Solute carrier (SLC) transporters play important roles in absorption and disposition of drugs in cells; however, the expression pattern of human SLC transporters in the skin has not been determined. In the present study, the expression patterns of 28 human SLC transporters were determined in the human skin. Most of the SLC transporter family members were either highly or moderately expressed in the liver, while their expression was limited in the skin and small intestine. Treatment of human keratinocytes with a reactive metabolite of ibuprofen significantly reduced cell viability. Expression array analysis revealed that S100 calcium binding protein A7A (S100A7A) was induced nearly 50-fold in dermal cells treated with ibuprofen acyl-glucuronide. Determination of the expression of drug-metabolizing enzymes as well as drug transporters prior to the administration of drugs would make it possible to avoid the development of idiosyncratic skin diseases in individuals.

UDP-glucuronosyltransferase (UGT) 1A1 is the sole enzyme that can metabolize bilirubin. Human infants physiologically develop hyperbilirubinemia as the result of inadequate expression of UGT1A1 in the liver. Although phototherapy using blue light is effective in preventing jaundice, sunlight has also been suggested, but without conclusive evidence, to reduce serum bilirubin levels. We investigated the mRNA expression pattern of human UGT1A1 in human skin, human skin keratinocyte (HaCaT) cells, and skin of humanized UGT1 mice. The effects of UVB irradiation on the expression of UGT1A1 in the HaCaT cells were also examined. Multiple UGT1A isoforms, including UGT1A1, were expressed in human skin and HaCaT cells. When HaCaT cells were treated with UVB-exposed tryptophan, UGT1A1 mRNA and activity were significantly induced. Treatment of the HaCaT cells with 6-formylindolo[3,2-b]carbazole, which is one of the tryptophan derivatives formed by UVB, resulted in an induction of UGT1A1 mRNA and activity. In neonates, the expression of UGT1A1 was greater in the skin; in adults, UGT1A1 was expressed mainly in the liver. Treatment of humanized UGT1 mice with UVB resulted in a reduction of serum bilirubin levels, along with increased UGT1A1 expression and activity in the skin. Our data revealed a protective role of UGT1A1 expressed in the skin against neonatal hyperbilirubinemia. Sunlight, a natural and free source of light, makes it possible to treat neonatal jaundice while allowing mothers to breast-feed neonates.

Stereoselectivity of the human reduced folate carrier (RFC1) was examined in Caco-2 cells using methotrexate (l-amethopterin or l-MTX) and its antipode (d-amethopterin or d-MTX) as model substrates. The initial uptake rate of folic acid (FA) was concentration dependent, with a K(m) value of approximately 0.6 microM. The Eadie-Hofstee plot of the RFC1-mediated FA uptake revealed a single component for FA uptake into Caco-2 cells, demonstrating that only RFC1 is involved in FA uptake. l-MTX inhibited FA uptake in a competitive manner with a K(i) value of approximately 2 microM, similar to the K(m) value of l-MTX. d-MTX also competitively inhibited FA uptake with a K(i) value being approximately 120 microM, indicating that the affinity of d-MTX is ca. 60-fold less than that of l-MTX. The stereoselectivity of human RFC1 observed in the present study was consistent not only with the stereoselectivity of rabbit RFC1 observed in rabbit intestinal brush border membrane vesicles but also with the reported differences in oral absorption of amethopterin enantiomers in humans.

Stereoselectivity of the human reduced folate carrier (RFC1) in Caco-2 cells was examined using methotrexate (L-amethopterin, L-MTX) and its antipode (D-amethopterin, D-MTX) as model substrates. The initial uptake rate of L-MTX into Caco-2 cells followed Michaelis-Menten kinetics with a Km value of approximately 1 microM. The Eadie-Hofstee plot of the RFC1-mediated L-MTX uptake showed that it was mediated by a single transport system, RFC1. Dixon plots revealed that L-MTX uptake was inhibited competitively by folic acid (FA), L-MTX and D-MTX, with Ki values of approximately 0.8, 1.5 and 180 microM, respectively. The results showed that the affinities of FA and L-MTX to RFC1 were approximately 120-fold greater than that of D-MTX. The uptake of L- and D-MTX into Caco-2 cells was also measured using LC-MS/MS analysis, which revealed that the L-MTX uptake was at least 7-fold greater than that of D-MTX. The present study revealed significant stereoselectivity of RFC1 toward amethopterin enantiomers with the L-isomer being much more favored.