ERK3 Is Involved in Regulating Cardiac Fibroblast Function
ERK3/MAPK6, an atypical MAPK, activates MAP kinase-activated protein kinase (MK)-5 in selected cell types. MK5 haplodeficient mice show reduced hypertrophy and attenuated increase in Col1a1 mRNA in response to increased cardiac afterload. In addition, MK5 deficiency alters cardiac fibroblast function. This study was to determine the effect of reduced ERK3 on cardiac hypertrophy following transverse aortic constriction (TAC) and fibroblast biology. Three wk post-surgery, ERK3, but not ERK4 or p38, was co-immunoprecipitated with MK5 from both sham and TAC heart lysates. The increase in left ventricular mass and myocyte diameter was lower in TAC-ERK3+/- than TAC-ERK3+/+ hearts, whereas ERK3 haploinsufficiency did not alter systolic or diastolic function. Furthermore, the TAC-induced increase in Col1a1 mRNA abundance was diminished in ERK3+/- hearts. ERK3 immunoreactivity was detected in atrial and ventricular fibroblasts but not myocytes. In both quiescent fibroblasts and activated myofibroblasts isolated from adult mouse heart, siRNA-mediated knockdown of ERK3 reduced the TGF-{beta}-induced increase in Col1a1 mRNA. In addition, intracellular type 1 collagen immunoreactivity was reduced following ERK3 depletion in quiescent fibroblasts but not myofibroblasts. Finally, knocking down ERK3 impaired motility in both atrial and ventricular myofibroblasts. These results suggest that ERK3 plays an important role in multiple aspects of cardiac fibroblast biology.