Pyruvate Upregulates Hepatic FGF21 Expression by Activating PDE and Inhibiting cAMP–Epac–CREB Signaling Pathway

Fibroblast growth factor 21 (FGF21) functions as a polypeptide hormone to regulate glucose and lipid metabolism, and its expression is regulated by cellular metabolic stress. Pyruvate is an important intermediate metabolite that acts as a key hub for cellular fuel metabolism. However, the effect of pyruvate on hepatic FGF21 expression and secretion remains unknown. Herein, we examined the gene expression and protein levels of FGF21 in human hepatoma HepG2 cells and mouse AML12 hepatocytes in vitro, as well as in mice in vivo. In HepG2 and AML12 cells, pyruvate at concentrations above 0.1 mM significantly increased FGF21 expression and secretion. The increase in cellular cAMP levels by adenylyl cyclase activation, phosphodiesterase (PDE) inhibition and 8-Bromo-cAMP administration significantly restrained pyruvate-stimulated FGF21 expression. Pyruvate significantly increased PDE activities, reduced cAMP levels and decreased CREB phosphorylation. The inhibition of exchange protein directed activated by cAMP (Epac) and cAMP response element binding protein (CREB) upregulated FGF21 expression, upon which pyruvate no longer increased FGF21 expression. The increase in plasma pyruvate levels in mice induced by the intraperitoneal injection of pyruvate significantly increased FGF21 gene expression and PDE activity with a reduction in cAMP levels and CREB phosphorylation in the mouse liver compared with the control. In conclusion, pyruvate activates PDEs to reduce cAMP and then inhibits the cAMP–Epac–CREB signaling pathway to upregulate FGF21 expression in hepatocytes.

Cardioprotective Mechanisms of Hypothyroidism on Ischemia/Reperfusion in Rats and Effects of Carvedilol: Energetic Study

Hypothyroidism is considered a cardiac risk factor, but there is controversial evidence about its effects on coronary disease. The aim of this work was to evaluate the influence of hypothyroidism in rat hearts exposed to 2 degrees of stunning due to ischemia and reperfusion (I/R) as well as the underlying mechanisms. Hypothyroid (HypoT) rats were obtained by drinking 0.02% methimazole during 15 days. Isolated hearts were perfused and introduced in a flow calorimeter to measure contractile performance (P), total heat rate (Ht), and muscle economy (P/Ht). Hearts were exposed to 2 models of I/R, moderate and severe (respectively 20 or 30 minutes I/45 minutes R). Moreover, free cytosolic and mitochondrial calcium changes were measured by confocal fluorometry on cardiomyocytes. Comparison to euthyroid (EuT) hearts was done. Hypothyroidism was cardioprotective, but HypoT hearts were more sensitive than EuT hearts to the preischemic blockade of mitochondrial transporters mNCX and mKATPchannels. Moreover, the postischemic recovery of P and P/Ht in HypoT hearts was strongly reduced by inhibition of the cellular pathways of PI3K/Akt and protein kinase C (PKC), and it was increased by nitric oxide synthase (NOS) inhibition. However, physiological concentrations of adrenaline reduced the cardioprotection of HypoT, but oral treatment with 20 mg/kg/day carvedilol prevented it. Results show that hypothyroidism reduces the mitochondrial Ca2+overload during I/R by mKATPchannel activation and Ca2+extrusion through mNCX, while the PI3K/Akt and PKC pathways are involved in that cardioprotection. Contrarily, NOS activation and adrenaline blunt such cardioprotection, but carvedilol prevented the adrenergic dysfunction. These results would explain why hypothyroidism is a clinical risk factor in angor patients under adrenergic exacerbation but reduced the incidence of acute episodes of coronary syndrome in hospitalized patients. Results suggest that a treatment with carvedilol could be a potential therapeutic agent to prevent cardiac postischemic dysfunction in hypothyroid patients.

Cardioprotection of stevioside on stunned rat hearts: A mechano-energetical study

BACKGROUND

The sweetener and hypoglycemic properties of stevioside (STV) are well known, as the main component of the plant Stevia rebaudiana. Given its extensive use in diabetic patients, it was of interest to evaluate its effects on the most frequent cardiovascular disease, the coronary insufficiency.

PURPOSE

To study whether STV could be cardioprotective against ischemia-reperfusion (I/R) in a model of "stunning" in rat hearts.

STUDY DESIGN

A preclinical study was performed in isolated hearts from rats in the following groups: non-treated rats whose hearts were perfused with STV 0.3 mg/ml and their controls (C) exposed to either moderate stunning (20 min I/45 min R) or severe stunning (30 min I/45 min R), and a group of rats orally treated with STV 25 mg/kg/day in the drink water during 1 week before the experiment of severe stunning in the isolated hearts were done.

