Ectopic Myoglobin Expression Is Associated with a Favourable Outcome in Head and Neck Squamous Cell Carcinoma Patients

Myoglobin in Brown Adipose Tissue: A Multifaceted Player in Thermogenesis

Brown adipose tissue (BAT) plays an important role in energy homeostasis by generating heat from chemical energy via uncoupled oxidative phosphorylation. Besides its high mitochondrial content and its exclusive expression of the uncoupling protein 1, another key feature of BAT is the high expression of myoglobin (MB), a heme-containing protein that typically binds oxygen, thereby facilitating the diffusion of the gas from cell membranes to mitochondria of muscle cells. In addition, MB also modulates nitric oxide (NO•) pools and can bind C16 and C18 fatty acids, which indicates a role in lipid metabolism. Recent studies in humans and mice implicated MB present in BAT in the regulation of lipid droplet morphology and fatty acid shuttling and composition, as well as mitochondrial oxidative metabolism. These functions suggest that MB plays an essential role in BAT energy metabolism and thermogenesis. In this review, we will discuss in detail the possible physiological roles played by MB in BAT thermogenesis along with the potential underlying molecular mechanisms and focus on the question of how BAT-MB expression is regulated and, in turn, how this globin regulates mitochondrial, lipid, and NO• metabolism. Finally, we present potential MB-mediated approaches to augment energy metabolism, which ultimately could help tackle different metabolic disorders.

Endogenous myoglobin expression in mouse models of mammary carcinoma reduces hypoxia and metastasis in PyMT mice

Myoglobin (MB) is expressed in different cancer types and may act as a tumor suppressor in breast cancer. The mechanisms by which basal MB expression level impacts murine mammary tumorigenesis are unclear. We investigated how MB expression in breast cancer influences proliferation, metastasis, tumor hypoxia, and chemotherapy treatment in vivo. We crossed PyMT and WapCreTrp53^flox mammary cancer mouse models that differed in tumor grade/type and onset of mammary carcinoma with MB knockout mice. The loss of MB in WapCre;Trp53^flox mice did not affect tumor development and progression. On the other hand, loss of MB decreased tumor growth and increased tissue hypoxia as well as the number of lung metastases in PyMT mice. Furthermore, Doxorubicin therapy prevented the stronger metastatic propensity of MB-deficient tumors in PyMT mice. This suggests that, although MB expression predicts improved prognosis in breast cancer patients, MB-deficient tumors may still respond well to first-line therapies. We propose that determining the expression level of MB in malignant breast cancer biopsies will improve tumor stratification, outcome prediction, and personalized therapy in cancer patients.

Differential proteomic expression in indolent versus transforming oral lichen planus

Oral lichen planus (OLP) confers an approximately 1% risk of transformation to oral squamous cell carcinoma (OSCC). Early identification of high-risk OLP would be very helpful for optimal patient management. We aimed to discover specific tissue-based protein biomarkers in patients with OLP who developed OSCC compared to those who did not. We used laser capture microdissection- and nanoLC-tandem mass spectrometry to assess protein expression in fixed lesional mucosal specimens in patients with indolent OLP (no OSCC after at least 5-year follow-up, n = 6), transforming OLP (non-dysplastic epithelium with lichenoid inflammation marginal to OSCC, n = 6) or normal oral mucosa (NOM, n = 5). Transforming OLP protein profile was enriched for actin cytoskeleton, mitochondrial dysfunction and oxidative phosphorylation pathways. CA1, TNNT3, SYNM and MB were overexpressed, and FBLN1 was underexpressed in transforming OLP compared with indolent OLP. Integrin signalling and antigen presentation pathways were enriched in both indolent and transforming OLP compared with NOM. This proteomic study provides potential biomarkers, such as CA1 overexpression, for higher-risk OLP. While further validation studies are needed, we propose that epithelial-mesenchymal transition may be involved in OLP carcinogenesis.

Pro-Apoptotic and Anti-Invasive Properties Underscore the Tumor-Suppressing Impact of Myoglobin on a Subset of Human Breast Cancer Cells

The expression of myoglobin (MB), well known as the oxygen storage and transport protein of myocytes, is a novel hallmark of the luminal subtype in breast cancer patients and correlates with better prognosis. The mechanisms by which MB impacts mammary tumorigenesis are hitherto unclear. We aimed to unravel this role by using CRISPR/Cas9 technology to generate MB-deficient clones of MCF7 and SKBR3 breast cancer cell lines and subsequently characterize them by transcriptomics plus molecular and functional analyses. As main findings, loss of MB at normoxia upregulated the expression of cell cyclins and increased cell survival, while it prevented apoptosis in MCF7 cells. Additionally, MB-deficient cells were less sensitive to doxorubicin but not ionizing radiation. Under hypoxia, the loss of MB enhanced the partial epithelial to mesenchymal transition, thus, augmenting the migratory and invasive behavior of cells. Notably, in human invasive mammary ductal carcinoma tissues, MB and apoptotic marker levels were positively correlated. In addition, MB protein expression in invasive ductal carcinomas was associated with a positive prognostic value, independent of the known tumor suppressor p53. In conclusion, we provide multiple lines of evidence that endogenous MB in cancer cells by itself exerts novel tumor-suppressive roles through which it can reduce cancer malignancy.

