RECEPTOR-MEDIATED UPTAKE OF LOW-DENSITY-LIPOPROTEIN BY B16 MELANOMA-CELLS IN-VITRO AND IN-VIVO IN MICE

Selective delivery of cytotoxic anti-neoplastic drugs can diminish the severe side-effects associated with these drugs. Many malignant tumou rs express high levels of low-density lipoprotein (LDL) receptors on t heir membranes. Therefore, LDL may be used as a carrier to obtain sele ctive delivery of anti-neoplastic drugs to tumours. The present study was performed to investigate the feasibility of the murine B16 tumour/ mouse model for the evaluation of LDL-mediated tumour therapy. LDL bin ds with high affinity to LDL receptors on cultured B16 cells (K-d, 5.9 +/-2.3 mu g ml(-1); B-max 206+/-23 ng LDL mg(-1) cell protein). After binding and internalisation, LDL was very efficiently degraded: 724+/- 19 ng LDL mg(-1) cell protein h(-1). Chloroquine and ammonium chloride completely inhibited the degradation of LDL by the B16 cells, indicat ing involvement of lysosomes. LDL receptors were down-regulated by 70% after preincubation of B16 cells with 300 mu g ml(-1) LDL, indicating that their expression is regulated by intracellular cholesterol. To e valuate the uptake of LDL by the B16 tumour in vivo, tissue distributi on studies were performed in C57/Bl mice inoculated with B16 tumours. For these experiments, LDL was radiolabelled with tyramine cellobiose, a non-degradable label, which is retained in cells after uptake. Ar 2 4 h after injection of LDL, the liver, adrenals and the spleen were fo und to be the major organs involved in LDL uptake, with tissue-serum ( T/S) ratios of 0.82+/-0.08, 1.17+/-0.20 and 0.69+/-0.08 respectively. Of all the other tissues, the tumour showed the highest uptake of LDL (T/S ratio of 0.40+/-0.07). A large part of the LDL uptake was recepto r mediated, as the uptake of methylated LDL was much lower. Although t he LDL uptake by the liver, spleen and adrenals is higher than that by the tumour, the LDL receptor-mediated uptake by these organs may be s electively down-regulated by methods that do not affect the expression of LDL receptors on tumour cells. It is concluded that the B16 tumour -bearing mouse constitutes a good model to evaluate the effectiveness of LDL-mediated delivery of cytotoxic (pro)drugs to tumours in vivo.