Beta-adrenergic receptor-mediated DNA synthesis in neonatal rat cardiac fibroblasts proceeds via a phosphatidylinositol 3-kinase dependent pathway refractory to the antiproliferative action of cyclic AMP

Nestin identifies a subpopulation of rat ventricular fibroblasts and participates in cell migration

Fibroblast progenitor cells migrate to the endocardial region during cardiogenesis, and the migration of ventricular fibroblasts to the ischemically damaged region of the infarcted adult heart is a seminal event of reparative fibrosis. The intermediate filament protein nestin is implicated in cell migration and expression identified in a subpopulation of scar-derived myofibroblasts. The present study tested the hypothesis that fibroblast progenitor cells express nestin, and the intermediate filament protein drives the migratory phenotype of ventricular fibroblasts. Transcription factor 21 (Tcf21)- and Wilms tumor 1 (WT1)-fibroblast progenitor cells identified in the epicardial/endocardial regions of the E12.5- to E13.5-day embryonic mouse heart predominantly expressed nestin. Nuclear Tcf21/WT1 staining was identified in neonatal rat ventricular fibroblasts (NNVFbs), and a subpopulation coexpressed nestin. Nuclear Tcf21/WT1 expression persisted in adult rat ventricular fibroblasts, whereas nestin protein levels were downregulated. Nestin-expressing NNVFbs exhibited a unique phenotype as the subpopulation was refractory to cell cycle reentry in response to selective stimuli. Nestin(-)- and nestin(+)-scar-derived rat myofibroblasts plated in Matrigel unmasked a migratory phenotype characterized by the de novo formation of lumen-like structures. The elongated membrane projections emanating from scar myofibroblasts delineating the boundary of lumen-like structures expressed nestin. Lentiviral short-hairpin RNA (shRNA)-mediated nestin depletion inhibited the in vitro migratory response of NNVFbs as the wound radius was significantly larger compared with NNVFbs infected with the empty lentivirus. Thus, nestin represents a marker of embryonic Tcf21/WT1(+)-fibroblast progenitor cells. The neonatal rat heart contains a distinct subpopulation of nestin-immunoreactive Tcf21/WT1(+) fibroblasts refractory to cell cycle reentry, and the intermediate filament protein may preferentially facilitate ventricular fibroblast migration during physiological/pathological remodeling.NEW & NOTEWORTHY Tcf21/WT1(+)-fibroblast progenitor cells of the embryonic mouse heart predominantly express the intermediate filament protein nestin. A subpopulation of Tcf21/WT1(+)-neonatal rat ventricular fibroblasts express nestin and are refractory to selective stimuli influencing cell cycle reentry. Scar-derived myofibroblasts plated in Matrigel elicit the formation of lumen-like structures characterized by the appearance of nestin(+)-membrane projections. Lentiviral shRNA-mediated nestin depletion in a subpopulation of neonatal rat ventricular fibroblasts suppressed the migratory response following the in vitro scratch assay.

Calcitriol Suppressed Isoproterenol-induced Proliferation of Cardiac Fibroblasts via Integrin β3/FAK/Akt Pathway

OBJECTIVE

Cardiac fibroblasts (CFs) proliferation and extracellular matrix deposition are important features of cardiac fibrosis. Various studies have indicated that vitamin D displays an anti-fibrotic property in chronic heart diseases. This study explored the role of vitamin D in the growth of CFs via an integrin signaling pathway.

METHODS

MTT and 5-ethynyl-2'-deoxyuridine assays were performed to determine cell viability. Western blotting was performed to detect the expression of proliferating cell nuclear antigen (PCNA) and integrin signaling pathway. The fibronectin was observed by ELISA. Immunohistochemical staining was employed to evaluate the expression of integrin β3.

RESULTS

The PCNA expression in the CFs was enhanced after isoproterenol (ISO) stimulation accompanied by an elevated expression of integrin beta-3 (β3). The blockade of the integrin β3 with a specific integrin β3 antibody reduced the PCNA expression induced by the ISO. Decreasing the integrin β3 by siRNA reduced the ISO-triggered phosphorylation of FAK and Akt. Both the FAK inhibitor and Akt inhibitor suppressed the PCNA expression induced by the ISO in the CFs. Calcitriol (CAL), an active form of vitamin D, attenuated the ISO-induced CFs proliferation by downregulating the integrin β3 expression, and phosphorylation of FAK and Akt. Moreover, CAL reduced the increased levels of fibronectin and hydroxyproline in the CFs culture medium triggered by the ISO. The administration of calcitriol decreased the integrin β3 expression in the ISO-induced myocardial injury model.

CONCLUSION

These findings revealed a novel role for CAL in suppressing the CFs growth by the downregulation of the integrin β3/FAK/Akt pathway.

