PTEN/PI3K/Akt Pathway Regulates the Side Population Phenotype and ABCG2 Activity in Glioma Tumor Stem-like Cells

Abstract

Summary In normal brain, the side population (SP) phenotype is generated by ABC transporter activity and identifies stem cell and endothelial cell sub-populations by dye exclusion. By drug efflux, the ABCG2 transporter provides chemoresistance in stem cells and contributes to the blood brain barrier (BBB) when active in endothelial cells. We investigated the SP phenotype of mouse and human gliomas. In glioma endothelial cells, ABC transporter function is impaired, corresponding to disruption of the BBB in these tumors. By contrast, the SP phenotype is increased in non-endothelial cells that form neurospheres and are highly tumorigenic. In this cell population, Akt, but not its downstream target mTOR, regulates ABCG2 activity, and loss of PTEN increases the SP. This Akt-induced ABCG2 activation results from its transport to the plasma membrane. Temozolomide, the standard treatment of gliomas, although not an ABCG2 substrate, increases the SP in glioma cells, especially in cells missing PTEN.

c-Jun N-terminal kinase has a pivotal role in the maintenance of self-renewal and tumorigenicity in glioma stem-like cells

“…Previous studies have shown that the PI3K/Akt pathway is important for the maintenance of cancer stem-like cells in breast cancer, 41 prostate cancer, 42 medulloblastoma 43 and glioma. 37 In line with these previous observations, our data also demonstrate that the PI3K pathway is critical for stemness of glioma cells. PI3K activity was high in sphere-cultured glioma cells, and inhibition of PI3K activity with LY294002 suppressed the expression of stem cell markers.…”

“…In addition, inhibition of JNK also decreased mRNA levels of b-catenin in sphere-forming glioma cells (Supplementary Figure S4D). Taken together, these data indicate that JNK signaling is PI3K is an upstream kinase of JNK that governs the self-renewal of glioma stem-like cells As phosphoinoside 3-kinase (PI3K)/Akt pathway has been shown to regulate glioma stemness, 37 we investigated whether this pathway is linked to JNK signaling in the maintenance of stemness of glioma cells. As expected, sphere-forming glioma cells showed significantly higher levels of lipid phosphatase activity and phosphorylated Akt compared with monolayer-cultured glioma cells (Supplementary Figure S5).…”

Standard of care therapy for malignant glioma and its effect on tumor and stromal cells

“…However, the potential contribution of microglia produced NO to the tumor stem cell phenotype that has yet to be explored. It has also been shown that glioma stem-like cells express multi-drug resistance proteins such as ABCG2 and P-gp (Lu and Shervington, 2008;Bleau et al, 2009;Wang et al, 2010); hence, these cells tend to be highly resistant to cytotoxic drug therapy (Charles and Holland, 2009 (Dix et al, 1999). Signaling through CXCL12-CXCR4 is known to recruit Tregs to bone marrow and may also be involved in the recruitment of Tregs to brain tumors (Zhou et al, 2002;Grauer et al, 2007).…”

“…Clinical studies have shown that B50% of these patients die within the first 2 years of therapy (Stupp et al, 2009). Glioma stem-like cells are resistant to temozolomide partly due to elevated MGMT expression (Bleau et al, 2009). Promoter methylation has been shown to be concordant in GBM samples and matching glioma stem cells (Sciuscio et al, 2011); although the effectiveness of temozolomide is not concordant between matched samples.…”

“…Glioma stem cells express ABCG2 and temozolomide exposure can actually increase the glioma stem cell side population suggesting that treatment can increase their resistance to drug therapy. In fact, exposing these cells to temozolomide in culture results in more aggressive and treatment resistant tumors when implanted into mice (Bleau et al, 2009). There has been some work that has looked at the effect of temozolomide on a few stromal cell types.…”