Low density lipoprotein receptors in rat adipose cells: subcellular localization and regulation by insulin

The distribution of LDL receptors within subcellular compartments of isolated rat adipose cells and the effects of insulin on their expression have been assessed. By immunoblotting with specific anti-rat LDL receptor antibodies, LDL receptors were 2.3- and 4.5-fold enriched in endoplasmic reticulum-rich high-density microsomes (HDM) and Golgi complex-rich low-density microsomes (LDM), respectively, compared to plasma membranes (PM). This distribution was similar in cultured cells in which total receptors were increased 2.5-fold compared to freshly isolated cells. After correction for enzyme recoveries, LDL receptors were distributed approximately 4% in HDM, approximately 73% in LDM, and approximately 23% in PM. Insulin decreased total LDL receptors in adipose cells approximately 44%, with a 48% and 49% decrease in HDM and LDM, respectively, without any changes in PM. In contrast, insulin caused an increase of glucose transporters in PM while also decreasing glucose transporters in LDM. When adipose cells were depleted of potassium to inhibit receptor-mediated endocytosis, insulin again caused a decrease of LDL receptors in LDM but now increased LDL receptors in PM. Insulin increased the rate of LDL receptor synthesis approximately 24%, but decreased their half life approximately 40%. Thus, in isolated adipose cells the majority of LDL receptors appear to be located in an intracellular compartment that co-sediments with the Golgi complex rather than located in the PM. The LDL receptors localized in intracellular compartments seem to be functionally regulated as insulin acutely diminishes the number of receptors by apparently accelerating their rate of degradation through, as yet, incompletely determined mechanisms.