Plasminogen activators in human thyrocytes: regulation and role
R Susarla1, JC Watkinson2 & MC Eggo1
The plasminogen activator (PA) system plays a central role in biological processes that involve extracellular proteolysis including tissue remodelling, blood clotting, angiogenesis and metastases. The thyroid gland is highly vascular and undergoes extensive remodelling in goitre. We found that primary cultures of human thyroid follicular cells secrete large amounts of PAs (2.5-110 U per ml per day per 105 cells), both urokinase-like (uPA) and tissue-type (tPA), and the plasminogen activator inhibitor-1 (PAI-1) (200-500 ng per ml per day per 105cells). PA production was inversely related to thyroid function (assayed by 125I uptake), which is dependent on TSH. PA activity (PAA) and production were stimulated by EGF treatment and by activation of protein kinase C using TPA (2-20 fold) (p<0.01) while PAI-1 levels were unaffected. Aprotinin, which inhibits PAA, increased 125I uptake by the cells whereas EGF and TPA were strongly inhibitory. These inhibitory effects were not abrogated by aprotinin suggesting no causative link. TPA effects were rapid and increased secretion of uPA and tPA was seen at 4h and 8h respectively when examined by Western blotting. The majority of immunoreactive uPA and tPA was secreted into the medium and was not sequestered in the cell layer where the follicles are formed. This is consistent with basolateral secretion into the circulation. The p42/44 MAPK inhibitor, PD98059, decreased PAA and increased 125I uptake. TPA stimulated p42/44 MAPK phosphorylation in our cells but PD98059 did not prevent TPA-induced increases in PAA and its inhibitory effects on 125I uptake. A specific inhibitor of PKC beta was however more effective at blocking TPA effects on PAA (10.3 vs 89.0 U per mg protein, p<0.01) suggesting that the predominant pathway mediating increased PA production, is PKC activation. We conclude thyroidal production of PAA may be for local remodelling in goitre but it may also serve an endocrine role.