Relationship between insulin stimulation and endogenous regulation of 2‐deoxyglucose uptake in 3T3‐L1 adipocytes

The occurrence of the endogenous regulatory response to high rates of 2-deoxyglucose (2-DG) uptake, as previously described for C6 glioma cells during incubation with 2 mM 2-DG (Lange et al.: J. Cell. Physiol., 1989), was studied in 3T3-L1 preadipocytes and adipocytes, and the influence of insulin on this endogenous uptake regulation was examined. In contrast to 3T3-L1 preadipocytes, insulin-sensitive differentiated 3T3-L1 adipocytes displayed the time-dependent cyclic pattern of 2-DG uptake rates characteristic of the membrane-limited and endogenously regulated cellular state of hexose utilization. Although insulin induced a threefold stimulation of 2-DG tracer uptake in adipocytes, the hormone did not additionally stimulate the uptake rates or affect the periodic response: maximum and minimum levels of uptake remained unchanged. Scanning electron microscopy (SEM) revealed that the acquirement of the differentiated state is accompanied by a conspicuous transformation of the smooth surface of undifferentiated 3T3-L1 cells into a surface covered by numerous microvilii of uniform size and appearance. Treatment with insulin (10 mU/ml; 10 minutes) converted these microvilii into voluminous saccular membrane protrusions of the same type as had been formed during incubation of 3T3-L1 adipocytes with 2 mM 2-DG, and which have previously been shown to be involved in the endogenous uptake regulation of C6 glioma cells (Lange et al.: J. Cell. Physiol., 1989). These insulin-induced saccated membrane areas appeared to become integrated into the cell surface. Accordingly, insulin treatment caused a twofold increase of the intracellular distribution space of 3-O-methylglucose (3-OMG) in 3T3-L1 adipocytes. This insulin-induced increase of the 3-OMG distribution space exhibited the same time (t1/2 = 2–2.5 minutes) and dose dependence (EC50 = 20 nM) as the insulin-induced stimulation of 3-OMG transport. Glucose deprivation during the differentiation period inhibited the outgrowth of microvilii from the cell surface. Glucose starvation (18 hours at <0.5 mM) induced a conspicuous reduction of the length of microvilii on differentiated 3T3-L1 cells. In this state, the stalks of the microvilii are almost invisible and the enlarged spherical tips of the microvilii (with an average diameter of 370 nm compared to 230 nm of fed cells) appeared to protrude directly out of the cell surface. Starvation-induced shortening of microvilli was accompanied by a threefold increase of the basal 3-OMG transport rate and a greater than twofold increase of the intracellular 3-OMG distribution space as compared to fed cells (10 mM; 18 hours). Starved cells showed nearly the same transport rates and 3-OMG distribution spaces as insulin-treated fed cells. 3-OMG uptake by unstimulated 3T3-L1 adipocytes, preincubated for 2 hours in glucose-free medium, displayed a biphasic time course exhibiting high rates during the first 5–10 seconds followed by a constant low rate for the next 20–25 seconds. These results are compatible with the idea of a small cellular entrance compartment for hexose transport, as previously formulated for C6 glioma cells (Lange et al.: J. Cell. Physiol., 1989), which is formed by the internal space of surface protrusions such as microvilli and lamellae and which is separated from the cellular main compartment by a diffusion barrier. In 3T3-L1 adipocytes, insuliti appeared to enlarge this entrance compartment, thereby reducing the effectiveness of the diffusion barrier. Both the endogenous mechanism of uptake regulation and the down-regulation of transport by high glucose feeding (glucose-curb) are supposed to control the rate of hexose uptake in a similar way by modulating the diffusional coupling of this entrance compartment with the cytoplasmic main compartment.