Histone and High Mobility Group Protein Phosphorylation in the Thyroid: Regulation by Cyclic Nucleotides*

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Abstract
A variety of cyclic nucleotide analogs and other agents that affect thyroid cyclic nucleotide metabolism were used to investigate the role of cAMP and cGMP in regulating nuclear protein phosphorylation in calf thyroid slices labeled in vitro with [32P]orthophosphate. Two major groups of acid-soluble proteins were studied. Group I consisted of protein whose phosphorylation is stimulated by TSH [histones H1 and H3, high mobility group (HMG) protein 14, and the HMG 14/17-like protein PS.3]; group II included representatives of a spectrum of proteins whose phosphorylation is unaffected by TSH (histones H2A, H2B and H4, HMG 17, the HMG 14/17-like protein PS.2 and the nonhistone protein AS.1). The effects of TSH (50 mU/ml) on the 32P labeling of group I proteins were partially reproduced by (Bu)2[dibutyryl]cAMP (1 mM), 8-bromo-cAMP (1 mM), and butyrate (2 mM), and closely mimicked by 8-(4-chlorophenylthio)cAMP (1 mM), forskolin (25 .mu.M), and butyrate (10 mM). (Bu)2cGMP (1 mM), 8-bromo-cGMP (1 mM), and carbachol (50 .MU.M) had no effect on protein phosphorylation. NaNO2 (20 mM), which markedly increase cGMP concentration in calf thyroid slices, decreased the 32P labeling of group I proteins and also affected, to varying extents, the phosphorylation of the group II proteins. The phosphodiesterase inhibitor methylisobutylxanthine (0.5 mM) had generally minor effects on 32P labeling; however, it did counteract the effects of NaNO2 on group I protein phosphorylation. The hypothesis that TSH-depending phosphorylation of group I proteins is mediated by cAMP is supported, but little evidence of cGMP regulation of histone or HMG protein phosphorylation is provided.