GDH promotes isoprenaline-induced cardiac hypertrophy by activating mTOR signaling via elevation of α-ketoglutarate level

Numerous studies reveal that metabolism dysfunction contributes to the development of pathological cardiac hypertrophy. While the abnormal lipid and glucose utilization in cardiomyocytes responding to hypertrophic stimuli have been extensively studied, the alteration and implication of glutaminolysis are rarely discussed. In the present work, we provide the first evidence that glutamate dehydrogenase (GDH), an enzyme that catalyzes conversion of glutamate into ɑ-ketoglutarate (AKG), participates in isoprenaline (ISO)-induced cardiac hypertrophy through activating mammalian target of rapamycin (mTOR) signaling. The expression and activity of GDH were enhanced in cultured cardiomyocytes and rat hearts following ISO treatment. Overexpression of GDH, but not its enzymatically inactive mutant, provoked cardiac hypertrophy. In contrast, GDH knockdown could relieve ISO-triggered hypertrophic responses. The intracellular AKG level was elevated by ISO or GDH overexpression, which led to increased phosphorylation of mTOR and downstream effector ribosomal protein S6 kinase (S6K). Exogenous supplement of AKG also resulted in mTOR activation and cardiomyocyte hypertrophy. However, incubation with rapamycin, an mTOR inhibitor, attenuated hypertrophic responses in cardiomyocytes. Furthermore, GDH silencing protected rats from ISO-induced cardiac hypertrophy. These findings give a further insight into the role of GDH in cardiac hypertrophy and suggest it as a potential target for hypertrophy-related cardiomyopathy.

Lipidated peptides derived from intracellular loops 2 and 3 of the urotensin II receptor act as biased allosteric ligands

Over the last decade, the urotensinergic system, composed of one G protein-coupled receptor and two endogenous ligands, has garnered significant attention as a promising new target for the treatment of various cardiovascular diseases. Indeed, this system is associated with various biomarkers of cardiovascular dysfunctions and is involved in changes in cardiac contractility, fibrosis and hypertrophy contributing, like the angiotensinergic system, to the pathogenesis and progression of heart failure. Significant investment has been made toward the development of clinically relevant UT ligands for therapeutic intervention, but with little or no success to date. This system therefore remains to be therapeutically exploited. Pepducins and other lipidated peptides have been used as both mechanistic probes and potential therapeutics; therefore, pepducins derived from the human urotensin II receptor might represent unique tools to generate signaling bias and study hUT signaling networks. Two hUT-derived pepducins, derived from the second and the third intracellular loop of the receptor (hUT-Pep2 and [Trp¹, Leu²]hUT-Pep3, respectively) were synthesized and pharmacologically characterized. Our results demonstrated that hUT-Pep2 and [Trp¹, Leu²]hUT-Pep3 acted as biased ago-allosteric modulators, triggered ERK_1/2 phosphorylation and to a lesser extent, IP₁ production and stimulated cell proliferation yet were devoid of contractile activity. Interestingly, both hUT-derived pepducins were able to modulate human urotensin II (hUII)- and urotensin II-related peptide (URP)-mediated contraction albeit to different extents. These new derivatives represent unique tools to reveal the intricacies of hUT signaling and also a novel avenue for the design of allosteric ligands selectively targeting hUT signaling potentially.

Cardiac Fibroblasts Mediate a Sexually Dimorphic Fibrotic Response to β-Adrenergic Stimulation

Background

Biological sex is an important modifier of cardiovascular disease and women generally have better outcomes compared with men. However, the contribution of cardiac fibroblasts (CFs) to this sexual dimorphism is relatively unexplored.

Methods and Results

Isoproterenol (ISO) was administered to rats as a model for chronic β‐adrenergic receptor (β‐AR)‐mediated cardiovascular disease. ISO‐treated males had higher mortality than females and also developed fibrosis whereas females did not. Gonadectomy did not abrogate this sex difference. To determine the cellular contribution to this phenotype, CFs were studied. CFs from both sexes had increased proliferation in vivo in response to ISO, but CFs from female hearts proliferated more than male cells. In addition, male CFs were significantly more activated to myofibroblasts by ISO. To investigate potential regulatory mechanisms for the sexually dimorphic fibrotic response, β‐AR mRNA and PKA (protein kinase A) activity were measured. In response to ISO treatment, male CFs increased expression of β1‐ and β2‐ARs, whereas expression of both receptors decreased in female CFs. Moreover, ISO‐treated male CFs had higher PKA activity relative to vehicle controls, whereas ISO did not activate PKA in female CFs.

Conclusions

Chronic in vivo β‐AR stimulation causes fibrosis in male but not female rat hearts. Male CFs are more activated than female CFs, consistent with elevated fibrosis in male rat hearts and may be caused by higher β‐AR expression and PKA activation in male CFs. Taken together, our data suggest that CFs play a substantial role in mediating sex differences observed after cardiac injury.

