EFFECTS OF EPIDERMAL GROWTH-FACTOR ON BASOLATERAL IODIDE UPTAKE AND APICAL IODIDE PERMEABILITY IN FILTER-CULTURED THYROID EPITHELIUM

The effect of epidermal growth factor (EGF) on vectorial iodide (I-) t ransport was studied in an in vitro model of the polarized porcine thy roid follicular epithelium in Transwell culture chambers. In this mode l, two mechanisms responsible for the unidirectional transport of I- f rom the basal to the apical chamber compartments, corresponding to the interstitium to lumen direction in vivo, are present in opposite plas ma membrane domains of the tight cell monolayer: a basolateral I- pump and an apical I- efflux mechanism, both regulated by TSH. The culture s were treated with EGF (10 ng/ml) with or without TSH (0.1 mU/ml) for 48 h, and then analyzed for 1) cellular uptake of I-125(-), 2) transe pithelial I-125- flux ((FTEI-)-I-125; basoapical and apicobasal direct ions) during continuous exposure to I-125(-) in either the basal or ap ical medium, and 3) bidirectional (epical and basal) efflux in I-125(- )-loaded cells. Iodination was prevented by methimazole (0.5 mM). EGF increased cell number 25-50%, but did not disturb the tightness or str uctural polarity of the original cell monolayer. EGF reduced the basal rate and inhibited the TSH-induced long term up-regulation of (FTEI-) -I-125 across the cell layer in the basoapical direction. However, the radioactivity content of EGF-treated cultures exposed to I-125(-) in the basal medium was 3-5 times higher than that in controls; apical up take of I-125(-) was negligible. The radioiodide accumulated in EGF-tr eated cells was predominantly released (similar to 80%) in the basal d irection, whereas in controls, the ratio of apical to basal I-125(-) e fflux was 3:2. Acute stimulation of EGF-treated cultures with TSH (10 mU/ml) or forskolin (50 mu M) caused, as in controls, an increase in a pical, but not basal, I-125(-) efflux within minutes; the peak value o f stimulated apical efflux was 10-fold over the prestimulatory level o f basoapical (FTEI-)-I-125 in the same culture. Moreover, the steady s tate level of basoapical (FTEI-)-I-125 after the transient efflux peak was higher than that before stimulation and, in fact, approached the corresponding flux in untreated cells. In contrast, in control and TSH -pretreated cultures, the I-125(-) efflux peak was less pronounced, an d the pre- and poststimulatory levels of basoapical (FTEI-)-I-125 were about equal. (FTEI-)-I-125 in the apicobasal direction was always low and unresponsive to acute stimulation regardless of pretreatment. In conclusion, EGF-treated porcine thyroid epithelial cells in Transwell culture, despite being released from contact inhibition of growth, are able to concentrate I- and release their I- content in response to ac ute TSH stimulation. Moreover, the polarized cell surface expression o f the I- uptake and efflux mechanisms is maintained. A decreased apica l I- permeability, although still sensitive to acute TSH stimulation, may explain the high I- content of EGF-treated cells. Together, the fi ndings indicate that thyroid function is not generally depressed along with EGF-stimulated growth.