LOW-DENSITY-LIPOPROTEIN RECEPTORS IN RAT ADIPOSE-CELLS - SUBCELLULAR-LOCALIZATION AND REGULATION BY INSULIN

The distribution of LDL receptors within subcellular compartments of i solated rat adipose cells and the effects of insulin on their expressi on have been assessed. By immunoblotting with specific anti-rat LDL re ceptor antibodies, LDL receptors were 2.3- and 4.5-fold enriched in en doplasmic reticulum-rich high-density microsomes (HDM) and Golgi compl ex-rich low-density microsomes (LDM), respectively, compared to plasma membranes (PM). This distribution was similar in cultured cells in wh ich total receptors were increased 2.5-fold compared to freshly isolat ed cells. After correction for enzyme recoveries, LDL receptors were d istributed similar to 4% in HDM, similar to 73% in LDM, and similar to 23% in PM. Insulin decreased total LDL receptors in adipose cells sim ilar to 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 rece ptor-mediated endocytosis, insulin again caused a decrease of LDL rece ptors in LDM but now increased LDL receptors in PM. Insulin increased the rate of LDL receptor synthesis similar to 24%, but decreased their half life similar to 40%. Thus, in isolated adipose cells the majorit y of LDL receptors appear to be located in an intracellular compartmen t 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.