Carboxyl Group-Modified Myoglobin Induces TNF-α-Mediated Apoptosis in Leukemia Cells

Previous studies have shown that chemical modification may increase the activity of proteins or confer novel activity to proteins. Some studies have indicated that myoglobin (Mb) is cytotoxic; however, the underlying mechanisms remain unclear. In this study, we investigated whether chemical modification of the carboxyl group by semicarbazide could promote the Mb cytotoxicity in human leukemia U937 cells and the underlying mechanism of semicarbazide-modified myoglobin (SEM-Mb)-induced U937 cell death. The semicarbazide-modified Mb (SEM-Mb) induced U937 cell apoptosis via the production of cleaved caspase-8 and t-Bid, while silencing of FADD abolished this effect. These findings suggest that SEM-Mb can induce U937 cell death by activating the death receptor-mediated pathway. The SEM-Mb inhibited miR-99a expression, leading to increased NOX4 mRNA and protein expression, which promoted SIRT3 degradation, and, in turn, induced ROS-mediated p38 MAPK phosphorylation. Activated p38 MAPK stimulated miR-29a-dependent tristetraprolin (TTP) mRNA decay. Downregulation of TTP slowed TNF-α mRNA turnover, thereby increasing TNF-α protein expression. The SEM-Mb-induced decrease in cell viability and TNF-α upregulation were alleviated by abrogating the NOX4/SIRT3/ROS/p38 MAPK axis or ectopic expression of TTP. Taken together, our results demonstrated that the NOX4/SIRT3/p38 MAPK/TTP axis induces TNF-α-mediated apoptosis in U937 cells following SEM-Mb treatment. A pathway regulating p38 MAPK-mediated TNF-α expression also explains the cytotoxicity of SEM-Mb in the human leukemia cell lines HL-60, THP-1, K562, Jurkat, and ABT-199-resistant U937. Furthermore, these findings suggest that the carboxyl group-modified Mb is a potential structural template for the generation of tumoricidal proteins.

Myoglobin: From physiological role to potential implications in cancer

Myoglobin (MB) belongs to the well-studied globin proteins superfamily. It has been extensively studied for its physiological roles in oxygen storage and transport for about a century now. However, the last two decades shed the light on unexpected aspects for MB research. Myoglobin has been suggested as a scavenger for nitric oxide and reactive oxygen species (ROS). Furthermore, MB was found to be expressed and regulated in different tissues, beyond the muscle lineage, including cancers. Current evidence suggest that MB is directly regulated by hypoxia and might be contributing to the metabolic rewiring in cancer tissues. In this article, we first discuss the MB physiological roles and then focus on the latter potential roles and regulatory networks of MB in cancer.

Expression of Myoglobin in Normal and Cancer Brain Tissues: Correlation With Hypoxia Markers

Background

Myoglobin (MB) is increasingly recognized as a key player in cancer growth and metastasis. Low oxygen tensions, commonly associated with highly aggressive and recurrent cancers, have been shown to regulate its expression in several cancers such as lung, neck, prostate and breast cancer. However, it is not yet known whether it contributes to the growth and spread of brain cancers especially Glioblastoma multiforme (GBM).

Methods

Here we investigate the expression of MB, and its correlation with the hypoxia markers carbonic anhydrase IX (CAIX) and lactate dehydrogenase A (LDHA), in human tissue microarrays of multiple organ tumors, brain tumors, and GBM tumors, and their respective cancer-adjacent normal tissues. Correlation between MB protein expression and tumor grade was also assessed.

Results

We show that MB protein is expressed in a wide variety of cancers, benign tumors, cancer-adjacent normal tissues, hyperplastic tissue samples and normal brain tissue, and low oxygen tensions modulate MB protein expression in different brain cancers, including GBM. Enhanced nuclear LDHA immune-reactivity in GBM was also observed. Finally, we report for the first time a positive correlation between MB expression and brain tumor grade.

