Transcriptional Characterization of Glioma Neural Stem Cells Diva ...

6.7 Isoform Differential Expression Results
cleotide exchange factor GAPVD1, a gene fundamental for the activation
of RAB5A during the engulfment of apoptotic cells [238] (Fig 6.19).
· miR-137 is one of the most down-regulated microRNAs in glioblas-
toma compared with normal brain tissue. In our microRNA microarray
dataset, miR-137 is up-regulated in GNS cell lines with respect to NS cell
lines. Since miR-137 directly targets CDK6, if over-expressed in glioblas-
toma it would induce cell-cycle arrest. The level of miR-137 increases
upon differentiation of glioma neurosphere cultures and if over-expressed
in these cells it leads to the expression of markers consistent with neu-
ronal differentiation. These data suggest that the lower expression of
this microRNA in glioblastoma, and the higher expression in GNS cells
with respect to NS cells, reflects the lack of tumour cell differentiation
in the former and the presence of regulatory mechanisms downstream of
miR-137 in the latter, since cancer stem cells are the least differentiated
within the lineage hierarchy according to the cancer stem cell hypothe-
sis [364]. In our microRNA prediction analysis, miR-137 was predicted
to target the serine/threonine-protein kinase 40 STK40 gene, a nega-
tive regulator of NFKB and p53-mediated gene transcription [200] and
the transcription factor TCF4, which associated with β catenin mediates
Wnt signaling by trans-activating downstream target genes [11].
· miR-34a expression is down-regulated in glioblastoma and in p53-null
mutant gliomas since non p53-null mutants, expressed in many tumours
and that can possess gain-of-function activities, do not regulate tran-
scription of miR-34a. This suggests miR-34a as a transcriptional target
of P53 [278]. In our microRNA microarray dataset miR-34a is found to
be down-regulated as well. Furthermore, miR-34a potently inhibits the
protein expression of MET, a hepatocyte growth factor receptor encod-
ing a tyrosine-kinase, as well as MET 3'UTR reporter activity in glioma,
medulloblastoma cells and astrocytes. miR-34a also inhibits Notch-1
and Notch-2 protein expression and their 3'UTR reporter activities, as
well as CDK6 protein expression in glioma cells. Transient transfection
of miR-34a into brain tumour cell lines inhibited cell proliferation, cell
cycle progression, cell survival, and cell invasion but did not affect hu-
man astrocyte cell survival and cell cycle. miR-34a transfection, also,
did not affect the protein levels of PDGFRA in any tested cell line, al-
though miR-34a has predicted seed matches in the 3'UTR of PDGFRA.
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