Inhibition of c-ras reactivates thyroglobulin synthesis in middle-T transformed thyroid cells.

Differentiated thyroid cells expressing polyoma Middle-T became transformed and tumorigenic when injected into syngenic animals. The expression of thyroglobulin was greatly reduced and no longer responsive to thyrotropin (TSH) and to cAMP. Inhibition of endogenous c-ras by the expression of two transdominant negative mutant H-ras genes, Asn17 and Leu61-Ser186, reactivated thyroglobulin synthesis. Reactivation of thyroglobulin synthesis by c-ras inhibition was not observed in the absence of TSH. These findings indicate that MT elicits dedifferentiation of thyroid cells by activating endogenous c-ras and that c-ras interferes with TSH or cAMP signaling.