Erk1-Mediated Development of Left Ventricular Cardiac Hypertrophy in a P21 Activated Kinase-1 (Pak1) Knockout Mouse Model

Abstract

Pak1, a serine/threonine protein kinase, plays a critical role in cardiac excitation-contraction. We hypothesize that Pak1 prevents or attenuates the development of left ventricular (LV) cardiac hypertrophy. Six wild-type (WT) and six Pak1-knockout (KO) mice were randomized into four groups of three mice each to receive subcutaneously 25μg/g/day of isoproterenol (ISO) or saline (CTRL) for seven days. Transthoracic echocardiography showed in the Pak1-KO/ISO group vs. WT/ISO group, respectively: i) increased LV fractional shortening (%)(61.65 ± 2.72 vs. 48.02 ± 4.01; p = 0.048); ii) reduced LV chamber volume (μL) in diastole (36.77 ± 4.19 vs. 73.92 ± 3.95; p = 0.003) and systole (4.14 ±0.54 vs. 16.24 ± 2.19; p = 0.0058); iii) increased isoproterenol-induced LV cardiac hypertrophy in Pak1-KO mice (40% increase in LV mass, p = 0.049) vs. WT mice (15.2 % increase in LV mass, p = 0.07); iv) enhanced early filling deceleration time (mm/s2) (−80460 ± 13620 vs. −21550 ± 3560; p = 0.014). We also generated novel evidence that Erk1 and Pak1 establish protein-protein interaction in whole cardiac tissue, as assessed by co-immunoprecipitation. Western immunoblotting of Erk1 phosphorylation in whole cardiac tissue showed maximal Erk1 activation in Pak1-KO/ISO mice vs. all other groups (phosphoErk1/totalErk1 ratio in Pak1-KO/ISO 0.80 ± 0.005 vs.: i) WT/CTRL 0.042 ± 0.02171, p<0.0001; ii) WT/ISO 0.3086 ± 0.0924, p=0.006; iii) Pak1-KO/CTRL 0.6020 ± 0.046, p=0.013), whereas Erk1 phosphorylation was consistently reduced in lysates obtained from adenovirally-infected adult rat cardiomyocytes that express constitutively active Pak1 vs. dominant negative Pak1 (phosphoErk1/actin ratio: 7.06% vs. 73.73%, respectively). In conclusion, Pak1-null mice develop LV cardiac hypertrophy due to increased Erk1 activation, indicating a role for Pak1 as a natural inhibitor of Erk1 and a novel anti-hypertrophic signaling molecule.