Functional expression of the human HepG2 and rat adipocyte glucose transporters in Xenopus oocytes: Comparison of kinetic parameters

Abstract

Facilitated glucose transport is a ubiquitous characteristic of animal cells carried out by a family of membrane glycoproteins. Two members of this gene family are the well characterized human erythrocyte protein that has been cloned from the HepG2 cell line and the insulin-sensitive transporter that has been cloned from adipocytes and muscle tissue. In the present study the HepG2 and adipocyte glucose transporters were functionally expressed in Xenopus oocytes after injection of synthetic mRNAs produced by transcription in vitro from cloned cDNAs. Both 2-deoxyglucose uptake and 3-O-methylglucose transport were increased several-fold over basal levels in mRNA-injected oocytes. Increased uptake of 2-deoxyglucose was inhibited completely in the presence of cytochalasin B, and 3-O-methylglucose transport was blocked by D-glucose but not by L-glucose. The half-saturation constant and turnover number for 3-O-methylglucose transport at 22 degrees C via the HepG2 transporter were estimated to be 21 mM and 2.2 x 10(3) s-1 under equilibrium exchange conditions. The half-saturation constant for 3-O-methylglucose transport via the adipocyte transporter under the same conditions was estimated to be 1.8 mM. These data prove the functional identity of the cloned HepG2 and adipocyte cDNAs and indicate that the HepG2 and adipocyte transporters display similar kinetic behavior when expressed in the frog oocyte membrane as compared with their native membrane environments. Thus, the difference in the equilibrium exchange kinetic parameters for glucose transport in the erythrocyte and the adipocyte is a result of the expression of two distinct glucose transporter proteins.