Glucocorticoids curtail stimuli-induced CREB phosphorylation in TRH neurons through interaction of the glucocorticoid receptor with the catalytic subunit of protein kinase A.

PURPOSE: Corticosterone prevents cold-induced stimulation of thyrotropin-releasing hormone (Trh) expression in rats, and the stimulatory effect of dibutyryl cyclic-adenosine monophosphate (dB-cAMP) on Trh transcription in hypothalamic cultures. We searched for the mechanism of this interference.

METHODS: Immunohistochemical analyses of phosphorylated cAMP-response element binding protein (pCREB) were performed in the paraventricular nucleus (PVN) of Wistar rats, and in cell cultures of 17-day old rat hypothalami, or neuroblastoma SH-SY5Y cells. Cultures were incubated 1h with dB-cAMP, dexamethasone and both drugs combined; their nuclear extracts were used for chromatin immunoprecipitation; cytosolic or nuclear extracts for coimmunoprecipitation analyses of catalytic subunit of protein kinase A (PKAc) and of glucocorticoid receptor (GR); their subcellular distribution was analyzed by immunocytochemistry.

RESULTS: Cold exposure increased pCREB in TRH neurons of rats PVN, effect blunted by corticosterone previous injection. Dexamethasone interfered with forskolin increase in nuclear pCREB and its binding to Trh promoter; antibodies against histone deacetylase-3 precipitated chromatin from nuclear extracts of hypothalamic cells treated with tri-iodothyronine but not with dB-cAMP + dexamethasone, discarding chromatin compaction as responsible mechanism. Co-immunoprecipitation analyses of cytosolic or nuclear extracts showed protein:protein interactions between activated GR and PKAc. Immunocytochemical analyses of hypothalamic or SH-SY5Y cells revealed diminished nuclear translocation of PKAc and GR in cells incubated with forskolin + dexamethasone, compared to either forskolin or dexamethasone alone.

CONCLUSIONS: Glucocorticoids and cAMP exert mutual inhibition of Trh transcription through interaction of activated glucocorticoid receptor with protein kinase A catalytic subunit, reducing their nuclear translocation, limiting cAMP-response element binding protein phosphorylation and its binding to Trh promoter.