Glycogen Phosphorylase: A Novel Biomarker in Doxorubicin-Induced Cardiac Injury Free

In this issue of Clinical Cancer Research, Yarana and colleagues suggested a new diagnostic molecular biomarker that could potentially be used for the early detection of cardiac injury after chemotherapy in patients with malignancies (1). In this regard, it is very common that, at least in some significant part, therapy for human malignancies includes the use of systemic therapies, that is, chemotherapy. In fact, it is predicted that nearly 20 million Americans will be cancer survivors in less than 10 years, emphasizing the need for a greater understanding of the causes and mechanisms accounting for cell killing and/or cellular damage in normal tissues, such as neuropathy, cardiomyopathy, or cognitive dysfunction (1). Among those adverse effects, cardiovascular disease is the leading cause of long-term toxicity as well as treatment-related deaths in cancer survivors. However, there is a lack of sensitive and/or predictive molecular biomarkers that address this critical issue in cancer therapy prior to the onset of symptoms (2). In this novel and seminal article, it is shown that glycogen phosphorylase, brain/heart (PYGB) may serve as an early detectable molecular biomarker for cardiac damage and, as such, may be very useful as a clinical early intervention tool to protect patients from treatment-induced cardiovascular disease.

Multiple studies have shown that systemic agents, such as doxorubicin, can cause not only DNA damage but also induce cell apoptosis and significantly alter cellular oxidative metabolism, and it seems likely that all of these damaging cellular processes are mechanistically connected (3). Interestingly, doxorubicin, as well as many other systemic anticancer agents, can significantly decrease the cellular antioxidant capacity and damage the mitochondrial electron transportation efficiency (4). It has been proposed that normal cells, including cardiomyocytes, exhibiting oxidative damage when exposed to doxorubicin appear to directly induce DNA damage, and this is the process that connects exposure to metabolic protein dysfunction and oxidative and/or metabolic cellular stress. In this regard, normal cells treated with systemic agents appear to exhibit prolonged elevated reactive oxygen species (ROS) levels, and the increase in ROS would inevitably result in the oxidation of many biomolecules, including mitochondrial lipids, protein, and mtDNA, that could consequently lead to organelle dysfunction as well as cell death. These results and observations have long led to the idea that cancer patients undergoing chemotherapy may exhibit an increase in oxidative stress–induced normal tissue damage that eventually leads to normal tissue toxicity, including in the heart tissue. Therefore, monitoring the current oxidative stress levels, via molecular biomarkers for oxidative stress and/or altered cellular metabolism, in the normal tissue could represent a novel and precise method to detecting chemotherapy-induced normal tissue injury.

To validate and subsequently qualify a molecular biological marker for cardiac damage, there are a few prerequisites, including that the proposed biomarkers need to (i) be easily obtained, (ii) accurately reflect the chemotherapy-induced damage, and (iii) be relatively stable. It is well-known that cells have a sophisticated mechanism to main cellular homeostasis. In this regard, this article suggests that extracellular vesicles (EV) may be a candidate that meets these criteria. As mentioned, Yarana and colleagues hypothesize that the lipid bilayer makes EVs very stable in the extracellular environment, and human serums contains high levels of EVs. The most important reason that EVs may be a potential molecular biomarker for normal tissue cardiomyocyte damage is that EVs carry origination-specific proteins, lipids, and other important biomolecules that could reflect the current biological conditions of the releasing cells.

Glycogen phosphorylase is one of the phosphorylase enzymes and catalyzes the rate-limiting step in glycogenolysis. Many studies suggested that glycogen phosphorylases are sensitive biomarkers of myocardial ischemia, acute coronary syndromes, and hypertrophic cardiomyopathy (5). In this study, Yarana and colleagues demonstrated that one specific glycogen phosphorylase, PYGB, could be used as a new perspective marker to assess the cardiac injury risk in patients after chemotherapy. Yarana and colleagues investigated the protein content of EVs and found that EVs present after doxorubicin treatment exhibit signatures of cardiac tissue and high levels of protein-bound 4-hydroxynonenal (4HNE). Further proteomic profiling data revealed that doxorubicin-treated EVs distinctively contained a brain/heart–specific glycogen phosphorylase (PYGB). Their data also showed that there is a decrease of PYGB level in heart tissue, but not in brain tissue because doxorubicin treatment could not pass the blood–brain barrier, suggesting this glycogen phosphorylase could be a specific indicator for the doxorubicin-damaged heart tissue.

Thus, monitoring PYGB in circulating EVs can accurately ascertain the condition of cells after chemotherapy, which increased 4HNE-adducted EVs with PYGB, representing the high oxidative stress status and a possible glycogenolysis dysfunction as well as early sign of cardiac tissue damage (Fig. 1). In addition, mice with MnSOD overexpression or MnP-pretreated mice exhibited lesser EV release from heart and decreased 4HNE levels as compared with the control group. Interestingly, DRZ, a similar antioxidant treatment, also reduced the EV release but with a weaker result. Because MnSOD specifically catalyzed the reaction of superoxide to hydrogen peroxides, it will be interesting to see whether future studies can explore the connection between particular ROS with cardiac damage. Furthermore, some studies mentioned that MnSOD may directly participate in other cellular processes, including cell-cycle checkpoint and apoptosis (6). It will also be interesting to investigate whether MnSOD, the primary mitochondrial antioxidant, could have a specific role other than superoxide dismutase after chemotherapy. Understanding those questions can further improve the new biological rationale of early detection and intervention for chemotherapy-induced cardiac injuries.

No potential conflicts of interest were disclosed.

Conception and design: Y. Zhu, D. Gius

Writing, review, and/or revision of the manuscript: Y. Zhu, D. Gius