Peroxisome Proliferator-Activated Receptors (PPARs) as Potential Inducers of Antineoplastic Effects in CNS Tumors

Ferroptosis and PPAR-gamma in the limelight of brain tumors and edema

Human malignant brain tumors such as gliomas are devastating due to the induction of cerebral edema and neurodegeneration. A major contributor to glioma-induced neurodegeneration has been identified as glutamate. Glutamate promotes cell growth and proliferation in variety of tumor types. Intriguently, glutamate is also an excitatory neurotransmitter and evokes neuronal cell death at high concentrations. Even though glutamate signaling at the receptor and its downstream effectors has been extensively investigated at the molecular level, there has been little insight into how glutamate enters the tumor microenvironment and impacts on metabolic equilibration until recently. Surprisingly, the 12 transmembrane spanning tranporter xCT (SLC7A11) appeared to be a major player in this process, mediating glutamate secretion and ferroptosis. Also, PPARγ is associated with ferroptosis in neurodegeneration, thereby destroying neurons and causing brain swelling. Although these data are intriguing, tumor-associated edema has so far been quoted as of vasogenic origin. Hence, glutamate and PPARγ biology in the process of glioma-induced brain swelling is conceptually challenging. By inhibiting xCT transporter or AMPA receptors in vivo, brain swelling and peritumoral alterations can be mitigated. This review sheds light on the role of glutamate in brain tumors presenting the conceptual challenge that xCT disruption causes ferroptosis activation in malignant brain tumors. Thus, interfering with glutamate takes center stage in forming the basis of a metabolic equilibration approach.

Recent Insights on the Role of PPAR-β/δ in Neuroinflammation and Neurodegeneration, and Its Potential Target for Therapy

Peroxisome proliferator-activated receptor (PPAR) β/δ belongs to the family of hormone and lipid-activated nuclear receptors, which are involved in metabolism of long-chain fatty acids, cholesterol, and sphingolipids. Similar to PPAR-α and PPAR-γ, PPAR-β/δ also acts as a transcription factor activated by dietary lipids and endogenous ligands, such as long-chain saturated and polyunsaturated fatty acids, and selected lipid metabolic products, such as eicosanoids, leukotrienes, lipoxins, and hydroxyeicosatetraenoic acids. Together with other PPARs, PPAR-β/δ displays transcriptional activity through interaction with retinoid X receptor (RXR). In general, PPARs have been shown to regulate cell differentiation, proliferation, and development and significantly modulate glucose, lipid metabolism, mitochondrial function, and biogenesis. PPAR-β/δ appears to play a special role in inflammatory processes and due to its proangiogenic and anti-/pro-carcinogenic properties, this receptor has been considered as a therapeutic target for treating metabolic syndrome, dyslipidemia, carcinogenesis, and diabetes. Until now, most studies were carried out in the peripheral organs, and despite of its presence in brain cells and in different brain regions, its role in neurodegeneration and neuroinflammation remains poorly understood. This review is intended to describe recent insights on the impact of PPAR-β/δ and its novel agonists on neuroinflammation and neurodegenerative disorders, including Alzheimer’s and Parkinson’s, Huntington’s diseases, multiple sclerosis, stroke, and traumatic injury. An important goal is to obtain new insights to better understand the dietary and pharmacological regulations of PPAR-β/δ and to find promising therapeutic strategies that could mitigate these neurological disorders.

Corrigendum to "PPARs in Human Neuroepithelial Tumors: PPAR Ligands as Anticancer Therapies for the Most Common Human Neuroepithelial Tumors"

[This corrects the article DOI: 10.1155/2010/427401.].

A novel treatment strategy for glioblastoma multiforme and glioma associated seizures: increasing glutamate uptake with PPARγ agonists

The established role of glutamate in the pathogenesis of glioma-associated seizures (GAS) led us to investigate a novel treatment method using an established drug class, peroxisome proliferator activated receptor (PPAR) gamma agonists. Previously, sulfasalazine has been shown to prevent release of glutamate from glioma cells and prevent GAS in rodent models. However, raising protein mediated glutamate transport via excitatory amino acid transporter 2 (EAAT2) has not been investigated previously to our knowledge. PPAR gamma agonists are known to upregulate functional EAAT2 expression in astrocytes and prevent excitotoxicity caused by glutamate excess. These agents are also known to have anti-neoplastic mechanisms. Herein we discuss and review the potential mechanisms of these drugs and highlight a novel potential treatment for GAS.

PPARgamma and RXRgamma ligands act synergistically as potent antineoplastic agents in vitro and in vivo glioma models

Glioblastoma represent the most common primary brain tumor in adults and are currently considered incurable. We investigated antiproliferative and anti-invasive mechanisms of 6-OH-11-O-hydroxyfenantrene (IIF), a retinoid X receptor ligand, and pioglitazone (PGZ), a peroxisome proliferator-activated receptor gamma activator, in three different glioblastoma cell lines. A dose-dependent reduction of tumor invasion and strong decrease of matrix metalloproteinases 2 and 9 expression was observed, especially when a combination therapy of IIF and PGZ was administered. Combined treatment also markedly reduced proliferation and induced apoptosis in all glioma cell lines tested. This was in particular accompanied by decrease of antiapoptotic proteins Bcl2 and p53, while simultaneously pro-apoptotic cytochrome c, cleaved caspase 3, Bax and Bad levels increased. These in vitro findings were further substantiated in a murine glioma model in vivo, where oral administration of PGZ and IIF resulted in significantly reduced tumor volume and proliferation. Of note, treatment with nuclear receptor ligands was not only effective when the treatment was initiated shortly after the intraparenchymal seeding of the glioma cells, but even when initiated in the last third of the observation period. Collectively, our results demonstrate the effectiveness of a combined treatment of ligands of proliferator-activated receptor and retinoid X receptor against glioblastoma.