Transcript levels for extracellular matrix proteins are altered in MK5-deficient cardiac ventricular fibroblasts

MK5 is a protein serine/threonine kinase activated by p38 MAPK and the atypical MAPKs ERK3 and ERK4. Although little is known of the physiological role of MK5 in the heart, both hypertrophic growth and the increase in collagen 1-α₁ mRNA induced by increased afterload are attenuated in hearts of MK5 haploinsufficient (MK5^+/-) mice. MK5 transcripts are detected at high levels in the left ventricular myocardium; however, MK5 immunoreactivity is detected in adult cardiac fibroblasts, but not myocytes. The present study was to determine if MK5 has a potential role in remodeling of the extracellular matrix. Ventricular fibroblasts were isolated from MK5^+/+, MK5^+/-, or MK5^-/- mice and maintained in culture on either compliant (8 kPa) or rigid substrates to obtain quiescent fibroblasts or activated myofibroblasts, respectively. In quiescent fibroblasts, reduced MK5 had little effect: BMP7 and TGF-β1 mRNA was increased in MK5^+/- and MK5^-/-.cells, respectively. Ang-II altered the abundance of numerous transcripts in an MK5-sensitive manner. Both collagen 1-α₁ mRNA and secreted type 1 collagen immunoreactivity were increased by Ang-II in wild type but not MK5-deficient fibroblasts. The effects of deleting MK5 were quite different in myofibroblasts: both the abundance of collagen 1-α₁ mRNA and secreted type 1 collagen immunoreactivity elevated in the absence of added Ang-II and addition of Ang-II failed to evoke a further increase in either. In addition, whereas type I collagen immunoreactivity was distributed throughout the cytosol of wild-type myofibroblasts, it was perinuclear in MK5^-/- myofibroblasts. Furthermore, in MK5-deficient myofibroblasts the abundance of collagen 3-α₂, Timp3, Smad 6, Smad 7, TGF-β3, and snail homolog 1 transcripts was increased whereas integrin β3, latent TGF-β binding protein 1, thrombospondin 1, hepatocyte growth factor, and interleukin 13 were decreased. Finally, fibroblast contraction was decreased upon knocking down MK5. These results indicate that MK5 may be involved in fibroblast-mediated regulation of extracellular matrix homeostasis.