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LIVER AND BILIARY TRACT

Pancreatic beta cells and islets take up thiamin by a regulated carrier-mediated process: studies using mice and human pancreatic preparations

¹Veterans Affairs Medical Center, Long Beach, California and Departments of Medicine and Physiology/Biophysics, University of California College of Medicine, Irvine, California; and ²Center of Biomolecular Interaction, University Bremen, Bremen, Germany

Submitted 10 March 2009 ; accepted in final form 25 April 2009

Thiamin is essential for the normal function of the endocrine pancreas, but very little is known about uptake mechanism(s) and regulation by beta cells. We addressed these issues using mouse-derived pancreatic beta-TC-6 cells, and freshly isolated primary mouse and human pancreatic islets. Results showed that thiamin uptake by beta-TC-6 cells involves a pH (but not Na⁺)-dependent carrier-mediated process that is saturable at both the nanomolar (apparent K_m = 37.17 ± 9.9 nM) and micromolar (apparent K_m = 3.26 ± 0.86 µM) ranges, cis-inhibited by thiamin structural analogs, and trans-stimulated by unlabeled thiamin. Involvement of carrier-mediated process was also confirmed in primary mouse and human pancreatic islets. Both THTR-1 and THTR-2 were found to be expressed in these mouse and human pancreatic preparations. Maintaining beta-TC-6 cells in the presence of a high level of thiamin led to a significant (P < 0.01) decrease in thiamin uptake, which was associated with a significant downregulation in level of expression of THTR-1 and THTR-2 at the protein and mRNA levels and a decrease in transcriptional (promoter) activity. Modulators of intracellular Ca²⁺/calmodulin- and protein-tyrosine kinase-mediated pathways also altered thiamin uptake. Finally, confocal imaging of live beta-TC-6 cells showed that clinical mutants of THTR-1 have mixed expression phenotypes and all led to impairment in thiamin uptake. These studies demonstrate for the first time that thiamin uptake by the endocrine pancreas is carrier mediated and is adaptively regulated by the prevailing vitamin level via transcriptional mechanisms. Furthermore, clinical mutants of THTR-1 impair thiamin uptake via different mechanisms.

THTR-1; THTR-2; thiamin uptake; transport regulation; TRMA