| Abstract: | Cholesterol metabolism was examined in aortic smooth muscle cells from atherosclerosis-susceptible White Carneau pigeons that have been shown to lack a functional LDL receptor pathway. In cells incubated in the presence of whole serum or low density lipoprotein (LDL) the rate of cholesterol synthesis from 1-14C]acetate or of HMG-CoA reductase activity was 20-100 times greater than for mammalian cells incubated under the same conditions. Unexpectedly, cholesterol synthesis decreased by nearly 50% after preincubation for 24 hr with lipoprotein-deficient serum (LPDS). This occurred without a change in cellular cholesterol content. Neither the high rate of cholesterol synthesis nor the effect of LPDS could be accounted for by differences in cell turnover or state of growth. Cholesterol added in ethanol was ineffective in altering cellular cholesterol synthesis or esterification even though a near doubling in cellular free cholesterol content occurred. Cholesterol synthesis and esterification were, however, able to be regulated with 25-OH cholesterol and mevalonolactone, as indicated by their ability to suppress cholesterol synthesis and to stimulate cholesterol esterification. In spite of the high rate of endogenous cholesterol synthesis, cellular cholesterol content was maintained at a constant level by the efficient efflux of the newly synthesized cholesterol from the cell. Unlike mammalian cells that require a cholesterol acceptor in the medium for efflux to occur, cholesterol efflux from pigeon cells occurred in the absence of a cholesterol acceptor. This suggests either that pigeon cells utilize a different mechanism for cholesterol efflux or that they produce their own cholesterol acceptor. As a result of a lack of a functional LDL receptor pathway, pigeon smooth muscle cells do not maintain cholesterol homeostasis through the controlled uptake of exogenous LDL cholesterol, as do mammalian cells. Rather, pigeon smooth muscle cells would appear to regulate cholesterol concentrations at the level of either cholesterol synthesis or efflux. |
