Human thyroid cells in culture take up and organify I-125 when cultured in TSH (acting through cAMP) and insulin. They also secrete urokinase (uPA) and tissue-type (tPA) plasminogen activators (5_100 IU/10(6) cells/day). TSH and insulin both decreased secreted PA activity, (PAA), uPA and tPA protein and their mRNAs. Autocrine fibroblast growth factor increased secreted PAA and inhibited thyroid cell 1 2 uptake. Epidermal growth factor (EGF) and the protein kinase C (PKC) activator, TPA significantly increased PAA and inhibited thyroid differentiated function, (TPA > EGF). For TPA, effects were rapid, increased PAA secretion and decreased 1 251 uptake being seen at 4 h whereas for EGF, a 24 h incubation was required. qRT-PCR showed significantly increased mRNA expression of uPA with lesser effects on tPA. Aprotinin, which inhibits PAA, increased 12 1 uptake but did not abrogate the effects of TPA and EGF. The MEKK inhibitor, PD98059 partially reversed the effects; of EGF and TPA on PAA, and largely reversed the effects of EGF but not TPA on differentiated function. PKC inhibitors bisindoylmaleimide 1, and the specific PKC beta inhibitor, LY379196 completely reversed the effects of TPA on 125, uptake and PAA whereas EGF effects were unaffected. TPA inhibited follicle formation and this effect was blocked by LY379196 but not PD98059. We conclude that in thyroid cells, MAPK activation inversely correlates with 1251 uptake and directly correlates with PA expression, in contrast to the effects of cAMP. TPA effects on iodide metabolism, dissolution of follicles and uPA synthesis are mediated predominantly through PKC beta whereas EGF exerts its effects through MAPK but not PKC beta.