Angiotensin II receptor internalization and signaling in isolated rat hepatocytes

Angiotensin II blockade: how its molecular targets may signal to mitochondria and slow aging. Coincidences with calorie restriction and mTOR inhibition

Caloric restriction (CR), renin angiotensin system blockade (RAS-bl), and rapamycin-mediated mechanistic target of rapamycin (mTOR) inhibition increase survival and retard aging across species. Previously, we have summarized CR and RAS-bl's converging effects, and the mitochondrial function changes associated with their physiological benefits. mTOR inhibition and enhanced sirtuin and KLOTHO signaling contribute to the benefits of CR in aging. mTORC1/mTORC2 complexes contribute to cell growth and metabolic regulation. Prolonged mTORC1 activation may lead to age-related disease progression; thus, rapamycin-mediated mTOR inhibition and CR may extend lifespan and retard aging through mTORC1 interference. Sirtuins by deacetylating histone and transcription-related proteins modulate signaling and survival pathways and mitochondrial functioning. CR regulates several mammalian sirtuins favoring their role in aging regulation. KLOTHO/fibroblast growth factor 23 (FGF23) contribute to control Ca(2+), phosphate, and vitamin D metabolism, and their dysregulation may participate in age-related disease. Here we review how mTOR inhibition extends lifespan, how KLOTHO functions as an aging suppressor, how sirtuins mediate longevity, how vitamin D loss may contribute to age-related disease, and how they relate to mitochondrial function. Also, we discuss how RAS-bl downregulates mTOR and upregulates KLOTHO, sirtuin, and vitamin D receptor expression, suggesting that at least some of RAS-bl benefits in aging are mediated through the modulation of mTOR, KLOTHO, and sirtuin expression and vitamin D signaling, paralleling CR actions in age retardation. Concluding, the available evidence endorses the idea that RAS-bl is among the interventions that may turn out to provide relief to the spreading issue of age-associated chronic disease.

Angiotensin II, mitochondria, cytoskeletal, and extracellular matrix connections: an integrating viewpoint

Malfunctioning mitochondria strongly participate in the pathogenesis of cardiovascular damage associated with hypertension and other disease conditions. Eukaryotic cells move, assume their shape, resist mechanical stress, accommodate their internal constituents, and transmit signals by relying on the constant remodeling of cytoskeleton filaments. Mitochondrial ATP is needed to support cytoskeletal dynamics. Conversely, mitochondria need to interact with cytoskeletal elements to achieve normal motility, morphology, localization, and function. Extracellular matrix (ECM) quantity and quality influence cellular growth, differentiation, morphology, survival, and mobility. Mitochondria can sense ECM composition changes, and changes in mitochondrial functioning modify the ECM. Maladaptive ECM and cytoskeletal alterations occur in a number of cardiac conditions and in most types of glomerulosclerosis, leading to cardiovascular and renal fibrosis, respectively. Angiotensin II (ANG II), a vasoactive peptide and growth factor, stimulates cytosolic and mitochondrial oxidant production, eventually leading to mitochondrial dysfunction. Also, by inducing integrin/focal adhesion changes, ANG II regulates ECM and cytoskeletal composition and organization and, accordingly, contributes to the pathogenesis of cardiovascular remodeling. ANG II-initiated integrin signaling results in the release of transforming growth factor-β1 (TGF-β1), a cytokine that modifies ECM composition and structure, induces reorganization of the cytoskeleton, and modifies mitochondrial function. Therefore, it is possible to hypothesize that the depression of mitochondrial energy metabolism brought about by ANG II is preceded by ANG II-induced integrin signaling and the consequent derangement of the cytoskeletal filament network and/or ECM organization. ANG II-dependent TGF-β1 release is a potential link between ANG II, ECM, and cytoskeleton derangements and mitochondrial dysfunction. It is necessary to emphasize that the present hypothesis is among many other plausible explanations for ANG II-mediated mitochondrial dysfunction. A potential limitation of this proposal is that the results compiled here were obtained in different cells, tissues, and/or experimental models.

