Rapid assessment of protein-tyrosine phosphatase expression levels by RT-PCR with degenerate primers

PTP-epsilon, a tyrosine phosphatase expressed in endothelium, negatively regulates endothelial cell proliferation

The vascular endothelium is a dynamic interface between the blood vessel and circulating factors and, as such, plays a critical role in vascular events like inflammation, angiogenesis, and hemostasis. Whereas specific protein tyrosine kinases have been identified in these processes, less is known about their protein tyrosine phosphatase (PTP) counterparts. We utilized a RT-PCR/differential hybridization assay to identify PTP-epsilon as a highly abundant endothelial cell PTP. PTP-epsilon mRNA expression is growth factor responsive, suggesting a role for this enzyme in endothelial cell proliferation. Overexpression of PTP-epsilon decreases proliferation by 60% in human umbilical vein endothelial cells (HUVEC) but not in smooth muscle cells or fibroblasts. In contrast, overexpression of PTP-epsilon (D284A), a catalytically inactive mutant, has no significant effect on HUVEC proliferation. These data provide the first functional characterization of PTP-epsilon in endothelial cells and identify a novel pathway that negatively regulates endothelial cell growth. Such a pathway may have important implications in vascular development and angiogenesis.

Protein tyrosine phosphatase gene expression analysis in Swiss 3T3 fibroblasts

The aim of this study was to identify protein tyrosine phosphatases (PTPs) expressed in Swiss 3T3 fibroblasts and to examine their expression levels as well as to characterize quantitative aspects of RT-PCR based on degenerate deoxyoligonucleotides. By using an RT-PCR assay based on degenerate deoxyoligonucleotide primers, expression of mRNAs for two cytoplasmic- and six transmembrane-type PTPs in Swiss 3T3 cells was detected. The sequences of two of them are new. Among nine analyzed PTPs expressed to widely varied extends, only three have mRNA levels high enough to be seen on Northern blots with 10 microg of total RNA per lane. The frequencies with which the examined PTPs are represented among the PCR amplification products, correlate stronger with the primer fidelity, defined as the number of mismatches between the primer- and the cDNA target-sequences, rather than with the PTP expression levels. In conclusion, an RT-PCR assay based on degenerate primers can be successfully used to sample the expressed PTPs and to identify new members of this gene family. However, reliable quantification of their mRNA levels can only be achieved using the classical approaches, like Northern, RNase protection assay or nondegenerate quantitative RT-PCR.

The neuronal nitric oxide synthase PDZ motif binds to -G(D,E)XV* carboxyterminal sequences

PDZ motifs are small protein-protein interaction modules that are thought to play a role in the clustering of submembranous signalling molecules. The specificity and functional consequences of their associative actions is still largely unknown. Using two-hybrid methodology we here demonstrate that the PDZ motif of neuronal nitric oxide synthase (nNOS) can mediate the binding to several other proteins in brain. Peptide library screening showed that proteins bearing a carboxy-terminal G(D,E)XV* sequence are preferred targets for the nNOS amino-terminal PDZ motif. Potential nNOS targets include a melanoma-associated antigen, cyclophilins and the alpha1C-adrenergic receptor.

Protein-tyrosine phosphatases expressed in mouse epidermal keratinocytes

The importance of growth factors acting via receptor-type protein-tyrosine kinases in the continuous renewal of the epidermis from the keratinocyte stem cell population has been well established. Protein-tyrosine phosphatases (PTPases), which dephosphorylate phosphotyrosine-containing proteins, may therefore be expected to play an equally important role in the control of epidermal growth and differentiation. In this study, we have made an inventory of the various PTPases that are expressed during mouse keratinocyte proliferation and maturation. A panel of 13 different PTPases probes was obtained by combining a set of PTPase cDNAs previously cloned from mouse brain and a set of PTPase probes obtained from a normalized keratinocyte PTPase cDNA library. This PTPase cDNA panel, spanning probes for receptor-type as well as cytoplasmic-type family members, was used to monitor RNA expression levels in keratinocyte fractions isolated from murine epidermis and in keratinocyte cell cultures. No overt changes were observed in PTPase mRNA levels in all strata of mouse epidermis, but comparison of cultured cells with freshly isolated keratinocytes revealed several conspicuous differences. In the cultured Balb/MK cell line, absence of PTP delta expression and upregulation of PTP kappa and, to a lesser extent, PTP gamma mRNA ratios were observed compared to the freshly isolated cells. These results provide a basis for further research on the impact of PTPase activity on epidermal growth control.

Molecular cloning of a mouse epithelial protein-tyrosine phosphatase with similarities to submembranous proteins

Protein-tyrosine phosphatases (PTPases) form an important class of cell regulatory proteins. We have isolated overlapping cDNA clones that together comprise an 8 kb transcript encoding a novel murine PTPase which is expressed in various organs. Sequence analysis revealed an open reading frame of 2,460 amino acid residues. The predicted protein, PTP-BL, is a large non-transmembrane PTPase that exhibits 80% homology with PTP-BAS, a recently described human PTPase. PTP-BL shares some intriguing sequence homologies with submembranous proteins. It contains a band 4.1-like motif also present in the tumor suppressors neurofibromatosis 2 and expanded, five 80 amino acid repeats also present in the discs-large tumor suppressor, and a single catalytic phosphatase domain. No obvious homologies to other proteins were found for the N-terminal region of the protein other than human PTP-BAS. RNA in situ hybridization experiments show that the PTP-BL gene is expressed in epithelial cells, predominantly in kidney, lung, and skin. These data suggest a cell cortical localization for PTP-BL in epithelial cells and a possible role in the morphology and motility of epithelial tissues.