Activation of ERK1,2 in RAW264.7 cells by MWCNTs. A) Western blot analysis using an antibody specific for phosphorylated ERK (p-ERK1,2) showing that MWCNTs (50 μg/ml) increased ERK activation in a concentration dependent manner in RAW 264.7 cells. CBNPs did not cause a consistent concentration-dependent activation of ERK. Total levels of ERK1,2 protein and β-actin were not changed by MWCNTs or CBNPs. B) Western blot analysis showing time course of ERK phosphorylation by MWCNTs. Numeric values above each Western blot represent the fold-increase in relative densitometric intensity of each p-ERK1,2 band relative to control (zero) nanoparticle treatment which was normalized to 1.0.