Tissue distribution and functional expression of a cDNA encoding a novel mixed lineage kinase

Hypertrophy is an adaptive response of the heart to myocardial injury or he modynamic overload that may progress and contribute to cardiac decompensati on and eventually to heart failure. The signaling pathways controlling this response in the cardiac myocyte are poorly understood. A data mining effor t of a human failed heart cDNA library was undertaken in an effort to ident ify novel signaling molecules involved in cardiac hypertrophy. This effort identified a novel kinase (MLK7) homologous to the mixed lineage kinase fam ily of proteins. The mixed lineage kinases are mitogen-activated protein ki nase kinase kinases (MAPKKKs) which activate stress activated protein kinas e/c-Jun N-terminal kinase (SAPK/JNK) and p38 kinase pathways. They contain a catalytic domain with homology to both serine/threonine and tyrosine-spec ific kinases and a dual leucine zipper. MLK7 is identical to leucine zipper and sterile-alpha motif protein kinase (ZAK) through the leucine zipper do main but has a completely divergent COOH-terminus and shares approximately 40% homology with the other MLKs overall. Expression of MLK7 mRNA is most a bundant in skeletal muscle and heart, with expression restricted to the car diac myocyte. The recombinant histidine tagged MLK7 expressed and purified from insect cells exhibited serine/ threonine kinase activity in vitro with myelin basic protein as substrate. When expressed in cardiac myocytes, MLK 7 activated SAPK/JNK1, and ERK and p38 to a lesser extent. Additionally. ML K7 altered fetal gene expression and increased protein synthesis in cardiac myocytes. These data suggest that MLK7 is a new member of the mixed lineag e kinase family that modulates cardiac SAPK/JNK pathway and may play a role in cardiac hypertrophy and progression to heart failure. (C) 2001 Academic Press.