Submitted on March 10, 2009
Revised on April 9, 2009
Accepted on April 25, 2009
Pancreatic beta cells/islets take up thiamin by a regulated carrier-mediated process: Studies using mice and human pancreatic preparations
Lisa Mee1, Svetlana M Nabokina2, V. Thillai Sekar3, Veedamali S Subramanian4, Kathrin Maedler5, and Hamid M. Said2*
1 University of California Irvine
2 UCI/VA Medical Center
3 Unviersity of California Irvine
4 University of California
5 University Bremen
* To whom correspondence should be addressed. E-mail: hmsaid{at}uci.edu.
Thiamin is essential for the normal function of the endocrine pancreas, but 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 Km= 37.17 ± 9.9 nM) and micromolar (apparent Km= 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 down-regulation 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 (PTK)- 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. Further, clinical mutants of THTR-1 impair thiamin uptake via different mechanisms.
