Dipeptidyl Peptidases: Substrates and Therapeutic Targeting in Human Health and Disease - Proteinases as Drug Targets - page 22

intact SDF-1 is functioning as a potent skin-specific chemoattractant for
infiltrating Sezary syndrome T cells. 228 Targeted restoration of DP4 expression
on the skin surface may help control this syndrome. A detailed review on the
proteolytic processing of chemokines by DP4 and other proteases can be found
in Wolf et al. 229
DP8 cleavage of chemokines has also been demonstrated in vitro (Table 1.2).
Screening of 27 chemokines for DP8 cleavage identified, in preference, SDF1-
a/b4IP-10 44 I-TAC as in vitro DP8 substrates. 192 DP8 cleavage had a
relatively lower turnover than that for DP4. 192 It can be hypothesized that DP9
will display a similar ability to cleave these chemokines. Future studies directly
comparing the catalytic eciencies of DP8/9 cleavage of chemokines will be of
interest to reveal if subtle differences exist between these two. It should be noted
that not all peptidic/protein-based molecules with a proline in P2 are substrates
of DP4 or DP8/9. Seven out of 16 chemokines containing a proline residue in
the penultimate position failed to be cleaved by either DP4 or DP8. 192 Addi-
tionally, many natural cytokines including interleukin(IL)-1b, IL-2, IL-3, and
IL-6, tumor necrosis factor-b, granulocyte macrophage colony-stimulating
factor (GM-CSF), and granulocyte colony-stimulating factor (G-CSF), have
an evolutionary conserved proline residue within the second position of their
N-termini. However, despite this feature making them ideal candidates as
DP4 substrates, no cleavage by DP4 is observed against full-length intact forms
of these molecules, 230 suggesting that their N-terminus is inaccessible in the

intact proteins.
Recently, the first natural substrate of DP9, the renal ubiquitous (RU)1 (34-42)
antigenic peptide, was identified by Geiss-Friedlander et al. 202 (Table 1.2).
RU1 (34-42) is produced in the cytoplasm by proteasome activity, then enters the
ER, and then associates with MHC class I molecules for presentation on the
cell surface where it is recognized by cytolytic T lymphocytes. 231,232 Following
the in vitro confirmation of both DP8 and DP9 cleavage of RU1 (34-42) , Geiss-
Friedlander et al. used siRNA to silence DP8 and DP9, demonstrating the
increased presentation of RU1 (34-42) antigen on the cell surface following loss
of active DP9 expression. 202 This finding points towards a possible role for DP9
in the regulation of MHC class I antigen presentation that may have important
consequences in immune-related disorders. A possible role for DP8 in this
process involving different antigenic peptides cannot be excluded at this stage.

1.6.5 Tissue Repair and Cancer
Both DP4 and FAP are known to play conflicting roles in cancer and appear to
be of importance in the development of fibrinolytic disorders and fibrosis.
Recent identification of a 2 AP as a likely physiological substrate for FAP is in
keeping with the pro-fibronyltic role of this enzyme and its importance in the
development of fibrogenic disorders. As previously discussed, soluble FAP
cleaves the Pro12-Asn13 bond of the 464 residue human a 2 -AP with Met at its
N-termini (Met-a 2 AP) (MEPLGRQLTSGP-NQEQVSPLT . . .) to yield the