The Biological Role of Nestin(+)-Cells in Physiological and Pathological Cardiovascular Remodeling

The intermediate filament protein nestin was identified in diverse populations of cells implicated in cardiovascular remodeling. Cardiac resident neural progenitor/stem cells constitutively express nestin and following an ischemic insult migrate to the infarct region and participate in angiogenesis and neurogenesis. A modest number of normal adult ventricular fibroblasts express nestin and the intermediate filament protein is upregulated during the progression of reparative and reactive fibrosis. Nestin depletion attenuates cell cycle re-entry suggesting that increased expression of the intermediate filament protein in ventricular fibroblasts may represent an activated phenotype accelerating the biological impact during fibrosis. Nestin immunoreactivity is absent in normal adult rodent ventricular cardiomyocytes. Following ischemic damage, the intermediate filament protein is induced in a modest population of pre-existing adult ventricular cardiomyocytes bordering the peri-infarct/infarct region and nestin⁽⁺⁾-ventricular cardiomyocytes were identified in the infarcted human heart. The appearance of nestin⁽⁺⁾-ventricular cardiomyocytes post-myocardial infarction (MI) recapitulates an embryonic phenotype and depletion of the intermediate filament protein inhibits cell cycle re-entry. Recruitment of the serine/threonine kinase p38 MAPK secondary to an overt inflammatory response after an ischemic insult may represent a seminal event limiting the appearance of nestin⁽⁺⁾-ventricular cardiomyocytes and concomitantly suppressing cell cycle re-entry. Endothelial and vascular smooth muscle cells (VSMCs) express nestin and upregulation of the intermediate filament protein may directly contribute to vascular remodeling. This review will highlight the biological role of nestin⁽⁺⁾-cells during physiological and pathological remodeling of the heart and vasculature and discuss the phenotypic advantage attributed to the intermediate filament protein.

NPY1-36 and PYY1-36 activate cardiac fibroblasts: an effect enhanced by genetic hypertension and inhibition of dipeptidyl peptidase 4

Cardiac sympathetic nerves release neuropeptide Y (NPY)1-36, and peptide YY (PYY)1-36 is a circulating peptide; therefore, these PP-fold peptides could affect cardiac fibroblasts (CFs). We examined the effects of NPY1-36 and PYY1-36 on the proliferation of and collagen production ([(3)H]proline incorporation) by CFs isolated from Wistar-Kyoto (WKY) normotensive rats and spontaneously hypertensive rats (SHRs). Experiments were performed with and without sitagliptin, an inhibitor of dipeptidyl peptidase 4 [DPP4; an ectoenzyme that metabolizes NPY1-36 and PYY1-36 (Y1 receptor agonists) to NPY3-36 and PYY3-36 (inactive at Y1 receptors), respectively]. NPY1-36 and PYY1-36, but not NPY3-36 or PYY3-36, stimulated proliferation of CFs, and these effects were more potent than ANG II, enhanced by sitagliptin, blocked by BIBP3226 (Y1 receptor antagonist), and greater in SHR CFs. SHR CF membranes expressed more receptor for activated C kinase (RACK)1 [which scaffolds the Gi/phospholipase C (PLC)/PKC pathway] compared with WKY CF membranes. RACK1 knockdown (short hairpin RNA) and inhibition of Gi (pertussis toxin), PLC (U73122), and PKC (GF109203X) blocked the proliferative effects of NPY1-36. NPY1-36 and PYY1-36 stimulated collagen production more potently than did ANG II, and this was enhanced by sitagliptin and greater in SHR CFs. In conclusion, 1) NPY1-36 and PYY1-36, via the Y1 receptor/Gi/PLC/PKC pathway, activate CFs, and this pathway is enhanced in SHR CFs due to increased localization of RACK1 in membranes; and 2) DPP4 inhibition enhances the effects of NPY1-36 and PYY1-36 on CFs, likely by inhibiting the metabolism of NPY1-36 and PYY1-36. The implications are that endogenous NPY1-36 and PYY1-36 could adversely affect cardiac structure/function by activating CFs, and this may be exacerbated in genetic hypertension and by DPP4 inhibitors.

Dynamic mass redistribution analysis of endogenous β-adrenergic receptor signaling in neonatal rat cardiac fibroblasts

Label-free systems for the agnostic assessment of cellular responses to receptor stimulation have been shown to provide a sensitive method to dissect receptor signaling. β-adenergic receptors (βAR) are important regulators of normal and pathologic cardiac function and are expressed in cardiomyocytes as well as cardiac fibroblasts, where relatively fewer studies have explored their signaling responses. Using label-free whole cell dynamic mass redistribution (DMR) assays we investigated the response patterns to stimulation of endogenous βAR in primary neonatal rat cardiac fibroblasts (NRCF). The EPIC-BT by Corning was used to measure DMR responses in primary isolated NRCF treated with various βAR and EGFR ligands. Additional molecular assays for cAMP generation and receptor internalization responses were used to correlate the DMR findings with established βAR signaling pathways. Catecholamine stimulation of NRCF induced a concentration-dependent negative DMR deflection that was competitively blocked by βAR blockade and non-competitively blocked by irreversible uncoupling of Gs proteins. Subtype-selective βAR ligand profiling revealed a dominant role for β2AR in mediating the DMR responses, consistent with the relative expression levels of β2AR and β1AR in NRCF. βAR-mediated cAMP generation profiles revealed similar kinetics to DMR responses, each of which were enhanced via inhibition of cAMP degradation, as well as dynamin-mediated receptor internalization. Finally, G protein-independent βAR signaling through epidermal growth factor receptor (EGFR) was assessed, revealing a smaller but significant contribution of this pathway to the DMR response to βAR stimulation. Measurement of DMR responses in primary cardiac fibroblasts provides a sensitive readout for investigating endogenous βAR signaling via both G protein-dependent and –independent pathways.

