Aberrant splicing isoforms of MEK5 exhibit loss of function and are prone to be degraded through proteasome. (A) Overexpression of different MEK5 isoforms in H929 and MM.1S cells. (B) Effects of overexpression of different MEK5 isoforms on the lethality of MM.1S or H929 cells caused by shRNA targeting the MEK5 3'UTR (shMEK5 3). (C) Effects of overexpression of different MEK5 isoforms on subcutaneous tumor growth inhibition by shRNA targeting the MEK5 3'UTR. (D) Tumor volume of H929 subcutaneous tumors in panel C. (E) Protein of different MEK5 isoforms overexpressed in COS7 treated with cycloheximide (CHX) detected by western blotting. (F) The protein levels of MEK5 isoforms in panel E were evaluated and plotted on the graph. (G) Protein level of different MEK5 isoforms in COS7 cells treated with MG132 or chloroquine (CQ) detected by western blotting. (H) The protein levels of MEK5 isoforms in panel G were evaluated and plotted on the graph. ∗P < .05, ∗∗P < .01, ∗∗∗P < .001.