The expression of the Na+/glucose cotransporter (SGLT1) in response to thyroid hormone [3,5,3'-tri-iodo-l-thyronine (T3)] was investigated in the enterocytic model cell line Caco-2/TC7. In differentiated cells, T3 treatment induces an average 10-fold increase in glucose consumption as well as a T3 dose-dependent increase in SGLT1 mRNA abundance. Only cells grown on glucose-containing media, but not on the non-metabolizable glucose analogue alpha-methylglucose (AMG), could respond to T3-treatment. The Vmax parameter of AMG transport was enhanced 6-fold by T3 treatment, whereas the protein abundance of SGLT1 was unchanged. The role of Na+ recycling in the T3-related activation of SGLT1 activity was suggested by both the large increase in Na+/K+ATPase protein abundance and the inhibition, down to control levels, of AMG uptake in ouabain-treated cells. Further investigations aimed at identifying the presence of a second cotransporter that could be expressed erroneously in the colon cancer cell line were unsuccessful: T3-treatment did not modify the sugar-specificity profile of AMG transport and did not induce the expression of SGLT2 as assessed by reverse transcription-PCR. Our results show that T3 can stimulate the SGLT1 cotransport activity in Caco-2 cells. Both transcriptional and translational levels of regulation are involved. Finally, glucose metabolism is required for SGLT1 expression, a result that contrasts with the in vivo situation and may be related to the fetal phenotype of the cells.

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Expression of the fructose transporter GLUT5 in Caco-2 cells is controlled by the carbohydrate content of the culture media [Mesonero, Matosin, Cambier, Rodriguez-Yoldi and Brot-Laroche (1995) Biochem. J. 312, 757-762] and by the metabolic status of the cells [Mahraoui, Takeda, Mesonero, Chantret, Dussaulx, Bell, and Brot-Laroche (1994) Biochem. J. 301, 169-175]. In this study we show that, in fully differentiated Caco-2/TC7 cells, thyroid hormone and glucose increase GLUT5 mRNA abundance in a dose-dependent manner. Using Caco-2/TC7 cells stably transformed with various fragments of the GLUT5 promoter inserted upstream of the luciferase reporter gene, we localized the sequences that confer 3,3',5-l-tri-iodothyronine (T3)- and/or glucose-sensitivity to the gene. Glucose responsiveness is conferred by the -272/+41 fragment of the promoter, but it is only with the -338/+41 region that transcription of the luciferase reporter gene is stimulated by T3. This 70 bp fragment from position -338 to -272 of the GLUT5 gene is able to confer T3/glucose-responsiveness to the heterologous thymidine kinase promoter. Electrophoretic-mobility-shift assays demonstrate that thyroid hormone receptors alpha and beta are expressed in Caco-2/TC7 cells. They further show that the -308/-290 region of the GLUT5 promoter binds thyroid hormone receptor/retinoid X receptor heterodimers, and that glucose and/or T3 exert a deleterious effect on the binding of the nuclear protein complex.