A role of metallothionein-3 in radiation-induced autophagy in glioma cells

Abstract

Although metallothionein-3 (MT3), a brain-enriched form of metallothioneins, has been linked to Alzheimer's disease, little is known regarding the role of MT3 in glioma. As MT3 plays a role in autophagy in astrocytes, here, we investigated its role in irradiated glioma cells. Irradiation increased autophagy flux in GL261 glioma cells as evidenced by increased levels of LC3-II but decreased levels of p62 (SQSTM1). Indicating that autophagy plays a cytoprotective role in glioma cell survival following irradiation, measures inhibiting autophagy flux at various steps decreased their clonogenic survival of irradiated GL261 as well as SF295 and U251 glioma cells. Knockdown of MT3 with siRNA in irradiated glioma cells induced arrested autophagy, and decreased cell survival. At the same time, the accumulation of labile zinc in lysosomes was markedly attenuated by MT3 knockdown. Indicating that such zinc accumulation was important in autophagy flux, chelation of zinc with tetrakis-(2pyridylmethyl)ethylenediamine (TPEN), induced arrested autophagy in and reduced survival of GL261 cells following irradiation. Suggesting a possible mechanism for arrested autophagy, MT3 knockdown and zinc chelation were found to impair lysosomal acidification. Since autophagy flux plays a cytoprotective role in irradiated glioma cells, present results suggest that MT3 and zinc may be regarded as possible therapeutic targets to sensitize glioma cells to ionizing radiation therapy. Patients with glioblastoma, the most malignant form of glioma, usually succumb within a few years after the diagnosis despite aggressive chemo and radiotherapies following surgery 1. Such disheartening therapeutic outcomes desperately call for more effective measures. However, untoward biological characteristics of glioma such as aggressive local invasion and resistance to anticancer therapy, present a daunting obstacle in achieving this goal. Dysregulation of autophagy has been implicated in cancer cell proliferation, invasion, high histological grades, and poor prognosis in a wide spectrum of cancers. These findings highlight the possibility that autophagy-related molecules may prove useful as prognostic markers and/or therapeutic targets. In addition, many of anticancer drugs along with ionizing radiation (IR) are known to activate autophagy in cancer cells. The functional significance of autophagy activation associated with cancer therapeutics is still controversial. A growing body of evidence supports the possibility that autophagy plays a self-defensive cytoprotective role against toxicity induced by anti-cancer therapies 2-4 , although opposing data favoring the cytotoxic effect of autophagy are also available 2,3. Despite these controversies, the current consensus is that the modulation of autophagy may have significant effects in anti-cancer therapeutics. Although a number of intrinsic or extrinsic pathways of autophagy have been delineated, the final destination of all these pathways is the lysosome, where degradation of cargo contents occurs. For this reas...

MiR-450a-5p strengthens the drug sensitivity of gefitinib in glioma chemotherapy via regulating autophagy by targeting EGFR

“…Therefore, in our case, we propose that the autophagy and apoptosis induced by miR-450a-5p synergistically promote cell death, which then increases the gefitinib chemosensitivity to glioma cells. It is also believed that the radiation treatment could cause protective autophagy allowing glioma cell survival [41]. With our current evidences, it's still not clear how the combined treatment of gefitinib and miR-450a-5p would affect glioma cells post radiation.…”

Role for Metallothionein-3 in the Resistance of Human U87 Glioblastoma Cells to Temozolomide

“…MT3 has been shown to play a role in the maintenance of autophagy flux both in cultured astrocytes 22 and in irradiated mouse and human-derived glioma cells. 18 Hence, MT3 could contribute to the survival of glioma cells through different routes and according to the cell type and/or the challenging insult. In the case of 1321N1 astrocytoma, MT3 silencing per se resulted into an increase in labile zinc and an increase in the basal apoptotic rate of the cells.…”

“… 12 − 15 Specifically, MT2A has been linked to chemoresistance in breast cancer 16 and osteosarcoma 17 cell lines, whereas MT3 has been associated with resistance to irradiation in both murine and human glioma cell lines. 18 …”

“… 5 , 7 Masiulionytė and colleagues have suggested that MT3 gene could be used to prognosticate a patient’s survival. 7 In this context, MT3 may potentially activate transcription factors 21 or promote autophagy 18 , 22 by acting as a zinc donor and protect against oxidative stress because of its high cysteine content. 23 However, the data on the role of MT3 in carcinogenesis are not unequivocal.…”

Drug Sequestration in Lysosomes as One of the Mechanisms of Chemoresistance of Cancer Cells and the Possibilities of Its Inhibition

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