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and reducing cell adhesion by modifying turnover of E-cadherin/catenin complex
[ 38 ]. Proper maintenance of E-cadherin/catenin complex is necessary to prevent
extensive cell overlap, thus maintaining cell adhesion [ 39 ]. PrP by involving in
E-cadherin mediated adherens junction (AJ) formation in A431 cells regulating
phosphorylation of catenin, thus affecting cell adhesion [ 40 ]. More importantly, PrP
expression is positively correlated with an increased risk of metastasis in colorectal
cancer. Knocking down PrP expression or treatment of the cancer stem cells with
PrP-specific monoclonal antibodies significantly reduces cancer cell metastasis via
ERK2 pathway [ 41 ]. PrP is expressed in the breast cancer cell line MCF7/Adr
which showed a reduced migration in vitro when PrP is downregulated by siRNA
[ 42 ]. Knocking down PrP expression reduces the formation of uropod and mono-
cyte adhesion but increases cell motility on endothelial cell under shear stress most
likely through RhoA-mediated pathway [ 43 ]. This observation is especially inter-
esting considering that when cancer cells migrate out of the primary site, cancer
cells must reduce adhesion but increase migration capability. The functions can be
achieved simply by PrP downregulation. Considering that KKRPK the five posi-
tively charged amino acids at the far most N-terminus of PrP has been shown to bind
glycosaminoglycans (GAGs), such as heparin or sulfated heparan [ 44 ], it is not
surprising that PrP participates in cell adhesion and migration. We recently proved
that CHO cells expressing GPI-anchored unglycosylated PrP adhere and migrate
faster than GPI-anchored glycosylated PrP and this migration depends on the inter-
actions between GAG and the N-terminal KKRPK domain of PrP because cells
expressing similar level of KKRPK truncated unglycosylated PrP migrate much
slower [ 45 ]. Unlike previous reports which did not define other binding partners in
the interaction, we proved that VEGFR2 is the receptor involved in the protein com-
plex containing PrP and GAG because activation of VEGFR2 signaling is greatly
reduced when the cells were treated with EFGR2-specific inhibitor or when
VEGFR2 was downregulated [ 45 ]. We now provide further evidence that VEGFR2
is directly involved in migration of these CHO cell. Knocking down VEGFR2 or
treating the unglycosylated PrP expressing CHO cells with VEGFR2-specific inhib-
itor significantly reduced cell migration using wound-healing assays under serum-
free condition (Fig. 13.2). In addition to regulation of migration by the GPI-anchored
PrPC, we identified an isoform of PrP in pancreatic cancer, and melanoma cells also
contribute to cell migration [ 30 , 46 ]. In this isoform, the GPI anchor peptide signal-
ing sequence (GPI-PSS) was not cleaved due to GPI anchor synthesis deficiency. To
identify the mechanism for the production of pro-PrP in most pancreatic cancer cell
lines, we found a pancreatic cancer cell line AsPC-1 expressing GPI-anchored PrPC;
profiling of GPI anchor synthesis machinery that identified several enzymes showed
a downregulated expression in BxPC-3 cells. To prove that the lower expression of
PIG F and PGAP1 in BxPC-3 cells is responsible for the production of pro-PrP, we
transfected PGAP1 alone into BxPC-3 cells, and the pro-PrP isoform was not
affected. However, when PIG F but not PIG P, another downregulated protein in
BxPC-3 cells, was co-expressed with PGAP1, the pro-PrP isoform was successfully
converted into GPI-anchored PrP and thus cleavable by phosphor-specific lipase C
(PI-PLC) [ 47 ]. The generation of pro-PrP obviously enhances the mobility of the
cancer cells due to pro-PrP retaining its GPI-PSS, which can interact with filamin A
(FLNa), a multifunctional cytoskeletal linker [ 30 , 46 ]. When comparing AsPC-1, a
X. Yang et al.