Richard Southworth

Disruption of Hexokinase II–Mitochondrial Binding Blocks Ischemic Preconditioning and Causes Rapid Cardiac Necrosis

Rationale: Isoforms I and II of the glycolytic enzyme hexokinase (HKI and HKII) are known to associate with mitochondria. It is unknown whether mitochondria-bound hexokinase is mandatory for ischemic preconditioning and normal functioning of the intact, beating heart. Objective: We hypothesized that reducing mitochondrial hexokinase would abrogate ischemic preconditioning and disrupt myocardial function. Methods and Results: Ex vivo perfused HKII+/− hearts exhibited increased cell death after ischemia and reperfusion injury compared with wild-type hearts; however, ischemic preconditioning was unaffected. To investigate acute reductions in mitochondrial HKII levels, wild-type hearts were treated with a TAT control peptide or a TAT-HK peptide that contained the binding motif of HKII to mitochondria, thereby disrupting the mitochondrial HKII association. Mitochondrial hexokinase was determined by HKI and HKII immunogold labeling and electron microscopy analysis. Low-dose (200 nmol/L) TAT-HK treatment significantly decreased mitochondrial HKII levels without affecting baseline cardiac function but dramatically increased ischemia-reperfusion injury and prevented the protective effects of ischemic preconditioning. Treatment for 15 minutes with high-dose (10 &mgr;mol/L) TAT-HK resulted in acute mitochondrial depolarization, mitochondrial swelling, profound contractile impairment, and severe cardiac disintegration. The detrimental effects of TAT-HK treatment were mimicked by mitochondrial membrane depolarization after mild mitochondrial uncoupling that did not cause direct mitochondrial permeability transition opening. Conclusions: Acute low-dose dissociation of HKII from mitochondria in heart prevented ischemic preconditioning, whereas high-dose HKII dissociation caused cessation of cardiac contraction and tissue disruption, likely through an acute mitochondrial membrane depolarization mechanism. The results suggest that the association of HKII with mitochondria is essential for the protective effects of ischemic preconditioning and normal cardiac function through maintenance of mitochondrial potential.

Targeting hexokinase II to mitochondria to modulate energy metabolism and reduce ischaemia-reperfusion injury in heart

Mitochondrially bound hexokinase II (mtHKII) has long been known to confer cancer cells with their resilience against cell death. More recently, mtHKII has emerged as a powerful protector against cardiac cell death. mtHKII protects against ischaemia‐reperfusion (IR) injury in skeletal muscle and heart, attenuates cardiac hypertrophy and remodelling, and is one of the major end‐effectors through which ischaemic preconditioning protects against myocardial IR injury. Mechanisms of mtHKII cardioprotection against reperfusion injury entail the maintenance of regulated outer mitochondrial membrane (OMM) permeability during ischaemia and reperfusion resulting in stabilization of mitochondrial membrane potential, the prevention of OMM breakage and cytochrome C release, and reduced reactive oxygen species production. Increasing mtHK may also have important metabolic consequences, such as improvement of glucose‐induced insulin release, prevention of acidosis through enhanced coupling of glycolysis and glucose oxidation, and inhibition of fatty acid oxidation. Deficiencies in expression and distorted cellular signalling of HKII may contribute to the altered sensitivity of diabetes to cardiac ischaemic diseases. The interaction of HKII with the mitochondrion constitutes a powerful endogenous molecular mechanism to protect against cell death in almost all cell types examined (neurons, tumours, kidney, lung, skeletal muscle, heart). The challenge now is to harness mtHKII in the treatment of infarction, stroke, elective surgery and transplantation. Remote ischaemic preconditioning, metformin administration and miR‐155/miR‐144 manipulations are potential means of doing just that.

PET imaging of cardiac hypoxia: Opportunities and challenges

Myocardial hypoxia is a major factor in the pathology of cardiac ischemia and myocardial infarction. Hypoxia also occurs in microvascular disease and cardiac hypertrophy, and is thought to be a prime determinant of the progression to heart failure, as well as the driving force for compensatory angiogenesis. The non-invasive delineation and quantification of hypoxia in cardiac tissue therefore has the potential to be an invaluable experimental, diagnostic and prognostic biomarker for applications in cardiology. However, at this time there are no validated methodologies sufficiently sensitive or reliable for clinical use. PET imaging provides real-time spatial information on the biodistribution of injected radiolabeled tracer molecules. Its inherent high sensitivity allows quantitative imaging of these tracers, even when injected at sub-pharmacological (≥pM) concentrations, allowing the non-invasive investigation of biological systems without perturbing them. PET is therefore an attractive approach for the delineation and quantification of cardiac hypoxia and ischemia. In this review we discuss the key concepts which must be considered when imaging hypoxia in the heart. We summarize the PET tracers which are currently available, and we look forward to the next generation of hypoxia-specific PET imaging agents currently being developed. We describe their potential advantages and shortcomings compared to existing imaging approaches, and what is needed in terms of validation and characterization before these agents can be exploited clinically.

