Opposite Effects of Insulin and Catecholamines on LDL-Receptor Activity in Human Mononuclear Leukocytes

The mechanisms by which insulin and catecholamines affect low-density lipoprotein (LDL)-receptor activity were studied in freshly isolated human mononuclear leukocytes. Incubation of cells for up to 24 h in a lipid-free medium resulted in an increase in the specific binding, accumulation, and degradation of ¹²⁵I-labeled LDL. Insulin stimulated the ability of the cells to bind, accumulate, and degrade the lipoprotein with high affinity, which may be caused by an increase in the LDL-receptor number without altering binding affinity. (–)-Epinephrine inhibited the specific binding, accumulation, and degradation of ¹²⁵I-LDL. This effect appears to be mediated by a decrease in the number of LDL receptors and not by a change in the binding affinity. (–)-Norepinephrine, the unspecific β-adrenergic agonist (–)-isoproterenol, and the β₂-specific agonist terbutaline mimicked the effect of epinephrine on LDL-receptor activity. Catecholamines and β-adrenergic agonists yielded sigmoidal log-concentration effect curves. The action of epinephrine was attenuated by the β-antagonist (dl)-propranolol. These results demonstrate that insulin stimulates and catecholamines suppress the specific binding, accumulation, and degradation of ¹²⁵I-LDL in human mononuclear leukocytes. The catecholamine action appears to be mediated by β₂-adrenergic receptors. A suppression of LDL-receptor activity resulting from deficiency of insulin and elevated plasma catecholamine concentrations in uncontrolled insulin-dependent diabetic patients may contribute to the increased levels of LDL cholesterol observed in these patients.