Biotin uptake in cultured cell lines

Recent advances in carrier-mediated intestinal absorption of water-soluble vitamins

Significant progress has been made in recent years toward understanding the mechanisms and regulation of intestinal absorption of water-soluble vitamins from the diet, especially those that are transported by a specialized carrier-mediated mechanism (i.e., ascorbic acid, biotin, folate, riboflavin, thiamin, and pyridoxine). The driving force involved in the uptake events and the molecular identity of the systems involved have been identified for a number of these vitamins. In addition, information about regulation of the uptake process of these micronutrients by intracellular and extracellular factors has been forthcoming. Furthermore, the 5' regulatory region of the genes that encode a number of these transporters has been characterized, thus providing information about transcriptional regulation of the transport events. Also of interest is the identification of existence of carrier-mediated mechanisms in human colonocytes that are capable of absorbing some of the vitamins that are synthesized by normal microflora of the large intestine. Although the contribution of the latter source of vitamins toward overall host nutrition is not clear and requires further investigations, it is highly likely that it does contribute toward the cellular homeostasis of these vitamins in the localized colonocytes.

Identification of the plasma membrane H+-biotin symporter of Saccharomyces cerevisiae by rescue of a fatty acid-auxotrophic mutant

Bakers' yeast is auxotrophic for biotin (vitamin H) and depends on the efficient uptake of this compound from the environment. A mutant strain with strongly reduced biotin uptake and with reduced levels of protein biotinylation was identified. The strain was auxotrophic for long-chain fatty acids, and this auxotrophy could be suppressed with high levels of biotin in the medium. After transformation of this mutant with a yeast genomic library, the unassigned open reading frame YGR065C was identified to complement this mutation. This gene codes for a protein with 593 amino acids and 12 putative transmembrane helices. Northern blot analysis revealed that, in wild-type cells, the corresponding mRNA levels were increased at low biotin concentrations. Likewise, cellular biotin uptake was increased with decreasing biotin availability. Expression of YGR065C under the control of the constitutive ADH1 promoter resulted in very high biotin transport rates across the plasma membrane that were no longer regulated by the biotin concentration in the growth medium. We conclude that YGR065C encodes the first biotin transporter identified for a non-mammalian organism and designate this gene VHT1 for vitamin H transporter 1.

Cellular uptake of biotin: mechanisms and regulation

This review describes our knowledge of biotin transport in the small intestine of humans and other mammals and presents recent findings in the area. Previous studies have shown that biotin transport across the brush border membrane of the small intestinal absorptive cells occurs via a carrier-mediated, Na+ gradient-dependent, electroneutral mechanism. Exit of biotin out of the enterocyte, i.e., transport across the basolateral membrane, also occurs via a carrier-mediated process, but the process is Na+ independent and electrogenic. Recent studies from our laboratory have shown that the uptake process of biotin in Caco-2 cells, a human-derived cultured intestinal epithelial cell line, are under the cellular regulation of both a protein kinase C- and a Ca/calmodulin-mediated pathway. In addition, the uptake process is shared by another water-soluble vitamin, pantothenic acid. For the first time, other recent studies have detected the existence of a Na+-dependent, carrier-mediated mechanism for biotin uptake at the apical membrane of colonocytes, which could theoretically mediate absorption of the biotin synthesized by colonic microflora. This system was again found to be shared by pantothenic acid, which is also synthesized by the normal microflora of the large intestine.