Binding and degradation of human 125I-HDL by rat adrenocortical cells

In order to learn more about the mechanism by which high density lipoprotein (HDL) cholesterol is taken up by the adrenal cortex, binding and degradation of human 125I-HDL by suspensions of intact rat adrenal cortical cells have been examined. Cellular accumulation of 125I-HDL was found to occur in two phases. Our results indicate that the initial phase of association results from reversible binding of 125I-HDL to a specific saturable set of membrane binding sites. Binding site affinity appears equal for both rat and human HDL while affinity for human LDL is approximately one order of magnitude less on the basis of apoprotein weight. In addition, isolated rat adrenal cortical cells were found to degrade human 125I-HDL at a rapid rate. Degradation, like binding, can be prevented by addition of excess unlabeled HDL suggesting that binding and degradation are linked. Thus, one mechanism that could account for adrenal uptake of HDL cholesterol is endocytosis, initiated by lipoprotein binding to the HDL specific membrane binding site.