Conclusion

Our data suggest that hypoxia regulate MB expression in different brain cancers (including GBM) and that its expression is associated with a more aggressive phenotype as indicated by the positive correlation with the brain tumor grade. Additionally, a role for nuclear LDHA in promoting aggressive tumor phenotype is also suggested based on enhanced nuclear expression which was observed only in GBM.

Lessons from the post-genomic era: Globin diversity beyond oxygen binding and transport

Vertebrate hemoglobin (Hb) and myoglobin (Mb) were among the first proteins whose structures and sequences were determined over 50 years ago. In the subsequent pregenomic period, numerous related proteins came to light in plants, invertebrates and bacteria, that shared the myoglobin fold, a signature sequence motif characteristic of a 3-on-3 α-helical sandwich. Concomitantly, eukaryote and bacterial globins with a truncated 2-on-2 α-helical fold were discovered. Genomic information over the last 20 years has dramatically expanded the list of known globins, demonstrating their existence in a limited number of archaeal genomes, a majority of bacterial genomes and an overwhelming majority of eukaryote genomes. In vertebrates, 6 additional globin types were identified, namely neuroglobin (Ngb), cytoglobin (Cygb), globin E (GbE), globin X (GbX), globin Y (GbY) and androglobin (Adgb). Furthermore, functions beyond the familiar oxygen transport and storage have been discovered within the vertebrate globin family, including NO metabolism, peroxidase activity, scavenging of free radicals, and signaling functions. The extension of the knowledge on globin functions suggests that the original roles of bacterial globins must have been enzymatic, involved in defense against NO toxicity, and perhaps also as sensors of O₂, regulating taxis away or towards high O₂ concentrations. In this review, we aimed to discuss the evolution and remarkable functional diversity of vertebrate globins with particular focus on the variety of non-canonical expression sites of mammalian globins and their according impressive variability of atypical functions.

Targeted Myoglobin Delivery as a Strategy for Enhancing the Sensitivity of Hypoxic Cancer Cells to Radiation

The effectiveness of cancer radiotherapy is frequently hindered by the hypoxia of the tumor microenvironment. Direct delivery of oxygen to hypoxic tumor tissues is an attractive strategy to overcome this hypoxia-associated radioresistance. Herein, we report the generation of submicron-sized particles comprising myoglobin fused to the crystal-forming domain of Cry3Aa protein for the targeted delivery of oxygen to cancer cells. We demonstrate that myoglobin-containing particles were successfully produced in Bacillus thuringiensis with the assistance of the Cry3Aa domain I. Furthermore, these particles could be genetically modified to incorporate the cell penetrating peptide TAT and cell targeting peptide A549.1, resulting in particles that exhibited improved cellular uptake and targeting toward A549 cells. Notably, these myoglobin-containing particles increased the intracellular oxygen levels of A549 cells and thereby sensitized them to radiation. These findings suggest that the targeted delivery of O₂-bound myoglobin could be an effective approach to enhance the efficacy of radiotherapy.

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The N-terminal domain of Cry3Aa can be used to generate sub-micron particles

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Genetic fusion of functional peptides to protein particles targets them to A549 cells

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Myoglobin is an effective oxygen carrier for delivery of O₂ into hypoxic cancer cells

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Targeted myoglobin delivery to hypoxic cancer cells increased their radiosensitivity

The role of myoglobin in epithelial cancers: Insights from transcriptomics

The muscle-associated respiratory protein myoglobin (MB) is expressed in multiple types of cancer, including breast and prostate tumors. In Kaplan-Meier analyses of the two tumor types, MB positivity is associated with favorable prognoses. Despite its well-characterized function in myocytes, the role of MB in cancer remains unclear. To study the impact of endogenous MB expression, small interfering RNA MB-knockdown cells were engineered using breast, prostate and colon cancer cell lines (MDA-MB468, LNCaP, DLD-1), and their transcriptomes were investigated using RNA-Seq at different oxygen levels. In MB-positive cells, increased expression of glycolytic genes was observed, which was possibly mediated by a higher activity of hypoxia-inducible factor 1α. In addition, the results of the gene set enrichment analysis suggested that MB contributed to fatty acid transport and turnover. MB-positive, wild-type-p53 LNCaP cells also exhibited increased expression of p53 target genes involved in cell cycle checkpoint control and prevention of cell migration. MB-positive cells expressing mutant p53 exhibited upregulation of genes associated with prolonged cancer cell viability and motility. Therefore, it was hypothesized that these transcriptomic differences may result from MB-mediated generation of nitric oxide or reactive oxygen species, thus employing established enzymatic activities of the globin. In summary, the transcriptome comparisons identified potential molecular functions of MB in carcinogenesis by highlighting the interaction of MB with key metabolic and regulatory processes.