A, Adventitial fibroblasts (AFs) were incubated with IL‐11 neutralizing antibody 1 hour before Ang II 24‐hour treatment. The protein levels of collagen, type I, α 1 (COL1a1) (1.00‐, 1.19‐, 2.10‐, and 0.95‐fold), α‐smooth muscle actin (ACTA2) (1.00‐, 1.00‐, 1.49‐, and 0.97‐fold), and Krüppel‐like factor 15 (KLF15) (1.00‐, 0.87‐, 0.66‐, and 0.63‐fold) were measured by Western blotting. N=4. The right panel was the quantity of the Western blot. B, AFs were incubated with IL‐11 neutralizing antibody 1 hour before Ang II 24‐hour treatment. The mRNA levels of COL1a1 (1.00‐, 1.09‐, 1.40‐, and 1.52‐fold), ACTA2 (1.00‐, 0.91‐, 1.80‐, and 1.58‐fold), and KLF15 (1.00‐, 0.98‐, 0.37‐, and 0.51‐fold) were measured by quantitative reverse transcription–polymerase chain reaction. N=5. C, AFs were incubated with ribosomal S6 kinase inhibitor (CMK) (10 µmol/L) or ERK1/2 inhibitor (ERKi) (10 µmol/L) 1 hour before recombination mouse IL‐11 (rmIL‐11) (5 ng/mL) 24‐hour treatment. The protein levels of phosphorylated p90RSK (p‐p90RSK) (1.00‐, 2.10‐, 0.93‐, and 0.74‐fold), p90RSK, phosphorylated ERK1/2 (p‐ERK1/2) (1.00‐, 10.71‐, 10.89‐, and 2.53‐fold), and ERK1/2 were measured by Western blotting. N=4. The right panel was the quantity of the Western blot. D, AFs were incubated with dimethyl sulfoxide, CMK (10 µmol/L), or ERKi (10 µmol/L) 1 hour before PBS or rmIL‐11 (5 ng/mL) 24‐hour treatment. The protein levels of COL1a1 (1.00‐, 1.40‐, 0.68‐, and 0.79‐fold) and ACTA2 (1.00‐, 1.13‐, 0.89‐, and 0.95‐fold) were measured by Western blotting. N=4. The right panel was the quantity of the Western blot. CON indicates control. *P< 0.05, **P< 0.01, and ***P< 0.001.