Life-threatening heart damage is an adverse side effect of the cancer drug doxorubicin, damage that also limits the use of newer chemotherapeutic agents such as trastuzumab and imatinib. The ability to protect the heart from these side effects would benefit patients, including cancer survivors who are at risk of developing heart damage years later, and it also could allow safer use of these drugs at higher doses.
In a paper published Feb. 25 in the Journal of Clinical Investigation Insight, University of Alabama at Birmingham researchers show that over-expression of the enzyme heme oxygenase-1 protects the heart from doxorubicin damage by regulating mitochondrial quality control in the heart. Using a mouse model, they describe the apparent mechanisms for this protection, and they note that the benefits of heme oxygenase-1, or HO-1, may extend beyond cancer patients.
"Given that oxidative stress and alterations in mitochondrial metabolism underlie many if not most forms of cardiac failure, including heart attack, congestive heart failure and cardiac remodeling," the authors write, "the findings highlighted in this study are broadly applicable and may point to HO-1 expression as a general therapeutic target for patients with cardiovascular disease arising from a multitude of etiologies."
Mitochondria are small organelles inside cells that are often called the powerhouses of the cell because they generate most of the cell's ATP, a chemical form of energy. Oxidative stress can damage mitochondria, allowing the release of dangerous reactive oxygen species, or ROS. Thus the cell has a protective recycling mechanism called mitophagy to recognize old or damaged mitochondria and mark them for digestion into mitochondrial components.
The failure to identify and remove dysfunctional mitochondria can be quite dangerous for the cell because those mitochondria spew ROS. "What they say about ROS is that it's just like plutonium," said James George, Ph.D., professor in the Division of Cardiothoracic Surgery, UAB Department of Surgery, and co-corresponding author of the paper with Anupam Agarwal, M.D., director of the Division of Nephrology, UAB Department of Medicine. "It's really useful in the right place, but God help you if it gets out of control."
The UAB research team, which included two researchers from Duke University School of Medicine, showed the protective effects HO-1 at a variety of levels: measurement of heart function, visualization by high-magnification microscopy, visualization by transmission electron microscopy of subcellular components, and changes in gene expression. In their mouse models, the murine HO-1 genes have been deleted, and the mice overexpress human HO-1 either globally or solely in heart muscle cells. In experiments where both models were used, similar results were found, suggesting that the protection seen is specific to heart muscle cells, independent of HO-1 expression in neighboring endothelial and vascular smooth muscle cells, or in mononuclear phagocytes in the heart.
At the level of heart function, the researchers found that HO-1 overexpression in doxorubicin-treated mice, as compared with wild-type doxorubicin-treated mice, protected the mice from a decline in heart ejection fraction, thinning of the left ventricle wall, and larger diameters of the left ventricle at both diastole and systole.
At the level of high-magnification microscopy, HO-1 overexpression in doxorubicin-treated mice, as compared with wild-type doxorubicin-treated mice, protected the hearts from cardiomyocyte vacuolization, hyper-eosinophilia, interstitial edema, fibrosis and an increased proportion of mononuclear phagocytes in the heart.
At the level of gene expression in heart muscle-cell mitochondria, HO-1 overexpression in doxorubicin-treated mice, as compared with wild-type doxorubicin-treated mice, prevented a significant reduction in mitochondrial DNA copies, suggesting an inability of wild-type mice to boost mitochondrial biogenesis after doxorubicin treatment. In contrast, HO-1 overexpression increased the expression of the mitochondrial DNA polymerase pol-gamma -- suggesting biogenesis -- as well as key components of the electron transport chain, NADH dehydrogenase I and cytochrome c oxidase III. HO-1 overexpression also increased the expression in heart cells of key initiators of the mitochondrial biogenesis pathway -- nuclear transcription factor NRF1 and its co-activator PGC-alpha -- as well as TFAM, a mitochondria-specific transcription factor that initiates the biogenesis pathway.
At the subcellular level, as examined by transmission electron microscopy, HO-1 overexpression in doxorubicin-treated mice prevented marked subcellular abnormalities that included dilation of the sarcoplasmic reticulum, an apparent increase in mitochondrial fragmentation and intercalated discs with deranged morphology. These subcellular changes in wild-type mice, seen 14 days after doxorubicin treatment, persisted at least 60 days, as did the protection against subcellular abnormalities in the HO-1 overexpressing mice. Higher numbers of mitochondria, having a decreased average size, were seen in the wild-type mice, suggesting that the mitochondria were fragmenting.
These results prompted a look into mitochondrial dynamics, the balance between mitochondria splitting apart and mitochondria fusing together. Oxidative stress causes fragmentation of mitochondria and subsequent clearance by mitophagy. Fragmentation can result from either decreased fusion or increased fission.
The researchers found that HO-1 overexpression in the heart muscle cells of doxorubicin-treated mice strikingly increased the expression of two fusion proteins, mitofusin 1 and 2. Overexpression of HO-1 also prevented the increase of Fis1, a fission-related protein that triggers mitophagy, that was seen in wild-type mice treated with doxorubicin. The wild-type mice had increased expression of mitofusin 1, but not mitofusion 2, compared with untreated wild-type controls.
"The clinical significance of these findings are multifold," the authors write. They include reducing chemotherapy-induced cardiac toxicity, realizing the full clinical utility of anthracycline drugs like doxorubicin, and also newer chemotherapeutic agents, and seeing if therapies based on HO-1 could be given to cancer survivors to prevent the development of later cardiac toxicity.
HO-1, they write, "… is a promising therapeutic target in humans, where mitochondrial pathophysiology is an underlying component of disease states of the heart and other organ systems."
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