Targeting ferroptosis: A novel therapeutic strategy for the treatment of mitochondrial disease-related epilepsy.

BACKGROUND: Mitochondrial disease is a family of genetic disorders characterized by defects in the generation and regulation of energy. Epilepsy is a common symptom of mitochondrial disease, and in the vast majority of cases, refractory to commonly used antiepileptic drugs. Ferroptosis is a recently-described form of iron- and lipid-dependent regulated cell death associated with glutathione depletion and production of lipid peroxides by lipoxygenase enzymes. Activation of the ferroptosis pathway has been implicated in a growing number of disorders, including epilepsy. Given that ferroptosis is regulated by balancing the activities of glutathione peroxidase-4 (GPX4) and 15-lipoxygenase (15-LO), targeting these enzymes may provide a rational therapeutic strategy to modulate seizure. The clinical-stage therapeutic vatiquinone (EPI-743, α-tocotrienol quinone) was reported to reduce seizure frequency and associated morbidity in children with the mitochondrial disorder pontocerebellar hypoplasia type 6. We sought to elucidate the molecular mechanism of EPI-743 and explore the potential of targeting 15-LO to treat additional mitochondrial disease-associated epilepsies. METHODS: Primary fibroblasts and B-lymphocytes derived from patients with mitochondrial disease-associated epilepsy were cultured under standardized conditions. Ferroptosis was induced by treatment with the irreversible GPX4 inhibitor RSL3 or a combination of pharmacological glutathione depletion and excess iron. EPI-743 was co-administered and endpoints, including cell viability and 15-LO-dependent lipid oxidation, were measured. RESULTS: EPI-743 potently prevented ferroptosis in patient cells representing five distinct pediatric disease syndromes with associated epilepsy. Cytoprotection was preceded by a dose-dependent decrease in general lipid oxidation and the specific 15-LO product 15-hydroxyeicosatetraenoic acid (15-HETE). CONCLUSIONS: These findings support the continued clinical evaluation of EPI-743 as a therapeutic agent for PCH6 and other mitochondrial diseases with associated epilepsy.

Bi-allelic CCDC47 Variants Cause a Disorder Characterized by Woolly Hair, Liver Dysfunction, Dysmorphic Features, and Global Developmental Delay.

Ca2+ signaling is vital for various cellular processes including synaptic vesicle exocytosis, muscle contraction, regulation of secretion, gene transcription, and cellular proliferation. The endoplasmic reticulum (ER) is the largest intracellular Ca2+ store, and dysregulation of ER Ca2+ signaling and homeostasis contributes to the pathogenesis of various complex disorders and Mendelian disease traits. We describe four unrelated individuals with a complex multisystem disorder characterized by woolly hair, liver dysfunction, pruritus, dysmorphic features, hypotonia, and global developmental delay. Through whole-exome sequencing and family-based genomics, we identified bi-allelic variants in CCDC47 that encodes the Ca2+-binding ER transmembrane protein CCDC47. CCDC47, also known as calumin, has been shown to bind Ca2+ with low affinity and high capacity. In mice, loss of Ccdc47 leads to embryonic lethality, suggesting that Ccdc47 is essential for early development. Characterization of cells from individuals with predicted likely damaging alleles showed decreased CCDC47 mRNA expression and protein levels. In vitro cellular experiments showed decreased total ER Ca2+ storage, impaired Ca2+ signaling mediated by the IP3R Ca2+ release channel, and reduced ER Ca2+ refilling via store-operated Ca2+ entry. These results, together with the previously described role of CCDC47 in Ca2+ signaling and development, suggest that bi-allelic loss-of-function variants in CCDC47 underlie the pathogenesis of this multisystem disorder.

Vitamin E hydroquinone is an endogenous regulator of ferroptosis via redox control of 15-lipoxygenase.