The neurogenic response of cardiac resident nestin(+) cells was associated with GAP43 upregulation and abrogated in a setting of type I diabetes

Background

Cardiac nestin⁽⁺⁾ cells exhibit properties of a neural progenitor/stem cell population characterized by the de novo synthesis of neurofilament-M in response to ischemic injury and 6-hydroxydopamine administration. The induction of growth associated protein 43 (GAP43) was identified as an early event of neurogenesis. The present study tested the hypothesis that the de novo synthesis of neurofilament-M by nestin⁽⁺⁾ cells was preceded by the transient upregulation of GAP43 during the acute phase of reparative fibrosis in the infarcted male rat heart. Secondly, a seminal feature of diabetes is impaired wound healing secondary to an inadequate neurogenic response. In this regard, an additional series of experiments tested the hypothesis that the neurogenic response of cardiac nestin⁽⁺⁾ cells was attenuated in a setting of type I diabetes.

Methods

The neurogenic response of cardiac nestin⁽⁺⁾ cells was examined during the early phase of reparative fibrosis following permanent ligation of the left anterior descending coronary artery in the adult male rat heart. The experimental model of type I diabetes was created following a single injection of streptozotocin in adult male rats. The impact of a type I diabetic environment on the neurogenic response of cardiac nestin⁽⁺⁾ cells was examined during myocardial infarction and following the administration of 6-hydroxydopamine.

Results

During the early phase of scar formation/healing, the density of GAP43/nestin⁽⁺⁾ fibres innervating the peri-infarct/infarct region was significantly increased, whereas neurofilament-M/nestin⁽⁺⁾ fibres were absent. With ongoing scar formation/healing, a temporal decrease of GAP43/nestin⁽⁺⁾ fibre density and a concomitant increase in the density of innervating neurofilament-M/nestin⁽⁺⁾ fibres were observed. The neurogenic response of cardiac nestin⁽⁺⁾ cells during scar formation/healing was inhibited following the superimposition of type I diabetes. The de novo synthesis of neurofilament-M by nestin⁽⁺⁾ cells after 6-hydroxydopamine administration was likewise attenuated in the heart of type I diabetic rats whereas the density of GAP43/nestin⁽⁺⁾ fibres remained elevated.

Conclusion

The transient upregulation of GAP43 apparently represents a transition event during the acquisition of a neuronal-like phenotype and a type I diabetic environment attenuated the neurogenic response of cardiac nestin⁽⁺⁾ cells to ischemia and 6-hydroxydopamine.

β-Adrenergic regulation of cardiac progenitor cell death versus survival and proliferation

RATIONALE

Short-term β-adrenergic stimulation promotes contractility in response to stress but is ultimately detrimental in the failing heart because of accrual of cardiomyocyte death. Endogenous cardiac progenitor cell (CPC) activation may partially offset cardiomyocyte losses, but consequences of long-term β-adrenergic drive on CPC survival and proliferation are unknown.

OBJECTIVE

We sought to determine the relationship between β-adrenergic activity and regulation of CPC function.

METHODS AND RESULTS

Mouse and human CPCs express only β2 adrenergic receptor (β2-AR) in conjunction with stem cell marker c-kit. Activation of β2-AR signaling promotes proliferation associated with increased AKT, extracellular signal-regulated kinase 1/2, and endothelial NO synthase phosphorylation, upregulation of cyclin D1, and decreased levels of G protein-coupled receptor kinase 2. Conversely, silencing of β2-AR expression or treatment with β2-antagonist ICI 118, 551 impairs CPC proliferation and survival. β1-AR expression in CPC is induced by differentiation stimuli, sensitizing CPC to isoproterenol-induced cell death that is abrogated by metoprolol. Efficacy of β1-AR blockade by metoprolol to increase CPC survival and proliferation was confirmed in vivo by adoptive transfer of CPC into failing mouse myocardium.

CONCLUSIONS

β-adrenergic stimulation promotes expansion and survival of CPCs through β2-AR, but acquisition of β1-AR on commitment to the myocyte lineage results in loss of CPCs and early myocyte precursors.

