Inosine: novel activator of brown adipose tissue and energy homeostasis

Extracellular purinergic molecules act as signaling molecules that bind to cellular receptors and regulate signaling pathways. Growing evidence suggests that purines regulate adipocyte function and whole-body metabolism. Here, we focus on one specific purine: inosine. Brown adipocytes, which are important regulators of whole-body energy expenditure (EE), release inosine when they are stressed or become apoptotic. Unexpectedly, inosine activates EE in neighboring brown adipocytes and enhances differentiation of brown preadipocytes. Increasing extracellular inosine, either directly by increasing inosine intake or indirectly via pharmacological inhibition of cellular inosine transporters, increases whole-body EE and counteracts obesity. Thus, inosine and other closely related purines might be a novel approach to tackle obesity and associated metabolic disorders by enhancing EE.

Calafiore cardioplegia offers better myocardial protection in adult and senescent rat hearts than Del Nido cardioplegia

OBJECTIVES

We compared the cardioprotective capacity of Del Nido cardioplegia and warm Calafiore blood cardioplegia in an experimental setting during 90 min of ischaemia.

METHODS

20 adult and 20 senescent rat hearts were isolated and mounted on a blood-perfused, pressure-controlled Langendorff apparatus. After a stabilization period, cardiac arrest (90 min) was induced by the administration of either Calafiore (Cala) or Del Nido solution (DNS). While Cala was given warm and intermittently, DNS was given as a cold single shot. During 90 min of reperfusion, cardiac function and metabolism were evaluated and biomarker levels were measured. After the end of the experiment, hearts were prepared for electronmicroscopic investigation.

RESULTS

Hearts exposed to Cala recovered faster during reperfusion compared with hearts administered DNS (Cala vs DNS at 30 min reperfusion: left ventricular developed pressure 72, SD: 22% of baseline (BL) versus 40, SD: 32% of BL, p < .001, and positive derived left ventricular pressure over time was better in both adult and senescent Cala groups (96, SD: 31% of BL) than in the DNS groups (39, SD: 27% of BL, p < .001). Ischaemic contractures were seen in the DNS groups starting after 30 min of ischaemia, whereas no rise in diastolic pressure was observed for the Cala groups. Accordingly, lactate production was higher after DNS (1.23 mg/dl (SD 0.87) than after Cala (0.33 mg/dl (SD 0.68), p = .015) at the beginning of reperfusion. Troponin I levels at the end of reperfusion were higher after DNS treatment in adult hearts (DNS: 287.9 SD: 147.7 ng/mL vs Cala 91.2: SD: 94.7 ng/mL, p = .02) and in senescent hearts (DNS: 376.5 (SD: 162.8) ng/ml versus Cala 104.7 (SD: 150.2) ng/ml, p = .025). Electron microscopy showed that the cellular oedema index was higher in adult DNS hearts (1.2 ± 0.2) than in adult Cala hearts (0.8 ± 0.1, p = .012), whereas the VS ratio was similar (0.18 ± 0.01 vs 0.17 ± 0.03).

CONCLUSION

Calafiore cardioplegia offers better myocardial protection from ischaemia/reperfusion-related damage in isolated perfused adult and senescent rat hearts than Del Nido cardioplegia.

Apoptotic brown adipocytes enhance energy expenditure via extracellular inosine

Brown adipose tissue (BAT) dissipates energy1,2 and promotes cardiometabolic health3. Loss of BAT during obesity and ageing is a principal hurdle for BAT-centred obesity therapies, but not much is known about BAT apoptosis. Here, untargeted metabolomics demonstrated that apoptotic brown adipocytes release a specific pattern of metabolites with purine metabolites being highly enriched. This apoptotic secretome enhances expression of the thermogenic programme in healthy adipocytes. This effect is mediated by the purine inosine that stimulates energy expenditure in brown adipocytes by the cyclic adenosine monophosphate-protein kinase A signalling pathway. Treatment of mice with inosine increased BAT-dependent energy expenditure and induced 'browning' of white adipose tissue. Mechanistically, the equilibrative nucleoside transporter 1 (ENT1, SLC29A1) regulates inosine levels in BAT: ENT1-deficiency increases extracellular inosine levels and consequently enhances thermogenic adipocyte differentiation. In mice, pharmacological inhibition of ENT1 as well as global and adipose-specific ablation enhanced BAT activity and counteracted diet-induced obesity, respectively. In human brown adipocytes, knockdown or blockade of ENT1 increased extracellular inosine, which enhanced thermogenic capacity. Conversely, high ENT1 levels correlated with lower expression of the thermogenic marker UCP1 in human adipose tissues. Finally, the Ile216Thr loss of function mutation in human ENT1 was associated with significantly lower body mass index and 59% lower odds of obesity for individuals carrying the Thr variant. Our data identify inosine as a metabolite released during apoptosis with a 'replace me' signalling function that regulates thermogenic fat and counteracts obesity.

