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Leukemic cells isolated from most patients with acute myelogenous leukemia (AML) have higher low density lipoprotein (LDL) uptake than normal mononuclear blood cells. Little is known, however, about the mechanism behind the elevated LDL uptake. We investigated if AML cells secrete factors that stimulate cellular LDL uptake. Mononuclear blood cells were isolated from peripheral blood from 42 patients with AML at diagnosis. Cellular LDL uptake was determined from the degradation rate of 125I-labelled LDL. Conditioned media from AML cells stimulated the LDL degradation in the leukemic cell lines KG1 and HL60, and in isolated AML cells. The stimulatory effect correlated with the LDL degradation in the AML cells directly after isolation from blood. Conditioned media also autostimulated LDL degradation in the AML cells themselves. Concentrations of IL-6 and IL-8 in AML cell conditioned media correlated with the LDL degradation in AML cells directly after isolation from blood. Addition of R-TNF-α, but not IL-6 or IL-8, stimulated LDL degradation in HL60, KG1, and AML cells. The LDL degradation in AML cells could be inhibited by a LDL receptor blocking antibody. AML cells secrete factors that stimulate LDL uptake in a paracrine and autocrine pattern which open up therapeutic possibilities to inhibit the uptake of LDL by administration of antibodies to these factors.

Hypocholesterolemia is a frequent finding in patients with acute myelogenous leukemia (AML) and in other types of malignancies. Since bile acids are major excretion products of cholesterol, the hepatic degradation of cholesterol to bile acids was investigated in AML patients by analyzing a circulating marker for bile acid synthesis. In addition, plasma levels of a marker for cholesterol synthesis were determined. The plasma levels of 7alpha-hydroxy-4-cholesten-3-one, reflecting bile acid production, were markedly lower in patients with AML than in healthy controls. The median levels were 3.3 and 18.5ng/ml (P

Altered cholesterol homeostasis has been noted in malignant cells, which led us to explore the regulation of cholesterol metabolism in normal and leukemic cells. The mean low-density lipoprotein (LDL) receptor and 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activities were fivefold and threefold higher in mononuclear blood cells from 33 patients with leukemia, compared with cells from 23 healthy subjects, whereas elevations in RNA levels were twofold and 40% only. The activities of the two proteins correlated in normal cells (r = .46), whereas an inverse correlation was found in leukemic cells (r = -.40). Relatively weak correlations were found between LDL receptor RNA levels and receptor activity in normal (r = .48) and leukemic cells (r = .49), and HMG-CoA reductase RNA levels correlated (r = .53) with reductase activity in leukemic cells only. The ratios of protein activities to RNA levels in cells were constant during consecutive blood samplings and similar in leukemic blood and bone marrow cells from the same individual. During cholesterol deprivation, protein activities increased more than RNA levels, and leukemic cells with high LDL receptor activity showed a partial resistance to the suppressing effect of sterols on LDL receptor gene expression. The results demonstrate that LDL receptor RNA levels alone can not explain variation in receptor activity, suggesting post-RNA regulation of LDL receptor expression, similar to what has been described for HMG-CoA reductase. Taken together, the present results suggest multilevel regulation of both proteins and demonstrate that each cell clone, normal or malignant, has a unique ratio of protein activity to RNA level. Leukemic cells, in contrast to normal cells, can meet increased cholesterol requirements by either elevated LDL receptor activity or increased cholesterol synthesis, which is of potential interest for diagnosis and specific treatment of leukemia.

Overexpression of a 170 kD membrane glycoprotein, P-glycoprotein (Pgp), which acts as an energy dependent efflux pump for cytotoxic drugs is believed to be one of the factors that is responsible for clinical drug resistance. Recent studies suggest that Pgp is also responsible for the intracellular transport of cholesterol from the plasma membrane to the endoplasmic reticulum. Leukemic cells from patients with acute myelogenous leukemia have an elevated uptake of low density lipoprotein (LDL) when compared with white blood cells from healthy individuals. Since elevated LDL receptor expression and multidrug resistance are both common events in leukemic cells, we investigated LDL receptor expression in sensitive and drug resistant human leukemic cell lines. We found a 2- to 10-fold higher uptake of LDL in five out of five drug resistant K562 cell lines. All three drug resistant HL60 cell lines studied also had higher uptake than the parental cells. The LDL receptor expression in vincristine resistant Pgp positive K562 cells was less sensitive to downregulation by sterols than in parental cells. There was no selective effect of the Pgp inhibitor PSC-833 or other Pgp modulators on LDL receptor activity in Pgp positive cells. Since also resistant Pgp, multidrug resistance protein 1, and breast cancer resistance protein negative cells exhibited an elevated LDL receptor activity, we conclude that overexpression of these proteins is not the mechanism behind the elevated LDL uptake in the drug resistant leukemic cell lines. The findings are of interest for the concept of using lipoproteins as carriers of cytotoxic drugs in cancer treatment.

Sterols regulate low density lipoprotein (LDL) receptor and 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase gene expressions by end product repression. Studies on cultured cells have shown that growing cells have a higher LDL uptake than quiescent cells and that incubation of cells with growth factors or mitogenic compounds leads to sterol-resistant upregulation of LDL receptor gene expression. The recent finding that elevated LDL receptor activity in acute myelogenous leukemia cells was characterized by a decreased sensitivity to downregulation by sterols raises the possibility that the mechanism behind this is related to the cellular growth rate. By using cultured human fibroblasts as a model system we therefore studied whether growth modulation of sterol sensitivity takes place in normal actively growing cells. Judging from the ability of sterols (25-hydroxycholesterol + cholesterol) to inhibit 125I-LDL degradation, we found that the sensitivity to sterols varied markedly between cells of different densities. The lowest sensitivity to sterols and highest 125I-LDL degradation rate were found in subconfluent cells, whereas sparse and confluent cells were the most sensitive ones. In contrast to the LDL receptor, HMG-CoA reductase sterol sensitivity did not appear to be growth regulated. We conclude that growth-dependent modulation of sterol sensitivity and LDL receptor activity takes place in normal human fibroblasts. Modulation of sterol sensitivity may be an important mechanism to ensure an adequate cholesterol supply in growing cells.

Leukemic cells from patients with acute myelogenous leukemia (AML) have higher low density lipoprotein (LDL) receptor activity than normal white blood and bone marrow cells. The underlying mechanism behind this is unclear. We studied the inhibitory effect of sterols on induction of LDL-receptor activity in leukemic cells from 27 patients with AML and in white blood cells from 13 healthy individuals. The high affinity degradation rate of 125I-labeled LDL was determined in mononuclear blood cells directly after isolation from blood and after incubation for 2 days in medium with 10% lipoprotein-deficient serum with or without various concentrations of 25-hydroxycholesterol + cholesterol. The median sterol concentration for 50% inhibition (IC50) of induction was more than five times higher for leukemic cells than for normal mononuclear cells. At the highest sterol concentration (0.400 microg/mL 25-hydroxycholesterol + 8 microg/mL cholesterol), the LDL-receptor activity was abolished in cells from all healthy individuals while the induction of LDL-receptor activity in cells from three AML patients was unaffected. The LDL-receptor activity of leukemic cells, directly after isolation from blood, correlated with IC50 values (r = 0.53, P = 0.007) and WBC counts (r = 0.72, P = 0.0001) but not with cellular cholesterol levels. The results demonstrate decreased feedback regulation of LDL-receptor activity by sterols in AML cells and support the conclusion that elevated LDL-receptor activity is associated with sterol resistance and cell proliferation. The findings are of potential interest for diagnosis and specific treatment of leukemia.