The plating of rat scar myofibroblasts on matrigel unmasks a novel phenotype; the self assembly of lumen-like structures

During tissue healing, the primary role of myofibroblasts involves the synthesis and deposition of collagen. However, it has also been reported that selective populations of myofibroblasts can acquire the phenotype and/or differentiate to other cells types. The present study tested the hypothesis that myofibroblasts isolated from the scar of the ischemically damaged rat heart can recapitulate an endothelial cell-like response when plated in a permissive in vitro environment. Scar myofibroblasts, neonatal and adult ventricular fibroblasts express smooth muscle α-actin, collagen α(1) type 1 and a panel of pro-fibrotic and pro-angiogenic peptide growth factor mRNAs. Myofibroblasts plated alone on matrigel led to the self assembly of lumen-like structures whereas neonatal and adult rat ventricular fibroblasts were unresponsive. Myofibroblasts labeled with the fluorescent cell tracker CM-DiI were injected in the viable myocardium of 3-day post-myocardial infarcted Sprague-Dawley rats and sacrificed 7 days later. Injected CM-DiI-labeled myofibroblasts were detected predominantly in the peri-infarct/infarct region, highlighting their migration to the damaged region. However, engrafted myofibroblasts in the peri-infarct/infarct region were unable to adopt an endothelial cell-like phenotype or lead to the de novo formation of CM-DiI-labeled blood vessels. The non-permissive nature of the infarct region may be attributed at least in part to the presence of growth-promoting stimuli as TGF-β and the β-adrenergic agonist isoproterenol inhibited the self assembly of lumen-like structures by myofibroblasts. Thus, when plated in a permissive in vitro environment, scar myofibroblasts can self assemble and form lumen-like structures providing an additional novel phenotype distinguishing this population from normal ventricular fibroblasts.

Moxonidine improves cardiac structure and performance in SHR through inhibition of cytokines, p38 MAPK and Akt

BACKGROUND AND PURPOSE

Regression of left ventricular hypertrophy by moxonidine, a centrally acting sympatholytic imidazoline compound, results from a sustained reduction of DNA synthesis and transient stimulation of DNA fragmentation. Because apoptosis of cardiomyocytes may lead to contractile dysfunction, we investigated in spontaneously hypertensive rats (SHR), time- and dose-dependent effects of in vivo moxonidine treatment on cardiac structure and function as well as on the inflammatory process and signalling proteins involved in cardiac cell survival/death.

EXPERIMENTAL APPROACH

12 week old SHR received moxonidine at 0, 100 and 400 µg·kg(-1)·h(-1) , s.c., for 1 and 4 weeks. Cardiac function was evaluated by echocardiography; plasma cytokines were measured by elisa and hearts were collected for histological assessment of fibrosis and measurement of cardiac proteins by Western blotting. Direct effects of moxonidine on cardiac cell death and underlying mechanisms were investigated in vitro by flow cytometry and Western blotting.

KEY RESULTS

After 4 weeks, the sub-hypotensive dose of moxonidine (100 µg) reduced heart rate and improved global cardiac performance, reduced collagen deposition, regressed left ventricular hypertrophy, inhibited Akt and p38 MAPK phosphorylation, and attenuated circulating and cardiac cytokines. The 400 µg dose resulted in similar effects but of a greater magnitude, associated with blood pressure reduction. In vitro, moxonidine inhibited norepinephrine-induced neonatal cardiomyocyte mortality but increased fibroblast mortality, through I(1)-receptor activation and differential effects on downstream Akt and p38 MAPK.

CONCLUSIONS AND IMPLICATIONS

While the antihypertensive action of centrally acting imidazoline compounds is appreciated, new cardiac-selective I(1)-receptor agonists may confer additional benefit.

Nestin expression is lost in ventricular fibroblasts during postnatal development of the rat heart and re-expressed in scar myofibroblasts

Studies have reported that the intermediate filament protein nestin was expressed in various non-stem/progenitor cells during development, downregulated during postnatal growth and re-expressed following injury. The present study tested the hypothesis that an analogous paradigm was prevalent for ventricular fibroblasts. In the neonatal rat heart, nestin protein levels were significantly higher than the adult heart and the isolation of cardiac cells revealed a selective expression in ventricular fibroblasts. In adult ventricular fibroblasts, nestin protein expression was markedly lower compared to neonatal ventricular fibroblasts. Following ischemic damage to the rat heart, nestin staining was detected in a subpopulation of scar myofibroblasts (37%) and the percentage of immunoreactive cells was greater than adult ventricular fibroblasts (7%) but significantly lower than neonatal ventricular fibroblasts (86%). Moreover, dissimilar rates of (3)H-thymidine uptake were observed among the fibroblast populations and may be related in part to the disparate percentage of nestin(+) cells. To assess the role of nestin in DNA synthesis, neonatal ventricular fibroblasts were infected with a lentivirus containing a shRNAmir directed against the intermediate filament protein. The partial depletion of nestin expression in neonatal ventricular fibroblasts significantly reduced basal DNA synthesis, in the absence of an apoptotic response. Thus, postnatal development of the rat heart was associated with a selective loss of nestin expression in ventricular fibroblasts and subsequent induction in a subpopulation of myofibroblasts following ischemic injury. The re-expression of nestin in scar myofibroblasts may represent an adaptive response to enhance their proliferative rate and accelerate the healing process.

