Karen J. Martell

hVH‐5: A Protein Tyrosine Phosphatase Abundant in Brain that Inactivates Mitogen‐Activated Protein Kinase

Abstract: A novel protein tyrosine phosphatase [homologue of vaccinia virus H1 phosphatase gene clone 5 (hVH‐5)] was cloned; it shared sequence similarity with a subset of protein tyrosine phosphatases that regulate mitogen‐activated protein kinase. The catalytic region of hVH‐5 was expressed as a fusion protein and was shown to hydrolyze p‐nitrophenylphosphate and inactivate mitogen‐activated protein kinase, thus proving that hVH‐5 possessed phosphatase activity. A unique proline‐rich region distinguished hVH‐5 from other closely related protein tyrosine phosphatases. Another feature that distinguished hVH‐5 from related phosphatases was that hVH‐5 was expressed predominantly in the adult brain, heart, and skeletal muscle. In addition, in situ hybridization histochemistry of mouse embryo revealed high levels of expression and a wide distribution in the central and peripheral nervous system. Some specific areas of abundant hVH‐5 expression included the olfactory bulb, retina, layers of the cerebral cortex, and cranial and spinal ganglia. hVH‐5 was induced in PC12 cells upon nerve growth factor and insulin treatment in a manner characteristic of an immediate‐early gene, suggesting a possible role in the signal transduction cascade.

A novel receptor-type protein tyrosine phosphatase is expressed during neurogenesis in the olfactory neuroepithelium

Tyrosine phosphorylation plays a central role in the control of neuronal cell development and function. Yet, few neuronal protein tyrosine phosphatases (PTPs) have been identified. We examined rat olfactory neuroepithelium for expression of novel PTPs potentially important in neuronal development and regeneration. Using the polymerase chain reaction with degenerate DNA oligomers directed to the conserved tyrosine phosphatase domain, we identified 6 novel tyrosine phosphatases. One of these, PTP NE-3, is a receptor-type PTP expressed selectively in both rat brain and olfactory neuroepithelium. In the olfactory neuroepithelium, PTP NE-3 expression is restricted to neurons and describes a novel pattern of expression with a high level in the immature neurons and a lower level in mature olfactory sensory neurons.

Metabolic, molecular genetic and toxicological aspects of the acetylation polymorphism in inbred mice

Over the past 10 years, much fascinating information has been obtained concerning the biochemistry, genetics, toxicological implications and molecular genetics of the N-acetylation polymorphism in mice. Using C57BL/6J (B6) mice as representative of rapid acetylation and A/J (A) mice as representing slow acetylation, it has been shown that the polymorphism observed in N-acetyltransferase (NAT) activity in liver also occurs in kidney, bladder, blood, and other tissues. The development of congenic acetylator mouse lines derived from B6 and A, have provided the necessary tools to study the role of the acetylation polymorphism, on either the B6 or A genetic background, free of nearly all other genetic differences between these strains. Eliminating genes which modify and complicate the differences due to the acetylator genes make the congenic lines very useful in toxicology studies, particularly those involving carcinogenesis. The molecular genetic basis of the acetylator polymorphism in B6 and A mice involves two Nat genes. Nat-1 encodes a protein termed NAT1 which is identical in rapid and slow acetylator strains. Nat-2, however, differs between rapid and slow strains by a single nucleotide change in the coding region. The corresponding NAT2 proteins differ by a single change at amino acid 99: an hydrophilic asparagine in rapid acetylator NAT2 to an hydrophobic isoleucine in NAT2 from slow acetylators. The mechanistic basis for the differences between rapid and slow acetylation in mice appears to be that NAT2 from the rapid B6 strain is 15-fold more stable at 37 degrees C and is transcribed/translated with a maximal efficiency twice that of the enzyme from slow acetylator A mice. Results discussed in this review indicate that mice provide an excellent system for studying the N-acetyltransferase polymorphism and also are useful for modelling several aspects of the human N-acetyltransferase polymorphism.

Polymorphic N-acetylation of 2-aminofluorene by cell-free colon extracts from inbred mice

The increased risk of rapid acetylator humans for the development of colorectal cancer has created interest in experimental animal models to study the relationship of N-acetyltransferase phenotype to colon cancer. Colon cytosols from inbred mouse lines were assayed for the ability to N-acetylate 2-aminofluorene to determine if the mouse model of the N-acetyltransferase polymorphism could be used to study this relationship. The results indicate that the colon acetylcoenzyme A: 2-aminofluorene-N-acetyltransferase activity parallels that of the liver. Colon activity from slow acetylator (A and B6.A) mouse lines is significantly lower than that of rapid acetylator (B6, B6.D, and A.B6) lines. p-Aminobenzoic acid N-acetyltransferase activity also differed between colon cytosols from rapid and slow acetylator strains. Isoniazid acetylation in colon and in liver did not differ between phenotypes. Northern blot analysis demonstrated the presence of mRNA for both NAT-1 and NAT-2 in mouse colon as well as in mouse liver. These results indicate that the N-acetyltransferase polymorphism is expressed in mouse colon when 2-aminofluorene or p-aminobenzoic acid is used as substrate and therefore the mouse may be a model for study of the effect of acetylator phenotype on development of colorectal cancer in humans.