Ferroptosis is a form of programmed cell death associated with inflammation, neurodegeneration, and ischemia. Vitamin E (alpha-tocopherol) has been reported to prevent ferroptosis, but the mechanism by which this occurs is controversial. To elucidate the biochemical mechanism of vitamin E activity, we systematically investigated the effects of its major vitamers and metabolites on lipid oxidation and ferroptosis in a striatal cell model. We found that a specific endogenous metabolite of vitamin E, alpha-tocopherol hydroquinone, was a dramatically more potent inhibitor of ferroptosis than its parent compound, and inhibits 15-lipoxygenase via reduction of the enzyme's non-heme iron from its active Fe3+ state to an inactive Fe2+ state. Furthermore, a non-metabolizable isosteric analog of vitamin E which retains antioxidant activity neither inhibited 15-lipoxygenase nor prevented ferroptosis. These results call into question the prevailing model that vitamin E acts predominantly as a non-specific lipophilic antioxidant. We propose that, similar to the other lipophilic vitamins A, D and K, vitamin E is instead a pro-vitamin, with its quinone/hydroquinone metabolites responsible for its anti-ferroptotic cytoprotective activity.

A human anti-IL-2 antibody that potentiates regulatory T cells by a structure-based mechanism.

Interleukin-2 (IL-2) has been shown to suppress immune pathologies by preferentially expanding regulatory T cells (Tregs). However, this therapy has been limited by off-target complications due to pathogenic cell expansion. Recent efforts have been focused on developing a more selective IL-2. It is well documented that certain anti-mouse IL-2 antibodies induce conformational changes that result in selective targeting of Tregs. We report the generation of a fully human anti-IL-2 antibody, F5111.2, that stabilizes IL-2 in a conformation that results in the preferential STAT5 phosphorylation of Tregs in vitro and selective expansion of Tregs in vivo. When complexed with human IL-2, F5111.2 induced remission of type 1 diabetes in the NOD mouse model, reduced disease severity in a model of experimental autoimmune encephalomyelitis and protected mice against xenogeneic graft-versus-host disease. These results suggest that IL-2-F5111.2 may provide an immunotherapy to treat autoimmune diseases and graft-versus-host disease.

Discovery of XL413, a potent and selective CDC7 inhibitor.

CDC7 is a serine/threonine kinase that has been shown to be required for the initiation and maintenance of DNA replication. Up-regulation of CDC7 is detected in multiple tumor cell lines, with inhibition of CDC7 resulting in cell cycle arrest. In this paper, we disclose the discovery of a potent and selective CDC7 inhibitor, XL413 (14), which was advanced into Phase 1 clinical trials. Starting from advanced lead 3, described in a preceding communication, we optimized the CDC7 potency and selectivity to demonstrate in vitro CDC7 dependent cell cycle arrest and in vivo tumor growth inhibition in a Colo-205 xenograft model.

High-content, high-throughput analysis of cell cycle perturbations induced by the HSP90 inhibitor XL888.

BACKGROUND: Many proteins that are dysregulated or mutated in cancer cells rely on the molecular chaperone HSP90 for their proper folding and activity, which has led to considerable interest in HSP90 as a cancer drug target. The diverse array of HSP90 client proteins encompasses oncogenic drivers, cell cycle components, and a variety of regulatory factors, so inhibition of HSP90 perturbs multiple cellular processes, including mitogenic signaling and cell cycle control. Although many reports have investigated HSP90 inhibition in the context of the cell cycle, no large-scale studies have examined potential correlations between cell genotype and the cell cycle phenotypes of HSP90 inhibition. METHODOLOGY/PRINCIPAL FINDINGS: To address this question, we developed a novel high-content, high-throughput cell cycle assay and profiled the effects of two distinct small molecule HSP90 inhibitors (XL888 and 17-AAG [17-allylamino-17-demethoxygeldanamycin]) in a large, genetically diverse panel of cancer cell lines. The cell cycle phenotypes of both inhibitors were strikingly similar and fell into three classes: accumulation in M-phase, G2-phase, or G1-phase. Accumulation in M-phase was the most prominent phenotype and notably, was also correlated with TP53 mutant status. We additionally observed unexpected complexity in the response of the cell cycle-associated client PLK1 to HSP90 inhibition, and we suggest that inhibitor-induced PLK1 depletion may contribute to the striking metaphase arrest phenotype seen in many of the M-arrested cell lines. CONCLUSIONS/SIGNIFICANCE: Our analysis of the cell cycle phenotypes induced by HSP90 inhibition in 25 cancer cell lines revealed that the phenotypic response was highly dependent on cellular genotype as well as on the concentration of HSP90 inhibitor and the time of treatment. M-phase arrest correlated with the presence of TP53 mutations, while G2 or G1 arrest was more commonly seen in cells bearing wt TP53. We draw upon previous literature to suggest an integrated model that accounts for these varying observations.

