MITOGEN-ACTIVATED PROTEIN-KINASE PHOSPHATASE-1 INHIBITS THE STIMULATION OF GENE-EXPRESSION BY HYPERTROPHIC AGONISTS IN CARDIAC MYOCYTES

The effect of constitutive expression of mitogen-activated protein kin ase (MAPK) phosphatase 1 (MKP-1) on gene expression in response to hyp ertrophic agonists was examined in cultured neonatal rat ventricular m yocytes. Luciferase (LUX) reporter genes linked to promoters for atria l natriuretic factor, ventricular myosin light chain 2, beta-myosin he avy chain, skeletal muscle alpha-actin (SkM alpha-actin) and serum res ponse element-regulated c-fos (c-fos-SRE) were transfected into cardio myocytes. Phenylephrine (PE; 10 mu M), phorbol 12-myristate 13-acetate (1 mu M) and endothelin 1 (10 nM) stimulated the expression of these various reporter genes by 2.5-20-fold. MKP-1 inhibited these effects b y 60-85%. In contrast, MKP-1 had no effect on the expression of a cons titutively active Rous sarcoma virus-LUX reporter gene. A catalyticall y inactive mutant MKP-1CS (cysteine-->serine mutation) and the broad-s pecificity protein tyrosine phosphatase 1B (PTP-1B) had no significant effect on any reporter gene tested. MKP-1 had much less effect on the morphological features accompanying agonist-induced cardiac hypertrop hy. PE(10 mu M) increased myocyte area by 59% but this effect was only decreased by one-third by MKP-1 and was also partly decreased (by 25% ) by expression of PTP-1B. PE also altered cell shape but this was una ffected by MKP-1. There was also no clear effect of MKP-1 on the organ ization of the contractile apparatus into sarcomeric structures in the presence of 10 mu M PE. We conclude that the transcriptional response s accompanying cardiac myocyte hypertrophy are dependent on an MKP-1-s ensitive step, presumably the activation of one or members of the MAPK family, but that cell size, shape and myofibrillar organization are m uch less sensitive to inhibition by MKP-1.