Regulation of gliomagenesis and stemness through acid sensor ASIC1a.

Glioblastoma multiforme (GBM) is the most prevalent and aggressive type of adult gliomas. Despite intensive therapy including surgery, radiation, and chemotherapy, invariable tumor recurrence occurs, which suggests that glioblastoma stem cells (GSCs) render these tumors persistent. Recently, the induction of GSC differentiation has emerged as an alternative method to treat GBM, and most of the current studies aim to convert GSCs to neurons by a combination of transcriptional factors. As the tumor microenvironment is typically acidic due to increased glycolysis and consequently leads to an increased production of lactic acid in tumor cells, in the present study, the role of acid­sensing ion channel 1a (ASIC1a), an acid sensor, was explored as a tumor suppressor in gliomagenesis and stemness. The bioinformatics data from The Cancer Genome Atlas revealed that ASIC1 expression levels in GBM tumor tissues were lower than those in normal brain, and glioma patients with high ASIC1 expression had longer survival than those with low ASIC1 expression. Our immunohistochemistry data from tissue microarray revealed that ASIC1a expression was negatively associated with glioma grading. Functional studies revealed that the downregulation of ASIC1a promoted glioma cell proliferation and invasion, while upregulation of ASIC1a inhibited their proliferation and invasion. Furthermore, ASIC1a suppressed growth and proliferation of glioma cells through G1/S arrest and apoptosis induction. Mechanistically, ASIC1a negatively modulated glioma stemness via inhibition of the Notch signaling pathway and GSC markers CD133 and aldehyde dehydrogenase 1. ASIC1a is a tumor suppressor in gliomagenesis and stemness and may serve as a promising prognostic biomarker and target for GBM patients.

TRPM7 Induces Tumorigenesis and Stemness Through Notch Activation in Glioma.

We have reported that transient receptor potential melastatin-related 7 (TRPM7) regulates glioma stem cells (GSC) growth and proliferation through Notch, STAT3-ALDH1, and CD133 signaling pathways. In this study, we determined the major contributor(s) to TRPM7 mediated glioma stemness by further deciphering each individual Notch signaling. We first determined whether TRPM7 is an oncotarget in glioblastoma multiforme (GBM) using the Oncomine database. Next, we determined whether TRPM7 silencing by siRNA TRPM7 (siTRPM7) induces cell growth arrest or apoptosis to reduce glioma cell proliferation using cell cycle analysis and annexin V staining assay. We then examined the correlations between the expression of TRPM7 and Notch signaling activity as well as the expression of GSC markers CD133 and ALDH1 in GBM by downregulating TRPM7 through siTRPM7 or upregulating TRPM7 through overexpression of human TRPM7 (M7-wt). To distinguish the different function of channel and kinase domain of TRPM7, we further determined how the α-kinase-dead mutants of TRPM7 (α-kinase domain deleted/M7-DK and K1648R point mutation/M7-KR) affect Notch activities and CD133 and ALDH1 expression. Lastly, we determined the changes in TRPM7-mediated regulation of glioma cell growth/proliferation, cell cycle, and apoptosis by targeting Notch1. The Oncomine data revealed a significant increase in TRPM7 mRNA expression in anaplastic astrocytoma, diffuse astrocytoma, and GBM patients compared to that in normal brain tissues. TRPM7 silencing reduced glioma cell growth by inhibiting cell entry into S and G2/M phases and promoting cell apoptosis. TRPM7 expression in GBM cells was found to be positively correlated with Notch1 signaling activity and CD133 and ALDH1 expression; briefly, downregulation of TRPM7 by siTRPM7 decreased Notch1 signaling whereas upregulation of TRPM7 increased Notch1 signaling. Interestingly, kinase-inactive mutants (M7-DK and M7-KR) resulted in reduced activation of Notch1 signaling and decreased expression of CD133 and ALDH1 compared to that of wtTRPM7. Finally, targeting Notch1 effectively suppressed TRPM7-induced growth and proliferation of glioma cells through cell G1/S arrest and apoptotic induction. TRPM7 is responsible for sustained Notch1 signaling activation, enhanced expression of GSC markers CD133 and ALDH1, and regulation of glioma stemness, which contributes to malignant glioma cell growth and invasion.

TRPM7 Induces Mechanistic Target of Rap1b Through the Downregulation of miR-28-5p in Glioma Proliferation and Invasion.

Objectives: Our previous findings demonstrate that channel-kinase transient receptor potential (TRP) ion channel subfamily M, member 7 (TRPM7) is critical in regulating human glioma cell migration and invasion. Since microRNAs (miRNAs) participate in complex regulatory networks that may affect almost every cellular and molecular process during glioma formation and progression, we explored the role of miRNAs in human glioma progression by comparing miRNA expression profiles due to differentially expressed TRPM7. Methods: First, we performed miRNA microarray analysis to determine TRPM7's miRNA targets upon TRPM7 silencing in A172 cells and validated the miRNA microarray data using A172, U87MG, U373MG, and SNB19 cell lines by stem-loop RT-qPCRs. We next determined whether TRPM7 regulates glioma cell proliferation and migration/invasion through different functional domains by overexpressing wild-type human TRPM7 (wtTRPM7), two mutants with TRPM7's α-kinase domain deleted (Δkinase-DK), or a point mutation in the ATP binding site of the α-kinase domain (K1648R-KR). In addition, we determined the roles of miR-28-5p in glioma cell proliferation and invasion by overexpressing or under expressing miR-28-5p in vitro. Lastly, we determined whether a Ras-related small GTP-binding protein (Rap1b) is a target of miR-28-5p in glioma tumorigenesis. Results: The miRNA microarray data revealed a list of 16 downregulated and 10 upregulated miRNAs whose transcripts are significantly changed by TRPM7 knock-down. Cell invasion was significantly reduced in two TRPM7 mutants with inactive kinase domain, Δkinase, and K1648R transfected glioma cells. miR-28-5p overexpression suppressed glioma cells' proliferation and invasion, and miR-28-5p under expression led to a significant increase in glioma cell proliferation and migration/invasion compared to that of the controls. miR-28-5p suppressed glioma cell proliferation and migration by targeting Rap1b. Co-transfection of siRap1b with miR28-5p inhibitor reduced the glioma cell proliferation and invasion, caused by the latter. Conclusions: These results indicate that TRPM7's channel activity is required for glioma cell growth while the kinase domain is required for cell migration/invasion. TRPM7 regulates miR-28-5p expression, which suppresses cell proliferation and invasion in glioma cells by targeting Rap1b signaling.