Abstract
Cultured IM-9 lymphocytes were preincubated with 1 microM 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 less than 0.05). By 1 hr of chase, the difference reached 41 +/- 14% for the proreceptor and 84 +/- 28% for the alpha subunit (P less than 0.01); values returned to normal by 2 hr. At 4 hr of chase, labeling of the alpha subunit of down-regulated cells was diminished 36 +/- 9% below control (P less than 0.05). The increased biosynthetic rate of the proreceptor was more prominent when the chase medium contained 25 microM 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.