Sugar Transport in Chick Embryo Fibroblasts: III. EVIDENCE FOR POST-TRANSCRIPTIONAL AND POST-TRANSLATIONAL REGULATION OF TRANSPORT FOLLOWING SERUM ADDITION

Abstract

Abstract The addition of fetal calf serum to the culture medium of chick embryo fibroblasts resulted in a biphasic increase in the rate of 2-deoxy-d-glucose uptake. The first increase in the rate of sugar uptake occurred within 10 min of serum addition and was not blocked by cycloheximide. A second increase in the rate of transport was noted by 1 hour following serum addition, and the increase continued for about 5 hours. The second increase was blocked by cycloheximide, but neither increase was blocked by actinomycin D. Transport rather than phosphorylation was responsible for the serum-stimulated increase in 2-deoxy-d-glucose uptake. No difference could be detected in the level of hexokinase activity between fibroblasts in serum-deficient medium and those in serum-containing medium. Studies with the transport inhibitor, cytochalasin B, indicated that transport was the rate-limiting step in 2-deoxy-d-glucose uptake at early time points. Analysis of the initial rates of 2-deoxy-d-glucose or 3-O-methyl-d-glucose transport by Lineweaver-Burk plots showed that increases in sugar uptake following the addition of fetal calf serum to the culture medium were due to increases in the Vmax for transport, with the Km remaining constant.