Real-time monitoring of cAMP in brown adipocytes reveals differential compartmentation of β1 and β3-adrenoceptor signalling

Objective

3′,5′-Cyclic adenosine monophosphate (cAMP) is a central second messenger governing brown adipocyte differentiation and function. β-adrenergic receptors (β-ARs) stimulate adenylate cyclases which produce cAMP. Moreover, cyclic nucleotide levels are tightly controlled by phosphodiesterases (PDEs), which can generate subcellular microdomains of cAMP. Since the spatio-temporal organisation of the cAMP signalling pathway in adipocytes is still unclear, we sought to monitor real-time cAMP dynamics by live cell imaging in pre-mature and mature brown adipocytes.

Methods

We measured the real-time dynamics of cAMP in murine pre-mature and mature brown adipocytes during stimulation of individual β-AR subtypes, as well as its regulation by PDEs using a Förster Resonance Energy Transfer based biosensor and pharmacological tools. We also correlated these data with β-AR stimulated lipolysis and analysed the expression of β-ARs and PDEs in brown adipocytes using qPCR and immunoblotting. Furthermore, subcellular distribution of PDEs was studied using cell fractionation and immunoblots.

Results

Using pre-mature and mature brown adipocytes isolated from transgenic mice expressing a highly sensitive cytosolic biosensor Epac1-camps, we established real-time measurements of cAMP responses. PDE4 turned out to be the major PDE regulating cytosolic cAMP in brown preadipocytes. Upon maturation, PDE3 gets upregulated and contributes with PDE4 to control β₁-AR-induced cAMP. Unexpectedly, β₃-AR initiated cAMP is resistant to increased PDE3 protein levels and simultaneously, the control of this microdomain by PDE4 is reduced upon brown adipocyte maturation. Therefore we postulate the existence of distinct cAMP pools in brown adipocytes. One cAMP pool is formed by β₁-AR associated with PDE3 and PDE4, while another pool is centred around β₃-AR and is much less controlled by these PDEs. Functionally, lower control of β₃-AR initiated cAMP by PDE3 and PDE4 facilitates brown adipocyte lipolysis, while lipolysis activated by β₁-AR and is under tight control of PDE3 and PDE4.

Conclusions

We have established a real-time live cell imaging approach to analyse brown adipocyte cAMP dynamics in real-time using a cAMP biosensor. We showed that during the differentiation from pre-mature to mature murine brown adipocytes, there was a change in PDE-dependent compartmentation of β₁-and β₃-AR-initiated cAMP responses by PDE3 and PDE4 regulating lipolysis.

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Establishment of real-time cAMP analysis using a FRET biosensor in brown adipocytes.

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Real-time dynamics of cAMP generation from different β-adrenoceptor subtypes in pre- and mature brown adipocytes.

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Expression analysis of PDE families in pre- and mature brown adipocytes.

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Differences in PDE3- and PDE4-dependent regulation of β₁- and β₃-adrenoceptor initiated cAMP signalling in brown adipocytes.

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Differential compartmentation of β₁- and β₃-adrenoceptor signalling in brown adipocytes.

P6376The role of circulating lymphocytes on hypertensive heart disease

Aims and background

Hypertension is a disease accompanied by moderate inflammation. Physical activity and pharmacological intervention protect the organisms against hypertension-dependent end-organ damage. They should therefore interfere with the mobilization of lymphocytes. Different subsets of lymphocytes should specifically interfere with vascular and cardiac remodeling. In this study we correlated the amount of circulating lymphocytes with the expression of vascular and cardiac specific genes in the aorta and left ventricle to identify the tissue-specific effects of these cells.