Positive feedback of hepatic angiotensinogen expression in silver sea bream (Sparus sarba)

The renin-angiotensin system (RAS) is involved in the maintenance of fluid homeostasis in vertebrates. Production of the precursor protein, angiotensinogen, is regulated by other components within the RAS. Angiotensin II (Ang II) stimulates the production and secretion of angiotensinogen in many mammalian models. However, the existence of a similar positive feedback mechanism for angiotensinogen has not been demonstrated for any non-mammalian species. In the present study, we have cloned the angiotensinogen for silver sea bream (Sparus sarba) and investigated the role of Ang II on angiotensinogen expression. The nucleotide sequence of angiotensinogen for S. sarba only exhibits a fair resemblance to other fish angiotensinogens and shows 76.6% similarity to that of Takifugu rubripes and 57.2% similarity to that of Danio rerio. Angiotensinogen transcripts have been identified in the brain, liver, kidney, and various parts of the intestine of sea bream, an observation, which probably implies the presence of a local RAS at the tissue level. The liver is probably the major source of angiotensinogen, as it exhibits the highest angiotensinogen transcript abundance among different tissues. Differential angiotensinogen expression was found among different regions of the intestine where the pyloric caeca exhibits the highest expression. Putative Ang I is identified at the N-terminal of the deduced protein with a novel sequence [Asn1, Ile5, His9]-Ang I. Hepatic angiotensinogen expression in sea bream adapted to different salinities remained constant and this is probably due to desensitization of the angiotensin receptors by angiotensin. A positive feedback mechanism of angiotensinogen by Ang II has been demonstrated as exogenous Ang II increased the amount of angiotensinogen transcript in isolated hepatocytes in vitro. Blockade of endogenous RAS by the angiotensin converting enzyme (ACE) inhibitor, captopril, significantly lowered the hepatic expression of angiotensinogen in vivo. The effect of Ang II stimulation on angiotensinogen expression is more potent in fish than that in mammals. These data suggest that the positive feedback mechanism of angiotensinogen by Ang II has already evolved in teleosts and such mechanism may be involved in the maintenance of angiotensinogen secretion under resting and hypertensive conditions.

Fate of bradykinin on the rat liver when administered by the venous or arterial route

BACKGROUND AND AIM

Bradykinin (BK) infused into the portal vein elicits a hypertensive response via the B2 receptor (B2R) and is efficiently hydrolyzed by the liver. Our purpose was to characterize the mechanism of interaction between BK and the liver.

METHOD

BK, HOE-140 (a B2R antagonist), des-R(9)-BK (a B1R agonist) and enzyme inhibitors were used in monovascular or bivascular perfusions and in isolated liver cell assays.

RESULTS

Des-R(9)-BK did not elicit a portal hypertensive response (PHR); BK infused into the hepatic artery elicited a calcium-dependent PHR and a calcium-independent arterial hypertensive response (HAHR), with the latter being almost abolished by naproxen. BK has a predominant distribution in the extracellular space and an average hepatic extraction of 8% in the steady state. Hydrolysis products of infused BK (R(1)-F(5) and R(1)-P(7)) did not elicit PHR. Angiotensin converting enzyme (ACE) is concentrated in the perivenous region and B2R in the periportal region. Microphysiometry showed that BK (and not a B1 agonist) interacts with stellate cells and the endothelial sinusoidal/Kupffer cell fraction. This effect was inhibited by the B2R antagonist.

CONCLUSIONS

Events can be summarized as: the hypertensive action of BK on sinusoidal cells of the periportal region is followed by its hydrolysis by ACE which is primarily present in the perivenous region; there is no functional B1R in the normal liver; BK induces HAHR via eicosanoid release and PHR by a distinct pathway on the B2R. Our data suggest that BK may participate in the modulation of sinusoidal microvasculature tonus both in the portal and the arterial routes.

Angiotensin II induces focal adhesion kinase/paxillin phosphorylation and cell migration in human umbilical vein endothelial cells

In the present study, we demonstrated that Ang II provokes a transitory enhancement of focal adhesion kinase (FAK) and paxillin phosphorylation in human umbilical endothelial cells (HUVEC). Moreover, Ang II induces a time- and dose-dependent augmentation in cell migration, but does not affect HUVEC proliferation. The effect of Ang II on FAK and paxillin phosphorylation was markedly attenuated in cells pretreated with wortmannin and LY294002, indicating that phosphoinositide 3-kinase (PI3K) plays an important role in regulating FAK activation. Similar results were observed when HUVEC were pretreated with genistein, a non-selective tyrosine kinases inhibitor, or with the specific inhibitor PP2 for Src family kinases, demonstrating the involvement of protein tyrosine kinases, and particularly Src family of tyrosine kinases, in the downstream signalling pathway of Ang II receptors. Furthermore, FAK and paxillin phosphorylation was markedly blocked after treatment of HUVEC with AG1478, a selective inhibitor of epidermal growth factor receptor (EGFR) phosphorylation. Pretreatment of cells with inhibitors of PI3K, Src family tyrosine kinases, and EGFR also decreased HUVEC migration. In conclusion, these results suggest that Ang II mediates an increase in FAK and paxillin phosphorylation and induces HUVEC migration through signal transduction pathways dependent on PI3K and Src tyrosine kinase activation and EGFR transactivation.

