Hiroshi Akazawa

p53-induced inhibition of Hif-1 causes cardiac dysfunction during pressure overload

Cardiac hypertrophy occurs as an adaptive response to increased workload to maintain cardiac function. However, prolonged cardiac hypertrophy causes heart failure, and its mechanisms are largely unknown. Here we show that cardiac angiogenesis is crucially involved in the adaptive mechanism of cardiac hypertrophy and that p53 accumulation is essential for the transition from cardiac hypertrophy to heart failure. Pressure overload initially promoted vascular growth in the heart by hypoxia-inducible factor-1 (Hif-1)-dependent induction of angiogenic factors, and inhibition of angiogenesis prevented the development of cardiac hypertrophy and induced systolic dysfunction. Sustained pressure overload induced an accumulation of p53 that inhibited Hif-1 activity and thereby impaired cardiac angiogenesis and systolic function. Conversely, promoting cardiac angiogenesis by introducing angiogenic factors or by inhibiting p53 accumulation developed hypertrophy further and restored cardiac dysfunction under chronic pressure overload. These results indicate that the anti-angiogenic property of p53 may have a crucial function in the transition from cardiac hypertrophy to heart failure.

G-CSF prevents cardiac remodeling after myocardial infarction by activating the Jak-Stat pathway in cardiomyocytes

Granulocyte colony-stimulating factor (G-CSF) was reported to induce myocardial regeneration by promoting mobilization of bone marrow stem cells to the injured heart after myocardial infarction, but the precise mechanisms of the beneficial effects of G-CSF are not fully understood. Here we show that G-CSF acts directly on cardiomyocytes and promotes their survival after myocardial infarction. G-CSF receptor was expressed on cardiomyocytes and G-CSF activated the Jak/Stat pathway in cardiomyocytes. The G-CSF treatment did not affect initial infarct size at 3 d but improved cardiac function as early as 1 week after myocardial infarction. Moreover, the beneficial effects of G-CSF on cardiac function were reduced by delayed start of the treatment. G-CSF induced antiapoptotic proteins and inhibited apoptotic death of cardiomyocytes in the infarcted hearts. G-CSF also reduced apoptosis of endothelial cells and increased vascularization in the infarcted hearts, further protecting against ischemic injury. All these effects of G-CSF on infarcted hearts were abolished by overexpression of a dominant-negative mutant Stat3 protein in cardiomyocytes. These results suggest that G-CSF promotes survival of cardiac myocytes and prevents left ventricular remodeling after myocardial infarction through the functional communication between cardiomyocytes and noncardiomyocytes.

Mechanical stress activates angiotensin II type 1 receptor without the involvement of angiotensin II

The angiotensin II type 1 (AT1) receptor has a crucial role in load-induced cardiac hypertrophy. Here we show that the AT1 receptor can be activated by mechanical stress through an angiotensin-II-independent mechanism. Without the involvement of angiotensin II, mechanical stress not only activates extracellular-signal-regulated kinases and increases phosphoinositide production in vitro, but also induces cardiac hypertrophy in vivo. Mechanical stretch induces association of the AT1 receptor with Janus kinase 2, and translocation of G proteins into the cytosol. All of these events are inhibited by the AT1 receptor blocker candesartan. Thus, mechanical stress activates AT1 receptor independently of angiotensin II, and this activation can be inhibited by an inverse agonist of the AT1 receptor.

Developmental stage-specific biphasic roles of Wnt/β-catenin signaling in cardiomyogenesis and hematopoiesis

Although Wingless (Wg)/Wnt signaling has been implicated in heart development of multiple organisms, conflicting results have been reported regarding the role of Wnt/β-catenin pathway in cardiac myogenesis: Wg/armadillo signaling promotes heart development in Drosophila, whereas activation of Wnt/β-catenin signaling inhibits heart formation in avians and amphibians. Using an in vitro system of mouse ES cell differentiation into cardiomyocytes, we show here that Wnt/β-catenin signaling exhibits developmental stage-specific, biphasic, and antagonistic effects on cardiomyogenesis and hematopoiesis/vasculogenesis. Activation of the Wnt/β-catenin pathway in the early phase during embryoid body (EB) formation enhances ES cell differentiation into cardiomyocytes while suppressing the differentiation into hematopoietic and vascular cell lineages. In contrast, activation of Wnt/β-catenin signaling in the late phase after EB formation inhibits cardiomyocyte differentiation and enhances the expression of hematopoietic/vascular marker genes through suppression of bone morphogenetic protein signaling. Thus, Wnt/β-catenin signaling exhibits biphasic and antagonistic effects on cardiomyogenesis and hematopoiesis/vasculogenesis, depending on the stage of development.

Cardiac side population cells have a potential to migrate and differentiate into cardiomyocytes in vitro and in vivo

Side population (SP) cells, which can be identified by their ability to exclude Hoechst 33342 dye, are one of the candidates for somatic stem cells. Although bone marrow SP cells are known to be long-term repopulating hematopoietic stem cells, there is little information about the characteristics of cardiac SP cells (CSPs). When cultured CSPs from neonatal rat hearts were treated with oxytocin or trichostatin A, some CSPs expressed cardiac-specific genes and proteins and showed spontaneous beating. When green fluorescent protein–positive CSPs were intravenously infused into adult rats, many more (∼12-fold) CSPs were migrated and homed in injured heart than in normal heart. CSPs in injured heart differentiated into cardiomyocytes, endothelial cells, or smooth muscle cells (4.4%, 6.7%, and 29% of total CSP-derived cells, respectively). These results suggest that CSPs are intrinsic cardiac stem cells and involved in the regeneration of diseased hearts.

