The Gift of Light

Heart failure (HF) resulting from impaired left ventricular (LV) function is associated with significant morbidity and mortality in the United States and worldwide ¹. In the failing heart, there are changes in energy substrate metabolism whose etiology and effects are poorly understood ². These changes may contribute to deterioration in cardiac contractility as well as to increasing left ventricular remodeling that are the hallmarks of the failing heart. In HF following cardiac hypertrophy, there is a major “metabolic switch” in myocardial substrate metabolism from fatty acid to glucose oxidation ^2–5. A key marker of this switch is the coordinated down regulation of fatty acid oxidation (FAO) enzymes (e.g., MCAD and CPT-1 β) and mRNA levels in the human left ventricle with a concomitant increase in glucose uptake and oxidation ^{2, 6}. This switch is thought to represent a metabolic reprogramming toward substrate metabolism that is more commonly seen in the fetal heart ⁷. The fetus in a womb uses glucose from the mother as the major substrate for energy production; in fact, approximately 80% of fetal energy comes from glucose oxidation.

Moreover, recent studies have suggested that brown adipose tissue (BAT) may play an important role in obesity, type 2 diabetes, and HF ^8–13. BAT regulates basal and inducible energy expenditure in humans and is composed of cells that contain numerous lipid droplets, dense packing of mitochondria, and the expression of the mitochondria-associated uncoupling protein 1 (UCP1) ¹¹. UCP1 mediates the biochemical uncoupling of electron transport by allowing proton leakage across the inner mitochondrial membrane into the mitochondrial matrix, thus, short-circuiting the usual linkage of electron transport to adenosine triphosphate production that is catalyzed by ATP-synthase ¹¹. Instead, the chemical energy generated by cellular respiration in brown adipocytes is dissipated in the form of heat, which allows thermogenesis by mammals in cold environments and newborn humans. Traditionally, BAT was considered to be no longer present and possessed no significant physiological functions in adult humans; however, PET/CT imaging investigations have clearly demonstrated that discreet depots of BAT are still present in human adults ¹⁴. These studies have relied on the use of clinical PET/CT to correlate focal FDG supraclavicular uptake to tissue fat density rather than solely muscle and termed the finding as uptake in supraclavicular area fat or “USA fat.”

In this issue of Circulation: Cardiovascular Imaging, Panagia and colleagues ¹⁵ aimed to develop a multiplexed optical imaging platform to simultaneously assess the uptake of glucose and fatty acid analogs in multiple organs in a murine model of left ventricular hypertrophy (LVH). They hoped that this approach would leverage the strength of optical imaging to examine multiple processes in various tissues in a simple and high throughput fashion. In their study, the detection of glucose uptake was measured with 2-deoxy-2-[F-18]fluoro-D-glucose (2-FDG) via Cerenkov luminescence imaging, a technique that was first used to image tumors in mice but also enabled visualization of the heart ¹⁶. In addition, the uptake of free fatty acids (FFA) was quantified using a synthetic, long-chain fluorescent FFA (BODIPY FL C₁₆). Both of these imaging agents were injected at the same time intraperitoneally. Simultaneous uptake of these agents was measured in the myocardium, intrascapular BAT, epididymal white adipose tissue (WAT), and skeletal muscle of the quadriceps in mice with and without thoracic aortic banding (TAC). While imaging was performed ex vivo after the animals had been euthanized and the tissues harvested, the authors do show that Cerenkov images of 2-FDG uptake in the mouse heart could be acquired in vivo as well.

The authors report that within 5 weeks post-TAC, mice developed LVH and BAT activation with upregulation of β₃ adrenergic receptors and increased natriuretic peptide receptor ratio. Recent studies have suggested that communication between the heart and BAT is possibly mediated through sympathetic stimulation and cardiac hormones such as natriuretic peptides ^{8, 12, 13}. In addition, there may be reciprocal communication between BAT and the heart ⁸. The heart was shown to be significantly increased in weight at both the 5-week and 15-week time-points post-TAC when compared to the controls, while there was a significant reduction in BAT weight (40%) at the 15-week time-point only. It was noted that the pattern of BAT activation was highly influenced by the duration of hypertrophy and the transition from a preserved (at 5 weeks post-TAC) to a reduced ejection fraction (at 15 weeks post-TAC).

