Ribonucleotide reductase (RNR) consists of two non-identical subunits, R1 and R2 and is one of the key
enzymes involved in
DNA biosynthesis. RNR activity is considerably higher in malignant
tumors than in normal tissues in the rat suggesting that RNR may play an important role in the pathogenesis of human
tumors. In order to obtain immunological
reagents to study the localization and level of expression of RNR in various human tissues, a synthetic
peptide containing sequences corresponding to the COOH-terminal region of the human R2 subunit was used to generate rat
monoclonal antibodies. The generated rat
monoclonal antibodies (
IgG) inhibited RNR enzymatic activity purified from murine
P388 leukemia cells. These
antibodies were used to immunohistochemically examine the distribution of RNR in a small panel of 8 malignant and 4 benign human
breast tumors. Positive immunostaining for RNR was observed in the cytoplasm of human
breast carcinoma cells in which a specific 44 kDa specific band of R2 subunit was also detected by Western blot analysis. The immunostaining was blocked by preabsorption of the antibody with an excess amount of the synthetic
peptide immunogen. In 8 of 8
breast carcinomas, positive immunostaining for the R2 subunit was observed whereas noninvolved, adjacent breast tissue showed no staining with this antibody. In addition, few of the benign breast lesions exhibited staining with this antibody. These data indicate that these
antibodies can immunohistochemically detect RNR in frozen or
formalin-fixed,
paraffin- embedded tissues and that there is a differential expression of RNR between
breast tumors and non-involved breast tissue. Immunohistochemical detection of RNR using these
antibodies may therefore be useful for the diagnosis of human
breast tumors.