Thyroid hormone export in rat FRTL-5 thyroid cells and mouse NIH-3T3 cells is carrier-mediated, verapamil-sensitive, and stereospecific

Export of L-T-3 out Of the cell is one factor governing the cellular T-3 content and response. We previously observed in liver-derived cells that T-3 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 T-3 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 T-3 efflux mechanism that is verapamil inhibitable, saturable, and stereospecific. By contrast, T-3 efflux in HTC cells was slow and unaffected by verapamil. Neither FRTL-5; nor NIH-3T3 cells express mdr1b, but all three cell types express mrp1, as assessed by immunoblotting. Overexpression of MDR1 in NIH-3T3 cells did not enhance verapamil-inhibitable T-3 afflux. Photoaffinity labeling of FRTL-5 and NIH-3T3 cells with [I-125]L-T-3 revealed a labeled 90- to 100-kDa protein that was not present in HTC cells. Verapamil and excess nonradioactive L-T-3,but not D-T-3, inhibited labeling of this protein. The lack of correlation between T-3 efflux and MDR1 and mrp1 expression and the finding of a photoaffinity-labeled putative transport protein smaller than MDR1 or mrp1 protein (similar to 170 kDa) suggest that a novel protein is involved in the transport of T-3 out of cells.