Glutamate Carboxypeptidase II as a Therapeutic Target - Proteinases as Drug Targets - page 69

tissues is clearly species specific, which is relevant for the appropriate choice of
an animal model for in vivo studies of GCPII-based therapy.
3.3.2 Cellular Localization
GCPII expressed in epithelial cells (such as kidney, small bowel and prostate
epithelial cells) is located in the apical plasma membrane. N-glycosylation is
required for this subcellular targeting. Upon ligand (antibody) binding GCPII
localizes to clathrin-coated pits and recycling endosomal compartments. A
number of immunohistochemical studies have reported significant cytoplasmic
localization of GCPII in different tissues. 36,37,40,42,43
Interestingly, in the LNCaP prostate-tumor cell line, GCPII was shown to be
localized both on the plasma membrane and on the mitochondria. 52
3.4 GCPII Biology
GCPII knock-out mice were independently generated by two groups. Bacich
et al. reported that GCPII -/- mice are developmentally and behaviorally nor-
mal. 53 No major phenotype was observed, suggesting a compensatory activity
for the missing GCPII. This compensatory activity was previously described in
humans as NAALADase II (GCPIII), and expression of its mRNA in mouse
brain was later confirmed. Subsequent analysis of the knock-out animals
showed partial neuroprotection in the stroke model and experimental nerve
injury. 54 GCPII -/- cells showed growth impairment in an angiogenesis model. 55
In a separate set of experiments, Tsai et al. generated GCPII -/- animals by
deleting exons 9 and 10 of the FOLH1 gene. 56 This deletion led to embryonic
death. Tsai and coworkers performed another GCPII knock-out trial, this time
removing exons 1 and 2 (as Bacich et al. had done) using the Cre/LoxP sys-
tem. 57 When a mouse homozygous for this deletion was produced, no embryos
grew, confirming the results of the previous study by Tsai et al. At present, it is
dicult to explain these disparate results.
No specific substrate for GCPII has been identified outside the central nervous
system and brush border of the proximal small intestine. Since GCPII expression
is upregulated in prostate cancer cells and in the vasculature of a variety of solid
tumors, it is tempting to speculate about the mechanism by which GCPII
expression and/or activity confers a selective advantage to cancer cells. GCPII is
an exopeptidase that does not cleave extracellular matrix and apparently does
not shed to the serum (although some conflicting reports of GCPII detection in
human serum have been published; see above). Therefore, the putative pro-
oncogenic activity of GCPII does not seem to be related to the activity of other
metallopeptidases implicated in cancer growth and metastasis (exoskeleton
degradation, activation of membrane-tethered signaling molecules, etc.) and
might suggest a non-proteolytic function of the enzyme.
Yao et al. reported that GCPII expression gives prostate-derived cell lines a
proliferative advantage by increasing folate uptake. 58 This advantage could be
attenuated by a specific inhibitor of the enzyme, suggesting that GCPII acts as a