Insulin-regulatable phosphoproteins in 3T3-L1 adipocytes form detergent- insoluble complexes not associated with caveolin

Abstract

Whole body glucose homeostasis is dependent on the action of insulin. In muscle and adipose tissues, insulin stimulates glucose uptake by inducing the translocation of vesicles containing the glucose transporter GLUT4 to the cell surface. While the mechanisms of insulin-regulated GLUT4 translocation are not fully understood, some signaling intermediates have been implicated in this process. Interestingly, some of these intermediates, including IRS-1 and PI3K, have been localised to the same intracellular membrane fraction as the GLUT4 storage pool, designated here as the high-speed pellet (HSP) fraction. This raises the possibility that many of the downstream insulin signaling intermediates may be located within close proximity to intracellular GLUT4. The goal of this study was to test this hypothesis in 3T3-L1 adipocytes. A large proportion of adipocyte phosphoproteins co- fractionated in the HSP fraction. In an attempt to resolve insulin- regulatable phosphoproteins, we subjected 32P-labeled subcellular fractions to two-dimensional gel electrophoresis (2-DE). Insulin reproducibly stimulated the phosphorylation of 12 spots in the HSP fraction. Most of the HSP phosphoproteins were insoluble in the nonionic detergent Triton X-100, whereas integral membrane proteins such as GLUT4 and intracellular caveolin were soluble under the same conditions. These results suggest that insulin- regulatable phosphoproteins in adipocytes may be organized in microdomains within the cell and that this assembly may act as an efficient conductor of the signaling proteins to rapidly facilitate down-stream biological responses. Further study is required to establish the molecular basis for these detergent-insoluble signaling complexes.