Roles of Cardiac Transcription Factors in Cardiac Hypertrophy

Different cell types, equipped with unique structure and function, synthesize different sets of proteins on the basis of different patterns of gene expression, even though their genomes are identical. Cardiac transcription factors have been reported to control a cardiac gene program and thus to play a crucial role in transcriptional regulation during embryogenesis. Recently, postnatal roles of cardiac transcription factors have been extensively investigated. Consistent with the direct transactivation of numerous cardiac genes reactivated in response to hypertrophic stimulation, cardiac transcription factors are profoundly involved in the generation of cardiac hypertrophy or in cardioprotection from cytotoxic stress in the adult heart. In this review, the regulation of a cardiac gene program by cardiac transcription factors is summarized, with an emphasis on their potential role in the generation of cardiac hypertrophy.

Cytokine therapy prevents left ventricular remodeling and dysfunction after myocardial infarction through neovascularization

Pretreatment with a combination of granulocyte colony‐stimulating factor (G‐CSF) and stem cell factor (SCF) has been reported to attenuate left ventricular (LV) remodeling after acute myocardial infarction (MI). We here examined whether the cytokine treatment started after MI has also beneficial effects. Anterior MI was created in the recipient mice whose bone marrow had been replaced with that of transgenic mice expressing enhanced green fluorescent protein (GFP). We categorized mice into five groups according to the following treatment: 1) saline; 2) administration of G‐CSF and SCF from 5 days before MI through 3 days after; 3) administration of G‐CSF and SCF for 5 days after MI; 4) administration of G‐CSF alone for 5 days after MI; 5) administration of SCF alone for 5 days after MI. All the three treatment groups with G‐CSF showed less LV remodeling and improved cardiac function and survival rate after MI. The number of capillaries, which express GFP, was increased and the number of apoptotic cells was decreased in the border area of all the treatment groups with G‐CSF. Even if the cytokine treatment is started after MI, it could prevent LV remodeling and dysfunction after MI—at least in part—through an increase in neovascularization and a decrease in apoptosis in the border area.

Oxidative stress-induced signal transduction pathways in cardiac myocytes: involvement of ROS in heart diseases.

Reactive oxygen species (ROS) are proposed to contribute to the deterioration of cardiac function in patients with heart diseases. It has been reported that ROS are increased in the failing heart and involved in atherosclerosis, myocardial ischemia/reperfusion injury, and heart failure. Antioxidant enzymes are decreased in the decompensated heart, depressing defense mechanisms against oxidative stress. A variety of proteins, including receptors, ionic channels, transporters, and components of signal transduction pathways, are substrates of oxidation by ROS. ROS also function as signal transduction intermediates to induce transcription factor activation, gene expression, cell growth, and apoptosis. Recently, the upstream and downstream molecules of ROS in signal transduction pathways have been the subjects of intense investigation. These molecules include the mitogen-activated protein kinase family, the Rho family of small GTP binding proteins, the Src family of tyrosine kinases, Ras, and cytokines. The modulation of oxidative stress-induced signaling pathways is effective for preventing the progression of heart diseases.

Critical Roles of Muscle-Secreted Angiogenic Factors in Therapeutic Neovascularization

The discovery of bone marrow–derived endothelial progenitors in the peripheral blood has promoted intensive studies on the potential of cell therapy for various human diseases. Accumulating evidence has suggested that implantation of bone marrow mononuclear cells effectively promotes neovascularization in ischemic tissues. It has also been reported that the implanted cells are incorporated not only into the newly formed vessels but also secrete angiogenic factors. However, the mechanism by which cell therapy improves tissue ischemia remains obscure. We enrolled 29 “no-option” patients with critical limb ischemia and treated ischemic limbs by implantation of peripheral mononuclear cells. Cell therapy using peripheral mononuclear cells was very effective for the treatment of limb ischemia, and its efficacy was associated with increases in the plasma levels of angiogenic factors, in particular interleukin-1&bgr; (IL-1&bgr;). We then examined an experimental model of limb ischemia using IL-1&bgr;–deficient mice. Implantation of IL-1&bgr;–deficient mononuclear cells improved tissue ischemia as efficiently as that of wild-type cells. Both wild-type and IL-1&bgr;–deficient mononuclear cells increased expression of IL-1&bgr; and thus induced angiogenic factors in muscle cells of ischemic limbs to a similar extent. In contrast, inability of muscle cells to secrete IL-1&bgr; markedly reduces induction of angiogenic factors and impairs neovascularization by cell implantation. Implanted cells do not secret angiogenic factors sufficient for neovascularization but, instead, stimulate muscle cells to produce angiogenic factors, thereby promoting neovascularization in ischemic tissues. Further studies will allow us to develop more effective treatments for ischemic vascular disease.

Complement C1q Activates Canonical Wnt Signaling and Promotes Aging-Related Phenotypes

Wnt signaling plays critical roles in development of various organs and pathogenesis of many diseases, and augmented Wnt signaling has recently been implicated in mammalian aging and aging-related phenotypes. We here report that complement C1q activates canonical Wnt signaling and promotes aging-associated decline in tissue regeneration. Serum C1q concentration is increased with aging, and Wnt signaling activity is augmented during aging in the serum and in multiple tissues of wild-type mice, but not in those of C1qa-deficient mice. C1q activates canonical Wnt signaling by binding to Frizzled receptors and subsequently inducing C1s-dependent cleavage of the ectodomain of Wnt coreceptor low-density lipoprotein receptor-related protein 6. Skeletal muscle regeneration in young mice is inhibited by exogenous C1q treatment, whereas aging-associated impairment of muscle regeneration is restored by C1s inhibition or C1qa gene disruption. Our findings therefore suggest the unexpected role of complement C1q in Wnt signal transduction and modulation of mammalian aging.