12-O-TETRADECANOYLPHORBOL 13-ACETATE AND FIBROBLAST GROWTH-FACTOR INCREASE THE 30-KDA SUBSTRATE-BINDING SUBUNIT OF TYPE-II DEIODINASE IN ASTROCYTES

Type II 5'-deiodinase (D-II) catalyzes the intracellular conversion of thyroxine (T-4) to 3,5,3'-triiodothyronine (T-3) 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 (TP A), 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 af finity labeling with N-bromoacetyl[I-125]T-4 in astroglial cells. TPA (0.1 mu M), 20 ng/ml acidic FGF (aFGF), and 1 mM 8-bromo cyclic AMP al l 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). Glu cocorticoids acted synergistically with cAMP and aFGF and promoted the effect of TPA. Affinity labeling was competitively inhibited by bromo acetyl-T-4 > bromoacetyl-T-3 > T-4 > reverse T-3 > iopanoic acid > T-3 > 3,5,3'-triiodothyroacetic acid. The effect of TPA (0.1 mu M) was ma ximum at 8 h and then gradually decreased. aFGF (20 ng/ml) plus hepari n (17 mu g/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-deri ved growth factor induced small increases in the 30-kDa protein. Epide rmal growth factor had little effect. In vitro labeling of cAMP, TPA, and aFGF-stimulated cell sonicates resulted in an increase in the 30-k Da protein that paralleled the increase in D-II activity. These result s correlate well with our previous studies showing that several distin ct signaling pathways regulate D-II activity. They suggest that the re gulation of D-II in astrocytes by cAMP, TPA, and aFGF involves an accu mulation of the 80-kDa substrate binding subunit.