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-) transport was studied in an in vitro model of the polarized porcine thyroid follicular epithelium in Transwell culture chambers. In this model, two mechanisms responsible for the unidirectional transport of I- from the basal to the apical chamber compartments, corresponding to the interstitium to lumen direction in vivo, are present in opposite plasma membrane domains of the tight cell monolayer: a basolateral I- pump and an apical I- efflux mechanism, both regulated by TSH. The cultures 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 125I-, 2) transepithelial 125I- flux (FTE 125I-; basoapical and apicobasal directions) during continuous exposure to 125I- in either the basal or apical medium, and 3) bidirectional (apical and basal) efflux in 125I(-)-loaded cells. Iodination was prevented by methimazole (0.5 mM). EGF increased cell number 25-50%, but did not disturb the tightness or structural polarity of the original cell monolayer. EGF reduced the basal rate and inhibited the TSH-induced long term up-regulation of FTE 125I- across the cell layer in the basoapical direction. However, the radioactivity content of EGF-treated cultures exposed to 125I- in the basal medium was 3-5 times higher than that in controls; apical uptake of 125I- was negligible. The radioiodide accumulated in EGF-treated cells was predominantly released (approximately 80%) in the basal direction, whereas in controls, the ratio of apical to basal 125I- efflux was 3:2. Acute stimulation of EGF-treated cultures with TSH (10 mU/ml) or forskolin (50 microM) caused, as in controls, an increase in apical, but not basal, 125I- efflux within minutes; the peak value of stimulated apical efflux was 10-fold over the prestimulatory level of basoapical FTE 125I- in the same culture. Moreover, the steady state level of basoapical FTE 125I- 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 125I- efflux peak was less pronounced, and the pre- and poststimulatory levels of basoapical FTE 125I- were about equal. FTE 125I- 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 acute TSH stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)