Cardiotrophin-1 activates a distinct form of cardiac muscle cell hypertrophy: Assembly of sarcomeric units in series via gp130/leukemia inhibitory factor receptor-dependent pathways

Cardiotrophin-1 (CT-1) was recently isolated by expression cloning based on its ability to induce an increase in cell size in neonatal rat ventricular cardiomyocytes. Sequence similarity data suggested that CT-1 is a novel member of a family of structurally related cytokines sharing the receptor component gp130. The present study documents that gp130 is required for CT-1 signaling in cardiomyocytes, by demonstrating that a monoclonal anti-gp130 antibody completely inhibits c-fos induction by CT-1. Similarly, a leukemia inhibitory factor receptor subunit β (LIFRβ) antagonist effectively blocks the CT-1 induction of c-fos, indicating a requirement for LIFRβ in the hypertrophic response, as well. Upon stimulation with CT-1, both gp130 and the LIFRβ are tyrosine-phosphorylated, providing further evidence that CT-1 signals through the gp130/LIFRβ heterodimer in cardiomyocytes. CT-1 induces a hypertrophic response in cardiomyocytes that is distinct from the phenotype seen after α-adrenergic stimulation, both with regard to cell morphology and gene expression pattern. Stimulation with CT-1 results in an increase in cardiac cell size that is characterized by an increase in cell length but no significant change in cell width. Confocal laser microscopy of CT-1 stimulated cells reveals the assembly of sarcomeric units in series rather than in parallel, as seen after α-adrenergic stimulation. CT-1 induces a distinct pattern of immediate early genes, and up-regulates the atrial natriuretic factor (ANF) gene, but does not affect skeletal α-actin or myosin light chain-2v expression. As evidenced by nuclear run-on transcription assays, both CT-1 and α-adrenergic stimulation lead to an increase in ANF gene transcription. Transient transfection analyses document that, in contrast to α-adrenergic stimulation, the CT-1 responsive cis-regulatory elements are located outside of the proximal 3 kilobase pairs of the ANF 5′-flanking region. These studies indicate that CT-1 can activate a distinct form of myocardial cell hypertrophy, characterized by the promotion of sarcomere assembly in series, via gp13/LIFRβ-dependent signaling pathways.