BACKGROUND

Dominant-activating mutations in the RET protooncogene, a receptor tyrosine kinase, have been identified as a cause of medullary thyroid carcinoma. Such oncogenic RET mutations induce its ligand-independent constitutive trans-autophosphorylation. We investigated the role of endogenous oncogenic RET autophosphorylation in maintaining the neoplastic phenotype in medullary thyroid carcinoma cells and orthotopic medullary thyroid carcinomas in RET transgenic mice.

METHODS

We constructed adenoviral vectors expressing a dominant-negative truncated form of RET, termed RET(DeltaTK), and analyzed its effect on cell viability, apoptosis, and proliferation of TT medullary thyroid carcinoma cells. We investigated the effect of RET(DeltaTK) on downsteam signaling by assessing alterations in phosphorylation or in gene expression. The effect of RET(DeltaTK) in primary medullary thyroid carcinomas in transgenic mice was assessed by monitoring tumor growth. All statistical tests were two-sided.

RESULTS

Cell viability was reduced. Phosphorylation of Akt and extracellular signal-regulated kinase (ERK), components of downstream signal transduction pathways, was abolished, and cell cycle progression was reduced. Expression of cell cycle regulator cyclin D1 was decreased, and expression of cell cyle regulators p21(CIP1/WAF1) and p27(KIP1) was increased. Apoptosis was stimulated and concurrently the expression of BCL-2 was decreased. All in vitro experiments compared TT cells expressing RET(DeltaTK) with untreated control cells or control vector-treated cells. Furthermore, 2 weeks after injecting adenovirus-carrying RET(DeltaTK) into thyroid glands of transgenic mice with orthotopic medullary thyroid carcinoma, tumors were statistically significantly smaller than their initial size in mice treated with RET(DeltaTK) (43.6%, 95% confidence interval [CI] = 30.7% to 56.5%; P =.010; two-sided unpaired Student's t test), whereas tumors in mice treated with a control vector were larger than their initial size (139.8%, 95% CI = 120.3% to 159.3%; P<.001).

CONCLUSION

Selective disruption of oncogenic RET signaling in medullary thyroid carcinoma in vitro and in vivo is associated with loss of the neoplastic phenotype of medullary thyroid carcinoma and should be investigated further as the basis for new therapeutic approaches for this disease.