THE PROTEIN-KINASE-A PATHWAY INHIBITS C-JUN AND C-FOS PROTOONCOGENE EXPRESSION INDUCED BY THE PROTEIN-KINASE-C AND TYROSINE KINASE PATHWAYSIN CULTURED HUMAN THYROID-FOLLICLES

We have previously demonstrated antagonistic interactions between the major signal transduction pathways in human thyroid follicles: TSH act ing via protein kinase A (PKA) attenuated phorbol ester [acting via pr otein kinase C (PKC)] as well as epidermal growth factor (EGF)-protein tyrosine kinase (PTK)-mediated cell proliferation, whereas the PKC an d PTK pathways inhibited PKA-mediated cell differentiation. In view of the key role played by the protooncogenes c-jun and c-fos in the casc ade of events leading to cell proliferation and differentiation, we ex amined whether the antagonism we observed between the pathways could b e related to changes in the expression of these genes. The experimenta l model used was the same in vitro system as that used in the above st udy on cell growth and differentiation: thyroid follicles of human ori gin cultured in suspension under serum-free conditions. Both EGF (1-50 ng/mL) and the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TP A; 10(-11)-10(-7) mol/L) dose and time dependently stimulated c-jun an d c-fos messenger ribonucleic acid (mRNA) expression. The c-jun and c- fos mRNA stimulation elicited by TPA was reduced by the PKC inhibitors , chelerythrine and staurosporine, and could not be mimicked by 4 alph a-phorbol 12,13-didecanoate (a phorbol ester that fails to activate PK C), whereas the stimulation induced by EGF was diminished by the PTK i nhibitor, genistein. This indicates a PKC- and PTK-mediated pathway tr iggered by TPA and EGF, respectively. TSH induced an increase in c-jun . and c-fos mRNA, which, though significant, was small compared to tha t elicited by TPA or EGF. Addition of TSH (0.1-0.5 mU/mL), however, to either TPA or EGF dose dependently inhibited the c-jun. and c-fos mRN A elicited by these agents. The repressive action of TSH on the effect s of TPA and EGF mRNA were mimicked by forskolin and 8-bromo-cAMP, sug gesting that the TSH inhibitory action is PKA mediated. The TSH inhibi tory action seems to require de novo protein synthesis, as it was abro gated in the presence of cycloheximide. In conclusion, the present stu dy provides novel data on c-jun and c-fos gene expression and their mo dulation by the major signal transduction pathways operating in human thyrocytes. Moreover, using the same serum-free system of human thyroi d follicles cultured with the same agents and at the same doses as in our previous study on cell growth and differentiation, we found the TS H/PKA pathway to inhibit PKC- and EGF/tyrosine kinase-induced c-jun an d c-fos mRNA, i.e. antagonistic effects parallel to those previously o bserved measuring cell proliferation. The findings suggest an associat ion between human thyroid cell proliferation and c-jun and c-fos gene expression.