The DLC Suppressor Genes - Therapeutic Strategies in Cancer Biology and Pathology - page 154

Effects of Re-expression of DLC1
The re-expression of DLC1 alters cell morphology, cell proliferation and migra-
tion and apparently also restores the non-metastatic phenotype. With the unequivo-
cal demonstration that DLC genes are silenced by promoter hypermethylation,
demethylation has provided a viable means to obtaining re-expression of the
genes. Flavones have been tested and shown to restore the expression of DLC1
with attendant inhibitory effects on cell motility (Herzog et al., 2004; Ullmannova
and Popescu, 2007). The expression of numerous genes is altered two- to three-
fold by flavone (Herzog et al., 2004) and it would be an onerous responsibility to
attribute a major influential role to DLC1, a proverbial needle in a haystack exer-
cise. Ullmannova and Popescu (2007) again found that several genes were upreg-
ulated and DLC1 among them. Flavone enhanced the expression of DLC1 two- to
sixfold in breast cancer cell lines and around fivefold in HT-29 colon carcinoma
cell lines. In parallel they noticed the induction of apoptosis and cell cycle arrest at
G2-M accompanied by an increase in p21 waf/cip1 , which could have arrested the cell
cycle. The G2-M arrest could also been an effect of downregulation of Stathmin.
Ullmannova and Popescu (2007) found that it was downregulated in response to fla-
vone. Stathmin can sequester p53 which functions as a G2-M checkpoint regulator
(Cajone and Sherbet, 1999) and its downregulation would restore the normal p53
checkpoint function at G2-M transition.
Gene or cDNA transfection is another favourite ploy employed in such studies.
DLC1 expression is not detected in Raji and Daudi cell lines of Burkitt's lymphoma.
Upon re-expression, however, these cells displayed inhibition of growth and cell
migration, and induction of apoptosis (Feng et al., 2011b). Transfection of DLC1
cDNA into HCC cells and into metastatic cell lines induced apoptosis, and inhibited
growth, invasion and tumorigenesis (Goodison et al., 2005; Zhou et al., 2004). In
the study with HCC, restoration of DLC1 reduced tumour size but did not prevent
tumour formation. Inexplicably there seems to have been no attempt there to look
at pulmonary metastases. In a subsequent study, this group of workers showed that
re-expression of DLC1 did indeed reduce pulmonary metastasis in vivo , as assayed
by an unduly sophisticated method (Goodison et al., 2005). Kim et al. (2007)
employed the HCC cell line SNU-368 and transfected them with DLC1. The gene
transfer resulted in the inhibition of cell proliferation and increase in sub-G1 popula-
tion. Forced expression of DLC1 also inhibited cell migration which was apparently
linked to the function of membrane adhesive proteins, such as FAK and paxillin.
Apart from this, DLC1 can influence intercellular signalling involving
E-cadherin, a well-established suppressor of cell motility and a suppressor of aggres-
sive behaviour of tumours, has been shown to involve small GTPases. E-cadherin is
believed to activate them and they then regulate the effects of E-cadherin on intercel-
lular adhesion (Watanabe et al., 2009). Tripathi et al. (2013) have shown recently
that DLC1 induces the expression of E-cadherin by virtue of its Rho-GAP func-
tion and suppression invasion. E-cadherin is also known to activate other routes in
regulating intercellular adhesion. In intestinal epithelial cells, it can downregulate