title: A physiological role for glucuronidated thyroid hormones: preferential uptake by H9c2(2-1) myotubes.
abstract: Conjugation reactions are important pathways in the peripheral metabolism of thyroid hormones. Rat cardiac fibroblasts produce and secrete glucuronidated thyroxine (T4G) and 3,3',5-triiodothyronine (T3G). We here show that, compared to fibroblasts from other anatomical locations, the capacity of cardiofibroblasts to secrete T4G and T3G is highest. H9c2(2-1) myotubes, a model system for cardiomyocytes, take up T4G and T3G at a rate that is 10-15 times higher than that for the unconjugated thyroid hormones. T3 and T4, and their glucuronides, stimulate H9c2(2-1) myoblast-to-myotube differentiation. A substantial beta-glucuronidase activity was measured in H9c2(2-1) myotubes, and this confers a deconjugating capacity to these cells, via which native thyroid hormones can be regenerated from glucuronidated precursors. This indicates that the stimulatory effects on myoblast differentiation are exerted by the native hormones. We suggest that glucuronidation represents a mechanism to uncouple local thyroid hormone action in the heart from that in other peripheral tissues and in the systemic circulation. This could represent a mechanism for the local fine-tuning of cardiac thyroid hormone action.
authors.affiliation: Department of Pathobiology, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
authors.forename: Sabine M
authors.initials: SM
authors.lastname: van der Heide
authors.forename: Brian J L J
authors.initials: BJ
authors.lastname: Joosten
authors.forename: Bieuwke S
authors.initials: BS
authors.lastname: Dragt
authors.forename: Maria E
authors.initials: ME
authors.lastname: Everts
authors.forename: Peter H M
authors.initials: PH
authors.lastname: Klaren
mesh_terms: D000818:Animals; D002454:Cell Differentiation / Q000187:drug effects*; D002460:Cell Line; D005347:Fibroblasts / Q000166:cytology / Q000378:metabolism; D005965:Glucuronates / Q000378:metabolism / Q000494:pharmacology*; D018485:Muscle Fibers, Skeletal / Q000166:cytology / Q000378:metabolism*; D032446:Myoblasts / Q000166:cytology / Q000378:metabolism*; D009206:Myocardium / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D014284:Triiodothyronine / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology
chemical_list: D005965:Glucuronates; D014284:Triiodothyronine; C054786:triiodothyronine glucuronide