Biomedical Engineering Reference
In-Depth Information
Matrigel, and increases both spontaneous and induced apoptosis.
3
Moreover,
FAP-transfected melanoma cells show suppression of the malignant pheno-
type, including cell-cycle arrest.
132
An important consideration in performing
such experiments is immune-system viability, which recently has been shown to
be more active when FAP-expressing cells are removed from tumors in a mouse
model.
133
Thus, experiments in SCID mice or in vitro might be masking the full
influence of FAP-expressing cells.
5.3.2 Distribution of FAP
FAP is generally absent fromnormal adult tissues,
134
but is upregulated in sites of
wound healing and tissue damage such as cancers, fibrosis, and inflammation
(Figure 5.1). This unique tissue distribution has made it a potential marker and
therapeutic target for cancers.
135-137
FAP expression is highly induced in human
cancers.
138
Interestingly, FAP is upregulated in reactive stromal fibroblasts of
over 90%of human epithelial tumors, but not in benign tumors.
134
Since stromal
fibroblasts are a common feature of many epithelial carcinomas, including breast,
colorectal, lung, and ovarian cancers, FAP presents a potential therapeutic target
for epithelial cancers. In vivo, FAP is absent in epithelial, mesenchymal, neural,
and lymphoid cells, non-proliferating fibroblasts, and non-malignant tumors
such as fibroadenomas.
134,139
Nevertheless, soluble FAP has been isolated from
human plasma
124,125
and bovine serum.
140
FAP is upregulated at sites of tissue
regeneration, such as during scar formation in wound healing,
134
the resorbing
tadpole tail,
141
and during mouse embryogenesis.
142
FAP expression is also sig-
nificantly induced during inflammation, such as in fibroblast-like synovial cells
present in rheumatoid arthritis and osteoarthritis.
143,144
FAP is upregulated in tissue fibrosis such as at the remodeling interface in
pulmonary fibrosis.
145
FAP expression is also greatly induced in cirrhotic livers,
and FAP levels correlate with the histological severity of liver fibrosis.
146
In
cirrhotic livers, FAP is expressed by myofibroblasts and activated hepatic stel-
late cells at the portal-parenchymal interface where tissue remodeling occurs.
119
5.3.3 Structure of FAP
FAP has an overall 52% sequence identity with DPP-4 and, like DPP-4, exists
as both a membrane-bound and soluble form and is enzymatically active as a
dimer. The crystal structure of FAP (PDB code: 1Z68)
121
reveals a general
topology similar to that of DPP-4, composed of an N-terminal eight-blade b-
propeller domain and a C-terminal a/b-hydrolase domain.
121
A small pocket
within the cavity enclosed by these domains contains the catalytic triad com-
posed of residues Ser624, Asp702, and His734. Glu203 and Glu204, from the b-
propeller domain, also contribute to increased anity for substrates. These two
glutamate residues, along with Arg123, Tyr656, and Asn704, confer transition-
state stabilization and greatly influence FAP activity.
147
Unlike the DPP-4
active site, which has an acidic pocket due to Asp663 (Figure 5.2B), FAP has an
alanine residue at the corresponding position (Ala657). This lowered surface
