Binding and internalization of low-density lipoproteins in SCC25 cells and SV40 transformed keratinocytes. A morphologic study.

Binding of low-density lipoproteins (LDL) to the plasma membrane and internalization of low-density lipoprotein receptor complexes were investigated in an epithelial tumor cell derived from the tongue (SCC25) and in SV40-transformed keratinocytes (SVK14 cells). For light microscopic studies an immunofluorescence technique with antiapoprotein B as well as conjugation procedure by which a fluorochrome 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanide (DIL) was conjugated with LDL (LDL-DIL) was used. Binding of LDL to the plasma membrane at 4 degrees C was observed in most SCC25 cells but not in SVK14 cells. The internalization of LDL-DIL was absent in SVK14 cells and was excessive in SCC25 cells. In SCC25 cells, internalization of the LDL-DIL particles was heterogeneously distributed over various cells. When a pulse-chase experiment was performed with LDL-DIL, less LDL was internalized into the SCC25 cells in comparison with a continuous label experiment. For the ultrastructural studies LDL conjugated with colloidal gold was used. In the binding experiments at 4 degrees C most LDL-gold particles were attached to the plasma membrane outside coated pits. During internalization experiments with LDL-gold particles it was observed that within 5-15 min at 37 degrees C several LDL-gold particles were seen in electron-dense structures near the plasma membrane. The electron-dense structures containing LDL-gold, as observed after an internalization period of 5-15 min, may represent the first endosomal compartment as described for transferrin receptors in A431 cells. After a period of 30 min at 37 degrees C the LDL-gold particles were observed in electron-lucent vesicles (multivesicular bodies) and dense bodies. However coated vesicles containing LDL-gold particles were seen sporadically. It is concluded that the route of internalization of LDL into the SCC25 cells differs from that of other cell types. No internalization of LDL gold was found in SVK14 cells, thus, in this respect, the SVK14 cells resemble normal keratinocytes. The morphologic data are in good agreement with biochemical studies published earlier (Ponec M et al, J Invest Dermatol 83:436-440, 1984). Both investigations suggest that LDL receptor activity is modulated during the process of terminal differentiation.