Acidic fibroblast growth factor or endothelin-1 stimulate the MAP kinase cascade in cardiac myocytes.

Abstract

The mitogen-activated protein kinases (MAPK) (p42'pk, p44' P' and others [l]) are a family of Ser/Thr protein kinases that are activated in response to a variety of physiological stimuli (for a review see [2]). The activation of MAPKs results from the phosphorylation of both Tyr and Thr residues and this prompted the suggestion that MAPK may integrate the pathways from Ser/Thr protein kinase cascades and Tyr protein kinase cascades [3]. More recently a MAPK activator, now termed M A P kinase kinase (MAPKK), has been identified as a dual-specificity kinase that phosphorylates both the regulatory residues [4]. In the present study we investigated the activation of MAPK cascade by two agonists, endothelin-1 (ET-1) and acidic fibroblast growth factor (aFGF), that act via apparently different signalling pathways but produce responses in cultured cardiac myocytes that are typical of hypertrophy IS]. Cardiac myocytes were isolated from the hearts of 1to 3-day-old rats, maintained in culture for 7 d and exposed to ET-1 or aFGF in serum-free medium [6]. Where indicated, the cells were cultured in the presence of 1 pM phorbol ester for 24 h prior to exposure to either aFGF or ET-1 to downregulate cellular protein kinase C (PKC) isotypes. cytosolic extracts were prepared and the protein b a s e activity against myelin basic protein (MBP) was assayed [6]. ET-1 and aFGF stimulated MBPK activity in these extracts with maximal activation achieved at 5 min (2.9and 2.4-fold, respectively). Activity then fell rapidly and returned to control values within 90 min. Using immunoblot analysis with antisera specific for each PKC isotype (71, we showed that cultured neonatal cardiomyocytes express PKC-a, 6 , -E and - ._ ._ c 2 20