Taste of a pill: organic cation transporter-3 (OCT3) mediates metformin accumulation and secretion in salivary glands

Abstract

Drug-induced taste disturbance is a common adverse drug reaction often triggered by drug secretion into saliva. Very little is known regarding the molecular mechanisms underlying salivary gland transport of xenobiotics, and most drugs are assumed to enter saliva by passive diffusion. In this study, we demonstrate that salivary glands selectively and highly express OCT3 (organic cation transporter-3), a polyspecific drug transporter in the solute carrier 22 family. OCT3 protein is localized at both basolateral (blood-facing) and apical (saliva-facing) membranes of salivary gland acinar cells, suggesting a dual role of this transporter in mediating both epithelial uptake and efflux of organic cations in the secretory cells of salivary glands. Metformin, a widely used anti-diabetic drug known to induce taste disturbance, is transported by OCT3/Oct3 in vitro. In vivo, metformin was actively transported with a high level of accumulation in the salivary glands of wild-type mice. In contrast, active uptake and accumulation of metformin in salivary glands were abolished in Oct3(-/-) mice. Oct3(-/-) mice also showed altered metformin pharmacokinetics and reduced drug exposure in the heart. These results demonstrate that OCT3 is responsible for metformin accumulation and secretion in salivary glands. Our study uncovered a novel carrier-mediated pathway for drug entry into saliva and sheds new light on the molecular mechanisms underlying drug-induced taste disorders.

Keywords: Diabetes; Drug Transport; Membrane Transport; Metformin; Organic Cation Transporter 3; Pharmacokinetics; Salivary Gland; Transporter.

© 2014 by The American Society for Biochemistry and Molecular Biology, Inc.

Figures

FIGURE 1.

Mouse Oct3 transports metformin.A, time-dependent uptake of metformin (10 μm) in mOct3-expressing HEK293 cells and pcDNA5-transfected (control) cells. B, metformin transport kinetics by mOct3. Uptake was performed with varying metformin concentrations for 5 min in HEK293 cells stably transfected with mOct3. Transporter-specific uptake was calculated by subtracting the transport activity in control cells.

FIGURE 2.

Expression of Oct1–3, Mate1, and Pmat mRNAs in various mouse tissues determined by quantitative RT-PCR. Transporter expression was normalized to the expression of the mouse β-glucuronidase (Gusb) gene.

FIGURE 3.

Salivary glands predominantly express OCT3/Oct3.A, relative expression of mOct3 mRNA in various mouse tissues determined by quantitative RT-PCR. B and C, gene profiling of functionally related transporters in mouse (B) and human (C) salivary glands by quantitative RT-PCR. D, OCT3 mRNA expression in human parotid, submandibular, and sublingual glands. The results were normalized to the expression of the human or mouse β-glucuronidase (GUSB/Gusb) gene.

FIGURE 4.

Localization of OCT3/Oct3 in salivary gland epithelial cells.A, detection of OCT3 (panel i, green) in human submandibular glands. Staining of Na+/K+-ATPase (panel ii), a basolateral marker, and nuclei (panel iii) is shown in red and blue, respectively. B, detection of Oct3 (panel i, green) and nuclei (panel ii, blue) in salivary gland sections from Oct3+/+ (upper panels) and Oct3−/− (lower panels) mice. In overlays (A, panel iv, and B, panel iii), the arrow and arrowhead indicate basolateral and apical membranes of salivary gland epithelial cells, respectively.

FIGURE 5.

Oct3 impacts metformin plasma kinetics.A and B, plasma metformin concentration-time profiles from 0 to 480 min (A) and from 0 to 120 min (B) in female Oct3+/+ and Oct3−/− mice. C and D, plasma metformin concentration-time profiles from 0 to 480 min (C) and from 0 to 120 min (D) in male Oct3+/+ and Oct3−/− mice. Mice were given by oral gavage a dose of 15 mg/kg metformin containing 0.2 mCi/kg [14C]metformin. Animals were killed at each time point, and the plasma and tissue concentrations of metformin were measured by liquid scintillation counting. Data represent mean ± S.E. (n = three to six mice at each time point). A total of 60 Oct3+/+ and 53 Oct3−/− female mice and a total of 48 Oct3+/+ and 40 Oct3−/− male mice were used for the pharmacokinetic studies.

FIGURE 6.

Oct3 mediates metformin accumulation in salivary glands.A, metformin concentrations in salivary glands at various time points (0–480 min) in female and male Oct3+/+ and Oct3−/− mice. B and C, comparisons of plasma versus salivary gland metformin concentrations at various time points in Oct3+/+ (B) and Oct3−/− (C) mice.

FIGURE 7.

Model proposed for OCT3-mediated organic cation (OC+) transport in salivary gland epithelial cells. OCT3 on the basolateral membrane of epithelial cells mediates metformin uptake from the blood into the cells. Once metformin is highly concentrated inside the cells, OCT3 on the apical membrane facilitates efflux of metformin into the saliva. The solid arrows indicate the preferred direction of metformin transport when drug concentrations in the systemic circulation are high. The dashed arrows indicate the possible transport direction when systemic drug concentrations decline.