Abstract
Mitochondrial dysfunction is a hallmark of cancer biology. Tumor mitochondrial metabolism is characterized by an abnormal ability to function in scarce oxygen conditions through glycolysis (the Warburg effect), and accumulation of mitochondrial DNA defects are present in both hereditary neoplasia and sporadic cancers. Mitochondrial Lon is a major regulator of mitochondrial metabolism and the mitochondrial response to free radical damage, and plays an essential role in the maintenance and repair of mitochondrial DNA. Despite these critical cellular functions of Lon, very little has been reported regarding its role in glioma. Lon expression in gliomas and its relevance with patient survival was examined using published databases and human tissue sections. The effect of Lon in glioma biology was investigated through siRNA targeting Lon. We also tested the in vitro antitumor activity of Lon inhibitor, CC4, in the glioma cell lines D-54 and U-251. High Lon expression was associated with high glioma tumor grade and poor patient survival. While Lon expression was elevated in response to a variety of stimuli, Lon knockdown in glioma cell lines decreased cell viability under normal conditions, and dramatically impaired glioma cell survival under hypoxic conditions. Furthermore, the Lon inhibitor, CC4, efficiently prohibited glioma cell proliferation and synergistically enhanced the therapeutic efficacy of the chemotherapeutic agents, temozolomide (TMZ) and cisplatin. We demonstrate that Lon plays a key role in glioma cell hypoxic survival and mitochondrial respiration, and propose Lon as a promising therapeutic target in the treatment of malignant gliomas.
Highlights
Mitochondrial changes are postulated to represent a significant part of cancer cell biology
Lower Lon levels were observed in low-grade astrocytomas (LGA), suggesting that the transition between low-grade to high-grade astrocytomas might be associated with Lonmediated mitochondrial changes
Previous studies have shown that hypoxic adaptation is a prerequisite for glial tumor progression and that the degree of hypoxia correlates with tumor grade, increased risk of recurrence after surgery and clinical aggressiveness in glioma patients [27]
Summary
Mitochondrial changes are postulated to represent a significant part of cancer cell biology. Cancer cells must survive and adapt to challenging microenvironments; notably in conditions in which tumor growth makes oxygen and glucose scarce [1]. These restrictive conditions require fundamental changes in cellular metabolism, including the respiratory transition from oxidative phosphorylation to glycolysis known as the Warburg effect [2]. GBM can arise de novo, but can result from the progression of low-grade astrocytomas. This malignant transformation is mediated by high expression of the transcriptional www.impactjournals.com/oncotarget activator hypoxia-inducible factor 1α (HIF-1α), and hypoxia-induced phenotypic changes such as abnormal vascular proliferation and necrosis. Even though hundreds of genes are regulated by HIF1α, mitochondrial Lon is one of the few genes known to be directly up-regulated by HIF-1α [6]
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