Dose-Dependent Inhibition of Thyroid Differentiation by RAS Oncogenes

Abstract

Activating mutations in RAS protooncogenes are associated with several different histotypes of thyroid cancer, including anaplastic thyroid carcinoma. The latter is the most aggressive cancer of the thyroid gland, showing little or no expression of the differentiated phenotype. Likewise, expression of viral RAS oncogenes in FRTL-5 rat thyroid cells mimics such loss of differentiation. We established FRTL-5 cell lines stably expressing constitutively active forms of RAS, either of viral (v-Ha-RAS or v-Ki-RAS) or cellular (H-RAS(V12)) origin and generated a tamoxifen-inducible RAS oncoprotein to analyze the timing of RAS effects on thyroid differentiation. In RAS-transformed FRTL-5 cells, we measured the expression of many thyroid-specific genes by real-time PCR and observed that a clear loss of differentiation was only obtained in the presence of high RAS oncogene expression. In contrast, TSH-independent growth appeared to be induced in the presence of both low and high levels of oncogenic RAS expression. We also showed that inhibition of differentiation is an early RAS-induced phenomenon. Finally, we demonstrated that only high doses of RAS oncogenes are able to inhibit the activity of Titf1 and Pax8, two transcription factors essential for the maintenance of thyroid differentiation, and that the homeodomain of Titf1 is a target of the inhibitory action of RAS. Our results represent the first evidence of a dose-dependent effect of RAS oncogenes on thyroid epithelial differentiation.

An autoregulatory loop mediated by miR-21 and PDCD4 controls the AP-1 activity in RAS transformation

“…Compositional change of the AP-1 complex in a RAS-inducible cell system To investigate the dynamics of AP-1 regulation in RAS transformation, we have utilized a RAS-inducible cell line (FRTL-5/ER-RAS), in which the tamoxifen-responsive estrogen receptor-H-RAS V12 fusion protein is stably expressed in FRTL-5 rat thyroid cells (De Vita et al, 2005).…”

“…FRTL-5-ER/RAS cells were propagated as described (De Vita et al, 2005). The pIRESpuro-GFP and pIRESpuro-GFP/A-FOS expression vectors have been described (Bahassi el et al, 2004).…”

Thyrotrophin receptor signaling dependence of Braf-induced thyroid tumor initiation in mice

“…Overexpression of HRAS G12V also down-regulates expression of genes required for thyroid hormone biosynthesis in vitro (27), an effect that has been shown to be dose dependent (28). To determine whether endogenous expression of Hras G12V is sufficient to induce thyroid dysfunction and thyroid cell transformation, we crossed FRHras G12V mice, which have a latent Hras oncogenic allele expressed under the control of its native promoter, with TPO-Cre mice.…”

Reactive oxygen species‐dependent destruction of MEK and Akt in Manumycin stimulated death of lymphoid tumor and myeloma cell lines

“…2 Hence, it is not surprising that 30% of known human tumors and 90% of pancreatic carcinomas, one of the most aggressive and morbid types of cancer, harbor mutated or dysfunctional Ras. [3][4][5] Together these features make Ras an attractive target for cancer therapy.Ras maturation requires the post-translational transfer of a farnesyl hydrocarbon group to its C-terminal cysteine in a process termed prenylation, 6-8 which is essential for the subsequent localization of Ras at the plasma membrane where receptor signals regulate its on and off state. 9 Because prenylation is also required for subsequent Ras activity, considerable effort has focused on the screening of natural compounds and design of synthetic compounds that interfere with this post-translational event.…”

“…2 Hence, it is not surprising that 30% of known human tumors and 90% of pancreatic carcinomas, one of the most aggressive and morbid types of cancer, harbor mutated or dysfunctional Ras. [3][4][5] Together these features make Ras an attractive target for cancer therapy.…”