Adenosine receptors control HIV-1 Tat-induced inflammatory responses through protein phosphatase

Recently, adenosine has been proposed to be a "metabolic" switch that may sense and direct immune and inflammatory responses. Inflammation and pro-inflammatory cytokine production are important in development of HIV-1 associated dementia, a devastating consequence of HIV-1 infection of the CNS. The HIV-1 protein Tat induces cell death in the CNS and activates local inflammatory responses partially by inducing calcium release from the endoplasmic reticulum. Because activation of adenosine receptors decreases production of the pro-inflammatory cytokine TNF-alpha in several experimental paradigms both in vitro and in vivo, we hypothesized that adenosine receptor activation would control both increased intracellular calcium and TNF-alpha production induced by Tat. Treatment of primary monocytes with Tat significantly increased the levels of intracellular calcium released from IP3 stores. Activation of adenosine receptors with CGS 21680 inhibited Tat-induced increases of intracellular calcium by 90 +/- 8% and was dependent on protein phosphatase activity because okadaic acid blocked the actions of CGS 21680. Tat-induced TNF-alpha production was inhibited 90 +/- 6% by CGS 21680 and concurrent treatment with okadaic acid blocked the inhibitory actions of CGS 21680. Using a model monocytic cell line, CGS 21680 treatment increased cytosolic serine/threonine phosphatase. Together, these data indicate that A2A receptor activation increases protein phosphatase activity, which blocks IP3 receptor-regulated calcium release and reduction of intracellular calcium inhibits TNF-alpha production in monocytes.

Angiotensin type 1 receptor immunoreactivity in the thoracic spinal cord

The angiotensin II type 1 receptor (AT1R) in the central nervous system (CNS) plays a pivotal role in determining blood pressure. However, the relationship of the receptor to neurones in the spinal cord which are the final CNS contribution to sympathetic outflow is unknown. Here we first use RT-PCR to show that AT1A, AT1B and AT2 receptors are expressed in thoracic spinal cord of the rat. Using light microscopic immunohistochemistry we find that the AT1 receptor in the thoracic spinal cord is located on neurones and ependymal cells. Neurones with extensive immunostaining of somata and dendrites were located in the intermediolateral cell column (IML) and lamina X (the central autonomic area), regions associated with autonomic outflow, as well as in lamina V. Retrograde labelling and dual immunolabelling with nNOS revealed that those AT1R-immunopositive cells in the IML were sympathetic preganglionic neurones, while those in lamina X were unlikely to be. Punctate labelling resembling that of axonal fibres and terminals was evident in lamina II of the dorsal horn and throughout the cord. Electron microscopy in the IML and lamina X revealed that these puncta were presynaptic terminals, but also astrocyte processes. Immunolabelling was also evident beneath the plasma membrane in neuronal somata. These data show that the AT1R in the spinal cord is ideally located to influence autonomic outflow and hence participate in the CNS determination of blood pressure.

Receptor-dependent metabolism of platelet-activating factor in murine macrophages

Degradation of platelet-activating factor (PAF) was examined by incubating PAF with macrophages from PAF receptor-deficient mice. The degradation rate was halved as compared with wild-type mice. The reduction of the rate was comparable with the presence of a PAF antagonist WEB 2086 in wild-type cells. PAF was internalized rapidly (t(12) approximately 1 min) into wild-type macrophages. The PAF internalization was inhibited by the treatment of 0.45 m sucrose but was not affected by phorbol 12-myristate 13-acetate, suggesting that PAF internalizes into macrophages with its receptor in a clathrin-dependent manner. Internalized PAF was degraded into lyso-PAF with a half-life of 20 min. Treatment of concanavalin A inhibited the conversion of PAF into lyso-PAF, suggesting that uptake of PAF enhances PAF degradation. Lyso-PAF was subsequently metabolized into 1-alkyl-2-acyl-phosphatidylcholine. In addition, release of PAF acetylhydrolase from macrophages was enhanced when wild-type macrophages were stimulated with PAF but not from macrophages of PAF receptor-deficient mice. Thus, the PAF stimulation of macrophages leads to its degradation through both intracellular and extracellular mechanisms.