The formation of an insulin-responsive vesicular cargo compartment is an early event in 3T3-L1 adipocyte differentiation

Differentiating 3T3-L1 cells exhibit a dramatic increase in the rate of ins ulin-stimulated glucose transport during their conversion from proliferatin g fibroblasts to nonproliferating adipocytes. On day 3 of 3T3-L1 cell diffe rentiation, basal glucose transport and cell surface transferrin binding ar e markedly diminished. This occurs concomitant with the formation of a dist inct insulin-responsive vesicular pool of intracellular glucose transporter 1 (GLUT1) and transferrin receptors as assessed by sucrose velocity gradie nts. The intracellular distribution of the insulin-responsive aminopeptidas e is first readily detectable on day 3, and its gradient profile and respon se to insulin at this time are identical to that of GLUT1. With further tim e of differentiation, GLUT4 is expressed and targeted to the same insulin-r esponsive vesicles as the other three proteins. Our data are consistent wit h the notion that a distinct insulin-sensitive vesicular cargo compartment forms early during fat call differentiation and its formation precedes GLUT 4 expression. The development of this compartment may result from the diffe rentiation-dependent inhibition of constitutive GLUT1 and transferrin recep tor trafficking such that there is a large increase in, or the new formatio n of, a population of postendosomal, insulin-responsive vesicles.