Insulin receptors in isolated human adipocytes. Characterization by photoaffinity labeling and evidence for internalization and cellular processing

Abstract

We photolabeled and characterized insulin receptors in isolated adipocytes from normal human subjects and then studied the cellular fate of the labeled insulin-receptor complexes at physiologic temperatures. The biologically active photosensitive insulin derivative, B2(2-nitro-4-azidophenylacetyl)des-PheB1-insulin (NAPA-DP-insulin) was used to photoaffinity label the insulin receptors, and the specifically labeled cellular proteins were identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and autoradiography. At saturating concentrations, the binding of 125I-NAPA-DP-insulin to the isolated adipocytes at 16 degrees C was rapid (half-maximal in approximately 1 min and maximal in approximately 10 min) and approximately 25% of the specifically bound ligand was covalently linked to the cells by a 3-min exposure to long-wave (366 nm) ultraviolet light. Analysis of the photolabeled cellular proteins by PAGE in the absence of disulfide reductants revealed the specific labeling of a major protein band of Mr 330,000 and two less intense bands of Mr 295,000 and 260,000. Upon reduction of disulfide bonds with dithiothreitol, all three unreduced forms of the insulin receptor were converted into a major labeled Mr-125,000 band and a less intensely labeled Mr-90,000 band. The labeling of the Mr-125,000 receptor subunit was saturable and native porcine insulin effectively inhibited (half-maximal inhibition at 12 ng/ml) the photolabeling of this binding subunit by NAPA-DP insulin. When intact adipocytes photolabeled at 16 degrees C (a temperature that inhibits endocytosis) were immediately trypsinized, all of the labeled receptor bands were converted into small molecular weight tryptic fragments, indicating that at 16 degrees C all of the labeled insulin-receptor complexes remained on the cell surface. However, when the photolabeled cells were further incubated at 37 degrees C and then trypsinized, a proportion of the labeled receptors became trypsin insensitive, indicating that this fraction has been translocated to the cell interior and thus was inaccessible to the trypsin in the incubation medium. The intracellular translocation of the labeled receptors was observed within 2 min, became half-maximal by 10 min, and maximal by approximately 30 min of incubation at 37 degrees C. Cellular processing of the internalized insulin-receptor complexes also occurred, since incubation at 37 degrees C (but not 16 degrees C) resulted in the generation of a Mr-115,000 component from the labeled receptors. Inclusion of chloroquine, a drug with lysosomotropic properties, in the incubation media caused a time-dependent increase (maximal increase of 50% above control by 2 h at 37 degrees C) in the intracellular pool of labeled receptors. In contrast to these findings in human adipocytes, no appreciable internalization of insulin-receptor complexes and no chloroquine effect was observed in cultures human IM-9 lymphocytes during a 1-h incubation at 37 degrees C. We concluded that in isolated human adipocytes: (a) the subunit structure of insulin receptors is the same as that reported for several other tissues, (b) insulin-receptor complexes are rapidly internalized and processed at physiologic temperatures, and (c) the cellular processing of insulin-receptor complexes occurs at one or more chloroquine-sensitive intracellular site(s).