Blackwell Publishing Journal Backfiles 1879-2005
Abstract: Type II 5′-deiodinase (D-II) catalyzes the intracellular conversion of thyroxine (T4) to 3,5,3′-triiodothyronine (T3) in the brain., The D-II activity in astroglial cell cultures is induced by several pathways including cyclic AMP (cAMP), 12-O-tetradecanoylphorbol 13-acetate (TPA), and fibroblast growth factors (FGFs). We have examined the effect of TPA and FGFs on the 30-kDa substrate binding subunit of D-II, by affinity labeling with N-bromoacetyl-[128I]T4 in astroglial cells. TPA (0.1 μM), 20 ng/ml acidic FGF (aFGF), and 1 mM 8-bromo cyclic AMP all caused an increase in the 30-kDa protein. cAMP induced the greatest increase (fivefold) followed by TPA (3.2-fold) and FGF (2.8-fold). Glucocorticoids acted synergistically with cAMP and aFGF and promoted the effect of TPA. Affinity labeling was competitively inhibited by bromoacetyl-T4 〉 bromoacetyl-T3 〉 T4 〉 reverse T3 〉 iopanoic acid 〉 T3 〉 3,5,3-triiodothyroacetic acid. The effect of TPA (0.1 μM) was maximum at 8 h and then gradually decreased. aFGF (20 ng/ml) plus heparin (17 μg/ml) induced a maximal 30-kDa increase at 8 h, which stayed stable for up to 24 h. The effect of aFGF was concentration dependent. Of the other growth factors studied, only basic FGF and platelet-derived growth factor induced small increases in the 30-kDa protein. Epidermal growth factor had little effect. In vitro labeling of cAMP, TPA, and aFGF-stimulated cell sonicates resulted in an increase in the 30-kDa protein that paralleled the increase in D-II activity. These results correlate well with our previous studies showing that several distinct signaling pathways regulate D-II activity. They suggest that the regulation of D-II in astrocytes by cAMP, TPA, and aFGF involves an accumulation of the 30-kDa substrate binding subunit.
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