title: Silychristin, a Flavonolignan Derived From the Milk Thistle, Is a Potent Inhibitor of the Thyroid Hormone Transporter MCT8.
abstract: Thyroid hormones (THs) are charged and iodinated amino acid derivatives that need to pass the cell membrane facilitated by thyroid hormone transmembrane transporters (THTT) to exert their biological function. The importance of functional THTT is affirmed by the devastating effects of mutations in the human monocarboxylate transporter (MCT) 8, leading to a severe form of psychomotor retardation. Modulation of THTT function by pharmacological or environmental compounds might disturb TH action on a tissue-specific level. Therefore, it is important to identify compounds with relevant environmental exposure and THTT-modulating activity. Based on a nonradioactive TH uptake assay, we performed a screening of 13 chemicals, suspicious for TH receptor interaction, to test their potential effects on THTT in MCT8-overexpressing MDCK1-cells. We identified silymarin, an extract of the milk thistle, to be a potent inhibitor of T3 uptake by MCT8. Because silymarin is a complex mixture of flavonolignan substances, we further tested its individual components and identified silychristin as the most effective one with an IC50 of approximately 100 nM. The measured IC50 value is at least 1 order of magnitude below those of other known THTT inhibitors. This finding was confirmed by T3 uptake in primary murine astrocytes expressing endogenous Mct8 but not in MCT10-overexpressing MDCK1-cells, indicating a remarkable specificity of the inhibitor toward MCT8. Because silymarin is a frequently used adjuvant therapeutic for hepatitis C infection and chronic liver disease, our observations raise questions regarding its safety with respect to unwanted effects on the TH axis.
authors.affiliation: Institut für Experimentelle Endokrinologie (R.J.-N., F.M., E.K.W., L.S., J.K., K.R.), Charité-Universitätsmedizin Berlin, D-13353 Berlin, Germany; and Institut für Biochemie und Molekularbiologie (J.J., U.S.), Rheinische Friedrich-Wilhelms-Universität, D-53115 Bonn, Germany.
authors.forename: Jörg
authors.initials: J
authors.lastname: Johannes
authors.affiliation: Institut für Experimentelle Endokrinologie (R.J.-N., F.M., E.K.W., L.S., J.K., K.R.), Charité-Universitätsmedizin Berlin, D-13353 Berlin, Germany; and Institut für Biochemie und Molekularbiologie (J.J., U.S.), Rheinische Friedrich-Wilhelms-Universität, D-53115 Bonn, Germany.
authors.forename: Roopa
authors.initials: R
authors.lastname: Jayarama-Naidu
authors.affiliation: Institut für Experimentelle Endokrinologie (R.J.-N., F.M., E.K.W., L.S., J.K., K.R.), Charité-Universitätsmedizin Berlin, D-13353 Berlin, Germany; and Institut für Biochemie und Molekularbiologie (J.J., U.S.), Rheinische Friedrich-Wilhelms-Universität, D-53115 Bonn, Germany.
authors.forename: Franziska
authors.initials: F
authors.lastname: Meyer
authors.affiliation: Institut für Experimentelle Endokrinologie (R.J.-N., F.M., E.K.W., L.S., J.K., K.R.), Charité-Universitätsmedizin Berlin, D-13353 Berlin, Germany; and Institut für Biochemie und Molekularbiologie (J.J., U.S.), Rheinische Friedrich-Wilhelms-Universität, D-53115 Bonn, Germany.
authors.forename: Eva Katrin
authors.initials: EK
authors.lastname: Wirth
authors.affiliation: Institut für Experimentelle Endokrinologie (R.J.-N., F.M., E.K.W., L.S., J.K., K.R.), Charité-Universitätsmedizin Berlin, D-13353 Berlin, Germany; and Institut für Biochemie und Molekularbiologie (J.J., U.S.), Rheinische Friedrich-Wilhelms-Universität, D-53115 Bonn, Germany.
authors.forename: Ulrich
authors.initials: U
authors.lastname: Schweizer
authors.affiliation: Institut für Experimentelle Endokrinologie (R.J.-N., F.M., E.K.W., L.S., J.K., K.R.), Charité-Universitätsmedizin Berlin, D-13353 Berlin, Germany; and Institut für Biochemie und Molekularbiologie (J.J., U.S.), Rheinische Friedrich-Wilhelms-Universität, D-53115 Bonn, Germany.
authors.forename: Lutz
authors.initials: L
authors.lastname: Schomburg
authors.affiliation: Institut für Experimentelle Endokrinologie (R.J.-N., F.M., E.K.W., L.S., J.K., K.R.), Charité-Universitätsmedizin Berlin, D-13353 Berlin, Germany; and Institut für Biochemie und Molekularbiologie (J.J., U.S.), Rheinische Friedrich-Wilhelms-Universität, D-53115 Bonn, Germany.
authors.forename: Josef
authors.initials: J
authors.lastname: Köhrle
authors.affiliation: Institut für Experimentelle Endokrinologie (R.J.-N., F.M., E.K.W., L.S., J.K., K.R.), Charité-Universitätsmedizin Berlin, D-13353 Berlin, Germany; and Institut für Biochemie und Molekularbiologie (J.J., U.S.), Rheinische Friedrich-Wilhelms-Universität, D-53115 Bonn, Germany.
authors.forename: Kostja
authors.initials: K
authors.lastname: Renko
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D001253:Astrocytes / Q000166:cytology / Q000378:metabolism; D001692:Biological Transport / Q000187:drug effects; D002478:Cells, Cultured; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D061985:Madin Darby Canine Kidney Cells; D008297:Male; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism*; D018345:Mice, Knockout; D020944:Silybum marianum / Q000737:chemistry*; D027501:Monocarboxylic Acid Transporters; D010936:Plant Extracts / Q000494:pharmacology; D012838:Silymarin / Q000494:pharmacology*; D027981:Symporters; D013963:Thyroid Hormones / Q000378:metabolism* / Q000493:pharmacokinetics; D014284:Triiodothyronine / Q000378:metabolism / Q000493:pharmacokinetics
chemical_list: D026901:Membrane Transport Proteins; D027501:Monocarboxylic Acid Transporters; D010936:Plant Extracts; D012838:Silymarin; C484257:Slc16a2 protein, mouse; D027981:Symporters; D013963:Thyroid Hormones; D014284:Triiodothyronine; C015504:silychristin