The effect of glucose and fructose and fetal bovine serum on the expression of the fructose transporter GLUT5 was studied in clone PD7 of the human colon cancer cell line Caco-2, which has been characterized previously [Chantret, Rodoloswe, Barbat et al. (1994) J. Cell Sci. 107, 213-225; Mahraoui, Rodolosse, Barbat et al. (1994) Biochem. J. 298, 629-633]. Culture of the cells in dialysed serum and hexose-free media, down-regulated the expression of GLUT5, which was below detection within 3-4 days. This effect was reversed by fructose and glucose feeding of the cells. Fructose feeding yielded a 3-fold higher abundance of GLUT5 protein and mRNA as compared with that expressed in glucose-fed cells. Cells fed normal serum exhibited an inverse hierarchy of expression, with glucose being a better inducer than fructose for the expression of GLUT5. The GLUT5 mRNA and protein abundances obtained in fructose-fed cells did not depend on the type of serum. A linear relationship between cyclic AMP (cAMP) levels and GLUT5 mRNA abundance was found in cells fed dialysed serum, whereas in cells fed normal serum, mRNA abundances were not correlated to cAMP levels. These results indicate that glucose and fructose, together with serum-related factors and cAMP, have combined effects on the expression of GLUT5 in Caco-2 cells.

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Thyroid hormones (T3) have prominent effects on gastrointestinal development, stucture and function [l].The human colon carcinoma cell line Caco-2 is a valuable model for in vitro studies dealing with enterocyte function. They grow as polarized monolayers of cells and exhibit, after confluency, morphological and functional features of intestinal absorbing cells. Indeed Caco-2 cells express sucraseisomaltase (SI), the NdD-glucose cotransporter (SGLTl), and the facilitative hexose transporters GLUT2 and GLUTS, 4 proteins involved in the terminal digestion and absorption of sugars in normal enterocytes. Similarly to many transformed cells, they also express GLUT1 and GLUT3 [2-41. Sugars have been shown to modulate the expression of hexose transporters both in vivo [5, 61 and in vitro [4, 71. Because thyroid hormone is known to increase the metabolism of glucose in cells, the aim of this study was to examine wether T3 would affect the expression of sugar transporters in Cacod cells. We here present results obtained with chronically T3-treated TC71Caco-2 cells [4] where the glucose consumption rate and the mRNA and protein abundances of the hexose transporters, SI and villin, were measured.Glucose consumption rates increased 20 fold (day 21) in T3-treated cells as compared to controls. This dose-dependent effect occured only after confluency and was saturable. T3 did not affect the growth curve nor the protein content of the cells. However their glycogen stores were decreased by 30 % indicating an accelarated glucose catabolism. T3 effects therefore are correlated with the onset of differentiation which occurs after confluency in Caco-2 cells. Wether this correlation is due to the absence of T3-receptors or metabolic target protein expression in undifferentiated Caco-2 cells remains to be investigated.T3 treatment increased the mRNA abundance of GLUTl, GLUT5 and SGLTl. On the contrary GLUT2 and SI mRNA were decreased. The expression of villin was unchanged and used as reference for mRNA quantification (Fig. 2). These effects are restricted to the postconfluent phase of growth and do not depend on the switch on (GLUT2, GLUT5 and SGLTl) or off (GLUTl) of gene transcription in differentiating control cells. S 0.3 e days I confluency Figure 1. Glucose consumption rate of TC7/Caco-2 cells as a function of T3treatment, growth and differentiation. Culture conditions were as described in [4]. T3 was added at day 2 and renewed daily. Glucose consumption rates represent the disapearance of glucose from the culture media. SEM error bars for 3 independent cultures are smaller than symbols. 300 t n GLUT1 GLUT2 GLUT5 SGLTI SI Fgure 2 Increase of mRNA abundance of hexose transporters and SI as a function of T3-treatment. Histogram bars represent the relative amount of hexose transporter mRNA in T3-treated as compared to control cells.Homogenate and luminal (P2) membrane fractions from T3-treated and control Caco-2 cells were analysed for SGLTl and GLUT5 by western blotting. Both the membranes and cell homogenates contained a 6OKDa...