STIMULATION OF MITOGEN-ACTIVATED PROTEIN-KINASE BY THYROTROPIN IN PRIMARY CULTURED HUMAN THYROID-FOLLICLES

In the thyroid, thyrotropin (TSH) stimulates both growth and function, and stimulates the production of cAMP which reproduces most of the ef fects of TSH, Here, we report evidence that TSH stimulates the mitogen -activated protein (MAP) kinase cascade through a cAMP-independent pat hway, in human thyroid. TSH stimulated MAP kinase activity (4-9-fold t he basal level) measured in the cytosolic fractions of primary culture d thyroid follicles. Maximal activity was reached after 20 min and rem ained sustained for 1-3 h, TSH being as potent as EGF; EC(50) was 1.5 nM TSH. Only a single isoform of MAP kinase (p42) was detected in the follicles. p42 was phosphorylated on tyrosine residues and showed a re duced electrophoretic mobility in follicles stimulated by TSH, All the se effects on MAP kinase were decreased by preincubation of the follic les with human anti-TSH receptor antibodies. The stimulation of MAP ki nase by TSH was neither blocked by pertussis toxin nor reproduced by f orskolin, cholera toxin, or 8-bromo-cAMP. In conclusion, in human thyr oid cells, in contrast with previous observations on dog thyroid cells , TSH stimulates strongly MAP kinase through a pertussis toxin-insensi tive and cAMP-independent pathway.