Young Woo Sohn

EGFR-AKT-SMAD signaling promotes formation of glioma stem-like cells and tumor angiogenesis by ID3-driven cytokine induction

Aberrant activation of receptor tyrosine kinases (RTK) is causally linked to the pathobiological traits of glioblastoma and genesis of glioma stem-like cells (GSC), but the underlying mechanism is still unknown. Here, we show that epidermal growth factor receptor (EGFR) signaling regulates the proliferation, angiogenesis, and acquisition of GSC characteristics by inducing inhibitor of differentiation 3 (ID3) and ID3-regulated cytokines [GRO1 and interleukins (IL)-6 and 8] induction. We found that EGFR-mediated ID3 expression was regulated by Smad5, which was directly phosphorylated by AKT. Furthermore, ID3 alone imparted GSC features to primary astrocytes derived from Ink4a/Arf-deficient mouse, and EGFR-ID3-IL-6 signaling axis gave rise to tumor cell heterogeneity. Conversely, EGFR inhibitors suppressed EGFR-AKT-Smad5-driven induction of ID3, which led to a decrease in the tumorsphere forming ability of GSCs and U87MG cells that possess an active mutant EGFR, EGFRvIII, without obvious cytotoxic effects. However, these cells seemed to regain colonogenic ability after removal of the EGFR inhibitors. Together, the results delineate a novel integrative molecular mechanism in which the RTK-ID signaling pathway governs genesis and maintenance of GBM histopathologic features, such as GSCs-based tumor initiation, progression, and angiogenesis.

Telomerase activity-independent function of TERT allows glioma cells to attain cancer stem cell characteristics by inducing EGFR expression

Telomerase reverse transcriptase (TERT), the catalytic subunit of the enzyme telomerase, is robustly expressed in cancer cells. TERT enables cells to avoid chromosome shortening during repeated replication by maintaining telomere length. However, several lines of evidence indicate that many cancer cells exhibit shorter telomere length than normal tissues, implying an additional function of TERT in tumor formation and progression. Here, we report a telomerase activity-independent function of TERT that induces cancer stemness in glioma cells. Overexpression of TERT712, a telomerase activity-deficient form of TERT, in U87MG cells promoted cell self-renewal in vitro, and induced EGFR expression and formation of gliomas exhibiting cellular heterogeneity in vivo. In patients with glioblastoma multiforme, TERT expression showed a high correlation with EGFR expression, which is closely linked to the stemness gene signature. Induction of differentiation and TERT-knockdown in glioma stem cells led to a marked reduction in EGFR expression, cancer stemness, and anticancer drug resistance. Together, our findings indicate that TERT plays a crucial role in tumor progression by promoting cancer stemness through expression of EGFR.

Human telomerase catalytic subunit (hTERT) suppresses p53-mediated anti-apoptotic response via induction of basic fibroblast growth factor.

Although human telomerase catalytic subunit (TERT) has several cellular functions including telomere homeostasis, genomic stability, cell proliferation, and tumorigenesis, the molecular mechanism underlying anti-apoptosis regulated by TERT remains to be elucidated. Here, we show that ectopic expression of TERT in spontaneously immortalized human fetal fibroblast (HFFS) cells, which are a telomerase- and p53-positive, leads to increases of cell proliferation and transformation, as well as a resistance to DNA damage response and inactivation of p53 function. We found that TERT and a mutant TERT (no telomerase activity) induce expression of basic fibroblast growth factor (bFGF), and ectopic expression of bFGF also allows cells to be resistant to DNA-damaging response and to suppress activation of p53 function under DNA-damaging induction. Furthermore, loss of TERT or bFGF markedly increases a p53 activity and DNA-damage sensitivity in HFFS, HeLa and U87MG cells. Therefore, our findings indicate that a novel TERT-bFGF axis accelerates the inactivation of p53 and consequent increase of resistance to DNA-damage response.

