Abstract
The development of a hormone-responsive glucose transport activity during differentiation of 3T3-L1 murine fibroblasts to an insulin-sensitive adipocyte-like phenotype was studied. Glucose transport activity was insensitive to insulin or insulin-like growth factor I (IGF-I) before differentiation, and was increased by 8-10-fold after differentiation by both insulin and IGF-I via their own respective receptors. In contrast, in undifferentiated cells insulin and IGF-I stimulated a large increase of [3H]thymidine incorporation into DNA via IGF-I receptors, indicating that undifferentiated 3T3-L1 cells are equipped with fully functioning hormone (IGF-I) receptors. Thus the previously described increase in expression of insulin receptors during differentiation cannot solely account for the development of hormone-sensitive glucose transport in the 3T3-L1 cell. The total glucose transport activity reconstituted from membrane fractions was increased by about 3-fold during differentiation. In differentiated cells, more than 80% of the total reconstitutable glucose transport activity was detected in an intracellular compartment (200,000 g microsomes) as compared with about 20% in undifferentiated cells. Immunoblots with specific antiserum confirmed previous reports indicating that the adipose tissue/muscle glucose transporter (GT3) was exclusively present in the differentiated cells, whereas the erythrocyte/brain glucose transporter (GT1) was detected in both differentiated and undifferentiated cells. Upon differentiation, GT1 was redistributed from plasma membranes to the intracellular compartment. In addition, the newly formed GT3 was predominantly found (greater than 80% of total) in the microsomal fraction of differentiated cells. Both GT1 and GT3 appeared to be hormone-sensitive, since in differentiated cells insulin as well as IGF-I gave rise to their translocation from the intracellular compartment to the plasma membrane. These data suggest that, in addition to the specific expression of the GT3 transporter, the formation of a large pool of intracellular glucose transporters comprising both GT1 and GT3 contributes to the development of insulin sensitivity in the 3T3-L1 cell.