Michael S. Petronek

Ferroptosis is an iron-dependent type of cell death caused by excessive membrane lipid peroxidation. It remains a challenge to detect ferroptotic cells, especially in in vivo settings, hindering translational studies. Here we discover that a reducing reagent-resistant (3R) dimer of the transferrin receptor 1 (TfR1) is a highly specific indicator for lipid peroxidation. The 3R dimer is elevated in cells when ferroptosis is induced, either by pharmacological agents or by genetic perturbations. Furthermore, the 3R dimer can be used to detect disease-associated ferroptosis in tissue and in extracellular vesicles isolated from mouse or patient plasma. Given the near-ubiquitous expression of TfR1, we posit that the 3R dimer may function as a ferroptosis marker with widespread potential utility for basic and translational studies.

Sirtuins 1 & 2 (SIRTs 1 & 2) are deacetylases that are overexpressed in several types of cancer, thereby contributing to various hallmarks of cancer, such as increased proliferation and chemoresistance. Therefore, these enzymes represent potential therapeutic targets. Sirtinol is a small-molecule pharmacological inhibitor of SIRT1 and SIRT2 that shows promise as an anti-tumor drug in several types of cancer. Treatment with Sirtinol reduces tumor proliferation and invasiveness and enhances the cytotoxic effects of other targeted therapies. In addition to its enzyme-inhibitory activity, sirtinol has iron-chelating properties that may contribute to its cytotoxicity in tumor cells by disrupting iron metabolism and inducing oxidative stress. However, the detailed mechanism(s) by which Sirtinol may cause iron-induced oxidative stress to kill tumor cells remain unknown. Furthermore, Sirtinol’s anti-cancer activities and the mechanism(s) by which it induces cytotoxicity have not been extensively evaluated. This review focuses on the following topics: 1) the role of SIRTs 1 & 2 as tumor promoters, providing a rationale for developing inhibitors like sirtinol for these enzymes; 2) examine the connection between SIRTs 1 and 2 and iron metabolism; 3) we examine sirtinol’s anti-tumor properties in relation to its ability to function as a tridentate iron chelator.

Background: Glioblastoma (GBM) represents the most common and aggressive central nervous system malignancy where overall survival is a dismal 14.6 months despite an aggressive standard of care treatment regimen that includes maximally safe surgical resection, radiotherapy, and temozolomide. It has recently been established that GBM tumors have increased transferrin receptor expression and iron content which corresponds with worse patient prognosis. This has led to the hypothesis that iron metabolism is a viable therapeutic target. An emergent therapeutic strategy to target GBM iron metabolism is gallium maltolate (GaM), which is an orally bioavailable form of Ga3+ that functions as a redox inactive iron mimic to disrupt iron metabolism. As mitochondrial iron metabolism serves as a central connection point between iron and DNA metabolism, the goal of this study was to determine if GaM could disrupt this system. Methods: The effects of GaM on GBM cells were interrogated using conventional techniques including colony formation assays to assess clonogenicity, alkaline comet assays to evaluate DNA damage induction, and Mito-FerroGreen analysis to evaluate mitochondrial iron content. The effects of GaM on tumor growth in vivo were evaluated using a subcutaneous flank xenograft model system to determine the induction of DNA damage and an orthtopic model system to evaluate the combinatorial effects of GaM, radiation, and temozolomide on murine survival. Results: This study uncovered that maintenance of high mitochondrial iron content corresponds with increased proliferative capacity in GBM cells. This study also shows that GaM can deplete mitochondrial iron to induce DNA damage both in vitro and in a subcutaneous xenograft model. Furthermore, GaM enhances the efficacy of radiation and temozolomide when delivered concomitant and adjuvantly in an orthotopic GBM model system. Conclusion: These results suggest that GaM warrants further consideration for use in combination with standard of care GBM therapy.

Radiation therapy is a standard of care treatment for patients with glioblastoma. However, patients’ survival rate is dismal, with nearly all patients experiencing disease progression after treatment. Enriched iron content associated with increased transferrin receptor (TfR) expression is an indicator of poor glioblastoma patient outcomes; however, the underlying contributions to tumor progression remain elusive. The goal of this present study is to understand how iron metabolism in glioma contributes to radiation-induced glioblastoma cell motility. U251 and a doxycycline-inducible ferritin heavy chain overexpressing U251 (U251 FtH⁺) cell line were used. For in vitro studies, cells were irradiated with 2 Gy using a ³⁷Cs source, and after 72 h, atomic force microscopy (AFM) nanoindentation was employed to assess changes in cell stiffness following irradiation. Cell motility was studied using temporal confocal microscopy. For in vivo studies, U251 cells were grown in the rear flanks of female nude athymic mice, and the tumor was irradiated with five fractions of 2 Gy (10 Gy). The tumors were then imaged using a GE 7T small animal MRI to assess changes in T2* MRI, and colorimetric analysis of labile iron was performed using ferrozine. Following irradiation, a biomechanical shift characterized by decreased cell stiffness along with increased cell motility occurred in U251 cells, which corresponded to increased TfR expression. FtH overexpression completely reversed the enhanced cell motility following irradiation. Irradiation of U251 tumors induced the same iron metabolic shift. Interestingly, the change in labile iron in U251 tumors corresponded with an increase in T2* relaxation times, suggesting that T2* mapping may serve as a surrogate marker for assessing radiation-induced changes in iron metabolism.

Background/Objectives: Radiation therapy can often lead to structural and functional changes in the brain resulting in radiation-induced brain injury. This study investigates the MRI-detectable effects of whole-brain irradiation across all neuroanatomical structures in adult mice, with a specific focus on T2* MRI measurements, to evaluate regions that may be particularly sensitive to iron accumulation. Methods: One year following irradiation or sham treatment, mice were imaged with a 7T MRI to evaluate changes in regional volume and T2* relaxation times across more than 652 neuroanatomical using the DSURQE mouse brain atlas. Results: Statistical analysis identified 301 altered regions with respect to regional volume and 85 regions with respect to T2* relaxation showing significant differences relative to the control group (p < 0.05). Further data refinement, including the consolidation of redundant, bi-lateral structures revealed 18 subregions with significant changes in both volume and T2*. The data refinement revealed that the most represented system was the olfactory system (8/18 regions, 44%). The olfactory regions also showed the most pronounced changes and greatest correlation between the two metrics. Conclusions: These findings are suggestive that ionizing radiation may cause a pronounced disruption in the olfactory system that coincides with potential iron accumulation.

Glioblastoma tumors are the most common and aggressive adult central nervous system malignancy. Nearly all patients experience disease progression, which significantly contributes to disease mortality. Recently, it has been suggested that recurrent tumors may be characterized by a ferroptosis-prone phenotype with a significant decrease in glutathione peroxidase 4 (GPx4) expression. This led to the hypothesis that GPx4 expression negatively influences GBM cell growth. This study utilizes a doxycycline inducible GPx4 overexpression model to test this hypothesis. Consistently, the overexpression of GPx4 significantly impairs cell growth and colony formation while also causing an accumulation of cells in G1/G0 phase of the cell cycle. From a biophysical perspective, GPx4 overexpressing cells have significantly greater surface area, increased Young’s modulus, and experience anomalous sub-diffusion as opposed to normal diffusion associated with Brownian motion. Moreover, analysis of patient derived GBM cells reveal that cell growth rates, plating efficiency, and Young’s modulus are all inversely proportional to GPx4 expression. Therefore, GPx4 appears to be a biophysical regulator of GBM cell growth that warrants further mechanistic investigation in its role in GBM progression.