Inactivation of p53 function in cultured human mammary epithelial cells turns the telomere-length dependent senescence barrier from agonescence into crisis.

Cultured human mammary epithelial cells (HMEC) encounter two distinct barriers to indefinite growth. The first barrier, originally termed selection, can be overcome through loss of expression of the cyclin-dependent kinase inhibitor p16(INK4A). The resultant p16-, p53+ post-selection HMEC encounter a second barrier, termed agonescence, associated with critically shortened telomeres and widespread chromosomal aberrations. Although some cell death is present at agonescence, the majority of the population retains long-term viability. We now show that abrogation of p53 function in post-selection HMEC inactivates cell cycle checkpoints and changes the mostly viable agonescence barrier into a crisis-like barrier with massive cell death. In contrast, inactivation of p53 does not affect the ability of HMEC to overcome the first barrier. These data indicate that agonescence and crisis represent two different forms of a telomere-length dependent proliferation barrier. Altogether, our data suggest a modified model of HMEC senescence barriers. We propose that the first barrier is Rb-mediated and largely or completely independent of telomere length. This barrier is now being termed stasis, for stress-associated senescence. The second barrier (agonescence or crisis) results from ongoing telomere erosion leading to critically short telomeres and telomere dysfunction.

Secreted sulfatases Sulf1 and Sulf2 have overlapping yet essential roles in mouse neonatal survival.

BACKGROUND: Heparan sulfate proteoglycans (HSPGs) use highly sulfated polysaccharide side-chains to interact with several key growth factors and morphogens, thereby regulating their accessibility and biological activity. Various sulfotransferases and sulfatases with differing specificities control the pattern of HSPG sulfation, which is functionally critical. Among these enzymes in the mouse are two secreted 6-O-endosulfatases, Sulf1 and Sulf2, which modify HSPGs in the extracellular matrix and on the cell surface. The roles of Sulf1 and Sulf2 during normal development are not well understood. METHODS/RESULTS: To investigate the importance of Sulf1 and Sulf2 for embryonic development, we generated mice genetically deficient in these genes and assessed the phenotypes of the resulting secreted sulfatase-deficient mice. Surprisingly, despite the established crucial role of HSPG interactions during development, neither Sulf1- nor Sulf2-deficient mice showed significant developmental flaws. In contrast, mice deficient in both Sulf1and Sulf2 exhibited highly penetrant neonatal lethality. Loss of viability was associated with multiple, although subtle, developmental defects, including skeletal and renal abnormalities. CONCLUSIONS: These results show that Sulf1 and Sulf2 play overlapping yet critical roles in mouse development and are redundant and essential for neonatal survival.

p16INK4a modulates p53 in primary human mammary epithelial cells.

p16(INK4a) (p16) and p53 are tumor suppressor genes that are inactivated during carcinogenesis in many tumors. Here we show that p16 gene activity inversely modulates p53 status and function in primary human mammary epithelial cells. Reduced levels of p16 protein stabilize p53 protein through inhibition of proteolytic degradation, and this increase in p53 protein levels enhances the cellular response to radiation, represses proliferation, and transcriptionally activates downstream targets. Stabilization of p53 is mediated through the retinoblastoma/E2F/p14(ARF)/murine double minute-2 pathway. However, we have observed that p16 does not modulate p53 in fibroblasts, indicating a possible cell type-specific regulation of this pathway.

p16(INK4a) prevents centrosome dysfunction and genomic instability in primary cells.