Methods

50 female spontaneously hypertensive rats (SHR) and 6 female normotensive Wistar rats (Wis) were analyzed. They were grouped in 7 subgroups exposed to different treatment regimes (modification of activity, type of activity, aldosterone blockade, and a combination thereof. Indicator genes of mal-adaptive vascular and cardiac remodeling were quantified by qRT-PCR.

Results

SHRs had an increased number of circulating NK cells and monocytes, but lower numbers of T cells. T cells inversely correlated with the vascular expression of p22phox, a member of the NADPH oxidase complex. Thus, lower number of T-cells favors the vascular expression of p22phox and thereby oxidative stress. In the left ventricle, T cells are inversely correlated with the expression of arginase-1 and UCP-2 but positively correlated with Glut-4 expression. The lower number of T cells in SHR therefore favors the expression of proteins often associated with oxidative stress and glucose metabolism. Monocytes are positively correlated with key markers of left ventricular hypertrophy (such as ANP, collagen-1, and β-MHC). Thus, the increased number of monocytes in SHRs supports cardiac hypertrophy. NK cells are inversely correlated with the expression of anti-inflammatory cytokines in the aorta (IL-10, IL-15). Thus, high number of circulating NK cells seems to favor a pro-inflammatory phenotype in the vasculature.

Conclusion

In a standard model of essential hypertension the profile of circulating lymphocytes differs from that in normotensive rats. The different subsets of lymphocytes specifically interfere with specific processes in the ventricle and vessel. Collectively, the difference between hypertensive and normotensive rats (less T cells, more NK cells and monocytes) supports a mal-adaptive role under hypertensive conditions.

P6372Does modification of activity regimes optimize the effect of high physical activity on hypertensive heart disease?

Aims and background

Spontaneously hypertensive rats (SHR) are a suitable model of essential hypertension and allow analyze the progression of hypertension and hypertension-dependent end-organ damage. In this model, acute improvement of physical activity (free running wheel activity) exerts known beneficial effects (such as lowering oxidative stress). However, these initial beneficial effects are lost during the continuation of high physical activity and translate into mal-adaptive processes, suggesting that not any high physical activity exerts beneficial effects. It has been hypothesized that the skeletal muscle release myokines, that contribute to the beneficial effects of exercise. However, myokines, such as IL-6, are induced by an acute increase in work load not by continuous work load. Therefore, we analyzed whether modification of high physical activity, i.e. intermittent free running wheel activity, modify the long-term impact on long lasting hypertension.

Methods

38 female SHR aged 6 weeks (pre-hypertensive state) were randomly allocated to one of the following groups: Sedentary (S; standard holding condition) imitating the condition of sedentary life style, high activity (HA; life-long free running wheel) imitating the condition of active life style, temporary activity (TA; 6 months free running wheel and 3 months sedentary) imitating the loss of active life style during ageing, and finally intermittent activity (IA; 10 months repetitive access to running wheels every 4 weeks) imitating altered workloads. All rats were sacrificed at the age of 10 months.

Results

IA was the only treatment regime that effectively lowered blood pressure (P syst: 186±11 vs. 165±6 mmHg), improved ejection fraction (EF: 56±5 vs. 63±2%), and displayed clear molecular profile of adaptive myocardial hypertrophy rather than mal-adaptive hypertrophy. Moreover, only IA reduced the number of circulating monocytes (377±102 vs. 220±16 /μl), a cell population that immigrated in the left ventricle. The number of monocytes was directly correlated with the expression of MMP12, BNP, ANP, biglycan, collagen-1, actinin, β-MHC, and somatstatin but inversely related to β-adrenoceptor, Glut-4, and UCP3 expression. Finally, IA increased the skeletal expression of IL-6 and decreased the renal expression of AT1 receptors.

Conclusion

The data confirm the previous findings that not all type of physical activity beneficially affects hypertensive-dependent disease. In contrast, the data support the hypothesis that alterations in work load are required triggering the release of myokines from the skeletal muscle and identify the amount of circulating monocytes as a main trigger of mal-adaptive hypertrophy in these rats. The data are important with respect to optimize life style suggestions for patients with essential hypertension.

Acknowledgement/Funding

DFG (ERAGON and SFB 1213)