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-β (TGF-β). Using subconfluent monolayers of porcine thyroid follicular cells, the aims of this study were to establish whether the actions of TGF-β 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-β by follicular cells, as opposed to iodide-mediated autoregulation events.
Exposure of porcine thyroid follicular cells in subconfluent monolayer culture to TGF-β over a 7-day period reduced both IGF-I release and the incorporation of [met hyl-3H]thymidine into trichloroacetic acidprecipitable 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-β antiserum, and subsequently readded 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. The results of this study are consistent with the hypothesis that IGF-I and TGF-β are both produced by subconfluent thyroid follicular cells in vitro, and that the inhibitory action of TGF-β on follicular cell growth may involve a decrease in the thyroidal production of IGF-I. While the attenuating action of iodide on follicular cell proliferation may, in part, reflect an increased autocrine production of TGF-β, and a reduction by TGF-β of the growth response to IGF-I, these studies also provide evidence that the intrathyroidal actions of TGF-β include an attenuation of IGF-I biosynthesis.
Journal of Endocrinology (1991) 130, 3–9
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