Characterization of high density lipoprotein binding activity in rat adrenocortical cells.

Rat adrenocortical cells take up high density lipo- protein cholesterol for use as steroidogenic substrate. To better understand this unique uptake process, we have first charac- terized HDL binding. Infusion of human '251-labeled HDL into rats pretreated with 4-APP demonstrated that the adrenal and ovary accumulate HDL in a saturable fashion in vivo. Subsequent studies using isolated rat adrenocortical cells dem- onstrated that cellular uptake of HDL is comprised of two events. One event is characterized by reversible membrane binding and is complete by 60 min = 20 min). The second event is marked by irreversible apoprotein accumulation which continues for at least 3 hr. Reversibly bound material exhibits the same apoprotein distribution as unincubated HDL. Irreversible accumulation could not be attributed to intemal- ization or lysosomal accumulation inasmuch as it also occurred with partially purified plasma membranes and was not enhanced by addition of chloroquine. Reversible binding of human HDLS exhibited a saturable dependence on concentration (rC, = 27 pg protein/ml; N = 3.0 X lo6 sites/cell) similar to that previously reported for rat liver, ovary, and testis. Cell accu- mulation of HDL decreased by over 80% at 4°C compared to 37"C, did not require calcium, and was not diminished by prior cell treatment with trypsin or pr0nase.M These results indicate that rat adrenocortical cells possess plasma membrane recognition sites for HDL with different properties than those of the LDL receptor. Moreover, adrenal accumulation of HDL apoproteins does not lead to secondary lysosome formation.- Gwynne, J. T., T. Hughes, and B. Hesr. Characterization of high density lipoprotein binding activity in rat adrenocortical