Monitoring of In Vivo Function of Superparamagnetic Iron Oxide Labelled Murine Dendritic Cells during Anti- Tumour Vaccination

Dendritic cells (DCs) generated in vitro to present tumour antigens have been injected in cancer patients to boost in vivo anti-tumour immune responses. This approach to cancer immunotherapy has had limited success. For anti-tumour therapy, delivery and subsequent migration of DCs to lymph nodes leading to effective stimulation of effector T cells is thought to be essential. The ability to non-invasively monitor the fate of adoptively transferred DCs in vivo using magnetic resonance imaging (MRI) is an important clinical tool to correlate their in vivo behavior with response to treatment. Previous reports of superparamagnetic iron oxides (SPIOs) labelling of different cell types, including DCs, have indicated varying detrimental effects on cell viability, migration, differentiation and immune function. Here we describe an optimised labelling procedure using a short incubation time and low concentration of clinically used SPIO Endorem to successfully track murine DC migration in vivo using MRI in a mouse tumour model. First, intracellular labelling of bone marrow derived DCs was monitored in vitro using electron microscopy and MRI relaxometry. Second, the in vitro characterisation of SPIO labelled DCs demonstrated that viability, phenotype and functions were comparable to unlabelled DCs. Third, ex vivo SPIO labelled DCs, when injected subcutaneously, allowed for the longitudinal monitoring by MR imaging of their migration in vivo. Fourth, the SPIO DCs induced the proliferation of adoptively transferred CD4+ T cells but, most importantly, they primed cytotoxic CD8+ T cell responses to protect against a B16-Ova tumour challenge. Finally, using anatomical information from the MR images, the immigration of DCs was confirmed by the increase in lymph node size post-DC injection. These results demonstrate that the SPIO labelling protocol developed in this study is not detrimental for DC function in vitro and in vivo has potential clinical application in monitoring therapeutic DCs in patients with cancer.

Cardiac Hypoxia Imaging: Second-Generation Analogues of 64Cu-ATSM

Myocardial hypoxia is an attractive target for diagnostic and prognostic imaging, but current approaches are insufficiently sensitive for clinical use. The PET tracer copper(II)-diacetyl-bis(N4-methylthiosemicarbazone) (64Cu-ATSM) has promise, but its selectivity and sensitivity could be improved by structural modification. We have therefore evaluated a range of 64Cu-ATSM analogs for imaging hypoxic myocardium. Methods: Isolated rat hearts (n = 5/group) were perfused with normoxic buffer for 30 min and then hypoxic buffer for 45 min within a custom-built triple-γ-detector system to quantify radiotracer infusion, hypoxia-dependent cardiac uptake, and washout. A 1-MBq bolus of each candidate tracer (and 18F-fluoromisonidazole for comparative purposes) was injected into the arterial line during normoxia, and during early and late hypoxia, and their hypoxia selectivity and pharmacokinetics were evaluated. The in vivo pharmacokinetics of promising candidates in healthy rats were then assessed by PET imaging and biodistribution. Results: All tested analogs exhibited hypoxia sensitivity within 5 min. Complexes less lipophilic than 64Cu-ATSM provided significant gains in hypoxic-to-normoxic contrast (14:1 for 64Cu-2,3-butanedione bis(thiosemicarbazone) (ATS), 17:1 for 64Cu-2,3-pentanedione bis(thiosemicarbazone) (CTS), 8:1 for 64Cu-ATSM, P < 0.05). Hypoxic first-pass uptake was 78.2% ± 7.2% for 64Cu-ATS and 70.7% ± 14.5% for 64Cu-CTS, compared with 63.9% ± 11.7% for 64Cu-ATSM. Cardiac retention of 18F-fluoromisonidazole increased from 0.44% ± 0.17% during normoxia to 2.24% ± 0.08% during hypoxia. In vivo, normoxic cardiac retention of 64Cu-CTS was significantly lower than that of 64Cu-ATSM and 64Cu-ATS (0.13% ± 0.02% vs. 0.25% ± 0.04% and 0.24% ± 0.03% injected dose, P < 0.05), with retention of all 3 tracers falling to less than 0.7% injected dose within 6 min. 64Cu-CTS also exhibited lower uptake in liver and lung. Conclusion: 64Cu-ATS and 64Cu-CTS exhibit better cardiac hypoxia selectivity and imaging characteristics than the current lead hypoxia tracers, 64Cu-ATSM and 18F-fluoromisonidazole.

