Work described herein was funded by Cold Spring Harbor Laboratory.
Focal adhesion plaques are specialized regions of the plasma membrane through which cells in culture adhere to the external substrate (Burridge, K. et al., Ann Rev. Cell. Biol., 4:487-525 (1988); Burridge, K. and K. Fath, BioEssays, 10:104-108 (1989)). On their internal face these structures anchor actin stress fibers, which are important in determining cell shape. Similar, but less well-characterized structures have been implicated in attachment between neighboring cells and adherence to extracellular matrix in vivo. Oncogenic transformation is frequently accompanied by a less-adherent, rounded morphology resulting from reorganization of the cytoskeleton (Ben Ze""ev, A., Biochem. Biophys. Acta., 780:197-212 (1985; Felice, G. R. et al., Eur. J. Cell Biol., 52:47-49 (1990)). In Rous Sarcoma Virus (RSV) transformed cells, for instance, it has been postulated that a contributing factor is the aberrant phosphorylation by pp60v-src of tyrosyl residues in key focal adhesion proteins (Burr, J. G. et al., Proc. Natl. Acad. Sci. USA, 77:3484-3488 (1980); Parsons, J. T. and M. J. Weber, Curr. Topics in Microbiol. and Immunol., 147:79-127 (1989)). Phosphotyrosine has also been detected in focal adhesions (Maher, P. A. et al., Proc. Natl. Acad. Sci. USA, 82:6576-6580 (1985)) and apical junctions (Takata, K. and S. J. Singer, J. Cell Biol., 106:1757-1764 (1988)) in nontransformed cells, raising the possibility that tyrosine phosphorylation at these sites may regulate normal cellular function. Such phosphorylation events must be tightly controlled and an understanding of the mechanism(s) involved would be very useful in furthering our understanding of control of normal and neoplastic cell growth.
The present invention relates to DNA which encodes a protein homologous to the protein tyrosine phosphatases (PTPases) which catalyze the dephosphorylation of proteins in which tyrosyl residues have been phosphorylated through the action of a protein kinase. The protein, which appears as if it will localize to focal adhesions, is also the subject of the present invention. In particular, it relates to cDNA encoding a protein, referred to as PTPH1, which was obtained from HeLa cells and characterized. PTPH1 has also been identified in other cell types. The structure of PTPH1 includes three segments: 1) an N-terminal segment of approximately 320 residues, which shows homology with the N-terminal segments of the talin family in the region known to be important for localization to focal adhesions; 2) a central segment, in which there are sequences with the features of sites of phosphorylation by casein kinase 2 and p34cdc2, which may be important for regulation of phosphatase activity; and 3) a C-terminal segment of approximately 250 residues, which shows homology to the known members of the PTPase family. PTPH1 has a single putative catalytic domain.
Because of its homology with the talin family of proteins, which are known to participate in linkage of intracellular actin filaments to the extracellular matrix at focal adhesions, it is likely that PTPH1 localizes to the focal adhesions, which is a major site ofaction. for oncogenic protein tyrosine kinases (PTK). Thus, overexpression of PTPH1 may be a powerful approach to countering the effects of oncogenic protein tyrosine kinases, such as those of transforming viruses, and interfering with or reversing cell transformation. This would provide a means of preventing or reversing abnormally high levels of phosphotyrosine associated with any disease or condition, such as preventing or reversing malignancy associated with the activity of a protein tyrosine kinase. Such protein tyrosine kinase may be of viral origin or be a cellular protein tyrosine kinase whose normal cellular function is disrupted, resulting in abnormal phosphorylation of tyrosyl residues. Such a method of preventing or reversing malignancy caused by or associated with the activity of a protein tyrosine kinase is also the subject of the present invention. In the present method, DNA or RNA encoding PTPH1 or a functional equivalent of PTPHl is administered to an individual in an appropriate gene transfer vehicle which can infect mammalian cells and, once inside the mammalian cells, express and make available PTPH1 or its functional equivalent in sufficient quantities to overcome or counteract the protein tyrosine kinase activity. As a result, phosphorylation of tyrosine residues at abnormal levels is prevented or reversed, resulting in turn in prevention or reversal of malignancy of cells. Suitable gene transfer vehicles are those which contain DNA or RNA encoding PTPH1 or a PTPH1 functional equivalent, can infect mammalian cells and express the encoded protein within the infected mammalian cells. Such vehicles include recombinant retroviruses and recombinant vaccinia virus.
The method of the present invention is useful in treating or preventing a wide variety of conditions in which abnormally high levels of phosphotyrosine occur and particularly in treating or preventing malignancies in which tyrosyl phosphorylation by a protein tyrosine kinase occurs at an abnormal rate or level and in which dephosphorylation of tyrosyl residues by PTPH1 or its functional equivalent results in prevention or reversal of a malignant phenotype.