Tissue distribution, ontogeny and induction of the transporters Multidrug and toxin extrusion (MATE) 1 and MATE2 mRNA expression levels in mice

Abstract

Transporters are expressed in a wide variety of tissues where they perform the critical function of enabling anionic and cationic chemicals of exogenous and endogenous origin to cross otherwise impermeable cell membranes. The Multidrug and toxin extrusion (MATE) transporters mediate cellular efflux of a variety of organic cations, including many drugs. The purpose of the current study was to determine (1) constitutive expression levels of MATE mRNA in various tissues, (2) whether there are gender differences in the expression of MATEs, (3) the ontogenic expression pattern of MATE1 in kidney and (4) whether MATEs are pharmacologically inducible in liver via activation of known transcription factors. In both male and female mice, MATE1 mRNA levels were highest in the kidney, where male expression was higher than female. MATE2 mRNA expression levels were the highest in the testis, where high expression was localized to Sertoli cells, a critical cell type of the blood testis barrier. In female mice, MATE2 mRNA levels were expressed most highly in the colon. The ontogenic pattern of expression of MATE1 mRNA in the kidneys of both males and females was gradual, with levels increasing steadily from prenatal day − 2 to 45 days of age, and a gender difference appearing at day 30. Of the transcription factor activators examined (AhR, CAR, Nrf2, PPARα and PXR), none were capable of altering MATE1 or MATE2. The current findings support a potential role for MATE1 and MATE2 in a wide range of tissues and, notably, a unique role for MATE2 in the blood-testis barrier.

Introduction

Transporter proteins fulfill many vital physiologic roles, as they are expressed in a variety of tissues and organs throughout the human body. For example, in intestinal and renal epithelial cells, the transporter known as the “sodium pump,” or Na⁺, K⁺-ATPase helps maintain the osmotic balance of these cells, as well as provides a gradient for Na⁺-dependent Na⁺-glucose symport. Transporters are also responsible for driving bile salt-dependent and bile salt-independent bile flow, as well as the uptake and efflux of numerous structurally diverse endobiotics (e.g. bilirubin, bile acids, prostaglandins, sex steroids) and xenobiotics (e.g. aflatoxin B₁, digoxin, pravastatin, acetaminophen). Thus transporters help maintain electrolyte homeostasis, provide organ/tissue defense and likely contribute to the therapeutic efficacy of many drugs (Trauner and Boyer, 2003).

Among the endobiotics and xenobiotics that require transporter-mediated uptake and efflux, much is known regarding the handling of organic anions (OAs). Specifically, members of the Organic Anion Transporter (OAT) and Organic Anion-Transporting Polypeptide (OATP) families can transport OAs across cell membranes into tissues. Additionally, many OAT and OATP substrates as well as their metabolic products can undergo efflux out of cells for subsequent elimination from the body by members of the Multidrug Resistance-Associated Protein (MRP) and Multidrug Resistance (MDR) transporter families (Trauner and Boyer, 2003, Deeley et al., 2006, Anzai et al., 2006).

Similarly, members of the Organic Cation Transporter (OCT) family mediate the uptake of many Organic Cation (OC) endobiotics and xenobiotics across cell membranes into tissues. While relatively less is known regarding transporter-mediated OC efflux, evidence in the literature clearly supports the phenomenon of transporter-mediated OC efflux. Specifically, a family of proteins known as the Multidrug and toxin extrusion (MATE) transporters has been partially characterized in mice, rats and humans, and there appear to be two MATE genes in each of the species, MATE1 and MATE2 (Otsuka et al., 2005, Masuda et al., 2006, Terada et al., 2006).

In humans, MATE1 mRNA levels are highest in the liver, and are localized to the canalicular membrane of hepatocytes. MATE1 mRNA expression is also high in the kidneys, where it is localized to the apical membrane of the renal tubule. Similarly, MATE2 mRNA levels are by far at their highest in the kidneys, while relatively low in most other tissues. Example substrates for members of the MATE family include endogenous chemicals such as serotonin and testosterone and a number of drugs, including metformin and cisplatin (Otsuka et al., 2005, Terada et al., 2006, Masuda et al., 2006).

Previous work has demonstrated the tissue distribution of MATE family transporters in mice to be similar to that of humans (Masuda et al., 2006). However, a number of important issues regarding MATE expression in mice remain undefined. First, whether gender differences in mouse MATE1 and MATE2 exist is not known. The most obvious example where one could reasonably anticipate intergender disparities would be in the respective sex organs. Thus, gender differences in tissue distribution may reflect significant discrepancies in the physiologic role(s) of the transporter in question. Second, the ontogenesis of MATE1 and MATE2 mRNA expression has not been defined. This is a particularly important topic with respect to transporters because differences in expression during early development and adult maturation have been shown to result in differential susceptibility to injury (Guo et al., 2002, Johnson et al., 2002, Huang et al., 2000). Third, there is currently no information regarding the transcriptional regulation or pharmacologic inducibility of the MATE family of transporters. It is known that treatment with various microsomal enzyme inducers (MEIs) can result in induction of drug metabolizing enzymes (DMEs) and transporters, resulting in dramatic alterations in the pharmacokinetics of prescription drugs (Klaassen, 1974, Guo et al., 2002). Pharmacologic induction of both DMEs and transporters has been demonstrated to occur following activation of various nuclear receptors, including Aryl hydrocarbon receptor (AhR), Constitutive androstane receptor (CAR), Pregnane x receptor (PXR), Peroxisome proliferator-activated receptor α (PPARα) and Nuclear factor erythroid 2 p45-related factor 2 (Nrf2) (Cheng et al., 2005, Maher et al., 2005). The purpose of the current study was to determine (1) the constitutive expression levels of MATE mRNA in various tissues, (2) whether there are gender differences in tissue distribution and expression levels of MATEs (3) the ontogenic expression pattern of MATE1 in kidney and (4) whether MATEs are pharmacologically-inducible in liver via activation of the nuclear receptor transcription factors AhR, CAR, PXR, PPARα and Nrf2.