Morphological and biochemical studies on aging and autophagy

To maintain health in the elderly is a crucial objective for modern medicine that involves both basic and clinical researches. Autophagy is a fundamental auto-cannibalizing process that preserves cellular homeostasis and, if altered, either by excess or defect, greatly changes cell fate and can result in incapacitating human diseases. Efficient autophagy may prolong lifespan, but unfortunately this process becomes less efficient with age. The present review is focused on the close relationship between autophagy and age-related disorders in different tissues/organs and in transgenic animal models. In particular, it comments on the up to date literature on mechanisms responsible for age-related impairment of autophagy. Moreover, before discussing about these mechanisms, it is necessary to describe the metabolic autophagic regulation of autophagy and the proteins involved in this process. At the end, these data would summarize the autophagic link with aging process, as important tools in the future biogerontology scenario.

Myocardial AKT: the omnipresent nexus

One of the greatest examples of integrated signal transduction is revealed by examination of effects mediated by AKT kinase in myocardial biology. Positioned at the intersection of multiple afferent and efferent signals, AKT exemplifies a molecular sensing node that coordinates dynamic responses of the cell in literally every aspect of biological responses. The balanced and nuanced nature of homeostatic signaling is particularly essential within the myocardial context, where regulation of survival, energy production, contractility, and response to pathological stress all flow through the nexus of AKT activation or repression. Equally important, the loss of regulated AKT activity is primarily the cause or consequence of pathological conditions leading to remodeling of the heart and eventual decompensation. This review presents an overview compendium of the complex world of myocardial AKT biology gleaned from more than a decade of research. Summarization of the widespread influence that AKT exerts upon myocardial responses leaves no doubt that the participation of AKT in molecular signaling will need to be reckoned with as a seemingly omnipresent regulator of myocardial molecular biological responses.

Beta(2)-adrenergic receptor regulates cardiac fibroblast autophagy and collagen degradation

Autophagy is a physiological degradative process key to cell survival during nutrient deprivation, cell differentiation and development. It plays a major role in the turnover of damaged macromolecules and organelles, and it has been involved in the pathogenesis of different cardiovascular diseases. Activation of the adrenergic system is commonly associated with cardiac fibrosis and remodeling, and cardiac fibroblasts are key players in these processes. Whether adrenergic stimulation modulates cardiac fibroblast autophagy remains unexplored. In the present study, we aimed at this question and evaluated the effects of b(2)-adrenergic stimulation upon autophagy. Cultured adult rat cardiac fibroblasts were treated with agonists or antagonists of beta-adrenergic receptors (b-AR), and autophagy was assessed by electron microscopy, GFP-LC3 subcellular distribution, and immunowesternblot of endogenous LC3. The predominant expression of b(2)-ARs was determined and characterized by radioligand binding assays using [(3)H]dihydroalprenolol. Both, isoproterenol and norepinephrine (non-selective b-AR agonists), as well as salbutamol (selective b(2)-AR agonist) increased autophagic flux, and these effects were blocked by propanolol (b-AR antagonist), ICI-118,551 (selective b(2)-AR antagonist), 3-methyladenine but not by atenolol (selective b(1)-AR antagonist). The increase in autophagy was correlated with an enhanced degradation of collagen, and this effect was abrogated by the inhibition of autophagic flux. Overall, our data suggest that b(2)-adrenergic stimulation triggers autophagy in cardiac fibroblasts, and that this response could contribute to reduce the deleterious effects of high adrenergic stimulation upon cardiac fibrosis.

Infarct size is increased in female post-MI rats treated with rapamycin

Rapamycin represents a recognized drug-based therapeutic approach to treat cardiovascular disease. However, at least in the female heart, rapamycin may suppress the recruitment of putative signalling events conferring cardioprotection. The present study tested the hypothesis that rapamycin-sensitive signalling events contributed to the cardioprotective phenotype of the female rat heart after an ischemic insult. Rapamycin (1.5 mg/kg) was administered to adult female Sprague-Dawley rats 24 h after complete coronary artery ligation and continued for 6 days. Rapamycin abrogated p70S6K phosphorylation in the left ventricle of sham rats and the noninfarcted left ventricle (NILV) of 1-week postmyocardial-infarcted (MI) rats. Scar weight (MI 0.028 +/- 0.006, MI+rapamycin 0.064 +/- 0.004 g) and surface area (MI 0.37 +/- 0.08, MI+rapamycin 0.74 +/- 0.03 cm2) were significantly larger in rapamycin-treated post-MI rats. In the NILV of post-MI female rats, rapamycin inhibited the upregulation of eNOS. Furthermore, the increased expression of collagen and TGF-beta3 mRNAs in the NILV were attenuated in rapamycin-treated post-MI rats, whereas scar healing was unaffected. The present study has demonstrated that rapamycin-sensitive signalling events were implicated in scar formation and reactive fibrosis. Rapamycin-mediated suppression of eNOS and TGF-beta3 mRNA in post-MI female rats may have directly contributed to the larger infarct and attenuation of the reactive fibrotic response, respectively.

