Conditional Apoptosis Induced by Oncogenic Ras in Thyroid Cells

Abstract

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. Beginning 3-4 days after H-Ras v12 expression, cells underwent apoptosis. The H-Ras v12 effects on apoptosis were decreased by a mitogen-activated protein kinase kinase (MEK1) inhibitor and recapitulated by doxycycline-inducible expression of an activated MEK1 mutant (MEK1 S217E/S221E ). As reported elsewhere, acute expression of H-Ras v12 also induces mitotic defects in PCCL3 cells through ERK (extracellular ligand-regulated kinase) activation, suggesting that apoptosis may be secondary to DNA damage. However, acute activation of SAPK/JNK (stress-activated protein kinase/Jun N-terminal kinase) through acute expression of Rac1 v12 also triggered apoptosis, without inducing large-scale genomic abnormalities. H-Ras v12 -induced apoptosis was dependent on concomitant activation of cAMP by either TSH or forskolin, in a protein kinase A-independent manner. Thus, coactivation of cAMP-dependent pathways and ERK or JNK (either through H-Ras v12 , Rac1 v12 , or MEK1 S217E/S221E ) is inconsistent with cell survival. The fate of thyrocytes within the first cell cycles after expression of oncogenic Ras is dependent on ambient TSH levels. If both cAMP and Ras signaling are simultaneously activated, most cells will die. Those that survive will eventually lose TSH responsiveness and/or inactivate the apoptotic cascade through secondary events, thus enabling clonal expansion.