METHODS

The mechano-calorimetrical performance of isolated beating hearts was recorded during stabilization period with control Krebs perfusion inside a calorimeter, with or without 0.3 mg/ml STV before the respective period of I/R. The left ventricular maximal developed pressure (P) and total heat rate (Ht) were continuously measured.

RESULTS

Both, orally administered and perfused STV improved the post-ischemic contractile recovery (PICR, as % of initial control P) and the total muscle economy (P/Ht) after the severe stunning, but only improved P/Ht in moderate stunning. However, STV increased the diastolic pressure (LVEDP) during I/R in both stunning models. For studying the mechanism of action, ischemic hearts were reperfused with 10 mM caffeine-36 mM Na⁺-Krebs to induce a contracture dependent on sarcorreticular Ca²⁺ content, whose relaxation mainly depends on mitochondrial Ca²⁺ uptake. STV at 0.3 mg/ml increased the area-under-curve of the caffeine-dependent contracture (AUC-LVP). Moreover, at room temperature STV increased the mitochondrial Ca²⁺ uptake measured by Rhod-2 fluorescence in rat cardiomyocytes, but prevented the [Ca²⁺]m overload assessed by caffeine-dependent SR release.

CONCLUSIONS

Results suggest that STV is cardioprotective against I/R under oral administration or direct perfusion in hearts. The mechanism includes the regulation of the myocardial calcium homeostasis and the energetic during I/R in several sites, mainly reducing mitochondrial Ca²⁺ overload and increasing the sarcorreticular Ca²⁺ store.

Mitochondrial Bioenergetics During Ischemia and Reperfusion

During ischemia and reperfusion (I/R) mitochondria suffer a deficiency to supply the cardiomyocyte with chemical energy, but also contribute to the cytosolic ionic alterations especially of Ca²⁺. Their free calcium concentration ([Ca²⁺]m) mainly depends on mitochondrial entrance through the uniporter (UCam) and extrusion in exchange with Na⁺ (mNCX) driven by the electrochemical gradient (ΔΨm). Cardiac energetic is frequently estimated by the oxygen consumption, which determines metabolism coupled to ATP production and to the maintaining of ΔΨm. Nevertheless, a better estimation of heart energy consumption is the total heat release associated to ATP hydrolysis, metabolism, and binding reactions, which is measurable either in the presence or the absence of oxygenation or perfusion. Consequently, a mechano-calorimetrical approach on isolated hearts gives a tool to evaluate muscle economy. The mitochondrial role during I/R depends on the injury degree. We investigated the role of the mitochondrial Ca²⁺ transporters in the energetic of hearts stunned by a model of no-flow I/R in rat hearts. This chapter explores an integrated view of previous and new results which give evidences to the mitochondrial role in cardiac stunning by ischemia o hypoxia, and the influence of thyroid alterations and cardioprotective strategies, such as cardioplegic solutions (high K-low Ca, pyruvate) and the phytoestrogen genistein in both sex. Rat ventricles were perfused in a flow-calorimeter at either 30 °C or 37 °C to continuously measure the left ventricular pressure (LVP) and total heat rate (Ht). A pharmacological treatment was done before exposing to no-flow I and R. The post-ischemic contractile (PICR as %) and energetical (Ht) recovery and muscle economy (Eco: P/Ht) were determined during stunning. The functional interaction between mitochondria (Mit) and sarcoplasmic reticulum (SR) was evaluated with selective mitochondrial inhibitors in hearts reperfused with Krebs-10 mM caffeine-36 mM Na⁺. The caffeine induced contracture (CIC) was due to SR Ca²⁺ release, while relaxation mainly depends on mitochondrial Ca²⁺ uptake since neither SL-NCX nor SERCA are functional under this media. The ratio of area-under-curves over ischemic values (AUC-ΔHt/AUC-ΔLVP) estimates the energetical consumption (EC) to maintain CIC. Relaxation of CIC was accelerated by inhibition of mNCX or by adding the aerobic substrate pyruvate, while both increased EC. Contrarily, relaxation was slowed by cardioplegia (high K-low Ca Krebs) and by inhibition of UCam. Thus, Mit regulate the cytosolic [Ca²⁺] and SR Ca²⁺ content. Both, hyperthyroidism (HpT) and hypothyroidism (HypoT) reduced the peak of CIC but increased EC, in spite of improving PICR. Both, CIC and PICR in HpT were also sensitive to inhibition of mNCX or UCam, suggesting that Mit contribute to regulate the SR store and Ca²⁺ release. The interaction between mitochondria and SR and the energetic consequences were also analyzed for the effects of genistein in hearts exposed to I/R, and for the hypoxia/reoxygenation process. Our results give evidence about the mitochondrial regulation of both PICR and energetic consumption during stunning, through the Ca²⁺ movement.