Biomedical Engineering Reference
In-Depth Information
Human PHT2 was found to be widely expressed in various tissues, with mRNA
expression demonstrated throughout the gastrointestinal tract, with increased expres-
sion in the colon.
35
Furthermore, hPHT2 mRNA expression was shown in the brain,
colon, heart, kidney, leukocytes, liver, lung, ovary, pancreas, placenta, skeletal mus-
cle, small intestine, spleen, testis, and thymus, which was all confirmed by Southern
blot analysis.
35
Interestingly, Sakata et al. demonstrated rPHT2 mRNA expression in
lung, spleen, and thymus, with lower expression in brain, liver, adrenal gland, and
heart by RT-PCR.
21
Strong expression was also determined by in situ hybridization in
immunocytes, specifically eosinophils, macrophages, and other phagocytes. Further
confounding the elucidation of PHT2's function were observations by Sakata et al.
suggesting an intracellular localization of PHT2 in the Golgi, lysosomes, autophago-
somes, and vacuoles of HEK-293T and baby hamster kidney (BHK) cells.
21
Interest-
ingly, some cells had apparent PHT2 localization on the outer nuclear membrane.
21
Unfortunately, one cannot ascertain the functional significance of these transporters
by investigating their tissue and cellular localizations. Further, peptide transport may
also be potentially mediated by a number of other transporter proteins that exhibit
overlapping specificity for POT member substrates or play a role in mediating the
energy forces driving transport. The primary role of PepT1, PepT2, and PHT1 trans-
porters seems to be the active absorption and accumulation of peptide nutrients. The
role of PepT2 may be extended to the clearance of neuropeptides, while the role of
PHT2 remains to be elucidated. Further complicating matters is as yet unidentified
but functionally characterized basolateral transporters that facilitate the active tran-
scellular flux of these substrates. Interestingly, similar to differences in PepT1 and
PepT2 tissue localizations, the intestinal and renal basolateral transporters are also
different.
67
In addition, non-POT members may also serve as a confounding factor in
understanding peptide transport.
First described in 1994 by Dantzig et al., hPT1 shares only 16% identity and
41% similarity with the hPepT1 amino acid sequence.
13
,
14
The PT1 cDNA coding
region is approximately 2.5 kb long and encodes a 120-kDa protein consisting of
832 amino acids.
14
HPT1 was first identified as a result of a monoclonal antibody
that inhibited the transport of
-lactam antibiotics in Caco-2 cells.
14
Chinese hamster
ovary (CHO) cells subsequently transfected with HPT1 demonstrated dipeptide and
-lactam transport that was not competitively inhibited by amino acids.
68
Interest-
ingly, hPT1 function has not been fully characterized, although a rat isoform has been
shown to be regulated by dietary protein content.
69
Although the molecular, struc-
tural, and functional characterization of PT1 has yet to be completed (presumably
due to patent issues), the concept of a cadherin family member mediating peptide
transport is an important finding and is indicative of the potential overlap in substrate
affinities/selectivity between families.
Table 6.2 provides a brief summary of the molecular and functional characteristics
of the established human oligopeptide transporters and splice variants. There are other
reports of nonclassical peptide transporter behavior, but those transporters will not be
elaborated on here. In short, considerably more research is required to fully elucidate
all of the functional peptide transporter systems.
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