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Autophagy and cellular senescence mediated by Sox2 suppress malignancy of cancer cells

2013-02-06 16:25:53

PLoS One; 2013 Feb; 8(2):e57172


Yong-Yeon Cho, Dong Joon Kim, Hye Suk Lee, Chul-Ho Jeong, Eun-Jin Cho, Myong-Ok Kim, Sanguine Byun, Kun-Yeong Lee, Ke Yao, Andria Carper, Alyssa Langfald, Ann M. Bode, Zigang Dong



Introduction



Cancer cells, but not normal cells, acquire immortalization by escaping cellular senescence. Compared to normal cells, cancer cells usually require more nutrition to produce proteins and enzymes for their faster proliferation, which results in nutritional dependence of cancer cells. Survival is supported by nutrients obtained through the blood supply and by self digestion of intracellular organelles in a process referred to as autophagy.



Autophagy is a conserved catabolic process in eukaryotes that degrades long-lived proteins, organelles, and bulk cytoplasm through the lysosomal system to maintain homeostasis during starvation and normal growth control. Autophagy promotes cell survival by purging the cells of damaged organelles, toxic metabolites, and intracellular pathogens and by generating the intracellular building blocks required to maintain vital functions during nutrient-limited conditions. However, autophagy might also comprise an important strategy to treat cancer because autophagy can also promote cell death through excessive self-digestion and degradation of essential cellular constituents. Autophagy is different from apoptosis, a process that lacks autophagosomes and autolysosomes in dying cells. Notably, blockage of autophagy by knocking out beclin induces tumor development and inducing autophagy by treatment with natural compounds, such as resveratrol, suppresses cancer cell proliferation and malignancy. Autophagy is reportedly closely related with cellular senescence and inhibition of autophagy delays cellular senescence in mitotic human diploid cells. These observations indicate that autophagy might induce senescence in cancer cells because inhibition of autophagy enhances tumorigenic properties of MCF-7 cells. However, detailed mechanisms have not been clearly elucidated.



Furthermore, tamoxifen, an antagonist of the estrogen receptor in breast tissue, induces autophagy in human MCF-7 breast cancer cells mediated through sphingolipid metabolites. Notably, arsenic trioxide, imatinib (Gleevec) and rapamycin are compounds known to induce autophagic cell death in many human cancer cell lines. Importantly, ovary, breast and prostate cancers are associated with monoallelic loss of beclin1 in humans. Furthermore, induction of autophagy by beclin overexpression suppresses anchorage-independent colony growth of MCF-7 cells and proliferation by negative inhibition of p70S6 kinase.



Both classical and modernized cellular reprogramming are stressful processes that activate apoptosis and cellular senescence, which are the two primary barriers to cancer development and somatic reprogramming. The mTOR signaling pathway shares in the reprogramming of somatic cells, cellular senescence and autophagy formation. For example, well-characterized mTOR inhibitors and autophagy activators, including PP242, rapamycin and resveratrol, notably improve the speed and efficiency of iPS cell generation. Sox2, a transcription factor belonging to the HMG (High Mobility Group) superfamily, has a role in determination of cell fate, differentiation and proliferation. It is also a key regulator of embryonic stem (ES) cell self-renewal and reprogramming of terminally differentiated cells to iPS cells. Although the oncogenic potential of stem cell factors, such as Sox2, has been hypothesized to be based on the “stemness” similarity between stem cells and cancer cells, Sox2’s function in cancer cells is not clearly understood. Furthermore, recent studies have shown that the Sox2 protein level strongly corresponds with less differentiated basal cell-like breast carcinomas, and the Sox2 protein is frequently found to be down-regulated in gastric carcinomas. These types of studies have caused more controversy regarding the normal function and oncogenic potential of stem cell factors. Here, we provide evidence showing that expression of Sox2 in cancer cells induces autophagy of cancer cells by up-regulating ATG10 gene expression and inducing cellular senescence, resulting in reduced malignancy of cancer cells and inhibition of tumor growth ex vivo and in vivo.



Results



Ectopic Expression of Sox2 Induces Autophagy



Recent studies indicated that ectopic expression of Sox2 by retroviral infection into MCF-7 breast cancer cells increased both the size and number of colonies formed in soft agar. However, Sox2 is frequently down-regulated in gastric cancers and inhibits cell growth through cell cycle arrest and apoptosis. Therefore, the role of Sox2 in cancer is controversial. To explore the role of Sox2 and other iPS factors in cancer, we ectopically expressed these factors in HCT116 human colorectal cancer cells and found that Sox2, but not Nanog, Lin28 or Oct4, induced severe vacuole formation in the cytoplasm, which is an important marker of macroautophagy (Fig. 1A). We found that over 90% of infected cells formed different sized vacuoles in their cytoplasm and Western blotting and immunocytofluorescence assay results indicated that all the cells expressed the ectopic Sox2 protein (Fig. 1B). Further, we confirmed that severe vacuole formation coincided with acidic lysosomal activation in HCT116 colon cancer cells (Fig. 1C). Importantly, Sox2 overexpression induced LC3 (also known as ATG8b) foci formation, which is a key biomarker of autophagy (Fig. 1D). These results indicated that Sox2 overexpression induced autophagy.



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