Thyroid hormone export in rat FRTL-5 thyroid cells and mouse NIH-3T3 cells is carrier-mediated, verapamil-sensitive, and stereospecific.
Export of L-T3 out of the cell is one factor governing the cellular T3 content and response. We previously observed in liver-derived cells that T3 export was inhibited by verapamil, suggesting that it is due to either ATP-binding cassette/multidrug resistance (MDR1/mdr1b) or multidrug resistance-related (MRP1/mrp1) proteins. To test this hypothesis we measured T3 export in FRTL-5, NIH-3T3, and rat hepatoma (HTC) cells that varied in expression of these proteins. FRTL-5 and NIH-3T3 cells were found to contain a T3 efflux mechanism that is verapamil inhibitable, saturable, and stereospecific. By contrast, T3 efflux in HTC cells was slow and unaffected by verapamil. Neither FRTL-5 nor NIH-3T3 cells express mdrlb, but all three cell types express mrpl, as assessed by immunoblotting. Overexpression of MDR1 in NIH-3T3 cells did not enhance verapamil-inhibitable T3 efflux. Photoaffinity labeling of FRTL-5 and NIH-3T3 cells with [125I]L-T3 revealed a labeled 90- to 100-kDa protein that was not present in HTC cells. Verapamil and excess nonradioactive L-T3, but not D-T3, inhibited labeling of this protein. The lack of correlation between T3 efflux and MDR1 and mrpl expression and the finding of a photoaffinity-labeled putative transport protein smaller than MDR1 or mrp1 protein (approximately 170 kDa) suggest that a novel protein is involved in the transport of T3 out of cells.
Cavalieri
R R
RR
Nuclear Medicine Research Laboratory, Veterans Administration Medical Center, San Francisco, California, USA.
Simeoni
L A
LA
Park
S W
SW
Baxter
J D
JD
Scharschmidt
B F
BF
Ribeiro
R C
RC
Lomri
N
N
D016475:3T3 Cells; D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000235:genetics; D018528:ATP-Binding Cassette Transporters / Q000235:genetics; D000818:Animals; D001692:Biological Transport; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D002352:Carrier Proteins / Q000378:metabolism*; D002460:Cell Line; D015870:Gene Expression; D015151:Immunoblotting; D008114:Liver Neoplasms, Experimental / Q000378:metabolism; D051379:Mice; D027425:ATP-Binding Cassette, Sub-Family C Proteins; D019920:Photoaffinity Labels; D051381:Rats; D013961:Thyroid Gland / Q000378:metabolism*; D014284:Triiodothyronine / Q000378:metabolism*; D014700:Verapamil / Q000494:pharmacology*
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D018528:ATP-Binding Cassette Transporters; D002121:Calcium Channel Blockers; D002352:Carrier Proteins; D027425:ATP-Binding Cassette, Sub-Family C Proteins; D019920:Photoaffinity Labels; D014284:Triiodothyronine; D014700:Verapamil