DIFFERENT MOLECULAR MECHANISMS ARE INVOLVED IN THE MULTIHORMONAL CONTROL OF GLUCOSE-TRANSPORT IN FRTL5 RAT-THYROID CELLS

We investigated the molecular mechanisms by which TSH, insulin and IGF-1 modulate the glucose transport system in FRTL5 cells. We found that TSH, insulin and IGF-1 increased the glucose transporter Glut-1 specific mRNA levels 6, 8 and 5 fold over control, respectively. The effect on Glut-1 mRNA was evident after 2 hours, followed by an increased Glut-1 protein expression in whole cells, as judged by western blot analysis, after 5 hours of stimulation with all the hormones studied. In contrast, plasma membrane Glut-1 increased (300-400% over control) after 2 hours of stimulation with TSH (10 mU/ml), dibutyrryl-cAMP (1 mM), IGF-1 (10 ng/ml) and insulin (10 nM). These data indicate that the glucose transport system is under multihormonal control in FRTL5 cells. Two different mechanisms are involved in TSH, IGF-1 and insulin stimulation of the glucose transport: a) neosynthesis of Glut-1 by activation of gene expression; b) recruitment of carriers from the intracellular pool to the plasma membrane.