| Our hypothesis was that cardiac expression of tyrosine kinases and related kinases might contribute to cardiac-specific changes in gene expression, leading to development of the hypertrophic phenotype in cardiac myocytes. cDNA fragments of the mixed lineage kinase MUK/DLK, and VEGFR-2 were recovered from PCR screen of TGF-β-stimulated neonatal rat cardiac myocytes, suggesting expression of these genes in primary cultures. VEGF demonstrated hypertrophic effects on primary cardiac-derived fibroblasts and cardiac-derived transformed fibroblasts, but not on cardiac myocytes. MUK/DLK steady-state mRNA is expressed under basal conditions in adult rat kidney and brain, but not in rat heart. MUK/DLK is induced in vivo by coronary artery ligation, and after the onset of hypertension in SHR rats. Expression of MUK/DLK is induced in neonatal rat cardiac myocyte cultures by addition of β-adrenergic agonists, norepinephrine and isoproterenol, by TGF-β and T3, and 8-bromo-cAMP, whereas α-adrenergic agonists do not induce expression of MUK/DLK. Expression levels of MUK/DLK are enriched in high-density cardiac myocytes. Forced expression of the full-length cDNA for MUK/DLK into neonatal rat cardiac myocytes potentiates the induction of beta-myosin heavy chain CAT with isoproterenol treatment five-fold. The addition of a kinase-deficient mutant of MUK/DLK did not potentiate this induction, suggesting the necessity of its functional kinase domain for activation of this surrogate endpoint for cardiac hypertrophy. Thus, MUK/DLK may act as an amplification point for beta-adrenergic-dependent hypertrophic signaling in rat myocardium. |
