Glutamate Carboxypeptidase II as a Therapeutic Target - Proteinases as Drug Targets - page 64

3.2.1 GCPII mRNA
3.2.1.1 Variants
A number of GCPII variants have been described based on RTPCR analysis of
mRNA from prostate-cancer cell lines.
The most important variant is designated PSM 0 and uses an alternative splice
donor site in the first FOLH1 exon. The resulting protein PSM 0 lacks the
intracellular and transmembrane domains of GCPII. PSM 0 mRNA is the
predominant form in normal prostate, while GCPII prevails in prostate cancer
tissue. The relative ratio of GCPII/PSM 0 mRNA in prostate tissues is termed
the 'tumor index', 20 although later studies have questioned its potential clinical
significance. 21 A putative PSM 0 protein was later isolated and characterized. It
was shown that the first 59 N-terminal amino acids are absent, i.e. that PSM 0
starts with alanine 60 of the GCPII sequence. 22 The cytoplasmic localization of
PSM 0 was confirmed, and its molecular weight on SDS PAGE was reported as
95 kDa, suggesting substantial post-translational modification of the protein
sequence. 22 Mlcochova et al. recently showed that PSM 0 protein isolated from
LNCaP cells is N-glycosylated. This indicates that the isolated protein is likely
not a product of alternative splicing (which would lack the signal sequence
necessary to drive the glycosylation process) but rather a product of endo-
proteolytic hydrolysis of GCPII. 23
Four additional splice variants have been identified: PSM-C, PSM-D, PSM-
E, and PSM-F 24 (http://www.ncbi.nlm.nih.gov). The expression of PSM-D has
been reported to increase significantly in cancer metastases. PSM-E mRNA
expression has been shown to differ between normal and cancerous prostate
tissue and correlates well with the tumor Gleason score. 21,24
3.2.1.2 mRNA Expression
GCPII is predominantly expressed in prostate, as shown by numerous RT-PCR
experiments. 17,25-27 Other tissues express at least one order of magnitude
less mRNA (liver, brain, kidney, small intestine, spleen, and testis 17,26-28 ),
as confirmed by the EST (expressed sequence tag) profiles available in the
UniGene database (http://www.ncbi.nlm.nih.gov/UniGene).
3.3 GCPII Localization
GCPII is expressed in different amounts in a number of tissues, localized in
different organelles, and modulated by factors such as hormonal levels and
different glycosylation moieties.
In this context, it is not surprising that the results of GCPII localization
studies differ greatly and depend on the methodology used. Since the antibodies
(Table 3.1) used are often not fully characterized, the results may also be com-
promised by non-specific signals and involve GCPII paralogs as well as splice
variants. It follows that the available localization studies should be accepted
with caution and always related to the experimental conditions employed.