Aneuploidy, frequently observed in premalignant lesions, disrupts gene dosage and contributes to neoplastic progression. Theodor Boveri hypothesized nearly 100 years ago that aneuploidy was due to an increase in centrosome number (multipolar mitoses) and the resultant abnormal segregation of chromosomes. We performed immunocytochemistry, quantitative immunofluorescence, karyotypic analysis, and time-lapse microscopy on primary human diploid epithelial cells and fibroblasts to better understand the mechanism involved in the production of supernumerary centrosomes (more than two microtubule nucleating bodies) to directly demonstrate that the presence of supernumerary centrosomes in genomically intact cells generates aneuploid daughter cells. We show that loss of p16(INK4a) generates supernumerary centrosomes through centriole pair splitting. Generation of supernumerary centrosomes in human diploid epithelial cells was shown to nucleate multipolar spindles and directly drive production of aneuploid daughter cells as a result of unequal segregation of the genomic material during mitosis. Finally, we demonstrate that p16(INK4a) cooperates with p21 through regulation of cyclin-dependent kinase activity to prevent centriole pair splitting. Cells with loss of p16(INK4a) activity have been found in vivo in histologically normal mammary tissue from a substantial fraction of healthy, disease-free women. Demonstration of centrosome dysfunction in cells due to loss of p16(INK4a) suggests that, under the appropriate conditions, these cells can become aneuploid. Gain or loss of genomic material (aneuploidy) may provide the necessary proproliferation and antiapoptotic mechanisms needed for the earliest stages of tumorigenesis.

Genetic and epigenetic changes in mammary epithelial cells may mimic early events in carcinogenesis.

Studies of human mammary epithelial cells from healthy individuals are providing novel insights into how early epigenetic and genetic events affect genomic integrity and fuel carcinogenesis. Key epigenetic changes, such as the hypermethylation of the p16 (INK4a) promoter sequences, create a previously unappreciated preclonal phase of tumorigenesis in which a subpopulation of mammary epithelial cells are positioned for progression to malignancy (Romanov et al. , 2001, Nature , 409:633-637; Tlsty et al. , 2001, J. Mammary Gland Biol. Neoplasia , 6:235-243). These key changes precede the clonal outgrowth of premalignant lesions and occur frequently in healthy, disease-free women. Understanding more about these early events should provide novel molecular candidates for prevention and therapy of breast cancer that target the process instead of the consequences of genomic instability. This review will highlight some of the key alterations that have been studied in human mammary epithelial cells in culture and relate them to events observed in vivo and discussed in accompanying reviews in this volume.

Methylation of p16(INK4a) promoters occurs in vivo in histologically normal human mammary epithelia.

Cultures of human mammary epithelial cells (HMECs) contain a subpopulation of variant cells with the capacity to propagate beyond an in vitro proliferation barrier. These variant HMECs, which contain hypermethylated and silenced p16(INK4a) (p16) promoters, eventually accumulate multiple chromosomal changes, many of which are similar to those detected in premalignant and malignant lesions of breast cancer. To determine the origin of these variant HMECs in culture, we used Luria-Delbrück fluctuation analysis and found that variant HMECs exist within the population before the proliferation barrier, thereby raising the possibility that variant HMECs exist in vivo before cultivation. To test this hypothesis, we examined mammary tissue from normal women for evidence of p16 promoter hypermethylation. Here we show that epithelial cells with methylation of p16 promoter sequences occur in focal patches of histologically normal mammary tissue of a substantial fraction of healthy, cancer-free women.