We have previously shown that protein tyrosine kinases are present in the nuclei of hamster renal epithelial cells. In this study we have identified an endogenous, critically important nuclear matrix protein as a substrate for tyrosine phosphorylation by nuclear matrix tyrosine kinases (NM-PTKs) and its modulation by stilbene estrogen, diethylstilbestrol (DES). Analyses of tyrosine phosphorylation of nuclear matrix proteins by endogenous labeling revealed that NM-PTKs were able to phosphorylate the tyrosine residues of several proteins. Both amino acid analysis and immuno-precipitation of nuclear matrix proteins with the monoclonal antibody that recognizes wild-type p53 revealed that p53 was phosphorylated on its tyrosine residues by NM-PTK(s). Moreover, DES exposure resulted in the enhanced tyrosine phosphorylation of p53 to a tune of 3.6 fold. Several other proteins of the nuclear matrix were also differentially phosphorylated in response to DES exposure. The DES-mediated enhanced tyrosine phosphorylation of p53 coincided with increased cell proliferation of renal tubular epithelial cells of Syrian hamster. The results of this study suggest that p53 protein is phosphorylated at tyrosine residues and enhanced tyrosine phosphorylation of p53 by DES exposure may play a role in the proliferation of hamster renal tubular epithelial cells.

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