. cells. This was despite the fact that cells from one patient with AML of GS-9901 M4 subtype had a 50- to 100-fold higher 125I-labeled LDL degradation compared with normal cells. Immunoblots with antibodies against gp330/megalin and the LDL-receptor-related protein (LRP) and ligand blot using 125I-labeled 39-kd receptor-associated protein (RAP) could not detect gp330/megalin or VLDL receptors. The LRP was abundant in AML samples of M4 and M5b subtype, as determined from both RAP ligand blot and immunoblot using an LRP-specific antibody. It is concluded that LDL receptors are suppressed in AML cells. It is possible that the high degradation of 125I-labeled LDL present in type M4 and M5 AML cells may involve another lipoprotein receptor. Human cells have receptors for the major cholesterol-carrying lipoprotein in human plasma, low-density lipoprotein (LDL). 1,2 These LDL receptors transport cholesterol-rich lipoproteins containing apolipoprotein (apo)B or apoE into the cell. Tissues with a high demand for cholesterol, such as the adrenal gland and the corpus luteum, have high receptor numbers. 3,4 In contrast, LDL receptor expression is strongly suppressed in cells exposed to high LDL levels, such as mononuclear cells within the bloodstream. 5 When normal cells proliferate, the LDL receptor number is induced, 6 presumably due to an increased demand for cholesterol. A similar situation might be present in tumor cells. Thus, Ho et al 7 have shown that freshly isolated leukemic cells from patients with acute myelogenous leukemia (AML) and chronic myelogenous leukemia (CML) in blast crisis display a 3- to 100-fold higher specific LDL degradation as compared with mononuclear cells from normal subjects or tumor cells from patients with acute AKT lymphoblastic leukemia (ALL). In that study, five patients with AML and two patients with CML in blast crisis were all found to have increased LDL receptor activity. These findings were later confirmed by Vitols et al 8 in a large series of patients. They demonstrated that very high LDL degradation was practically always present in samples from CML in blast crisis and AML of subgroups M4 and M5. The cause for this increased specific LDL degradation remains unclear. 7-9 Two studies have shown, in two leukemia patients, that the elevated basal specific LDL degradation in AML cells is not suppressed after incubation with cholesterol and 25-hydroxycholesterol. 7,9 This fact was recently confirmed in a study on 27 leukemia patients by Tatidis et al. 10 This result is somewhat remarkable considering current concepts of LDL receptor regulation 1 but has been interpreted to be indicative of post-transcriptional regulation of the LDL receptor. 9 In all previous studies on LDL receptors in leukemia, the cellular high-affinity degradation of 125I-labeled LDL has been measured. 7,11 This is determined from the generation of 125I-labeled tyrosine after incubation of cells with 125I-labeled LDL at 37C in the absence and presence of excessive unlabeled LDL. The degradation of 125I-labeled LDL that can be displaced by unlabeled LDL is definitely then referred to as cellular LDL receptor activity. 7,11 This practical assay is very robust and offers therefore been used extensively to monitor LDL receptor function in cultured cells. However, it is an indirect assay, and in the past years it has been demonstrated that cell surface receptors other than the LDL receptor can also mediate specific, high-affinity degradation of 125I-labeled LDL. 12,13 We recently found that the gene manifestation of the LDL receptor and that of the rate-limiting enzyme of cholesterol GS-9901 synthesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase, were coordinately suppressed in human being renal cell carcinoma. 14 Because of this unpredicted finding, we GS-9901 examined the studies on LDL GS-9901 receptors GS-9901 in leukemia 7-10,15-18 and found several observations inconsistent with the current concept of an increased manifestation of LDL receptors in AML cells (observe Discussion). Remarkably, we recognized that all studies to day possess identified LDL receptor activity from the same indirect LDL degradation assay. We consequently decided to assay directly the manifestation of LDL receptors in leukemic cell membranes, with the aim of clarifying whether LDL receptor manifestation is indeed improved in AML cells. Using an established ligand blot assay, 19-22 we found that the manifestation of LDL receptors in membranes of mononuclear cells.