Impact of prevailing thiamin levels on thiamin pyrophosphate uptake in pancreatic acinar cells: do the shuttle!

THIAMIN IS A WATER-SOLUBLE B1 vitamin that regulates critical cellular processes, such as oxidative energy metabolism, adenosine triphosphate (ATP) production, and mitochondrial function; thus, it is referred to as the energy vitamin (3, 5). Based on these known roles of thiamin, deficiency of this vitamin can lead to serious cellular impairments, including reduced cellular energy, increased oxidative stress, and mitochondrial dysfunction (2). Pancreatic acinar cells (PACs) maintain a high degree of metabolic activity and therefore require larger amounts of thiamin (9); however, PACs are unable to synthesize their own thiamin and thus require thiamin via dietary or supplemental intake (16). Thiamin is taken up by the cell through thiamin transporter-1 and -2, is largely converted (80–90% of thiamin) into thiamin pyrophosphate (TPP), and is then transported into the mitochondria by the mitochondrial TPP transporter (MTPPT) (1). The MTPPT, encoded by the SLC25A19 gene, is a member of the mitochondrial carrier family (MCF), and previous work has shown that SLC25A19 promoter activity is driven by nuclear factor (NF)-Y function (7). Additionally, it has been shown that chronic exposure to alcohol and cigarette smoke affects mitochondrial TPP uptake in PACs (14, 15); however, it is unknown if extracellular thiamin levels affect this process as well. The function of the MTPPT shuttle is of clinical relevance since mutations in the transporter are linked to Amish congenital lethal microcephaly, as well as neuropathy and bilateral striatal necrosis (12, 13). Based on this information, this study by Sabui et al. (11), published in this issue of the American Journal of Physiology-Lung Cellular and Molecular Physiology, aimed to determine the effect of the prevailing level of thiamin on mitochondrial TPP uptake by PACs, using both PAC 266-6 (cultured PAC) and transgenic mice carrying the human SLC25A19 promoter. In this study, the effects of thiamin levels on mitochondrial [H]-TPP uptake were first shown in PACs maintained in either thiamin-deficient (thiamin-DEF) or oversupplemented (thiaminOS) conditions (11). These initial results indicate that mitochondrial [H]-TPP uptake was increased in PAC 266-6 maintained in thiamin-DEF conditions compared with thiamin-OS conditions. To corroborate these findings in an in vivo model, the authors fed mice either a thiamin-DEF or thiamin-OS diet and evaluated mitochondrial TPP uptake in hepatocytes. The authors argue that the use of hepatocytes for these studies is valid since 1) it is difficult to obtain a sufficient concentration of PAC mitochondria for studies, 2) PACs and hepatocytes share the same embryonic lineage, and 3) this method has been used by other researchers (4, 6). Similar to the in vitro study, the authors found that mitochondrial [H]-TPP uptake was increased in hepatocytes isolated from mice fed a thiamin-DEF diet versus mice fed a thiamin-OS diet. These findings are the first to indicate that prevailing thiamin levels regulate TPP shuttling in PACs. While the use of hepatocytes for these studies is experimentally valid, it may be beneficial in the future to look at isolated PACs; however, this may be dependent on the development of a method that isolates a larger concentration of PACs. As previously stated, thiamin regulates ATP production; therefore, the authors evaluated the effect of extracellular thiamin levels on PAC 266-6 ATP production. Using the same in vitro studies as described above, the authors found that PAC 266-6 maintained in thiamin-DEF conditions had a significant reduction in ATP levels when compared with cells maintained in thiamin-OS conditions. In consideration that TPP taken up by the mitochondria acts as a cofactor for pyruvate dehydrogenase, -ketoglutarate dehydrogenase, and branched-chain -keto acid dehydrogenase, it is not surprising that thiamin deficiency leads to a decrease in ATP production (8, 13). Since mitochondrial TPP uptake is regulated by the MTPPT, the authors examined the expression of MTPPT in PAC 266-6 maintained in either thiamin-DEF or thiamin-OS conditions. The results showed that PAC 266-6 cultured in thiamin-DEF conditions have a significant increase in both mRNA and protein levels of MTPPT when compared with thiamin-OS conditions. These findings were corroborated in vivo where the authors noted an increase in both mRNA and protein levels of MTPPT in hepatocytes isolated from mice fed a thiamin-DEF diet versus mice fed a thiamin-OS diet. These findings indicate that the increase in TPP uptake, in response to a thiamin-DEF surrounding, may be dependent on changes in expression of the MTPPT. Based on the fact that the levels of MTPPT mRNA expression increase in response to increased thiamin levels, the authors suggest that this may be mediated by transcriptional changes in SLC25A19. Similar mechanisms have been shown to mediate the adaptive regulation of a multitude of other water-soluble vitamins (10). To evaluate this hypothesis, the authors evaluated the activity of the human SLC25A19 promoter (transfected into PAC 266-6) in both thiamin-DEF and thiamin-OS conditions. The authors found that SCL25A19 Address for reprint requests and other correspondence: G. Alpini, Central Texas Veterans Univ. Health Care System, Texas A & M Health Science Ctr., Olin E. Teague Medical Ctr., 1901 S. 1st St., Bldg. 205, 1R60, Temple, TX, 76504 (e-mail: [email protected]). Am J Physiol Gastrointest Liver Physiol 313: G373–G375, 2017; First published August 24, 2017; doi:10.1152/ajpgi.00256.2017.