Internalization of low‐density‐lipoprotein‐specific receptors in leukemic guinea pig lymphocytes

Abstract

Leukemic guinea pig lymphocytes (L2C) have ten times as many low-density lipoprotein (LDL) receptors as healthy lymphocytes, but LDL accounts for only 38% of the cholesterol in L2C cells, compared to more than 95% in normal cells, Our data show that LDL fails to regulate cholesterol biosynthesis and that there is a defect in LDL internalization and receptor turnover in L2C cells. We also demonstrate that the degradation of LDL is not a limiting process. By discriminating between binding and internalization, we show that internalization in L2C is much slower than in normal cells and that the decrease in metabolism is related to the slow turnover of the LDL receptors.In human lymphocytes, as in many eukaryotic cells, cholesterol homeostasis depends on endogenous cholesterol synthesis via the 3-hydroxy-3-methylglutaryl-CoA reductase pathway and on the supply of cholesterol from the endocytosis of LDL after binding to LDL-specific receptors [l]. We have shown that in healthy guinea pig lymphocytes, cholesterol biosynthesis and LDL endocytosis account for 2.3% and 97.7%, respectively, of total cellular cholesterol needs. These cells, when freshly isolated, have about 2000 LDL-specific receptors per cell [2] and their cholesterol synthesis is directly regulated by cellular LDL metabolism [3]. On the other hand, guinea pig leukemic lymphocytes (L2C) are characterized by abnormally high cholesterol biosynthesis in vivo [4] and in vitro [5] but the regulation of cholesterol synthesis by LDL is defective [3]. Our previous results demonstrated that although L2C cells have 20000 LDL-specific receptors per cell, they have a very poor LDL metabolism compared to normal lymphocytes. The decreased rate of internalization and degradation relative to binding in the L2C cells is reflected in the internalization index, which is nearly four times as low in L2C cells as in normal cells [2]. Thus, in these cells, the de novo production of cholesterol corresponds to 62% of the whole cellular cholesterol supply, whereas only 38% comes from the LDL pathway [2]. We do not yet know enough about the impairment of LDL metabolism in cancer cells to ascertain whether the origin of this defect lies in the internalization or the intracellular degradation of LDL. Kraaijenhagen et al. [6] showed that the activities of all enzymes, except acid phosphatase, were significantly lower in leukemic B lymphocytes than in normal lymphocytes, while Anderson et al. [7] demonstrated that cancerous A-431 cells have an abnormally small capacity for internalizing LDL. We previously found that the uptake of '"I-LDL by L2C cells reaches a plateau in 4-5 h but we were unable to conclude which of the two processes (internalization or degradation) was defective [2]. In the present study, we have separately analyzed the rate of LDL internalization and the yield of its degradation by lysosomal enzymes, and the turnover of LDL receptors. Our data show that LDL internalization and/or the turnover of the receptors are the limiting steps. At equilibrium, degradation always ...

Low-Density Lipoprotein as a Vehicle for Targeting Antitumor Compounds to Cancer Cells

“…If this is borne out in future studies, LDL-based therapy will be even more valuable than it presently appears. Some tumors, however, do not internalize great amounts of LDL, e.g., ALL (mentioned above) (15), chronic lymphocytic leukemia (CLL) (34), several colon adenocarcinomas (35), Lewis rat renal carcinoma (36), cervical cancer EC-168 (37), epitheloid carcinoma A-431 (38), and guinea pig leukemic lymphocytes (39). The latter three have plenty of LDL receptors, but internalization is deficient.…”

LDL-mediated targeting of liposomes to leukemic lymphocytes in vitro.

“…The L2C leukemia affecting the cells used in this study, arose spontaneously in a strain 2 guinea pig (Congdon and Lorenz, 1954) and was passaged in syngeneic animals. L2C lymphocytes were harvested and purified by Lymphoprep gradient centrifugation, as previously described (Chuillon-Sainte-Marie et al, 1981). Leukemic (L2C) lymphocytes have LDL-specific receptors (Sainte-Marie et al, 1985).…”