Adaptive regulation of intestinal thiamin uptake: molecular mechanism using wild-type and transgenic mice carrying hTHTR-1 and -2 promoters.

Thiamin participates in metabolic pathways contributing to normal cellular functions, growth, and development. The molecular mechanism of the human intestinal thiamin absorption process involves the thiamin transporters-1 (hTHTR-1) and -2 (hTHTR-2), products of the SLC19A2 and SLC19A3 genes. Little is known about adaptive regulation of the intestinal thiamin uptake process or the molecular mechanism(s) involved during thiamin deficiency. In these studies, we addressed these issues using wild-type mice and transgenic animals carrying the promoters of the hTHTR-1 and -2. We show that, in thiamin deficiency, a significant and specific upregulation in intestinal carrier-mediated thiamin uptake occurs and that this increase is associated with an induction in protein and mRNA levels of mTHTR-2 but not mTHTR-1; in addition, an increase in the activity of the SLC19A3, but not the SLC19A2, promoter was observed in the intestine of transgenic mice. Similar findings were detected in the kidney; however, expression of both thiamin transporters and activity of both human promoters were upregulated in this organ in thiamin deficiency. We also examined the effect of thiamin deficiency on the level of expression of mTHTR-1 and mTHTR-2 messages and activity of the human promoters in the heart and brain of transgenic mice and found an increase in mTHTR-1 mRNA and a rise in activity of the SLC19A2 promoter in thiamin-deficient mice. These results show that the intestinal and renal thiamin uptake processes are adaptively upregulated during dietary thiamin deficiency, that expression of mTHTR-1 and mTHTR-2 is regulated in a tissue-specific manner, and that this upregulation is mediated via transcriptional regulatory mechanism(s).