Active Influx Transport is Mediated by Members of the Organic Anion Transporting Polypeptide Family in Human Epidermal Keratinocytes

Abstract

Normal human epidermal keratinocytes have been shown to express a cell-type-specific pattern of extrahepatic cytochrome P450 enzymes and efflux transport proteins showing that these cells metabolize and excrete a variety of xenobiotics. Recently transport proteins involved in the uptake of xenobiotics have been detected and here we analyzed the mRNA and protein expression profiles and functional activities of these proteins in human keratinocytes in comparison to primary liver cells. The transporters studied included the subtypes A, B, C, D, and E of the organic anion transporting polypeptide (OATP) family, which are responsible for the uptake of various anionic and neutral molecules and especially organic cations - including drugs. Constitutive expression of OATP-B, OATP-D, and OATP-E was shown for the first time in normal human epidermal keratinocytes on a molecular level using reverse transcription polymerase chain reaction and northern blot analysis, as well as in human skin tissue shown by tissue blot hybridization and immunohistochemistry. Expression of OATP-A and OATP-C was not detected in any of the keratinocyte samples. In contrast, liver tissue showed a significant expression of OATP-A and OATP-B as well as OATP-C, a weak expression of OATP-D, and no expression of OATP-E. These data revealed that normal human epidermal keratinocytes express a specific profile of transporters involved in drug influx. Using a newly developed uptake-transport assay, uptake of known and well-characterized OATP substrates like estradiol-17beta-glucuronide and estrone sulfate was inhibited in normal human epidermal keratinocytes by specific inhibitors such as taurocholate, verifying the functional capacity of the expressed OATPs. Human dermal fibroblasts seem to have a lower influx transport activity for estradiol-17beta-glucuronide, which correlates with the immunohistologic data. Even though the substrate specificity of the OATP isoforms is only partially known until now, our findings support the concept that uptake of large organic cations like drugs in keratinocytes is an active transport process mediated by members of the OATP family.

Characterization of SLC transporters in human skin

“…This has been reported in human keratinocytes by Schiffer et al [3] and in human skin by Li et al [18] and more recently by Fujiwara et al [19]. It should be noted that MATE1 and MATE2 transporters were not included in those studies.…”

“…Although constitutive expression of xenobiotic-metabolizing enzymes has been detected in normal keratinocytes [2,3] the levels of drug-metabolizing enzymes are generally much lower in the skin than those in the liver and intestine [4][5][6][7]. Thus, skin permeability rather than drug metabolism appears to be the major barrier to topical bioavailability [8,9].…”

“…Xenobiotic transporters (ABCs and SLCs) have broad specificity and are involved in both uptake (influx) and secretion (efflux) of their substrates, thereby affecting their cellular disposition. Constitutive expression of multidrug-resistance associated proteins (MRP, ABCC) or solute carrier organic anion transporting polypeptides (OATPs) has been detected in human keratinocytes [2,3,10,11]. The expression of ABCB1 (multidrug resistance protein 1, P-glycoprotein or P-gp) has only been found after keratinocyte treatment with dexamethasone [2].…”

“…Three others transporters SLCO2B1 (OATPB), SLCO3A1 (OATPD) and SLCO4A1 (OATPE) were included as a control as they are known to be expressed in human skin [3,18,19]. Furthermore, the modulation of expression of SLC47A1 and SLC47A2 (MATE family) transporters by rifampicin and UV light exposure was also evaluated in human skin in organ-culture.…”

Evaluation of Percutaneous Drug Permeation Using a Lateral Sectioning Approach

“…In general, LETD-mediated drug permeation has been explained in terms of passive diffusion of non-ionized compounds (Potts, 1992). However, several studies have focused on the possible involvement of transporters expressed in the skin, similar to the case for other tissues (Schiffer et al, 2003;Bonen et al, 2006;Li, 2006). Organic anion transporter 2 (OAT2) is thought to be a candidate for an exchanger involved in the uptake and/or efflux of flurbiprofen in the skin, and the flurbiprofen permeability in the absorptive direction was higher than that in the secretory direction (Ito et al, 2007).…”