STIMULATION OF MITOGEN-ACTIVATED PROTEIN-KINASE BY THYROTROPIN IN ASTROCYTES

We have recently reported the expression of the thyrotropin (TSH) rece ptor and the stimulation by TSH of type-II iodothyronine 5'-deiodinase in astrocytes. In these cells, TSH stimulated arachidonate release, b ut neither cAMP production, nor phosphatidylinositolbisphosphate hydro lysis, as described in the human thyroid gland. Here we report, in con trast to a recent observation made in dog thyroid cells, that TSH stim ulates mitogen-activated protein kinase (MAP kinase) in astrocytes. In deed, TSH increases the tyrosine phosphorylation of the two isoforms o f MAP kinase expressed in these cells, in correlation with both a slow er electrophoretic migration of the tyrosine phosphorylated species an d an enhanced enzymic activity measured on a specific substrate peptid e. This stimulation of MAP kinase by TSH was specifically inhibited by incubation of astrocytes in the presence of human blocking anti-(TSH receptor) IgG, and by immunoprecipitation of TSH with monoclonal anti- TSH IgG. In astrocytes, TSH was neither mitogenic by itself, nor modif ied significantly the basic-fibroblast-growth-factor-induced mitogenes is. The stimulation of MAP kinase by TSH was not affected by treatment with pertussis toxin, suggesting guanine-nucleotide-binding-regulator y protein i/o was not implicated in this TSH effect. Our model will al low the study of the stimulation of MAP kinase by TSH without interfer ence either from cAMP or from phosphoinositide signalling pathways.