Conditional apoptosis induced by oncogenic ras in thyroid cells

Mutations of ras are tumor-initiating events for many cell types, including thyrocytes. To explore early consequences after oncogenic Ras activation, we developed a doxycycline-inducible expression system in rat thyroid PCCL3 cells. Begining 3-4 days after H-Ras(v12) expression, cells underwent apop tosis. The H-Ras(v12) effects on apoptosis were decreased by a mitogen-acti vated protein kinase kinase (MEK1) inhibitor and recapitulated by doxycycli ne-inducible expression of an activated MEK1 mutant (MEK1(S217E/S221E)). AS reported elsewhere, acute expression of H-Ras(v12) also induces mitotic de fects in PCCL3 cells through ERK (extracellular ligand-regulated kinase) ac tivation, suggesting that apoptosis may be secondary to DNA damage. However , acute activation of SAPK/JNK (stress-activated protein kinase/Jun N-termi nal kinase) through acute expression of Rac1(v12) also triggered apoptosis, without inducing large-scale genomic abnormalities. H-Ras(v12)-induced apo ptosis was dependent on concomitant activation of cAMP by either TSH or for skolin, in a protein kinase A-independent manner. Thus, coactivation of cAM P-dependent pathways and ERK or JNK (either through H-Ras(v12) Rac1(v12), o r MEK1(S217E/S221E)) is inconsistent with cell survival. The fate of thyroc ytes within the first cell cycles after expression of oncogenic Ras is depe ndent on ambient TSH levels. If both cAMP and Ras signaling are simultaneou sly activated, most cells will die. Those that survive will eventually lose TSH responsiveness and/or inactivate the apoptotic cascade through seconda ry events, thus enabling clonal expansion.