Full-length MEK5 contributes to enhanced proliferation and survival of MM cells. (A) Full-length MEK5 was knocked down in H929 cells. Expression of caspase 3 and cyclin B1-CDK1 complex was detected by western blotting. (B) Knockdown of full-length MEK5 inhibited the proliferation of H929 cells. Cell viability was determined by CCK-8 assay at the indicated time points. (C) Knockdown of full-length MEK5 induced apoptosis in H929 and MM.1S cells, which was detected 48 hours after infection. (D) Knockdown of full-length MEK5 induced cell cycle arrest of H929 cells at the G2/M phase, which was detected 48 hours after infection. (E) Images of subcutaneous tumor tissues with MEK5 knockdown. H929 cells with full-length MEK5 knockdown were subcutaneously inoculated into the hind flanks of 6-week-old female NOG mice. The mice were euthanized 5 weeks after inoculation. (F) Tumor volume of the subcutaneous tumor in panel E after sacrificing the mice. (G) Images of subcutaneous tumor tissue with U266-inducible MEK5 knockdown cells. U266 cells with dox-inducible shMEK5 2 were subcutaneously inoculated into the hind flank of 6-week-old female NOG mice. The mice were euthanized 20 days after inoculation. (H) Tumor sizes were measured every 4 days. (I) RBM39 was knocked down in H929 or MM.1S cells overexpressing full-length MEK5. (J,K) Expression of full-length MEK5 partially rescued H929 or MM.1S cells from the lethality of an shRNA targeting RBM39. (L) Expression of full-length MEK5 partially rescued H929 subcutaneous tumors from the growth inhibition of shRBM39. (M,N) Tumor volume and tumor mass of H929 subcutaneous tumors in panel L. ∗P < .05, ∗∗P < .01, ∗∗∗P < .001, ∗∗∗∗P < .0001. CCK-8, Cell Counting Kit-8.