Imaging of BAT 15 weeks post-TAC surgery revealed an increase in glucose (p <0.01) and FFA (p< 0.001) uptake versus controls and an increase in UCP1 (p <0.01). Similar but less robust changes were seen in skeletal muscle, while glucose and FFA uptake in WAT remained unchanged. Glucose uptake via the glucose transporters (e.g., GLUTs) in the myocardium was significantly increased at 15 weeks post-TAC (>70%), but FFA uptake appeared to decrease and was not statistically significant. Although these findings add to our understanding of the relationship between BAT and LVH, it remains unclear whether BAT activation in LVH represents an adaptive or maladaptive event.

Despite the novel platform presented by the authors, several limitations must be acknowledged. Optical imaging techniques like the one described in this study will generally be confined for use in preclinical studies due to its limited penetration depth. The poor penetration will hold back its applications in humans, although the development of near-infrared (NIR) fluorescence imaging may allow this approach to delve deeper into the body. The other drawback of optical imaging is its resolution, which is good near the surface, but poorer for deeper structures such as the heart. In human subjects, optical imaging is likely to remain restricted to relatively superficial targets because of the absorbing and scattering properties of tissue in the visible and NIR portion of the electromagnetic spectrum.

PET imaging, on the other hand, is relatively depth independent due to its highly penetrating pair of back-to-back 180° gamma rays that are emitted from the body. PET is an imaging technology that is directed at the biochemical processes that underlie all body functions and their alterations in disease ¹⁷. Efforts are currently underway to build a new type of PET scanner that will allow for multiplex imaging so that two or more PET tracers can be injected at the same time. This would reduce the time and imaging costs to simultaneously acquire multiple biochemical and physiological information about an organ or body of interest. The current use of a “mouse hotel,” which can house at least four mice for preclinical PET studies demonstrates the high-throughput nature of microPET imaging ¹⁸. PET/CT and PET/MR will, thus, remain vital centerpieces for preclinical as well as clinical translation of molecular imaging agents in the study of human health and disease, and this is no more important than in the heart.

Another limitation of the current study is that the authors restricted themselves to using only 2-FDG for studying glucose uptake and metabolism. There are two major classes of glucose transporters in the body, the passive facilitative glucose transporters (GLUTs) and the active sodium-coupled glucose transporters (SGLTs) ¹⁹. 2-FDG is only transported through the GLUTs, but not through the SGLTs. New F-18 labeled PET tracers, like methyl-4FDG, have been shown to be excellent substrates specifically for the SGLTs ¹⁹. Uptake of methyl-4FDG into the heart may be useful for diagnostic purposes in heart failure as SGLT genes are expressed in cardiomyocytes. Imaging with both 2-FDG and methyl-4FDG will therefore give a more complete picture of how glucose is transported and metabolized in healthy and diseased tissue. It is exciting to highlight that while the current study was limited to the imaging of glucose and free fatty acids, the authors expect that 4–5 spectrally resolvable substrates could be imaged simultaneously with their approach. The authors should be commended for their efforts to synergistically merge the fields of optical imaging and energy metabolism to elucidate novel mechanisms that underlie cardiac pathophysiology.

Finally, it would have been interesting to include both male and female animals in this study in order to examine sex differences in cardiac metabolism in LVH and heart failure. Heart complications often take different forms in men and women ²⁰. Women are known to adapt differently than men to cardiac pressure overload. Female heart walls tend to stiffen and get thicker, decreasing the ability of blood to enter the left ventricle. Male heart walls get thinner, weakening pumping power. It would, thus, be interesting to compare the bioenergetics and metabolic changes in the failing hearts of males and females and see the role of metabolic imaging in guiding and evaluating therapeutic interventions.

In conclusion, the authors present a novel optical approach that can permit the imaging of multiple metabolic and energetic pathways in the heart and peripheral tissues in a preclinical mouse model of LVH. This method could provide important insights into HF pathogenesis and help resolve its outstanding metabolic mysteries. Moreover, the effects of metabolic therapies in LVH and HF (e.g., drugs that can modulate myocardial glucose and fatty acid metabolism) could be investigated in future studies. For human translation, however, PET/CT and PET/MR will remain the essential imaging modalities for studying the underlying biochemical processes in cardiac disease.