The role of the Grb2-p38 MAPK signaling pathway in cardiac hypertrophy and fibrosis

Role of 14-3-3-Mediated p38 Mitogen-Activated Protein Kinase Inhibition in Cardiac Myocyte Survival

14-3-3 family members are dimeric phosphoserine-binding proteins that regulate signal transduction, apoptotic, and checkpoint control pathways. Targeted expression of dominant-negative 14-3-3eta (DN-14-3-3) to murine postnatal cardiac tissue potentiates Ask1, c-jun N-terminal kinase (JNK), and p38 mitogen-activated protein kinase (MAPK) activation. DN-14-3-3 mice are unable to compensate for pressure overload, which results in increased mortality, dilated cardiomyopathy, and cardiac myocyte apoptosis. To evaluate the relative role of p38 MAPK activity in the DN-14-3-3 phenotype, we inhibited cardiac p38 MAPK activity by pharmacological and genetic methods. Intraperitoneal injection of SB202190, an inhibitor of p38alpha and p38beta MAPK activity, markedly increased the ability of DN-14-3-3 mice to compensate for pressure overload, with decreased mortality. DN-14-3-3 mice were bred with transgenic mice in which dominant-negative p38alpha (DN-p38alpha) or dominant-negative p38beta (DN-p38beta) MAPK expression was targeted to the heart. Compound transgenic DN-14-3-3/DN-p38beta mice, and to a lesser extent compound transgenic DN-14-3-3/DN-p38alpha mice, exhibited reduced mortality and cardiac myocyte apoptosis in response to pressure overload, demonstrating that DN-14-3-3 promotes cardiac apoptosis due to stimulation of p38 MAPK activity.

P38 alpha mitogen-activated protein kinase sensitizes cells to apoptosis induced by different stimuli

p38 alpha mitogen-activated protein (MAP) kinase is a broadly expressed signaling molecule that participates in the regulation of cellular responses to stress as well as in the control of proliferation and survival of many cell types. We have used cell lines derived from p38 alpha knockout mice to study the role of this signaling pathway in the regulation of apoptosis. Here, we show that cardiomyocytes and fibroblasts lacking p38 alpha are more resistant to apoptosis induced by different stimuli. The reduced apoptosis of p38 alpha-deficient cells correlates with decreased expression of the mitochondrial proapoptotic protein Bax and the apoptosis-inducing receptor Fas/CD-95. Cells lacking p38 alpha also have increased extracellular signal-regulated kinase (ERKs) MAP kinase activity, and the up-regulation of this survival pathway seems to be at least partially responsible for the reduced levels of apoptosis in the absence of p38 alpha. Phosphorylation of the transcription factor STAT3 on Ser-727, mediated by the extracellular signal-regulated kinase MAP kinase pathway, may contribute to the decrease in both Bax and Fas expression in p38 alpha-/- cells. Thus, p38 alpha seems to sensitize cells to apoptosis via both up-regulation of proapoptotic proteins and down-regulation of survival pathways.

Grb2 is required for the development of neointima in response to vascular injury

OBJECTIVE

Neointima formation occurs in arteries in response to mechanical or chemical injury and is responsible for substantial morbidity. In this work, the role of the intracellular linker protein Grb2 in the pathogenesis of neointima formation was examined. Grb2 is a critical signaling protein that facilitates the activation of the small GTPase ras by receptor tyrosine kinases.

METHODS AND RESULTS

Cultured rat aortic smooth muscle cells were treated with an antisense morpholino to Grb2 and these cells showed a reduced proliferative response to platelet-derived growth factor stimulation. Grb2-/- mice do not survive embryonic development. Grb2+/- mice appear normal at birth and are fertile but have defective signaling in several tissues. Cultured smooth muscle cells derived from Grb2+/- mice grew at a much slower rate than cells derived from Grb2+/+ mice. Grb2+/- and Grb2+/+ mice were subjected to carotid injury. After 21 days, Grb2+/+ mice developed robust neointima formation that, in some cases, resulted in an occlusive lesion. In contrast, Grb2+/- mice were resistant to the development of neointima

CONCLUSIONS

Grb2 is an essential component of the signaling cascade resulting in neointima formation after arterial injury.

Phosphoinositide 3-kinase(p110alpha) plays a critical role for the induction of physiological, but not pathological, cardiac hypertrophy

An unresolved question in cardiac biology is whether distinct signaling pathways are responsible for the development of pathological and physiological cardiac hypertrophy in the adult. Physiological hypertrophy is characterized by a normal organization of cardiac structure and normal or enhanced cardiac function, whereas pathological hypertrophy is associated with an altered pattern of cardiac gene expression, fibrosis, cardiac dysfunction, and increased morbidity and mortality. The elucidation of signaling cascades that play distinct roles in these two forms of hypertrophy will be critical for the development of more effective strategies to treat heart failure. We examined the role of the p110alpha isoform of phosphoinositide 3-kinase (PI3K) for the induction of pathological hypertrophy (pressure overload-induced) and physiological hypertrophy (exercise-induced) by using transgenic mice expressing a dominant negative (dn) PI3K(p110alpha) mutant specifically in the heart. dnPI3K transgenic mice displayed significant hypertrophy in response to pressure overload but not exercise training. dnPI3K transgenic mice also showed significant dilation and cardiac dysfunction in response to pressure overload. Thus, PI3K(p110alpha) appears to play a critical role for the induction of physiological cardiac growth but not pathological growth. PI3K(p110alpha) also appears essential for maintaining contractile function in response to pathological stimuli.