(A) Treatment with 100 nM trametinib for 72 hours induces differentiation in SMS-CTR (left) and RD (right) cells grown in complete medium as determined by immunofluorescence for MHC. Scale bars, 200 μm. (B) Volcano plot comparing gene expression in SMS-CTR treated with 100 nM trametinib or vehicle for 48 hours. Statistically significant differentially expressed genes (at least twofold change with P < 0.05) are represented by dark gray dots; genes that are not differentially expressed are represented by light gray dots. Differential expression of MYC (green, decreased expression in trametinib-treated cells), MYOG and MEF2C (blue, increased expression in trametinib-treated cells), and MYOD (black, unchanged expression) is highlighted. (C) Significance (nominal P value) versus normalized enrichment score (NES) plot for gene sets in the C2: canonical pathways molecular signatures database (top) and C3: transcription factor motif molecular signatures database (bottom). (D) Gene set enrichment analysis (GSEA) enrichment plot showing positive enrichment for a set of genes up-regulated during differentiation of human skeletal muscle myoblasts into myotubes in both RD (top) and SMS-CTR (bottom) treated with trametinib or transfected with MEK1 small interfering RNA (siRNA; RD only). For each of the enrichment plots shown here, the false discovery rate (FDR) q value and the nominal P value is < 0.05. (E) Expression of MYOG and MEF2C but not of MYOD is induced by trametinib treatment in RD (left) and SMS-CTR (right) as determined by immunoblot. Expression and phosphorylation of MYC are decreased by trametinib treatment in SMS-CTR as determined by immunoblot (far right). (F) ERK inhibition with 100 nM SCH772894 for 72 hours induces differentiation in SMS-CTR (left) and RD (right) cells grown in complete medium as determined by immunofluorescence for MHC. Scale bars, 200 μm. (G) GSEA enrichment plot showing positive enrichment for a set of genes up-regulated during differentiation of human skeletal muscle myoblasts into myotubes in SMS-CTR treated with the ERK inhibitor, SCH77894 (FDR and nominal P < 0.05).