Homologous Down-Regulation of the Insulin Receptor is Associated with Increased Receptor Biosynthesis in Cultured Human Lymphocytes (IM-9 Line)

Cultured IM-9 lymphocytes were preincubated with 1 μ M insulin, a condition resulting in a 56% reduction in cell surface insulin receptors. Cellular proteins were then metabolically labeled, and the radioactivity incorporated into the insulin proreceptor and receptor mature subunits was measured over a 4-hr chase period. As early as 30 min of chase, incorporation into the proreceptor was 28 ± 6% higher in down-regulated cells than in control cells (mean ± SEM, P < 0.05). By 1 hr of chase, the difference reached 41 ± 14% for the proreceptor and 84 ± 28% for the α subunit (P < 0.01); values returned to normal by 2 hr. At 4 hr of chase, labeling of the α subunit of down-regulated cells was diminished 36 ± 9% below control (P < 0.05). The increased biosynthetic rate of the proreceptor was more prominent when the chase medium contained 25 μ M monensin, an inhibitor of processing of the proreceptor into mature subunits. Similar effects occurred whether [3H]mannose or [3H]lysine was used as biosynthetic marker. The effect was specific for the insulin receptor. These data demonstrate that insulin receptor homologous down-regulation is associated with increased proreceptor biosynthesis and processing into mature subunits. This might represent a cellular mechanism compensating for insulin-induced receptor loss.