Blockade of EGFR signaling promotes glioma stem-like cell invasiveness by abolishing ID3-mediated inhibition of p27KIP1 and MMP3 expression

Aberrant epidermal growth factor receptor (EGFR) signaling is a typical oncogenic signature in glioblastoma. Here, we show that EGFR inhibition in primary glioma stem cells (GSCs) with oncogenic EGFRvIII and EGFRvIII-transduced glioma stem-like cells promotes invasion by decreasing ID3 levels. ID3 suppresses GSC invasiveness by inhibiting p27(KIP1)-RhoA-dependent migration and MMP3 expression. Xenograft and human glioblastoma specimens show that ID3 localizes within glioblastoma cores, whereas p27(KIP1) and MMP3 are predominantly expressed in glioma cells in invasive fronts. Together, our findings show that EGFR inhibition induces GSC invasiveness by abolishing ID3-mediated inhibition of p27(KIP1) and MMP3 expression.

Tumoral RANKL activates astrocytes that promote glioma cell invasion through cytokine signaling.

The invasiveness of glioblastoma is a major cause of poor prognosis and relapse. However, the molecular mechanism controlling glioma cell invasion is poorly understood. Here, we report that receptor activator of nuclear factor kappa-B (NFκB) ligand (RANKL) promotes glioma cell invasion in vivo, but not in vitro. Unlike the invasiveness under in vitro culture conditions, in vivo xenograft studies revealed that LN229 cells expressing high endogenous RANKL generated more invasive tumors than U87MG cells expressing relatively low endogenous RANKL. Consistently, RANKL-overexpressing U87MG resulted in invasive tumors, whereas RANKL-depleted LN229 generated rarely invasive tumors. We found that the number of activated astrocytes was markedly increased in the periphery of RANKL-high invasive tumors. RANKL activated astrocytes through NFκB signaling and these astrocytes in turn secreted various factors which regulate glioma cell invasion. Among them, transforming growth factor β (TGF-β) signaling was markedly increased in glioblastoma specimens and xenograft tumors expressing high levels of RANKL. These results indicate that RANKL contributes to glioma invasion by modulating the peripheral microenvironment of the tumor, and that targeting RANKL signaling has important implications for the prevention of highly invasive glioblastoma.

The ID1-CULLIN3 Axis Regulates Intracellular SHH and WNT Signaling in Glioblastoma Stem Cells.

Inhibitor of differentiation 1 (ID1) is highly expressed in glioblastoma stem cells (GSCs). However, the regulatory mechanism responsible for its role in GSCs is poorly understood. Here, we report that ID1 activates GSC proliferation, self-renewal, and tumorigenicity by suppressing CULLIN3 ubiquitin ligase. ID1 induces cell proliferation through increase of CYCLIN E, a target molecule of CULLIN3. ID1 overexpression or CULLIN3 knockdown confers GSC features and tumorigenicity to murine Ink4a/Arf-deficient astrocytes. Proteomics analysis revealed that CULLIN3 interacts with GLI2 and DVL2 and induces their degradation via ubiquitination. Consistent with ID1 knockdown or CULLIN3 overexpression in human GSCs, pharmacologically combined control of GLI2 and β-CATENIN effectively diminishes GSC properties. A ID1-high/CULLIN3-low expression signature correlates with a poor patient prognosis, supporting the clinical relevance of this signaling axis. Taken together, a loss of CULLIN3 represents a common signaling node for controlling the activity of intracellular WNT and SHH signaling pathways mediated by ID1.

LIM domain only 2 induces glioma invasion via cytosolic p27 KIP1

High-grade gliomas are considered the most malignant of brain tumors and have a poor prognosis. In a previous study, we showed that LIM domain only 2 (LMO2) regulates glioma stem cell properties and tumor angiogenesis and gave rise to highly invasive glioma xenografts. Glioma invasion in the surrounding parenchymal tissues is a major hurdle with respect to eliminating glioma by surgery. Invasive glioma cells are considered one of the main culprits for the recurrence of tumors after therapies. In the current study, we focused on determining the molecular mechanism(s) by which LMO2 regulates glioma cell migration and invasion. Forced expression of LMO2 in human U87MG glioma cells led to glioma invasion, as determined by in vivo xenograft assays and enhanced in vitro migration and invasion. LMO2 was associated with increased levels of cytosolic p27Kip1 protein. LMO2 possibly induced the stabilization and augmented interactions between p27Kip1 and RhoA. We knocked down the expression of p27Kip1, which led to a decrease in LMO2-driven glioma cell migration and invasion. Taken together, our findings indicate that LMO2 promotes glioma cell migration and invasion by increasing the levels of cytosolic p27Kip1.