The bone morphogenetic protein (BMP) pathway is a major signaling mechanism during cardiac development but it has no clear function in the post-natal heart. Here, we tested the hypothesis that BMP mediates the physiological effect of the cardiac chaperone H11Kinase/Hsp22 (H11K).
Expression of H11K increases during both cardiac ischemia and overload, and its cardiac-specific over-expression in a transgenic (TG) mouse is sufficient to provide major protection against ischemia and to promote cardiac cell growth, which involves the activation of phosphatidylinositol-3-kinase (PI3K) and of its effector Akt. We tested whether H11K-induced activation of PI3K is mediated by BMP. Microarray comparison between hearts from TG and wild type (WT) mice showed an up-regulation of the BMP receptor subunits Alk3 and BMPR-II, as well as of the BMP receptor ligand BMP4, which was confirmed at the protein level (P<0.01 vs WT). Activation of the BMP pathway in TG mice was confirmed by increased phosphorylation of the canonical BMP effectors Smad 1/5/8 (P<0.01 vs WT). The mechanism was further studied in isolated cardiac myocytes. Adeno-mediated over-expression of H11K was accompanied by significant 2–3-fold increase in PI3K activity, phospho-Akt, Smad 1/5/8 phosphorylation and cell growth as measured by [3H]phenylalanine incorporation, and by a 70% reduction in H2O2-mediated apoptosis (all values, P<0.01 vs control). All these changes mediated by H11K in myocytes were abolished upon addition of the BMP antagonist noggin. In pull-down experiments, H11K co-precipitated with both Alk3 and BMPR-II, and increased the association of these two subunits into a functional receptor. Accordingly, Smad 1/5/8 phosphorylation in presence of BMP4 was enhanced by 5-fold upon H11K over-expression, whereas it was decreased by 3-fold upon H11K knockdown (both, P<0.01 vs control), which shows that H11K potentiates the BMP receptor signaling pathway. Therefore, potentiation of the BMP receptor pathway by H11K promotes the activation of the PI3K/Akt pathway and dictates the physiological effects of H11K on cardiac cell growth and survival, which shows a novel role for BMP signaling in post-natal heart.
This research has received full or partial funding support from the American Heart Association, AHA National Center.
Protein turnover in cardiac cell growth and survival
