MAPK8 and breast cancer: The P38 MAPK inhibitor SB203580 increased FRA-1 expression in both invasive breast cancer cell lines, perhaps reflecting the established antagonism between the JNK and p38 pathways.28 In a time course, treatment with either the JNK or MEK inhibitor resulted in a reduction of basal FRA-1 protein levels over time (Supplementary Figure 5), consistent with what has been observed on MEK inhibitor treatment of colon cancer cells.29 In this experiment, SP600125 effectively blocked JNK activity as judged by phospho-c-JUN levels, but had no impact on phospho-ERK.