Differential uptake of FDG and DG during post-ischaemic reperfusion in the isolated, perfused rat heart

Abstract. Fluorine-18 2-fluoro-2-deoxyglucose (FDG) and 2-deoxyglucose (DG) are widely used as tracers of glucose uptake in the myocardium. Although there is agreement that the two analogues behave similarly to glucose under control conditions, there is growing evidence that some interventions (e.g. insulin stimulation or ischaemia/reperfusion) cause differential changes in their behaviour. The addition of a two-surface coil nuclear magnetic resonance (NMR) probe and a dual-perfusion cannula to our recently developed PET and NMR dual-acquisition (PANDA) system allows us to collect PET (FDG) images and phosphorus-31 NMR (2-deoxyglucose-6-phosphate) spectra simultaneously from each independently perfused coronary bed of the heart. We have used this technique to study the effect of regional ischaemia/reperfusion on FDG and DG uptake in the isolated, perfused rat heart. During control perfusion, FDG uptake was almost identical in both coronary beds. When one coronary bed was made ischaemic, FDG uptake ceased on that side but continued on the control side. Reperfusion failed to restore FDG uptake. In contrast, NMR spectra showed that, during reperfusion, the uptake and phosphorylation of DG did not differ between the two coronary beds. The results thus demonstrate that regional myocardial ischaemia/reperfusion has different effects on the uptake of FDG and DG in the isolated, perfused rat heart.

Lactate-induced translocation of GLUT1 and GLUT4 is not mediated by the phosphatidylinositol-3-kinase pathway in the rat heart

Abstract We have determined the effect of lactate on the translocation of GLUT1 and GLUT4 and on the myocardial uptake and phosphorylation of the glucose analogues 2-deoxy-D-glucose (DG) and 2-18F-fluoro-2-deoxy-D-glucose (18FDG). The involvement of phosphatidyl-inositol-3-kinase (PI3K) in this translocation was determined using wortmannin. Hearts from fed and fasted male Wistar rats were perfused in the presence of 11 mM glucose ± 10 mM lactate for two hours and the distribution of glucose transporters was determined using Western blot techniques. Two other groups of hearts from fed animals were perfused in the presence of 11 mM glucose ± 10 mM lactate for two hours followed by perfusion for a further 30 minutes in the presence of 4 mM 2-deoxy-D-glucose. Using 31P NMR spectroscopy, the accumulation of 2-deoxy-D-glucose-6-phosphate (DG6P) was monitored over time. Another group of hearts from fed animals was initially perfused in the presence of 11 mM glucose for 100 minutes and then the perfusate was changed to 11 mM glucose + 10 mM lactate for a further 120 minutes. Using PET, the accumulation of 2-18F-fluoro-deoxy-D-glucose-6-phosphate (18FDG6P) was monitored throughout the whole protocol. Lactate induced the translocation of both GLUT1 and GLUT4 to the plasma membrane (from 67 ± 1% to 82 ± 2% and from 16 ± 1% to 28 ± 2%, respectively (P < 0.05)) in hearts from fed animals; similar translocations were observed in hearts from fasted animals. Wortmannin did not inhibit the translocation of either GLUT1 or GLUT4. Glucose transporter translocation was accompanied by a significant inhibition of DG6P accumulation (4.24 ± 0.68 vs. 1.50 ± 0.38; P < 0.001) and a decrease in the rate of 18FDG6P accumulation. In conclusion, lactate causes translocation of GLUT1 and GLUT4 to the plasma membrane, via a non-PI3K-mediated pathway. Despite this externalisation of the GLUT transporters, a marked decrease in the accumulation of both DG6P and 18FDG6P was observed.

Pathophysiological Consequences of TAT-HKII Peptide Administration Are Independent of Impaired Vascular Function and Ensuing Ischemia

Rationale: We have shown that partial dissociation of hexokinase II (HKII) from mitochondria in the intact heart using low-dose transactivating transcriptional factor (TAT)-HKII (200 nmol/L) prevents the cardioprotective effects of ischemic preconditioning, whereas high-dose TAT-HKII (10 &mgr;mol/L) administration results in rapid myocardial dysfunction, mitochondrial depolarization, and disintegration. In this issue of Circulation Research, Pasdois et al argue that the deleterious effects of TAT-HKII administration on cardiac function are likely because of vasoconstriction and ensuing ischemia. Objective: To investigate whether altered vascular function and ensuing ischemia recapitulate the deleterious effects of TAT-HKII in intact myocardium. Methods and Results: Using a variety of complementary techniques, including mitochondrial membrane potential (&Dgr;&psgr;m) imaging, high-resolution optical action potential mapping, analysis of lactate production, nicotinamide adenine dinucleotide epifluorescence, lactate dehydrogenase release, and electron microscopy, we provide direct evidence that refutes the notion that acute myocardial dysfunction by high-dose TAT-HKII peptide administration is a consequence of impaired vascular function. Moreover, we demonstrate that low-dose TAT-HKII treatment, which abrogates the protective effects of ischemic preconditioning, is not associated with ischemia or ischemic injury. Conclusions: Our findings challenge the notion that the effects of TAT-HKII are attributable to impaired vascular function and ensuing ischemia, thereby lending further credence to the role of mitochondria-bound HKII as a critical regulator of cardiac function, ischemia-reperfusion injury, and cardioprotection by ischemic preconditioning.

