Pigeon aortic smooth muscle cells lack a functional low density lipoprotein receptor pathway

The low density lipoprotein (LDL) receptor pathway was studied in aortic smooth muscle cells from atherosclerosis-susceptible White Carneau pigeons and compared with rhesus monkey cells whose LDL receptor pathway has been previously characterized. Pigeon LDL was bound with high affinity in a saturable manner to both pigeon and monkey aortic smooth muscle cells. The kinetics of binding were different, however. LDL binding to pigeon cells exhibited positive cooperativity at low LDL concentrations and at least two classes of binding sites. The same pigeon LDL bound to monkey cells in a manner consistent with a single class of binding sites. Thus, these differences were a property of the pigeon cells and not the result of differences in the LDL. On the average, pigeon cells bound less than 50% the amount of LDL as monkey cells. Despite the surface binding to pigeon cells, little of the LDL was internalized, whereas pigeon LDL was actively internalized by monkey cells. Consistent with this observation, chloroquine and leupeptin had no effect on accumulation of LDL or on LDL degradation by pigeon cells, and incubation of pigeon cells with LDL produced no increase in cellular cholesteryl ester content. Binding of LDL to pigeon cells also differed from that of monkey cells by being unaffected by pretreatment with the proteolytic enzyme pronase, and by not requiring calcium. Binding was not specific for LDL since acetyl-LDL, and to a lesser degree HDL, were able to compete for LDL binding. Incubation with lipoprotein-deficient serum decreased LDL binding in pigeon cells while 25-OH cholesterol caused an increase in binding; both effects are opposite of that seen with the same LDL in mammalian cells. Preincubation with LDL or cholesterol dissolved in ethanol were without effect on LDL binding in pigeon cells, even though they produced significant increases in cellular free cholesterol content. In spite of the failure to internalize LDL, there was considerable degradation of LDL. This apparently occurred on the cell surface rather than by internalization and degradation within the lysosomes as occurs in mammalian cells. The functional significance of LDL binding to pigeon smooth muscle cells is unclear. The characteristics of binding resemble that of a nonspecific lipoprotein receptor referred to by others as the "lipoprotein receptor" or the "EDTA-insensitive receptor." It is apparent, however, that White Carneau pigeon aortic smooth muscle cells lack a functional LDL receptor pathway and in this way resemble cells from human beings with homozygous familial hypercholesterolemia or from Watanabe rabbits.