MK5 and ERK3 play overlapping but distinct roles in regulating cardiac fibroblast function

Abstract

Background and objectivesImmunoreactivity for MAP kinase‐activated protein kinase‐5 (MK5), a protein serine/threonine kinase, is detected in cardiac ventricular fibroblasts but not myocytes. p38 and ERK3/4 MAP kinases are activators of MK5. However, the regulation of MK5 remains controversial and its physiological functions are poorly understood. Following hemodynamic overload, the increase in collagen mRNA was attenuated in ERK3 and MK5 haploinsufficient mice. In addition, following myocardial infarction, scar size and collagen content were reduced in MK5+/− mice, compared with MK5+/+ mice. In addition, MK5−/− fibroblasts show reduced motility and proliferation. Thus, MK5 may play a role in fibroblast function. The present study was to examine the role of ERK3, a putative activator of MK5, in cardiac fibroblast function.MethodsCardiac ventricular fibroblasts were isolated from MK5+/+ mice and male Sprague‐Dawley rats. Subconfluent cultures of fibroblasts from passages 2 and 3 were used. Ventricular myocytes were prepared from adult MK5+/+ mice. Protein expression was determined by immunoblotting. The presence of ERK3‐MK5 complexes was determined by proximity ligation assay (PLA) and co‐immunoprecipitation assays. Cell motility and myofibroblast contraction were studied by scratch‐wound and collagen gel contraction assays, respectively.ResultssiRNA‐mediated knockdown of MK5 (MK5‐kd) resulted in reduced contraction of collagen gels, reduced cell spreading, and formation of fewer dendritic extensions compared to fibroblasts transfected with scrambled RNA. ERK3 immunoreactivity was detected in fibroblasts but negligible in cardiomyocytes. MK5 immunoprecipitates from fibroblast lysates contained ERK3 immunoreactivity. Proximity ligation assays revealed ERK3‐MK5 complexes in the cytoplasm, which were less abundant following knockdown of MK5. ERK3‐MK5 complexes were not observed in the nucleus. Cell migration, in response to serum and/or angiotensin II, was reduced upon siRNA‐mediated knockdown of ERK3. However, whereas the secretion of type 1 collagen and fibronectin was increased in MK5−/− or MK5‐kd cells relative to MK5+/+ fibroblasts, knocking down ERK3 did not affect collagen secretion.ConclusionMK5 and ERK3 immunoreactivity was detected in fibroblasts but not myocytes. Co‐immunoprecipitation and PLA suggest that ERK3 and MK5 form cytoplasmic complexes in cardiac fibroblasts. Suppressing ERK3 or MK5 expression decreased cell motility; however, type 1 collagen secretion was enhanced by reduced MK5 expression but unaffected by knockdown of ERK3. Hence, MK5 and ERK3 play overlapping but distinct roles in regulating cardiac fibroblast function.Support or Funding InformationThis study was supported by a grant from the Heart and Stroke Foundation of Canada.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.