64Cu-CTS: A Promising Radiopharmaceutical for the Identification of Low-Grade Cardiac Hypoxia by PET.

The subtle hypoxia underlying chronic cardiovascular disease is an attractive target for PET imaging, but the lead hypoxia imaging agents 64Cu-2,3-butanedione bis(N4-methylthiosemicarbazone) (ATSM) and 18F-fluoromisonidazole are trapped only at extreme levels of hypoxia and hence are insufficiently sensitive for this purpose. We have therefore sought an analog of 64Cu-ATSM better suited to identify compromised but salvageable myocardium, and we validated it using parallel biomarkers of cardiac energetics comparable to those observed in chronic cardiac ischemic syndromes. Methods: Rat hearts were perfused with aerobic buffer for 20 min, followed by a range of hypoxic buffers (using a computer-controlled gas mixer) for 45 min. Contractility was monitored by intraventricular balloon, energetics by 31P nuclear MR spectroscopy, lactate and creatine kinase release spectrophotometrically, and hypoxia-inducible factor 1-α by Western blotting. Results: We identified a key hypoxia threshold at a 30% buffer O2 saturation that induces a stable and potentially survivable functional and energetic compromise: left ventricular developed pressure was depressed by 20%, and cardiac phosphocreatine was depleted by 65.5% ± 14% (P < 0.05 vs. control), but adenosine triphosphate levels were maintained. Lactate release was elevated (0.21 ± 0.067 mmol/L/min vs. 0.056 ± 0.01 mmol/L/min, P < 0.05) but not maximal (0.46 ± 0.117 mmol/L/min), indicating residual oxidative metabolic capacity. Hypoxia-inducible factor 1-α was elevated but not maximal. At this key threshold, 64Cu-2,3-pentanedione bis(thiosemicarbazone) (CTS) selectively deposited significantly more 64Cu than any other tracer we examined (61.8% ± 9.6% injected dose vs. 29.4% ± 9.5% for 64Cu-ATSM, P < 0.05). Conclusion: The hypoxic threshold that induced survivable metabolic and functional compromise was 30% O2. At this threshold, only 64Cu-CTS delivered a hypoxic-to-normoxic contrast of 3:1, and it therefore warrants in vivo evaluation for imaging chronic cardiac ischemic syndromes.

Demonstration of the retention of 64Cu-ATSM in cardiac myocytes using a novel incubation chamber for screening hypoxia-dependent radiotracers.

ObjectiveWe have designed a low-cost, reusable incubation system that allows cells to be cultured in either plated or suspension culture under complete atmospheric control for radiotracer characterization. We demonstrate its utility here in the first quantification of the hypoxia-dependent accumulation of 64Cu-diacetyl bis(N4-methylthiosemicarbazone) (64Cu-ATSM) in adult rat ventricular myocytes (ARVMs). Materials and methodsARVMs were allowed to adhere overnight in 9 cm culture plates (2×105 cells/dish) or were used in suspension culture, placed inside the chamber and equilibrated with either oxic (95 or 21% O2/5% CO2) or anoxic gas (95% N2/5% CO2). 64Cu-ATSM of 100 kBq was administered, and the cells were incubated for 30 or 60 min. Cells were then harvested, counted and fractionated to determine intracellular 64Cu biodistribution. ResultsAfter 1 h, the average cellular 64Cu retention in plated ARVMs under oxygenated conditions was 23.9±2.5 mBq/cell (95% O2), increasing to 27.3±5.1 under 21% O2 (P<0.05) and to 36.1±3.1 under 0% O2 (P<0.05). When ARVMs were cultured in suspension, normoxic–hypoxic contrast was less marked but still significant [63.2±14.1 vs. 53.4±10.9% mBq/cell after 30 min (P<0.05)]. Sixty percent of tracer accumulated in the cytosol, and, although total cellular retention increased during hypoxia, there was no enrichment in any particular cellular compartment. ConclusionThis apparatus allows the conduction of radiotracer uptake studies in cells under complete atmospheric control, as evidenced by our first demonstration of the hypoxia-dependent uptake of 64Cu-ATSM in ventricular myocytes. It is ideally suited for screening, validating and characterizing novel hypoxia-selective radiotracers.