The inability of Spry2 to inhibit B‐Raf V600E activity is required for induction of resistance to B‐Raf inhibitor in B‐Raf V600E cells

Abstract

B-Raf is the most frequently mutated protein kinase in human cancer. Thus, inhibition of B-Raf kinase activity could benefit cancer patients. However, the clinical benefit of RAF inhibitor therapies is limited by the emergence of drug resistance. In this study, expression of Spry2, negative regulators of the MAPK pathway, was higher in A375P cells harboring the B-Raf V600E mutation compared to wild-type B-Raf-bearing cells (SK-MEL-2) that is resistant to B-Raf inhibitor. In particular, Spry2 had strongly reduced expression in A375P/Mdr cells with acquired resistance to B-Raf inhibitor. Furthermore, overexpression of Spry2 partially restored sensitivity to B-Raf inhibitor PLX4720 in two B-Raf inhibitor-resistant cells, indicating a positive role for Spry2 in the growth inhibition induced by B-Raf inhibitor. However, Spry2 overexpression in A375P/Mdr cells resulted in no significant change in the levels of pERK or pMEK.On the other hand, when Spry2 is silenced, the A375P cells were significantly more sensitive to B-Raf inhibitor. Moreover, long-term treatment of PLX4720 induced pERK reactivation following B-Raf inhibition in A375P cells, indicating that Spry2 might be bypassed in B-Raf mutant melanoma cells by the presence of B-Raf mutation. Taken together, these data suggest that induction of resistance to B-Raf inhibitor in B-Raf V600E cells can be attributed to the inability of Spry2 to inhibit B-Raf V600E activity. Grant Funding Source: Supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2013R1A1A2A10058897)