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of FACT decreased the formation of metastasis and delayed arrest whereas neither the GPI nor the C-terminal domain
tumor growth, it also proved to be an excellent cytotoxic are necessary for this function. [80]
adjuvant. Thus, these data suggest that the inhibition
[63]
both of FACT and Myc, could increase the anti-tumoral We previously showed that Gas1 possesses significant
effect of Gas1. Moreover, transgenic mice that express structural homology with the glial cell-derived neurotrophic
Src under the transcriptional control of the glial fibrillary factor (GDNF) family of receptors (GFRαs). Gas1 has two
acidic protein develop hypervascularized glioblastomas domains, called D-N and D-C, which have high similarity to
with morphological and molecular characteristics of human the D2 and D3 domains of the GFRαs. These domains have
GBM. [64,65] cysteines that participate in the formation of five disulfide
bridges. [81,82] It is noteworthy to mention that Gas1 binds
On the other hand, estrogens like estradiol, induce their to RET in either the presence or the absence of GDNF.
[82]
biological effects through binding to intra-cellular hormone- Based on the above information we and other research
specific estrogen receptors (ERα and ERβ), and this binding groups showed that Gas1 inhibits the signaling pathway
produces a conformational change in the receptors, causing induced by GDNF, an aspect that we will discuss later.
the activation of their transcriptional domains. Specifically,
estradiol reduced the levels of gas1 mRNA, however it is Interestingly, it has been reported that a soluble form
not yet known whether the gas1 promoter has an estrogen of Gas1 inhibits the proliferation of mesangial cells.
response element. Disintegrin and metalloproteinase (ADAM) 10 and 17 are
[66]
responsible of cleaving the Gas1 GPI anchor. [83,84] In glioma
Little is known about the transcription factors that up- cells, ADAM17 increases the shedding of soluble VEGF
regulate gas1. For example the transcription factor Tbox5 and activates the EGFR-PI3K-AKT pathway, contributing
increases the activity of the mouse gas1 promoter. to invasiveness, and angiogenesis. For its part, ADAM10
[85]
[67]
Moreover, microarray experiments indicate that gas1 promotes glioma cell migration by cleaving the adhesion
could be a retinoic acid responsive gene. [68,69] Since both molecule N-cadherin from the cell surface. On the other
[86]
retinoic receptors and Gas1 are expressed during embryonic hand, we constructed a lentiviral vector that produce a
development, [70-74] we insinuate that retinoic acid may soluble and secretable form of Gas1 (tGas1), lacking the
induce the expression of Gas1 to promote exit from the cell GPI consensus sequence. tGas1 induces cell arrest and
cycle and initiate the differentiation process. apoptosis of GBM cells and inhibit glioma tumor growth in
vivo. [87,88] This soluble form of Gas1 acts in both autocrine
Four miRNAs have been reported to interact with the and paracrine manners. However, previous data suggests
human gas1 transcript: miR-34a-5p, -148a-3p, -130b-5p that the full form of Gas1 (with GPI) can have paracrine
and -183-5p. [75,76] Only miR-34a, derived from the 5’ arm effects, since the GPI anchor of Gas1 could be cleaved by
of the pre-miRNA sequence (miR-34a-5p), has been shown ADAM 10 and 17 in gliomas.
to downregulate the translation of Gas1 when the miRNA
interacts with nucleotides located at position 812-832 from Gas1 is expressed during the early stages of development
the 3’ untranslated region of gas1, preventing the activity in the primitive streak, somites, heart, limb, otic vesicle,
of Gas1 on the phosphatidylinositol 3-kinase (PI3K)- kidney, lung, muscle, gonads, brain and placenta. [51,70,72,73] Its
AKT dependent cell survival pathway. [77,78] In fact, the expression is fundamental during embryonic development
repression of gas1 by miR-34a-5p promotes cell survival since Gas1 knockout (K.O.) mice die immediately after
and proliferation, preventing apoptosis by reducing the birth. [89-91] The K.O. mice develop several defects including
cleavage of Caspase-3 on papillary thyroid carcinoma cell decreased cell proliferation in cerebellum, morphological
cultures. [77] alterations in the gastrointestinal tract and microform
holoprosencephaly associated with multiple craniofacial
GAS1 PROTEIN STRUCTURE AND defects. [72,89-93] The defects in Gas1-/- mice, are associated
EXPRESSION with the loss of the signaling induced by Sonic hedgehog
(Shh). Interestingly, some patients with holoproscencephaly
The nucleotide sequence of the gas1 of both the human present mutations in the Gas1 gene with or without additional
and mouse genes reveals an open reading frame of 345 mutations on the Shh gene. [93,94] During development, Gas1
and 384 amino acids, respectively. The proteins encoded has both negative and positive effects on cell proliferation,
[49]
by these genes undergo post-translational modifications for example: in the limbs, Gas1 promotes the death of the
in the endoplasmic reticulum consisting of an N-linked interdigital tissue; whereas it promotes proliferation of
[95]
glycosylation, signal peptide cleavage and addition of a granular cell progenitors in the cerebellum. [96]
glycosylphosphatidylinositol (GPI) group at the C-terminal.
The mature form of the Gas1 protein has a molecular mass Previous studies using in situ hybridization showed the
of about 37 kDa and is anchored to the outer cell membrane expression of the gas1 gene in the brain of adult mice
by a GPI molecule. [49,51,79] The region of Gas1 from amino (http://www.brain-map.org/; www.genepaint.org; www.
acid 182 to amino acid 234 is essential to induce growth stjudebgem.org). Additionally, we reported that Gas1 is

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Journal of Cancer Metastasis and Treatment ¦ Volume 2 ¦ March 11, 2016 ¦