Prenyltransferase inhibitors induce apoptosis in proliferating thyroid cells through a p53-independent, CrmA-sensitive, and caspase-3-like protease-dependent mechanism

The inhibitors of protein prenylation have been proposed for chemotherapy o f tumors. Lovastatin, a 3-hydroxy-3-methylglutaryl-Coenzyme A (HMG-CoA) red uctase inhibitor, displays proapoptotic activity in tumor cells blocking th e synthesis of isoprenoids isoprenoids compounds. To test whether HMG-CoA r eductase inhibition can induce apoptosis in proliferating thyroid cells, we studied the effects of lovastatin in normal and neoplastic thyroid cells a nd in primary cultures from normal human thyroids. In an immortalized human thyroid cell line (TAD-2) and in neoplastic cells, lovastatin induced cell rounding within 24 h of treatment. After 48 h the cells were detached from the plate and underwent apoptosis, as evidenced by DNA fragmentation. Morp hological changes and apoptosis did not occur in serum-starved quiescent TA D-2 cells or in primary cultures of normal thyrocytes. Mevalonate, the prod uct of the HMG-CoA reductase enzymatic activity, and the protein synthesis inhibitor cycloheximide completely blocked the effects of lovastatin in a d ose-dependent fashion. The geranylgeranyl transferase GGTI-298 inhibitor mi micked the effects of lovastatin on cell morphology and induced cell death, whereas the farnesyl transferase inhibitor FTI-277 was less effective to i nduce both cell rounding and apoptosis. Resistance to lovastatin-induced ap optosis by expression of the viral serpine CrmA and by the peptide inhibito r of caspases, Z-DEVD-fmk, demonstrated the involvement of CrmA-sensitive, caspase-3-like proteases. Inhibition of endogenous p53 activity did not aff ect the sensitivity of thyroid cells to lovastatin, demonstrating that this type of apoptosis is p53 independent. We conclude that lovastatin is a potent inducer of apoptosis in proliferati ng thyroid cells through inhibition of protein prenylation. This type of ap optosis requires protein synthesis, is CrmA sensitive and caspase-3-like pr otease dependent, and is independent from p53.