Epidermal growth factor, but not thyrotropin, stimulates the expression of c-fos and c-myc messenger ribonucleic acid in porcine thyroid follicle cells in primary culture.

Previous studies have shown that TSH stimulates functional parameters in cultures of porcine follicle cells, but has no stimulatory effect on their growth. Epidermal growth factor (EGF), on the other hand, is a potent mitogen in such cells. The aim of the present investigation was to study the effect of EGF and TSH on the expression of proto-oncogene mRNA in porcine thyroid follicle cells. The addition of EGF caused a marked increase in the c-fos and c-myc mRNA levels. Stimulation of cells with 10 mU/ml TSH, however, failed to stimulate expression of the proto-oncogenes. The increase in c-fos mRNA caused by EGF was transient, with a maximum after 30 min. The c-myc transcript level increased after 30-60 min; the maximum level was at 90 min, and it decreased thereafter but remained at an elevated level throughout the 24 h of investigation. Half-maximal effects on proto-oncogene expression were obtained using 0.5-1 ng (c-fos) and 0.2-0.5 ng (c-myc) EGF/ml. Our data clearly demonstrate that increased expression of c-fos and c-myc mRNA is an early event in the prereplicative phase of EGF-stimulated porcine thyroid follicle cells. Since c-fos and c-myc expression may be necessary events in the prereplicative phase, the lack of effect of TSH on these functions may be closely related to its inability in stimulating the growth of porcine thyrocytes.