Cardiac fibroblasts: at the heart of myocardial remodeling

Cardiac fibroblasts are the most prevalent cell type in the heart and play a key role in regulating normal myocardial function and in the adverse myocardial remodeling that occurs with hypertension, myocardial infarction and heart failure. Many of the functional effects of cardiac fibroblasts are mediated through differentiation to a myofibroblast phenotype that expresses contractile proteins and exhibits increased migratory, proliferative and secretory properties. Cardiac myofibroblasts respond to proinflammatory cytokines (e.g. TNFalpha, IL-1, IL-6, TGF-beta), vasoactive peptides (e.g. angiotensin II, endothelin-1, natriuretic peptides) and hormones (e.g. noradrenaline), the levels of which are increased in the remodeling heart. Their function is also modulated by mechanical stretch and changes in oxygen availability (e.g. ischaemia-reperfusion). Myofibroblast responses to such stimuli include changes in cell proliferation, cell migration, extracellular matrix metabolism and secretion of various bioactive molecules including cytokines, vasoactive peptides and growth factors. Several classes of commonly prescribed therapeutic agents for cardiovascular disease also exert pleiotropic effects on cardiac fibroblasts that may explain some of their beneficial outcomes on the remodeling heart. These include drugs for reducing hypertension (ACE inhibitors, angiotensin receptor blockers, beta-blockers), cholesterol levels (statins, fibrates) and insulin resistance (thiazolidinediones). In this review, we provide insight into the properties of cardiac fibroblasts that underscores their importance in the remodeling heart, including their origin, electrophysiological properties, role in matrix metabolism, functional responses to environmental stimuli and ability to secrete bioactive molecules. We also review the evidence suggesting that certain cardiovascular drugs can reduce myocardial remodeling specifically via modulatory effects on cardiac fibroblasts.

TGF-beta1-induced cardiac myofibroblasts are nonproliferating functional cells carrying DNA damages

TGF-beta1 induces differentiation and total inhibition of cardiac MyoFb cell division and DNA synthesis. These effects of TGF-beta1 are irreversible. Inhibition of MyoFb proliferation is accompanied with the expression of Smad1, Mad1, p15Ink4B and total inhibition of telomerase activity. Surprisingly, TGF-beta1-activated MyoFbs are growth-arrested not only at G1-phase but also at S-phase of the cell cycle. Staining with TUNEL indicates that these cells carry DNA damages. However, the absolute majority of MyoFbs are non-apoptotic cells as established with two apoptosis-specific methods, flow cytometry and caspase-dependent cleavage of cytokeratin 18. Expression in MyoFbs of proliferative cell nuclear antigen even in the absence of serum confirms that these MyoFbs perform repair of DNA damages. These results suggest that TGF-beta1-activated MyoFbs can be growth-arrested by two checkpoints, the G1/S checkpoint, which prevents cells from entering S-phase and the intra-S checkpoint, which is activated by encountering DNA damage during the S phase or by unrepaired damage that escapes the G1/S checkpoint. Despite carrying of the DNA damages TGF-beta1-activated MyoFbs are highly functional cells producing lysyl oxidase and contracting the collagen matrix.

Heterodimers of adenylyl cyclases 2 and 5 show enhanced functional responses in the presence of Galpha s

Recent studies have demonstrated that adenylyl cyclase isoforms can form both homo- and heterodimers and that this may be the basic functional unit of these enzymes (see Cooper, D.M.F. and Crossthwaite, A.J. (2006) Trends. Pharmacol. Sci. 8:426-431). Here, we show that adenylyl cyclases 2 and 5 can form a functional heterodimeric complex in HEK293 cells using a combination of BRET, confocal imaging, co-immunoprecipitation and assays of adenylyl cyclase activity. The AC2/5 complex is formed constitutively and is stable in the presence of receptor or forskolin stimulation. The complex formed by AC2/5 is also much more sensitive to the presence of Galpha(s) and forskolin than either of the parent AC isoforms themselves. Finally, we also show that this complex can be detected in native tissues as AC2 and AC5 were localized to the same structures in adult mouse ventricular myocytes and neonatal mouse cardiac fibroblasts and could be co-immunoprecipitated from lysates of mouse heart.

Disparate regulation of signaling proteins after exercise and myocardial infarction

INTRODUCTION

The signaling proteins extracellular signal-regulated kinase (ERK1/2) and protein kinase B (PKB) were implicated in the development of pathological cardiac hypertrophy. The present study examined whether the progression of physiological eccentric cardiac hypertrophy was associated with ERK1/2 and PKB recruitment.

