Differential regulation of the HepG2 and adipocyte/muscle glucose transporters in 3T3L1 adipocytes. Effect of chronic glucose deprivation

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Abstract
Glucose transport in 3T3L1 adipocytes is mediated by two facilitated diffusion transport systems. We examined the effect of chronic glucose deprivation on transport activity and on the expression the HepG2 (GLUT 1) and adipocyte/muscle (GLUT 4) glucose transporter gene products in this insulin-sensitive cell line. Glucose deprivation resulted in a maximal increase in 2-deoxyglucose uptake of 3.6-fold by 24 h. Transport activity declined thereafter but was still 2.4-fold greater than the control by 72 h. GLUT 1 mRNA and protein increased progressively during starvation to values respectively 2.4-and 7.0-fold greater than the control by 72 h. Much of the increase in total immunoreactive GLUT 1 protein observed later in starvation was the result of the accumulation of a non-functional or mistargeted 38 kDa polypeptide. Immunofluorescence microscopy indicated that increases in GLUT 1 protein occurred in presumptive plasma membrane (PM) and Golgi-like compartments during prolonged starvation. The steady-state level of GLUT 4 protein did not change during 72 h of glucose deprivation despite a > 10-fold decrease in the mRNA. Subcellular fractionation experiments indicated that the increased transport activity observed after 24 h of starvation was principally the result of an increase in the 45-50 kDa GLUT 1 transporter protein in the PM. The level of the GLUT 1 transporter in the PM and low-density microsomes (LDM) was increased by 3.9- and 1.4-fold respectively, and the GLUT 4 transporter content of the PM and LDM was 1.7 and 0.6-fold respectively greater than that of the control after 24 h of glucose deprivation. These data indicate that newly synthesized GLUT 1 transporters are selectively shuttled to the PM and that GLUT 4 transporters undergo translocation from an intracellular compartment to the PM during 24 h of glucose starvation. Thus glucose starvation results in an increase in glucose transport in 3T3L1 adipocytes via a complex series of events involving increased biosynthesis, decreased turnover and subcellular redistribution of transporter proteins.