Insulin-like growth factor-I production and action in porcine thyroid follicular cells in monolayer: regulation by transforming growth factor-beta.

The regulation of thyroid follicular cell growth in vitro involves autocrine or paracrine actions of insulin-like growth factor-I (IGF-I), which are partially suppressed by transforming growth factor-beta (TGF-beta). Using subconfluent monolayers of porcine thyroid follicular cells, the aims of this study were to establish whether the actions of TGF-beta involve changes in the synthesis of, or response to, IGF-I. We also investigated the extent to which inhibitory actions of iodide on IGF-I-dependent proliferation of thyroid follicular cells may be attributable to the production of TGF-beta by follicular cells, as opposed to iodide-mediated autoregulation events. Exposure of porcine thyroid follicular cells in subconfluent monolayer culture to TGF-beta over a 7-day period reduced both IGF-I release and the incorporation of [methyl-3H]thymidine into trichloroacetic acid-precipitable cellular material, while preincubation of cells with NaI (0.1 mmol/l) for 24 h prior to the addition of TSH reduced the stimulatory effect of the latter on IGF-I release over the following 7 days. Preincubation of cells with iodide also reduced basal (i.e. autonomous) [methyl-3H]thymidine incorporation. This effect was partially reversed when, following initial exposure to follicular cells, iodide-containing preincubation medium was immunoadsorbed with a neutralizing TGF-beta antiserum, and subsequently re-added to the cells. Furthermore, similar immunoadsorption of iodide-free preincubation medium resulted in an enhancement of the control level of [methyl-3H]thymidine incorporation when the treated medium was returned to the original cultures.(ABSTRACT TRUNCATED AT 250 WORDS)