Differential responses of human papillary thyroid cancer cell lines carrying the RET/PTC1 rearrangement or a BRAF mutation to MEK1/2 inhibitors

Objectives: To examine the effects of 2 mitogenactivated protein kinase kinase (MEK1/2) inhibitors on papillary thyroid carcinoma (PTC) cell lines carrying the RET/PTC1 rearrangement or a BRAF mutation. In PTC, RET/PTC1 rearrangement or BRAF mutations results in constitutional activation of RET kinase or BRAF, respectively. Along the RET or BRAF signaling cascades, the activated RET kinase or BRAF activates MEK1/2, and then mitogen-activated protein kinases (extracellular signalrelated kinase 1/2 [ERK1/2]) is activated. Activated ERK1/2 enters the nucleus and phosphorylates a variety of transcription factors, resulting in cancer cell proliferation. The MEK1/2 inhibitors, PD98059 and U0126, have been shown to inhibit cell growth in other cancers. Design: In vitro study. Subjects: Papillary thyroid carcinoma cell lines carrying the RET/PTC1 rearrangement (BHP2-7) or a BRAF mutation (BHP5-16). Intervention: We treated PTC cells carrying the RET/ PTC1 rearrangement or a BRAF mutation with 2 MEK1/2 inhibitors (PD98059 and U0126). Main Outcome Measures: Using Western blot analysis, we detected the expression of phosphorylated ERK1/2 and expression of cleaved poly(ADP-ribose) polymerase (PARP) in cells after treatment with either inhibitors. Growth inhibition was monitored by the 3-(4,5dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide NTT) assay. Results: Using Western blot analysis, we detected the dephosphorylation of ERK1/2 in PTC cells carrying the RET/PTC1 rearrangement or a BRAF mutation after treating the cells with 2 MEK1/2 inhibitors (PD98059 and U0126). In addition, both PD98059 and U0126 completely inhibited the growth of the PTC cells carrying a BRAF mutation but partially inhibited the growth of the PTC cells carrying the RET/PTC1 rearrangement. Finally, we observed PARP cleavage only in cells with a BRAF mutation in the Western blot analysis. Conclusion: These data suggested that treatment with MEK1/2 inhibitors can be used as tools for inhibiting the growth of PTC cells.