METHODS AND RESULTS

Following 1 and 3 wk of voluntary exercise, female Sprague-Dawley rats ran a total distance of 55 +/- 10 and 195 +/- 19 km, respectively. Left ventricular hypertrophy was detected in 3-wk-exercised rats, albeit prepro-ANP protein expression was unchanged. ERK1/2 was not recruited in the left ventricle (LV) of either 1-wk-exercised rats or the hypertrophied LV of 3-wk-exercised rats. In 1-wk-exercised rats, PKB Thr308 and Ser473 phosphorylation were significantly reduced, whereas a selective increase of PKB Ser473 phosphorylation was observed in the hypertrophied LV of 3-wk-exercised rats. In both 1- and 3-wk-exercised rats, an upward electrophoretic mobility band shift of p70 ribosomal S6 kinase (p70 S6K) was detected. In 1-wk post-myocardial-infarcted (MI) female Sprague-Dawley rats, scar formation was associated with increased left ventricular end-diastolic pressure. In the hypertrophied noninfarcted left ventricle (NILV), ERK1/2, p70 S6K, PKB Ser473, and Thr308 phosphorylation were increased.

CONCLUSIONS

These data support the premise that ERK1/2 and PKB were differentially regulated during the development of eccentric physiological and pathological cardiac hypertrophy. It remains to be determined whether the chronic activation of either ERK1/2 and/or PKB in the NILV of post-MI rats may contribute in part to maladaptive cardiac remodelling.

The impact of blunted beta-adrenergic responsiveness on growth regulatory pathways in hypertension

The effects of vasodilator hormones acting through receptors linked to adenylyl cyclase are impaired in the hypertensive state. This has been ascribed to impaired receptor-G protein coupling. However, these receptors also act via effectors not linked to adenylyl cyclase activation. These "alternate" mechanisms may be especially important in growth regulation and might be unaffected (or enhanced) with G protein-coupled receptor-G protein uncoupling. Therefore, we assessed the effects of beta-adrenergic activation on 1) regulation of phosphatidylinositol 3-kinase (PI3 kinase) and extracellular signal-regulated kinase (ERK) activation-two tyrosine kinase-dependent enzymes linked to cell growth-and 2) microarray analysis in vascular smooth muscle cells from spontaneously hypertensive rats (SHR). Isoproterenol-stimulated phosphorylation of ERK1/2 was impaired in SHR. The effect of forskolin was unaltered. In contrast, both vasopressin and angiotensin 2-mediated stimulation of ERK activation was enhanced in SHR. In addition, beta-adrenergic-mediated inhibition of PI3 kinase activity was attenuated in SHR (whereas the effect of forskolin remained intact). In microarray studies, the effect of isoproterenol to regulate transcription was significantly impaired in SHR (as was the effect of forskolin). Together, these data support the hypothesis that the blunted vasodilator effects of hormones linked to adenylyl cyclase activation are an index of a more generalized impairment in modulating growth regulatory pathways. Furthermore, this study supports the hypothesis that the blunting of beta-adrenergic responses relating to increased G protein-coupled receptor kinase 2 expression reflects a "generalized uncoupling" of beta-adrenergic-mediated responses and do not support the concept of "enhanced coupling" of "alternate" pathways of beta-adrenergic growth regulatory pathways in the hypertensive state.

Nestin-expressing neural stem cells identified in the scar following myocardial infarction

Nerve fiber innervation of the scar following myocardial damage may have occurred either via the growth of pre-existing fibers and/or the mobilization of neural stem cells. The present study examined whether neural stem cells were recruited to the infarct region of the rat heart following coronary artery ligation. The neural stem cell marker nestin was detected in the infarct region of 1-week post-myocardial infarct (MI) male rats and cultured scar-derived neural-like cells. By contrast, nestin staining was undetected in either scar myofibroblasts or cardiac myocytes residing in the non-infarcted left ventricle. Reactive astrocytes were isolated from the infarct region and characterized by the co-expression of nestin, glial fibrillary acidic protein, and vimentin. Specific staining of oligodendrocytes and neurons was also detected in the infarct region and cultured scar-derived neural-like cells. Furthermore, neurofilament-M positive fibers were identified in the scar and tyrosine hydroxylase immunoreactivity was observed in peripherin-positive neurons. Neurite formation was induced in PC12 cells treated with the conditioned-media of primary passage scar-derived cells, highlighting the synthesis and secretion of neurotrophic factors. Nerve growth factor (NGF) and brain-derived neurotrophic factor were detected in myofibroblasts and neural cells, and both cell types expressed the NGF receptors trkA and p75. These data highlight the novel observation that neural stem cells were recruited to the infarct region of the damaged rat heart and may contribute in part to nerve fiber growth and subsequent innervation of the scar.